Brakes
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brakes.htm-53A
***Broken brake switch?,......see article on brake switches-38E
WARNING!!
NEVER, EVER put pressure on brake discs when changing tires or doing other wheel work! Support the wheel with, perhaps, pieces of 2 x 4 lumber under the tire/rim. Keep the disc(s) OFF the ground! If you do not heed this warning, you MAY warp the disc/carrier. Some folks have old oil drums, one end cut out, set vertically, for use as a fixture when working on disc wheels.
Brake bleeding should be considered a NORMAL YEARLY ""MUST DO"" maintenance item. It is easy, and hardly has any problems, if you do it right! Failure to annually (or at least every other year at worst case) perform a full system bleeding, in which all of the old discolored fluid is properly bled out of the system and replaced by fresh fluid, will eventually cost you a lot of money, and can cause an accident if bad enough. Once things start to get bad due to neglect, they tend to get worse and worse at an increasing rate. If you let things go REALLY a long time, then failure to change the fluid and bleed the brakes may well eventually cause the brakes to bind, seize up, and MIGHT toss you over the handlebars at a very inopportune time (I think ANY time is bad for that!!). See #9 below
In some instances, you might find your brake pistons jammed; or, partially
so. More subtle, and seen more often is to have them not
retract as well as they should, and that can let the pads be in
contact with the discs with too much pressure when they should
hardly be touching. There are usually two causes for
this. One is the O-ring that is around the piston and seals
the fluid from escaping. That O-ring, by deformation (designed to do that), is what is supposed to help retract the
pistons ever so slightly, once you release the brake
pressure. These O-rings have been known to get hard with
age....but it is the corrosion in the groove that is the primary
problem. Water eventually accumulates from absorption, to
the extent that the fluid contains a fair percentage of moisture,
and with the corrosion, the O-ring and piston will have sticky
problems. Another problem with old fluid...or just accumulated
carbon and rubber residue... is that they can plug up the small return
hole in the master cylinder, thus the caliper part of the system
will retain pressure, and the pistons will not retract
fully. See #9 below
Longevity of HOSES:
Although this particular problem used to be very rare, it is not all that rare nowadays. The BMW stock
hoses are VERY good and LAST for quite a few DECADES; ....IF.......
you do not
hang them by the
calipers. That BAD practice CAN excessively bend the ends
of the hoses INTERNALLY at the fittings. Internal hidden damage is
possible. What
happens is that the small diameter internal stiff plastic tube
(yep, the rubber hose covers that internal tube) kinks, and
produces a partial one-way flap.
There is a LOT of pressure from the master cylinder when using
hand or foot pressure. That pressure needs to relieve
itself when the lever is released...by a relatively small amount of fluid
coming back to the master cylinder. AFTER the major part of that
pressure is relieved (one hopes, by releasing the lever), the
final bit of pressure relief effect is from the caliper piston's O-ring
deformation, now pushing the piston back a really tiny amount. If
a flap or kink develops in the inner tube of the hose, the system
might NOT relieve pressure partially or completely. Even a partial
non-release will cause problems, the least of
which is, after awhile, a change in surface of the pads, and
thereby some squealing and possibly poor pad friction. The worst
situation would probably be enough heat
from enough pad friction to allow the possibility of brake fluid boiling (especially
if the fluid is old!), causing bubbles, causing
NO BRAKES!!!
Stainless Steel braided lines are NOT
generally any better overall and
potentially FAR WORSE, and I recommend you do not willy-nilly
replace stock hoses, no matter what type. There are exceptions, but that is a
good general rule. I discuss SS hoses in more
depth later in this article.
Brake fluids:
Use ONLY DOT3 or the slightly better DOT4.
Keep it OFF the paintwork, and keep a VERY WET RAG on your
paintwork when working with it. If, HORRORS!.. it gets on
the paintwork, wash it off with
water, INSTANTLY...that means RIGHT NOW! You do
NOT have 10 seconds! ALWAYS
have a WET RAG instantly available when using brake
fluid!
Don't use
"race" fluids...they require changing once or twice
EVERY year without fail, and offer NO advantages to a street
bike. There are so-called
'premium' DOT4 fluids; some are called RACE fluids; and there is
also, confusingly, a DOT 5.1 that is NOT silicone based and hence is
usable...but WHY?
I recommend inexpensive DOT3 for most folks who want the cheapest, and who do change fluids EVERY year and are not overly hard on brakes, with DOT4 for those who are hard on the brakes, getting them VERY hot. It IS BEST to change brake fluid yearly. I personally do it as part of my pre-Winter service. For longest brake system life, your brake fluid really needs yearly changing, as it attracts moisture, right through the non-leaking lines, molecular sized holes in everything....especially the master cylinder bolt and screw fastenings, etc. Bleed the brakes until clear fluid comes out. Best to use a fresh 8 ounce can each time; but not as much of a must as some say...just don't leave the cap off for appreciable amount of time.
If you regularly change fluid by the full bleeding method, until fluid is nice and clear, then you are UNlikely to EVER have to replace the master cylinder or calipers or brake hoses. If you DO open a system, for such as rebuilding a caliper, or changing a hose, use only brake fluid or specific brake cleaner, for cleaning parts. For very serious caliper overhauls, etc., you COULD use other solvents, heck, even detergent and water...but after drying, any reassembly means using brake fluid for lubrication of parts during assembly, ETC.
In MY opinion, the absolute limit for brake fluid changing for a bike with very MINimal use, is 18 months. Those who ride in a lot of humid weather, or who ride a lot; if the bike is parked in the sunlight, etc., might even consider 6 month fluid changes. Fluid is VERY cheap compared to new parts!
NEVER
use DOT 5 silicone fluid in your BMW braking system...NEVER!
DOT5 silicone fluid does not absorb moisture, so it allows moisture to
condense into droplets in your braking system and thereby probably rotting it
out faster; and it can, in freezing weather, FREEZE the brakes! In most
situations yearly bleeding will NOT get trapped moisture out of a silicon fluid system. Silicon fluids are also
generally not
compatible with the rubber parts in your bike's braking system (depending on year
of seal manufacture, and if Brembo or ATE). DOT5 silicone fluid has no advantage for
airheads. Silicone fluid is thin, so you MIGHT
think that it is easier to bleed on an opened system...but, the
truth is that silicone brake fluids tend to get millions of tiny bubbles, defeating that
idea; and, won't absorb water...but water gets inside from
various means, and forms globules, and corrodes the
parts! At very elevated
temperatures, silicone gets
compressible!.....a BAD thing.
Yes, there ARE reports of folks using silicone brake fluid for
years in Airheads, with no problems. I suspect that there are various
types of rubber parts in the system in use over the years. Brembo
themselves has always used 'natural rubber' (per Brembo) that is not
compatible with silicon fluid. That is how it probably STILL is. I am unsure on ATE
calipers. Best not to be a Test Rider.
Remember: moisture droplets can do REAL FAST damage, but small to moderate amounts of moisture dissolved in DOT3 or DOT4 acts so slowly that yearly fluid changes really do work well! The moisture does NOT dissolve in silicon fluids, it collects in droplets that increase their size slowly, and they can do real nasty damage.
Calipers and master cylinders:
The rear master cylinder repair kit, up to 9/1980, is 34-31-1-237-233;
thereafter is 34-21-1-242-791.
The 40 mm ATE swinging caliper repair kit is 34-11-2-301-709 (38 mm is -705).
The Brembo 38 mm caliper repair kit does not include the
piston. The kit has just the O-rings and cap seal.
That kit is 34-21-1-237-234.
Their is a product called by various names, but is a special
grease for use in assembling the calipers. Some brake kits come with a
tiny tube of the stuff. USE IT, otherwise you may have a lot of fun trying
to get the pistons in the calipers.
If overhauling calipers, it is best to not take the halves apart.
If you DO take them apart, you must replace the O-rings, and you
must have the halves perfectly clean, smooth, flat.
BMW does not supply the halves-sealing O-rings, which you will
then have to obtain from some other source. Caliper
pistons work best if fairly well polished. If you are
overhauling caliper internals, you may find that
cleaning/scraping old corroded caliper bore grooves can be done
by some sort of prepared screwdriver blade, or one can do it a
bit easier by heating the bore with a small gas torch....to hot,
not fiery!...once cooled, the residue will be much easier to
clean out. If your grooves clean easily, forget that
hint.
The early ATE calipers were quite different, and incorporated a strange looking flat spring and an O-ring, at the pads. These were anti-rattle, and anti-squeal parts. O-ring 34-11-1-233-120; spring 34-11-1-232-513.
There are places that can resleeve/overhaul
master cylinders.
Try: www.applehydraulics.com
All the rest of the details:
1. DRUM rear brakes; shaft O-rings...etc.:
There is, generally after 1980, up to 1985, one or more rubber
O-rings on the brake actuating shaft, in the SQUARE grooves in
that shaft. That shaft may have up to 7 grooves machined
into it. Do NOT put any O-rings into the
round cut grooves. Prior to 1980, most did not have
any O-rings, you simply cleaned the shaft, greased it, and there
was a felt just inside the right side actuating lever, at the
spline end. I like to use silicone grease
here, on the shaft (and O-rings if has them). 1981-1984 models used 4 O-rings, in the square
cut grooves. In 1985 BMW changed to a tube through
the rear drive, and the shaft is inside that tube, and all you
have to do is to be sure to grease
the shaft. You can use conventional petroleum grease for
that. Some anal types, and I am one of them,
feel that the shaft of any of these models should be removed, O-ring(s)
freshened if shaft uses them, and clean and fresh grease, at
EVERY or every OTHER tire change. It is possible to fit that
tube to the 1981-1984 models, but it isn't really
needed. The brake cam lever that has those
grooves can be updated, but you will likely have major problems getting the
correct one, so that is why I say to just change the
O-rings. NOTE that if
the rear drive housing cover is disturbed, that tube must be
realigned, or you will have recurring oil leaks. Do NOT allow the
cover to be misaligned. Try an old /5 front axle for
this alignment.
Sometime, perhaps after 1983?,
BMW modified the brake cam rods. The O-ring grooves became
shallower, and were spaced differently. It is all rather
confusing, and trying to upgrade/update can be a mess. I
suggest that all of the cam rods work fine, if you replace the
O-rings at tire changes.
1981+ rear drum brake pivot
shafts, the shaft that has the O-ring(s), will not accept the
O-rings, and it will be a bear to install them, if the diameter
measured at the bottom of the grooves is not a minimum of 10.41 mm.
There was a rear brake cam change as of 08/1989 production. The early O-rings were 10 x 2 mm, and were 36-21-1-239-134. Later, BMW changed that ordering number to 07-11-9-906-328. AFTER 08/89, the O-ring became 35-21-1-457-605, which were 12 x 1 mm. The early O-rings of 10 x 2 will NOT fit the later cam.
****Drum brake linings: Whilst
you certainly can use BMW brake shoes; you may want to consider
www.vintagebrake.com. Ask
Michael about sending your drums (ask him if he can handle the wheels?...which
would be better, drums distort faintly when respoked)...or you can precisely
measure the drums and give him the measurements(??)...... but in any case, send
him your old shoes for relining and upgrading the material.
2. Brake squealing:
This comes up rather often. All sorts of fixes are touted
(including glues and pastes for the disc brake types on the back
sides of the pads). Some have used BMW-Brembo CAR type
anti-squeal backing plates (dangerous on an Airhead with near
stock thickness pads, particularly with any air bubbles in the
system, which will expand and pressurize with heat).
Some 'fixes' will work for some time, as will, for a time, changing to a soft
material, disc changes, etc. While different models of our
airheads are specified with different brake material, they are
ALL designed to be used rather vigorously....specifically the
design is for the worst case...moderately extreme
conditions! That is, BMW does not want your disc brakes to
fail under relatively heavy usage on steep downhill mountain
passes. Pads tend to squeal from localized,
usually small, areas of the pad where the surface has chemically
and physically changed, let us just call it carbonized, whether
or not you can see any visual change.
The BMW Brembo CAR type anti-squeal plates are 34-21-1-116-006. Be SUPER CAUTIOUS on using them! I am very hesitant about even mentioning them here! You MUST have a system with no air bubbles in it; and you MUST have pistons that retract properly, and you MUST have considerably WORN PADS to even think about using these. These can be used on airheads ONLY if the pads are thinned in thickness first (either sanded or actually worn a fair amount)...otherwise they could seize. NO air in the system...air expands with heat. You do NOT want to be riding a bike with a sudden brake seizure!! These plates are particularly insidious for airheads if any bubbles of air are in the system which could expand when hot and cause the brakes to seize and lock. So, if you intend to use those plates (NOT recommended by me), have well worn pads first, remember that I told you not to do it, and NOT to install them.
NOTE: BMW has had bulletins
about squealing. They noted that the chemical structure of
pads and shoes changes with age and temperature, and the material
is designed for a very long life, INcluding
time. They are designed for high stability under EXTREME conditions.
Extreme conditions, to me, means using lots of brakes at a goodly speed down a
steep mountain pass, pulling a trailer, ...etc. BMW noted that deceleration
from lower speeds, such as city driving, does not allow the material to get as
hot as would braking from higher speeds. Thus, the pads become harder (at
the near outer area), causing squealing. YOU
NEED to do some MODESTLY vigorous braking now and then
to reduce,
or eliminate squealing. Speed to produce the right
temperatures is important here, not just braking hard from
30mph which is not generally going to be the best thing to do! In fact, you don't need to brake very hard. You need
those linings to HEAT UP, from MEDIUM hard braking, maybe from 70
mph to near zero. If you seldom use your brakes much,
particularly enough to heat them up real well, the SURFACE of the
pads (especially the outer area) will HARDEN. It is WORSE if your pads are
dragging a bit.
Squealing is the result of failure to use brakes reasonably vigorously, or
having dragging pads. MOST of the time, NOT ALWAYS HOWEVER, if I hear a
BMW near me that is squealing, I usually guess that the rider does not use his
brakes much. If you remove the pads, a very easy job, you can place them
on an upturned piece of fine grit sandpaper
(NO emery or other types!!) on a flat piece of
glass...whatever.....and make even-pressured figure eight
motions, and remove a SLIGHT amount of material. If the
pad has grooves filled with crud, clean the crud out! IT
IS IMPORTANT that the disc HOLES be thoroughly cleaned out...use
whatever means, including Q-tips, with a fast drying solvent
(regular brake cleaner is NOT very strong)...whatever.....and note that just
because the holes appear clean does not mean that they
are. If the holes are not cleaned, the crud in them
will re-contaminate the pads. Clean the surface of the disc, then
holes, then disc, then holes, back and forth until no more black crud comes out.
NOTE: If you change the
type of material ...in fact, I recommend this for any pad change,
even the original type of material,... you should not just clean the
discs, you should clean them up with maybe 500-600 grit SANDpaper
(NEVER emery or other nasty papers), as some INVISIBLE old pad
material may have been carbonized onto the disc surface....and
then clean up with a good solvent.
I have had many questions about REAR brake
squealing. Here is some additional information from
BMW, condensed for you. This information was in TWO BMW
bulletins, called SI's for their official name: Service Information, and their identifications
were:
34-013-88 (2296)
34-011-86 (2230)
This information applies ONLY to the
REAR DRUM brakes. The application here is for any drum
brake with shoes that are 25 mm wide (that is, 1 inch).
This means ALL rear drum brakes from 1985, and it also includes
some older bikes, such as the R80G/S and the R80ST. NOTE
that all bikes FROM 1987 should already have these modifications.
According to BMW, one should take the approach in
steps. Do step #1, if squealing not eliminated, do step #2.
The first step, #1, is to install rubber dampeners.
#34-21-1-457-602 is 55 mm long, and is ONLY for the K75 that has
sand-cast shoes.
#34-21-1-457-572 is 70 mm long, and is for K75 with pressure-cast
shoes, and the R bikes.
These rubber dampeners are installed ONLY on the REAR return
spring. The FLAT side of these is installed against the
brake shoe and pivot cam; the slant side is to face left (towards
wheel).
Step #2: If squealing not stopped:
Install later brake shoes, that have a revised place
to hook the spring to...it increases tension.
The UPPER shoe is 34-21-1-242-401; the LOWER shoe is
34-21-2-242-402.
How do YOU KNOW if you already have these shoes?
There is a circular stamping, with the numerals 86 in the
center. It is unclear to me if even later shoes might not
have later numerals in the stamping.
HINT! It
is important on drum brakes that the actuating cam contact the
full width of the shoe at exactly the same time. Use
a thin feeler gauge, and SLOWLY bring the cam into just barely
any contact with the shoe....and file the shoe METAL SLIGHTLY, to
ensure that the full width contacts at the same time....this is,
of course, done with the wheel/tire removed. This gives
better braking, and may reduce squealing too. Squealing
on drum models can be singing of the springs, hardened lining
surfaces (same vigorous braking use recommended), or unequal/angular
contacting of the brake shoe at the cam pivot....fixable with
CAREFUL filing using thin feeler gauges and just enough
actuation, to see if contact is EVEN, across the cam and shoe
points. Singing of springs can sometimes be fixed by
stuffing them with a bit of rubber, or using the snap-on BMW
rubber part. Some use some spaghetti tubing over the
springs.
3. Do NOT mix up the locations of pads,
that will cause you to have to break in the pads to the disc
again, and your braking will be poor for awhile. This
is particularly so with the swinging ATE brake calipers.
4. ATE models of the swinging caliper type disc brakes have an adjustable cam at the bottom of the brake caliper. This cam part must be cleaned and lightly greased and adjusted properly. You can use an ink marking pen on the inside of the disc, and adjust the cam to give equal erasure as that brake is very lightly applied, wheel rotated. If incorrect, braking is poor, and squealing more likely. An article on the Airheads Club website, under Technical Tips, tells more about adjusting ATE swinging calipers. http://www.airheads.org
5. Use of "HH+" rated material will produce easier and probably better braking, with less pressure needed at the handlebars, with the usual caveats on braking use. Even better braking is had with meonite, cast-iron (some like EBC have full-floating conversions, but in SS); but whilst I do not recommend cast iron over the stock material, and it is not easy to find (one Canadian producer??), it DOES work, but might be heavy, and affect handling. The degree of "better braking" with various changes varies! Often a modification does NOT help braking. Use of HH+ pads almost always helps, and is likely the first thing to try, but be sure the manufacturer says that their material is OK for the BMW discs (ask not just about 'compatibility' but ask about WEAR!!). Don't be stupid and install the new pads without first cleaning the disc!!...that means the holes, the outer disc surfaces, and using sandpaper. Brake pad material should be compatible with your disc material. If you use all BMW discs and pads, you don't have the problem of figuring it all out.
NOTE!......if you change pads, remember that initial braking can be poor, until the pads break-in. This is particularly so if going to a new type of pad containing, perhaps, copper or other metal. This can happen with BMW's own pads too!
Here I am, AGAIN, emphasizing this: It is VERY helpful to clean the discs on both sides with a very fine grit paper, I use sandpaper or silicon carbide in 500 grit, and I am VERY particular about using Q-tips and solvent to clean each and every hole in the discs with a strong solvent, and cleaning the disc, and holes, over and over, before installing the new pads. For those of you that just will not do a perfect cleaning job, use sandpaper, not silicon carbide. Do moderate stops for first 100 miles or so, and then the brakes should work well. If you fail to clean the small disc holes properly, the disc, which heats up in normal use, can release oily grunge, and contaminate the disc surface...and your nice new pads!
6. Single disc models can be
converted to dual-disc, typically by changing a lower fork
leg. This CAN greatly increase braking
power. Once the system is fully bled and the lever
feel is relatively hard and not spongy, and if
the lever does not pull back too far...a TYPICAL
situation, ....then there is NO need
to change the master cylinder piston/bore size. That
stock smaller master cylinder size will produce more
braking for a given squeeze on the lever, and this can be
completely safe, IF...IF....the lever does not come back too far
towards the grip. AGAIN, only a properly bled brake system
that has a nice hard lever feel will tell you this.
In every instance I have personally worked with, where I have added a second
disc and used the same type and size of caliper, no master
cylinder change of size was necessary....nor overly
desirable. Thus, if YOUR particular system works fine
with the added second disc with the original size master
cylinder, you have that extra advantage of that slightly smaller
MC bore size for more braking (compared to what BMW used for DUAL
discs), and no need to change the MC. Keep
in mind that putting another disc and carrier and a caliper on
the front suspension will NOT improve handling!...rather,
handling gets poorer!
NOTE: BMW selected the master
cylinder bore size on a particular bike for a number of
reasons. INcreased braking will come from using a smaller master
cylinder bore, and some have done this even to
an original dual-disc machine; such as changing to a single-disc MC. Be cautious if you
contemplate this sort of thing, you do NOT want that lever coming
back all the way to the grip under ANY circumstances.
Be SURE that bleeding is done properly, before analyzing the
amount the lever can come back. The size of the
master cylinder...the lever design for leverage, caliper size,
etc., was all selected by BMW for a certain FEEL at the
lever....one wants to be able to have a feel that is hard to
describe, but you know it when you feel it (sorry for the
pun). In addition to that hard-to-describe
feeling, you want to feel, via sensitive braking lever response,
the wheel slowdown, before any locking-up (IF that were
possible, typically not on an Airhead on dry pavement). BUT, in every case I know of, the
"feel" is fine with the extra disc added and the
original master cylinder. But, YOUR case may be
different. All disclaimers apply here!
Basically, ONE of the things I am recommending to you, if converting from single to dual-disc, do NOT go willy-nilly with a master cylinder
larger bore size change. Try the stock one, and properly
bleed the system before making
judgment! IF you are replacing a master cylinder on a stock brake
system due
to its having failed and not being re-buildable due to its
damage, typically deep pitting of the bore, you can consider a 1 mm , sometimes 2 mm, DEcrease in bore
diameter....BMW has many different master cylinders available,
especially for on-bars type. Unfortunately, those with the
G/S and ST, who have single discs, and who want more braking,
have a limited selection of master cylinders from BMW, and may
have to go aftermarket.
There are a number of
places that can rebuild MC, resleeving, etc. Here is
one:
www.applehydraulics.com
7. There is an "accepted" left and a right side to front wheels, having dual discs. Standard accepted assembly is for the nuts and the axle nut to be on the left side. That means left as you sit on the bike. Not making the mistake of reversing the wheel will avoid caliper adjustments (ATE only) or having to break in the pads again (ATE or Brembo) to conform to the fitment. It can also prevent you from having your front tire run the other way. Mark your wheels. Remember also, that tires are often marked for direction, via arrows on the carcass. There are even front wheels so marked on the rim.....some discs also have arrows for forward direction.
8.
A commonly done conversion is to convert an early under-gas-tank
master cylinder Airhead to use the on-bars type of master
cylinder. What confuses folks is just how and what
parts are needed. Many folks do a conversion that is
partial, but, for the truly interested, here is the complete list
of parts, etc:
a. 34-32-241-567 "distributor"
replaces the stock master cylinder, provides a hoses junction,
and a place to screw-in the brake switch.
b. 34-32-1-241-565 is the upper brake hose for USA type
bars....if you have low bars, you need the appropriate length of
upper brake hose.
c. 32-72-2-310-747 is the 14 mm throttle and master
cylinder assembly for years 1985 onwards....this allows the
keeping of your stock dual throttle cables.
d. 34-32-1-242-205 banjo bolt, to connect the upper
hose to the MC.
e. two each 07-11-9-963-072 crush gaskets for the
banjo bolt
f. 32-72-1-457-038 the cam gear for 32 mm carbs
for the handle bar MC assembly
g. 51-16-1-237-641 RIGHT side mirror, with SHORT
stem, for the USA bars, as the stock mirror will not fit the
wider handlebar assembly. For low bars, get the LONG
mirror, or, whatever matches your current LEFT mirror, unless you
are using bar-end mirrors, or? If the bike being
converted is such as an RT, which has the mirrors on the fairing,
then this information does not apply to you.
h. 07-11-9-919-112, plug, for the right side of the
'distributor'. This plug is NOT needed if you have
dual-discs.
i. If your throttle twist gear is worn, you will need
32-72-1-454-129
j. If you want a new brake switch, it is
61-31-1-244-334
It is OK to obtain some of these items USED, but I'd suggest a
NEW master cylinder assembly.
NOTE: Brembo calipers have always used
hydraulic fittings threads that are 1.0 x 10 mm.
The stock turn signal switch will attach to the new throttle/MC
assembly, and the stock lower brake hose attaches to the
'distributor'. Use the stock BMW upper
brake hose, don't use an uncovered SS braided hose. It is
possible to run a hose directly to the caliper, avoiding some
parts, it is YOUR decision.
Twin-cable throttle assemblies from a 79-80 R65,
or 1985+ models that have such, fit. You CAN use a
1981-1984 throttle assembly, but then you need to convert the
throttle cables to the one-into-two cables setup. I differ
with some folks on this, as I believe the one-into-two is a
BETTER, more stable, throttle setup. Master cylinders come
in many bore sizes. Smaller bores mean more lever
movement but more braking for a given hand pressure.
Lever FEEL is also involved, and that FEEL is not the same as
pressure. Generally, folks use 13 mm bore MC for
single disc conversions, 14 mm for dual-disc. I don't like
the 15 mm all that much. Two different under-tank splitters are
available. One has three holes, one has four
holes. You can decide what you want to do, and
with the brake switch in the lever....or in the splitter.
***When one contemplates a conversion, sometimes what comes to mind is to go even further than simply adding a disc or changing master cylinder bore sizes. Maybe you are thinking of upgrading the braking on a later model that already has the MC on the right bar area. One possible upgrade conversion is to go to two or four pot (piston) calipers. One popular method is to use a BMW Oilhead caliper. You can use the LEFT Oilhead Brembo caliper from the earlier Oilhead years, and it is then mounted to the RIGHT side of the airhead BREMBO mounts. You need to mill about 4 mm off the mounting surface, and the larger piston then becomes the leading instead of the trailing piston, leading to weird wearing patterns, but a substantial braking improvement. You can use the stock Oilhead brake pads. This conversion is popular in Europe. Detailed information is on the www.powerboxer.de website. Swapping can be done for both sides, using the Left-Right, as described above, and reversing for the LEFT side of the airhead (that means using the RIGHT Oilhead caliper at the LEFT airhead side). If I was doing this sort of conversion, I probably would try to get the two Oilhead calipers from such as a BMW salvage yard. The earlier Oilheads calipers have identical bolt-hole spacing , and only need that approx. 4 mm milled off the mounting tabs. The 4 pot caliper uses 32 and 34 mm pistons. For those interested, the 2 pot units on the GS are 48 mm.
Whilst the dual-size calipers
have some modest advantages, and the later calipers are stiffer....the major
increase in actual braking comes from the increase in TOTAL caliper piston
area. Calculate from the area of a circle: each piston has an area
of its radius squared times pi. That means that a 38 mm piston has
about 1134 square millimeters of area...as one example. Add up the total
caliper piston area, and compare to the unmodified system total piston area.
It is also possible to mount a K1100LT or RS 4 pot
caliper. You will probably find the hole centers to be 108
mm. These should mount relatively easily to airheads,
particularly with Brembo calipers.
9. Here is sort-of a repeat of earlier stuff: It is MANDATORY, IN MY OPINION for the
brake fluid to be changed/bled YEARLY, although some may get away
with 2 years. This is for corrosion prevention AND
prevention of moisture accumulation problems with lowering of
boiling point. Seizing of brakes can throw you off the bike!
If the fluid boils, and the boiling point is constantly lowering
as the fluid gets old and more moisture is in it, you have ZERO
brakes!! If you put your bike up for the
Winter, do this job during
PRE-Winter
servicing. Do NOT use DOT5 SILICONE
fluids. Do NOT confuse 5 with 5.1 (I ALSO recommend not using
5.1). For most folks DOT 3 is OK, although BMW
recommends DOT 4. Dot 4 is better, as it has a higher
boiling point when it has some same amount of moisture in
it. If you are a very vigorous and heavy user of brakes;
where the brakes get VERY hot,...this usually means big loads,
steep mountain passes and VERY vigorous brake use, you could
consider a premium DOT4 with a particularly high boiling point
for additional safety factor. CASTROL makes such a
fluid. BUT, be sure to change THAT fluid yearly if it
is one of these extended temperature types. I recommend 6 months for that sort
of race fluid. On
a practical basis, even with those who literally almost race down
mountains with sidecars, I have NOT heard of common inexpensive
DOT4, changed yearly, having ANY problems.
Glycol brake fluids are 'hygroscopic' (note
the g in that word), they absorb water....right through
lines, microscopic places, MC screws/covers, etc. Sunlight affects the
reservoir fluid if the reservoir is on the bars on your
model.
DO NOT use high pressure water spray while cleaning your
motorcycle, where that spray can get to the master
cylinder. Failure to change your fluid, especially
after it changes to light tan/straw color, will result in
expensive repairs eventually, and might result in serious brake
fading. Carefully clean the inside bottom of the master cylinder reservoir, when doing a change/bleed. I use
Q-tips. NO lint allowed in the master cylinder.
ONE fresh 8 ounce can is more than enough to do the bike (even if
twin discs front and a disc rear). Do NOT allow brake
fluid to get on the paintwork!!as it will INSTANTLY destroy paint!
(water and rag should be available right next to you as you bleed
or otherwise work on the system). I put
wet rags around the master cylinder area to protect against
accidental spills.
Do NOT use brake fluids specifically stating that they are for race use...UNLESS you change the fluid often. NEVER use anything but brake fluid when doing simple on-bike cleaning of brake hydraulic system parts; or, use specific brake cleaning fluids.
HINT: When doing a fluid change and bleeding, and especially if the fluid is rather dark in color, obtain a baby-type of ear syringe and syringe out all the fluid from the master cylinder....and use clean Q-tips to clean any carbon looking matter out of the corners of the MC reservoir (careful!!...NO LINT allowed in the chamber or port at bottom!!!!!!!)...then immediately fill the master cylinder to about 1/2 to 3/4 with FRESH fluid, then start your bleeding. Do NOT leave the m/c above the maximum fill line after you are all done. It is better to have the level a bit lower than the maximum line. If the rubber bellows has deteriorated, replace it. Do NOT overtighten the top cover on the handlebars master cylinder....there is an updated (rectangular models) cover available....early rectangular ones warped. When the improved rectangular cap/cover is used, a gasket under it was eliminated. My old hand-scribbled notes said that the updated cover was 32-72-1-454-945.
10. BMW stock-original type rubber hoses may well last MANY DECADES, if you never hang the calipers by the hoses, which is true ABUSE!....and pinches the tiny hidden internal hose, especially at the hose end fittings. It is RARE for the internals of a stock-original BMW hose to collapse, so long as the hose is not injured by that mentioned hanging. Stainless steel lines are NOT all that much better...many are MUCH worse than the stock hoses. The problem with nearly all SS hoses is that they won't pass the WHIPPING test...a test of constant flexing to simulate long term use; and, the 304 SS material will work-harden, then can break; also the bad braiding area can rupture...and you get a bubble situation that can blow out. SS hoses are NOT just SS braid covered standard rubber hydraulic line hoses. If you use SS hoses be SURE that they are plastic tubing covered...not hardly just to protect painted surfaces, but to help avoid the common SS hoses/lines failure modes. SOME premium hoses WILL pass whipping tests. ASK.
Some SS hoses have exceptionally small diameter internal plastic
tubes....lousy for fast braking release.
BMW hoses are not overly long, which tends to quite well minimize
any soft-feeling from using non-braided hoses. BMW hoses
are of the HIGHEST quality. With a stock hose
you can generally feel the hose to see if it is failing, especially near
the end fittings, while you apply heavy lever pressure. That is
not so easy with SS braiding, a drawback. SS
hoses require covering by clear plastic tubing of some sort,
where the hoses might touch paintwork, etc. Some SS
lines are far too stiff, not really meant for all the flexing
that your airhead does with its moving fork. Some, if not
many or most, SS hoses have far too little diameter in the inside
tubing (often Teflon), and that creates a harder lever feel, but
creates other problems as the lever pressure is released.
I like the LOOK of SS braid covered hoses, but not the potential
problems.
Installing new hoses of any type MAY lead to a lot of cussing and a LOT OF LABOR....as you try to eliminate bubbles of air.
NOTE that as described earlier in this article, that as our hoses age over the years, that there may be a tendency ....however small, but perhaps now increasing....for a hose to have an internal flapper problem in which the hose allows pressure to go to the caliper, but does not allow, or only slowly, pressure to be relieved (at lever release) and fluid to go FULLY back to the master cylinder. This will result in insufficient piston release. You can check for this by hand rotating the wheel and checking for anything more than lightest friction from the caliper pads. Of course, the problem could be the piston and O-ring, corrosion, etc....as described earlier.
11. BLEEDING:
Bleeding/changing fluid on brakes could be an entire article in
itself. Here is a method that works fine ASSUMING
that you already have a fairly well bled system:
Attach a clear piece of plastic tubing that fits modestly snugly
to the caliper bleed port, with a box-end wrench over the tubing,
and have the tubing go UPward into a loop, then downward to a catch bottle.
Be sure the master cylinder is at least 1/2 full. Be sure you have the area
below the master cylinder covered by a fairly water-wet rag, and more water and
rags standing by. This is particularly important for the on-bars master
cylinder. You do NOT want, ever, the slightest amount of brake fluid to
get on your paint...and the fuel tank is right there! Squeeze
the lever (hand or foot) and while KEEPING some pressure, loosen the bleed port
JUST enough so that the lever moves maybe 2/3 of the way to the stop (at the
most). Avoid moving the lever farther, which it will, if the bleed port is
left open too long. Before the lever moves all the way, tighten the bleed port lightly. Yes, this is a
two-handed operation if you do it by yourself.
Release the lever. Repeat
the process over and over until the fresh fluid is clear and no
bubbles appear in the clear plastic tubing. DO NOT allow the
bleed port to be open as lever is released, that can suck-in air, particularly
at the beginning of the process, before the tubing has an inch or two of fluid in it.
You should be observing the fluid coming out of the bleed port,
looking at COLOR....and for tiny or any size bubbles. Color
will eventually change to new fresh clear fluid, and NO bubbles
of air (no matter how tiny), as the system is bled. Replenish
the master cylinder BEFORE it gets empty...failure to do THAT
will cause you enormous bleeding problems as you suck air and force air into the
system!!
If you have opened the system, meaning removed a hose or fitting (NOT meaning just removing top cover from master cylinder), then the bleeding process can take a very long time, and in some instances a bubble of air can get trapped at some junction, and cause you to be VERY unhappy. Pressure and vacuum methods are available. In some instances, with the UNDER-tank master cylinder, you may have to move it to a different position to eliminate such a bubble of air. On the rear disc brake, rotating the caliper for the bleed port vertical is a must if the system is very spongy and you have not been able to bleed it otherwise. In fact, I recommend that even if the foot lever feels OK, that you do all rear bleeding with the bleeder port vertical. That will ensure better bleeding. While I think it best to ALWAYS bleed the rear disc brake by rotating the caliper for a vertical port, if the system has no spongy feeling, then you can do the yearly bleeding-to-clean-fluid, with the caliper in normal position, but the bleeding and fluid change is not as thorough. To rotate the bleeder port vertical, without removing the caliper (and the need to insert some sort of 5 mm object between the pistons), you need to unfasten the holders bolts.
Bleeding problems can also apply to SOME front caliper(s) if they are really spongy, and the bleeder port not quite vertical.
HINT: SOME BMW airheads with twin front discs have ONE brake line leading to one caliper, and a crossover line to the other caliper. If that is what you have, bleed the most distant caliper in the system, that is, the caliper farthest, line-wise, from the master cylinder, FIRST. That is a good policy for ANY brake system....bleed the farthest distant caliper first.
HINT: If you are changing a hose, or otherwise 'opening' the system, you may have lots of problems trying to bleed the system properly. Vacuum bleeding at the bleeder port may work, might not. There are PROBLEMS using vacuum devices, such as the popular MityVac. A whole story unto itself. Filling the hose, or any part upwards, as best possible, from the caliper bleed port, using a syringe/needle and clear tubing setup will help, as will tapping on every part to release bubbles, and letting the system sit overnight to allow microscopic bubbles to rise. You can try putting a stiff rubber band on the lever, to keep pressure or lever position, overnight, further allowing bubbles to rise. Under some circumstances you may have to repeat this. Don't use full lever-back position, if the MC is an old used one....you can push the rubber seal into a crudded area. Pressure bleeding is an option. It does not always work. With the UNDER TANK ATE type brakes master cylinder, you may have to move the master cylinder position in order to get out the last bubbles. The REAR brake will require the bleed hole to be VERTICAL!!
Think THRICE before willy-nilly opening up a system to install fancy SS hoses!
Here are more bleeding hints. Try normal bleeding, without vacuum or pressure equipment. If doing the front brakes on a bike with the bars-mounted master cylinder, turn the fork to full LEFT, so the master cylinder is the highest thing in the system...that sometimes dislodges a bubble, but might take overnight to reach the reservoir. Use the lever a number of times with the forks to the left, then let the bike sit overnight. I recommend TYING the lever to about 3/4 way back, letting the system sit overnight that way. I use a block of wood shaped properly, to prevent the lever from coming full back (unless the master cylinder is new or newly rebuilt, then no block needed).
If this does not do the job, the next thing is the short jabs at
the lever method, and tapping at the area of hose connections, at
MC outlet, etc...then again let the bike sit
overnight. Keep in mind that tiny
bubbles rise over time, and will rise to the highest level where
they can be trapped by sharp turns and so on. Another
thing to understand is that if the substantial collected air
bubble is at a very high point, that when you use the lever, the
fluid may flow UNDERNEATH the bubble! This is why the short
jab may well dislodge the bubble and cause it to return to the
MC, or, be expelled at the bleeder in some instances.
Be patient, try things, be patient!
SPECIAL NOTE: This happens mostly with the ATE
under-fuel-tank master cylinder...but can happen with the on-bars
MC. You can't get pressure in the system after working on it.
You bleed and bleed, you try overnight waiting, you try vacuum methods, pressure
methods...and still no lever pressure. You wonder what is what.
Well, the problem is very likely a bubble of air in the SWITCH at the master
cylinder. You will have to remove the switch, fill its cavity and
reassemble in such a way that you try not to introduce any
bubble.
12. Some squealing....and ESPECIALLY brake lever pulsing...will be had if the shoes do not contact the brake cam properly with drum brakes. This can be fixed by using a feeler gauge and very slight brake lever movement...to see if the cam is contacting evenly across its surface, and if not, filing the shoe metal to match perfectly. In addition, you can try filing down that point of the LOWER shoe by about .004". If the bushing area is all worn out, you will have to fix that.
13. BMW uses different types of brake materials in various models. It is up to you to make the decision regarding use of a different type of lining or pad material for your bike. Note cautions earlier in this article. Linings are rated by friction cold and friction hot (and a host of other things). Letters are used to identify RELATIVE friction values. "FG", for instance, has a lower friction (F) cold, more friction (G) hot...compared to some unidentifiable standard. The higher the letter, the higher the friction value. SOME pad types are not compatible with BMW discs...but I have not seen that problem much in recent years. NOTE that INcompatibility can take several forms. Incompatibility can mean great brakes and high wear! GENERALLY BMW has used FF and FG material. HH material has very high friction. A popular lining is HH+. BMW does not use HH. It is acceptable for most folks though, and increases braking...but, keep in mind the various ramifications about changing such things, including faster disc wear (sometimes that happens). It is important that the pads you are purchasing be rated to work with the disc material you are using...so ASK the seller or contact the maker. It is sometimes difficult to get reliable information about disc compatibility and wear from the various linings. Frankly, disc pads and discs last SO long, I am UNconcerned about life, but, of course, I am not buying you your discs. NOTE that it only takes a very tiny bit of dirt, grain of sand, etc., to get into the lining or on the disc to start a groove and land type wear on the disc (pad too of course). This is NOT a big problem, usually NO problem....which is contrary to what folks trying to sell you discs may state. Certainly, discs with grooves and lands are going to take time to have the new pads break-in to them....and in some instances the discs do need resurfacing....but this is tricky, and needs to be done on a surface grinder, and could be rather expensive, and certainly takes material off the disc...and there is a limit to the disc thinness...stamped right on the assemblies. Discs that look pretty bad can be fine. A bigger problem (?) might be if the discs get too much thinner than the minimum BMW allows.
You
should meticulously clean the disc
holes once in awhile, and I personally clean the disc surface
with a CLEAN rag and a strong evaporating solvent, rather
often...usually after each washing.
HOWEVER....when time comes to replace the pads, be aware that
modest braking in the first few hundred miles, will enable the
pad to better match your grooved/land disc. NEVER replace
pads without cleaning the holes and sandpapering the disc with fine grit paper! Only in the worst
instances should the disc be machined to eliminate these grooves. The official minimum thickness is stamped on
the assembly. You are on your own if you exceed
BMW guidelines (my standard disclaimer!). The very best
method of resurfacing a disc (if it HAS to be done)...perhaps to eliminate some PULSING
in the lever (disc warping is not often fixed by resurfacing)....is to go to a QUALIFIED machine shop that has
what is called a "Blanchard Grinder"....and remove a
few thousandths from each side of the disc. Mostly, I leave
the discs alone! BMW discs have an official wear
limit. The limit varies with the model. Limits are
part of the disc brake CARRIER casting, that is, the limit is
cast into the surface, something like: min 0.18. The limit marking is on the INside surface of the CARRIER, near
the outer edge. You sometimes have to look closely for it.
***If you have a
warped disc, I advise that you do NOT grind the disc, but FIRST try to use very
thin shims at the carrier bolts, which may well remove enough of the warping.
Grinding sometimes does not work well, if the disc metal has taken on a 'memory'
in the material itself!!
14. There have been some instances of SINGLE (only) front disc models having loose bolts where the disc assembly bolts to the wheel hub. Use 33-31-1-108-204 washers on BOTH sides of the bolt and torque to 22 Nm. If it was quite loose, check the flange, etc.
15. PRE-1981 final drives use a wider brake shoe, and a narrower pivot pin. Late shoes will NOT fit correctly on older drives. You may need to remove a ridge to make things work if you substitute shoes, etc....that is, a late used drum on earlier drive.
***Another way of stating some of this: If
you install a 1981-1984 rear drum onto an earlier drum brake
airhead, you need to change some parts, and you will need:
1 each 25 mm shoe 34-21-1-242-403
1 each 25 mm shoe 34-21-1-242-404
1 each cam 35-21-1-454-836
4 rubber O-rings 07-11-9-906-328.
BMW actually had a bulletin to that effect, brought about by BMW
discontinuing selling rear drives for pre-1981 bikes ('use the
1981-1984...and these parts....).
16. Caliper
rebuilding hints:
BREMBO (and some of this is applicable to ATE): Break the
torque on the caliper joining bolts before
removing the caliper. Calipers are hard to hold in a vise, this will prevent
scratching the
calipers. Use compressed air to CAREFULLY unseat the
pistons toward one another before separating the caliper.
Then use air to finish removing the pistons, while covering
the piston with a rag. Problems with resealing the halves after reassembly may
happen because the bolts were tightened so much that the aluminum caliper
halves distorted
slightly. Separated halves might never
join up again in exactly the same position; high and low
spots don't match exactly, and thus leakage occurs. The fix is
to resurface the halves
on a surface plate with some 1000
grit Wet-or-Dry paper, using the paper wetted. You can then finish with an
even finer grit if you want to. Best to use only brake fluid for cleaning
the halves before reassembly, then wiping down with a lint-free cloth. If
you use other, stronger liquids as solvents for cleaning, be sure you flush them
away with brake fluid before reassembly. The rebuild kit MAY contain an envelope of silicone
brake assembly grease. This is for the piston seal and
caliper bores. Use the compound very sparingly. I generally use brake
fluid as the lubricant on re-assembly, of all calipers; but, if you have
problems, try silicon brake assembly grease. Sometimes the pistons are
devils to install. Be extra careful that the caliper groove is totally
cleaned, and that the piston is smooth at the edges. Some auto parts stores carry the brake assembly grease,
if it did not come with your brake repair kit. Lubricate
the new square ring on the piston on all sides before assembly. Threads
should be clean and then lightly lubricated with brake fluid. Torque the bolts
on the Brembo caliper to 22 Foot-pounds. Once all is assembled and the
caliper installed and system bled, you can finish the job by washing off
hydraulic fluid drips from the caliper with plain water.
17. For those with the master cylinder on the bars, if you are replacing a 15 mm size, which is 32-72-2-302-370; you can consider, for somewhat more powerful braking, the 14 mm size, which is 32-72-2-310-785. This type of recommendation works with most master cylinder sizes. Watch out for excessive lever movement. I am NOT responsible if you have a problem.
18. Rebuilding master cylinders: Sometimes it is not cost
effective. Here is one place that can hone them,
etc...plenty more: www.autosportseattle.com.
You can also rebuild them yourself, and you might want to check
with EuroTech for parts.
You can contact Ted Porter at
www.Beemershop.com
Also, try
www.applehydraulics.com
Rebuilding ATE under-tank master cylinders:
Here is a pictorial, step-by-step procedure:
http://www.bmwwerkstatt.com/gallery2/main.php?g2_view=core.ShowItem&g2_itemId=967
19. This is a copy of what is in the brakes section of an article
on performance upgrades, on this website, slightly edited here::
Depending on use, both drum and disc brakes can be
adequate. For drum brakes, being sure that the cam contacts very
evenly and that the drum diameter is in specification, and for racing cooling is important. For drum and disc brakes,
you could specify a higher friction
type lining. HH+ suggested. For disc brakes, cleanliness of the
holes and disc and a proper system operation is very important. If
you have only one front disc, change the other fork leg and add a second disc
and caliper. You MIGHT find that the original master cylinder piston size
is OK, and better than the larger stock one used normally with twin discs. The REAR drum brake is better than the rear DISC
brake. The rear ends swap, on twin shock bikes....with minor work.
For those with the master cylinder on the bars, if you are replacing a 15 mm
size, such as 32-72-2-302-370; you can consider, for somewhat more powerful
braking, the 14 mm size, which is 32-72-2-310-785....as one example.
***Quite a number of Airhead owners have 'upgraded' their disc brake systems:
(A) An additional disc where only one was stock on that year and
model.
(B) Modified disc(s), such as some other than stock type of metal, perhaps
floating type of disc/carrier, and other changes, including drilled or undrilled
or grooved or not discs. NOTE that drilled or slotted discs are not
drilled or slotted primarily for sweeping off rain water, as is commonly
thought. The real purpose is to reduce the gas that is produced from the
resin binders in older (especially) pads being heated. The gas goes to the
pad surface, and thereby makes a 'hydroplaning' type of layer. There are two other purposes. Drilled discs help INcrease
the coefficient of friction. The holes' edges provide those edges to
promote 'bite'. The latest high friction pads minimize the effect
somewhat. The other purpose of drilling (or slots or even shallow grooves)
is to help remove brake disc debris...they have NOTHING MUCH to do with COOLING.
(C) Different master cylinder piston size.
(D) Different pad material
(F) Different caliper(s), such as more pistons, or staggered size pistons, etc.
I have seen a LOT of confusion over why an 'upgrade' brake caliper...to one with
more than one piston, where approximately the same total pistons area as the
stock one, has not improved braking.
Sometimes these modified systems have the unequal piston size calipers mounted
to their Airhead such that the stock LARGER piston, normally being the exit
section (tire rotating normal direction) is mounted backwards from what the
manufacturer intended. This, with the gassing, see item (2), results in
less braking. This type of problem can be somewhat minimized by
being sure you are using drilled discs. Staggered size pistons calipers are
staggered in size specifically to help with this gas problem. Very
modern motorcycles with the latest type of discs and calipers and especially pad
compounds, are much less susceptible to this problem of gas-hydroplaning, and may
not even have drilled discs. Beware of using the wrong pads!
Revisions:
12/20/2007: Incorporate all previous revisions.
Completely revise the entire article, mostly for clarity.
Add more information
on rebuild kits.
01/15/2008: hyperlink to 38E added
05/11/2008: Edit entire article
12/28/2008: Minor updates, for clarity.
01/17/2009: add to #13 and add new item, #19
11/10/2009: add hyperlinks to item #18.
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