and in some instances reliability; for BMW Airhead Boxer Engines, Electricals, Suspension, ETC.
© Copyright, 2013, R. Fleischer
NOTE!!...this is NOT the only place on this website that information on modifications are listed!
I am not interested in having long multiple E-mails, nor telephone conversations, etc., with those intending to modify their bikes. What I have to say is on this page. If you want to do inquiries on the Airheads LIST I may or may not respond; but others might. What is in the article that follows is strictly information for which YOU are responsible in reading and acceptance (OR NOT). What YOU DO is not MY responsibility.
Some people modify the Airheads using parts from other makes of motorcycles. I am not about to get into the Franken-Beemer and ruining a good Airhead, or any other arguments here. I am simply listing information.
I am putting this link here, so you can get an idea what goes into true out and out racing performance when you are "a fanatic" (pick YOUR choice of words here).
Never finished as far as it could be, given enough money, this BMW racebike is well-described in this web article, and is for your reading pleasure.
In the article that I wrote, after this section, I try to keep away from pushing the limits, so as to leave a reliable bike, that can be driven on the street, for many happy miles; not one that needs rebuilding after a race; nor constant attention, etc.
Snowbum's suggestions, information, etc,
for improving performance:
NOTE!!: The author is not promoting any of the following items. The author is not guaranteeing safety, longevity, nor anything else. Every sort of legal disclaimer is made! In SOME instances, pros and cons that I KNOW OF are detailed.
Many will be looking at this article for engine performance increases, although that is hardly the only thing here. Before you make a possibly huge error in $$$, labor and results, DO understand that increasing the torque/horsepower potentially can reduce reliability or create a more peaky engine. Faster acceleration CAN mean that the moving parts, in particular the flywheel and clutch assembly are lighter. That leads to more vibration from a variety of other effects that are masked by heavier flywheel and clutch assemblies. This is quite noticeable with the 1981+ bikes, just from the stock factory changes in 1981. That makes carburetion more critical for smooth balance. If you increase compression ratio, you may have problems with gasoline octane. If you change camshafts you will find that the lower rpm areas are lacking in power, and must do something else to bring that power back up. You will find a need to use higher rpm most of the time. Fuel economy will likely decrease a substantial amount in some instances. However, raising the compression ratio may IMPROVE mileage. As you 'hop up' the motorcycle more and more you will find that you have placed vastly more stress on some components, such as the valves. If you go too far, you may find that the engine needs a lot more maintenance, and even may be so unreliable as to be a real headache. Going way far, your engine may also be too close to disaster. As a general rule in 'hotrodding' engines, when you modify one thing, you need to modify other things in compensation. There is a limit to what modifications can be reasonably safely done to the Airhead engine. After a certain point is reached, the engine casting parts will move in relationship to each other, causing problems. Even well before that point... once you exceed a certain point in hop-up, you will be faced with a FAR less tractable engine, one that you must pay a lot more attention to, as far as rpm, gear selection, fuel grade, and increased maintenance.....to mention only a FEW things that will occur. Be cautious about modifications. Some modifications, such as dual-plugging and slight to moderate CR increase, may well be rather useful, others are possibly not. If you are after a 'sleeper' bike, one that will run away from other Airheads, and maybe even Oilheads; or, are considering racing an Airhead; remember that the bike world is full of even smaller displacement bikes that will outperform ANY hopped-up Airhead.
I. Electrical System (these items in blue, just below, are hyperlinks):
Bosch electronic voltage regulator
Bosch mechanical voltage regulator
Wehrle electronic voltage regulator
Testing voltage regulators
Diode boards and grounding wires
GEN lamp resistor
Ignition system: Elsewhere's on this website are several articles on the ignition system, the automatic advance, etc. You are referred to those articles. HERE I will say a bit about changes for increased power and acceleration. Firstly, if you have close to 100 octane fuel available, you will obtain better acceleration by using a faster advance. In the earliest /5 bikes, BMW used an automatic advance that peaked at only about 2000 rpm. That would be better. If you have a pre-1979 airhead, you can fit the early /5 automatic advance, or simply use one or two lighter strength springs in a later version (to 1978); or, simply remove some metal from both automatic advance ears, until you obtain the advance you want. You could even tailor your own advance curve. It is a lot more work to do this sort of thing on a 1979 and later, as the canister has to be disassembled, and unless you fashion a jig of some sort, you will be assembling and disassembling a considerable amount. There are aftermarket ignitions available, from the Dyna type to the Boyer....to very pricey crankshaft driven types. All of these have advantages and disadvantages. For those 1981+ bikes that came with a single dual output coil, use of TWO of the 6 volt Bosch coils that have a lightning bolt on the side of them, in series connection on the primaries, instead of a stock single coil with dual outputs, may be advisable IF...IF...your bike has a modestly hopped-up engine. If you are replacing an early grey-bodied single coil (dual output type) anyway, you can use the later type, OR, these two mentioned 6v coils. For use with pump grades of gasoline, on a somewhat hotted up engine, it may be best, especially if the compression ratio is 9.2 or higher, to use a dual-plug conversion. In fact, to obtain the full advantage of a dual-plugged Airhead, one of the things to always consider is raising the compression ratio, IF PRACTICAL.
If you plan to stay with points: Points bikes
can have an amplifier or booster added to the points to reduce electrical wear
on the points. Dyna makes those and there are Velleman kits sold too. Pay attention to coil primary
resistances when using points. Typically you do not change the coil(s). If the primary resistance is too low, such as from the wrong type of coil,
points can burn up rapidly, or if using a booster (points amplifier) the booster
might not handle the current. Using dual-plugging and points will usually mean changing the coils to higher primary resistance types so that boosters can be used to reduce points current.
Points amplifiers, sometimes call points boosters, are made by a number of manufacturer's. They will GREATLY increase points life. Numerous makers.
Accell and Dyna are two of the popular makers.....and the mentioned Velleman kits, and there are others...see below:
NOTE: http://www.qkits.com/ www.apogeekits.com and maybe others...
Velleman is probably the actual maker of a number of kits sold by others, using the model number K2543, this is a kit. It it is rated at 4 amperes, but with the heat sink that comes with it, I think it will handle MORE, if placed in a relatively cool place on the motorcycle. http://www.vellemanusa.com
A problem can occur if you have coils that draw more amperes than the points boosters/amplifiers are rated for. Many have used them in this somewhat overloaded condition, if they are kept reasonably cool. The Velleman seems to hold up.
Firstly, the clutches built before 1981 require a heavy amount
of hand pressure, that is sometimes complained about by those with less muscular
hands. A pulley/small chain affair, an EZ-clutch conversion, is
available from such as:
Craig Vechorik; Benchmark Works, 662-325-2103. This is a very simple device that fits at the rear of the transmission, easy to install. It roughly halves the effort at the bar lever. There are others, see my CLUTCH article.
CC products may still be selling a heavy duty clutch that is quite different, as far as the friction disc goes, than the stock item. CC Products may still have various types of lightened flywheels and heavy-duty clutch parts available. Luftmeister, now long gone, used to have such clutch parts, and others. It is possible that Matt Capri at South Bay Triumph still has some of those items for sale.
Check Southland Clutch, that advertises in Airmail....101 E. 18th St., National City, CA 91950 (619) 477-2105.
The 1981 up clutch and flywheel are quite a bit lighter than earlier models, and it is difficult to approach the weight of these later components by modifying the older design.
NOTE that PROPER lightening of the early flywheel WILL help considerably. See: Lightened-Flywheel
The early clutches can use beefing-up if a large increase in horsepower is going to be had. Hard surfacing of the early parts is a good idea. Generally the later parts already are hardened. The later clutch parts will fit (including the carrier) but you have to change the input shaft of the transmission (or, shorten its spline length). The lighter the flywheel/carrier/clutch assembly, the more vibration the engine will produce, but the faster you can increase rpm, and the better the shifting of the gearbox. As the assembly gets lighter, the carburetors become more critical to adjust for engine smoothness. Perhaps that is best said that a small amount of carburetor UNbalance, is seen more easily with SOME additional vibration. The big advantages of a lighter assembly are more rapid acceleration and faster and easier shifting. As noted above, some various types of special clutch discs were once available from Luftmeister. Also note that those clutches tended to be rather grabby, and a finer touch with throttle and clutch hands is necessary for a smooth take-off, especially when the clutch is COLD. Those pulling trailers may well be advised to beef up their clutches some....although most get along with the stock parts just fine. Other sources can be such as Southland Clutch, that advertises in Airmail....101 E. 18th St., National City, CA 91950 (619) 477-2105. firstname.lastname@example.org They can machine your plate, etc., and provide a stronger clutch if you need it.
For the more technically inclined...here is some information. Pressure can be increased in various ways to handle increased power or heavy 'slipping clutch' use. The following paragraph on INcreasing effective clutch pressure was taken from a posting by Jim Roche, somewhat edited by me here, and I tend to agree with it.
1970-1980: THREE methods, and in all
cases I recommend hard surfacing:
1. Bring spring closer to the pressure plate. Place a .035"-.065" hard steel donut ring shim between the flywheel and the spring base. Such large transmission and differential 'shims' are available from such as RingPower heavy equipment shops; or, see any competent local mechanic's junk box of old shimming items. For this modification, the flywheel need not be removed, and the weight added is small.
2. Bring pressure ring and pressure plate closer to the spring. Remove the flywheel (you were going to lighten it anyway, right??) and machine off .040"to .085" of its face surface. This is the face area where the 6 clutch bolts hold the clutch assembly to the flywheel.
3. Bring the pressure plate alone closer to the spring by installing a THICKER driven disc between the pressure plate and the pressure ring. Install a 3 or 4 'wing' metallic 'competition' type clutch disc, which is thicker than the original. Its extra thickness means that washers, about .040" thick, must be placed between the pressure ring and the flywheel face. Some very slight interference with the transmission case might be seen for a short while, or you can relieve it.
These later parts are already hardened. Some improvement can be had by machining the fulcrum ring contact point .030"- .050" to the outside...which increases spring pressure considerably. Hard surfacing is a good idea for modified parts.
HINT!....Early models had the clutch actuating lever at the rear of the transmission held to the two bosses of the transmission cover by a PIN, that used a single C clip. That C-clip fit on the pin at the INside of the lower boss. If the clip came loose, the pin could come upwards, and come out of the lower boss, and the next clutch application could, and often did, break off a transmission boss ear...necessitating a transmission overhaul....or some inert gas welding at a minimum. A cure is to remove the old pin and clip and install these parts:
#23-13-1-241-484 pin, that has a flange, and won't fly out. This is used with a clip that is part 51-23-1-864-963. For the full details, see my clutch article.
LIGHTENED FLYWHEELS: CLICK HERE!
III. Carburetion, valve gear, cylinder heads, and camshaft:
KRAUSER 4 VALVE HEAD
If you are trying to get better fuel mileage, and only that, I suggest a very carefully done tuneup (valve adjustment, ignition timing, carburetor synchronization after checking all adjustments including fuel level, needle and jets, etc). Worn needles and needle jets (especially from 1985) WILL CAUSE poor fuel mileage. I do not suggest hopping up the engine, or, only a wee small thing or two, as you might be forced to use high test gasoline exclusively, which will offset fuel mileage gains. It can also take a lot of miles to pay for changing heads or modifying them, etc. One thing you SHOULD look at is the rear drive ratio. This can affect fuel mileage by as much as 10 mpg. If you had an R80 engine, you might consider changing to a lower ratio. Some R80 bikes had as high a numerical ratio as 3.36:1 on the rear drive. Going to 3.09 will help mileage, as will, moreso, 3.00, and 2.91. Think this through carefully, as to cost, and effect. If you lower the ratio too much, you may find 5th gear is not all that usable. HOWEVER....the engine will withstand lower cruising rpm to a fair extent....and use of light throttle at, say, 3000-3500 rpm is completely OK.
If you go to dual-plugging, and raise the compression ratio to 9.0:1 (approximately) (if on a\ bike with a lower compression to start with), and in some instances to 9.2:1, you can use 87 or 89 octane fuel if the combustion chamber is smooth and clean, and if carburetion, ignition, etc., are all in very proper condition. This WILL increase fuel mileage a few mpg. You may be able to use 89-93 octane with 9.5 or even 9.8 compression ratio and dual-plugging.
Mostly, people want MORE horsepower, so will deal mostly with that here:
For high rpm operation....well into and even above normal redline rpm, you want to rework the valve gear. There is NO reason to use RPM up as high as redline with a stock engine.
You can start by using the '336' type camshaft if you are going to be using rpm consistently above 6000 or so. This is a GOOD camshaft for sporty to modest racing performance. The combination of the 336 camshaft and a lightened clutch will give you less grunt coming off the line, and some of the usual minor hopping up feeling/problems. That is helped considerably by a higher compression ratio. I do NOT recommend that cam...or any cam other than the stock one....unless you have at least 9.2, and preferably higher. Going to high ratio rockers is risky, you MUST know what you are doing. You can lighten the valve system by using later pushrods (or even aftermarket ones), and elongating the slit in the followers. Use the biggest intake valves that you can fit PROPERLY....probably 45 mm, and grind the inside of the seat. Remove the unused unneeded area of threads on the valve adjusters. Clean up & polish the rocker arms. Use the later needle bearing rocker arms, etc. (Use the 1985+ type parts, and modify them). Other lightening techniques can be utilized, just be careful. For a true race engine, very hopped up, you will want to go to more exotic valve gear parts...spring buckets to keep the spring cool, lightweight spring retainers and keepers, etc. Note that if you modify to a different camshaft from stock, use different pistons or heads, or any modifications of these types at all, YOU are responsible to ensure that there is no interference between the various parts during engine operation. There is even a 4 valve head available (Krauser). They are tricky to get working correctly, but will perform very well, with a large increase in power.
Some other suggestions:
Use the 40 mm exhaust system, dual crossover, and if for the street, modify the mufflers slightly by drilling a 3/4 inch hole from the outlet end with a LONG extended drill, through ...way inside baffle. Drill two each 3/8" angular holes, one on each side of the muffler OUTlet, about 3/4 inch or 1 inch forward of the outlet tip. For a race machine, the exhaust system should not be those heavy and even modified, stock-type mufflers. Some would recommend a traditional taper megaphone, with a reverse taper cone. Use a 2 into 2 system. Consider the old Axtell system....6° taper megaphones, ending in 45° reverse taper cone, the hole of which is half again as large as the entrance. This is not at all a bad idea for a 8000 rpm engine with a fair amount of hopping-up. As always, the top end is the secret to power, and a dyno is recommended.
Probably the 1977 40 mm exhaust heads and pistons would be a
good starting place....but you CAN use any of the heads....and modify them. For
the larger engines like the 800, 900, or 1000 cc engines, you need the
appropriate carburetors and the proper INternal size of intake adapter (that's
the screw-in metal adapter that screws into the intake port of the head).
***If you have a later eighties and nineties R100 engine bike, there are some worthwhile changes that improve power, improve fuel mileage, and have no real negative points. The USA later R100 engine bikes have 32 mm carbs, smaller valves, smaller intake adaptors (matching the 32 mm carbs), etc. One easy change is to go to 9.5 pistons, and fit early 80's heads from a R100 engine. Use 40 mm carburetors and change the throttle assembly (or parts inside) to be equivalent to the EUROPEAN 1991-1995 R100GS. DO NOT change the camshaft unless you understand the over-all effects. I highly recommend dual-plugging, as it will reduce the necessity for super-premium fuels.
I suggest you do NOT use lightened piston pins, unless part of a package from a major Airhead modification manufacturer. Motoren Israel, some German companies, etc. I prefer heavy wrist pins, actually.
Modifications can be made to ensure a faster shifting transmission. Removal of any kickstart mechanism is a good idea. Modification to prevent pawl spring breakage is simple, and should be done. The 4 speed transmission is not a good idea. Neither is the 1974 5 speed transmission. 1981+ transmissions will fit earlier clutches if the input shaft of the transmission is changed. The reverse is also true. Modifications for racing can include using the later stronger transmission case, ensuring you have the shift kit parts (see transmission article) and undercutting and possibly other modifications to the gears and gear dogs. Special gear ratios are available for the transmission. Think carefully before going that route.
V. Rear drive:
Select the ratio to match your engine and your expected speeds and rpm.
VII. Front and rear suspension:
Quite a number of other brands of forks are adaptable to the Airhead. This has been done for accidents reasons; has been done to get a higher performance front end (stiffer, better brakes, or other reasons), etc. KTM forks have been adapted. Suzuki DR650 forks will probably match up to the BMW bearings with little problems. ETC.
Do a very carefully done fork bracing.... and install a billet top triple clamp (especially do THAT). Be SURE you get a billet top triple clamp that is properly made. Modify the front forks for improved valving. Using aftermarket springs, or modify the BMW heavy duty ones, by cutting at a calculated (HAH!) length, using a washer between the two pieces. Install a spacer inside one of the springs, NOT BOTH. I suggest that you cut at 50% of spring length. The spacer needs to take up a bit of the length of the one half. This will make a two-stage progressive spring, with twice the rate after the spacer'd section binds up. Obviously you can do 25% cut too, or anything you prefer. The 50% was suggested as the effect is large. I'd do 25% or 30% unless you had lots of springs. Most will not experiment in this way, which is tricky, so use TRUE progressive springs, of proper rate, setting the sag properly, and you will be fine. Use the MORE STRINGENT BMW valve ring (ONE per fork tube)....see the BMW parts lists.
If a /5 bike, braze up two of the holes.
a REAL 10
weight suspension or fork fluid. Select from my
viscosity.htm article. Be SURE you spend a LOT of
time on stiction and alignment, see Randy Glass' article, posted on Duane
Ausherman's website now, instead of his own. (see URL.HTM
Rear suspension: Twin shock Airheads can be modified for MONOSHOCK, EASILY. The Monoshock rear end just bolts right up. I suggest you modify the frame and make a curved plate with a slot, so that the single spring/shock unit is adjustable; which, with the usual adjustments of the spring perch and shock valving on most units, will offer you much improved performance. If keeping the twin-shock rear end, invest in QUALITY shocks.
Potentially it is possible to install a R100GS setup, with its transmission, Paralever, rear end, etc. I suggest you do not....unless you really want to and have a large budget. Yes, it is possible to change the transmission cover, etc.
Upgrading a front drum brake is possible, by careful assembly and use of appropriate friction materials. It is possible to install discs, at some typically large expense....with wheel changes usually needed (but not always). There are innumerable possibilities.
are costly and complex.
Example: You might be thinking of grafting on a R90S dual disc brake setup;....let us say you have a 1974 /6 you want to do that to. You will find the TUBES spacing to be different! That means you need need triple clamps, although the old tubes are OK. You will need 1975-1980 dual disc ATE Lowers, the 17 mm axle, etc. You CAN use the ATE calipers. Finding an entire R90S front end, complete, is not likely.
Another example: You can convert a single disc ATE system to dual disc by using 1979 parts...a right slider lower, the caliper, the disc. You probably can use the existing MC.
Quite a number of Airhead owners have 'upgraded' their disc brake systems:
(1) An additional disc where only one was stock on that year and model. This will improve braking SOME.
(2) Modified disc(s), such as some other than stock type of metal, perhaps floating type of disc/carrier, and other changes. 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.
(3) Different master cylinder piston size, 1 or 2 mm smaller than stock for the same number of discs.
(4) Different pad material. HH+ pads work very well on most discs. If using cast iron aftermarket discs, consult the maker.
(5) 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!
(6) Some strange things you might run into include the REAR DRUM BRAKE fitment. PRE-1981 rear drives with drum brakes used WIDER shoes and narrower pivot pin; so the later shoes won't fit. A late model brake drum might need a ridge machined off due to wear from narrower shoes.
(7) Installation of 4 spot calipers from such as an Oilhead or K-bike is USUALLY rather simple, and results in VASTLY improved braking. If done in a DUAL disc setup, you can have truly modern performing brakes. You often can continue to use the stock master cylinder. The installation of dual discs and 4 spot calipers IS MY FAVORITE METHOD...as it gives a HUGE increase in REAL braking power, for the least amount of money. If done correctly, the brakes still have great FEEL, and are NOT overly sensitive. Of course, there are always those that go way overboard on a modification.
This is a complex subject, and individual recommendations can be made.
Modifications will depend on use, and year of your frame. One of the weak points is the subframe, and suitable beefups above the battery are a good idea. Another beef-up is to remove the top cover over the starter motor, and make a bridge/ladder/lattice type of strong adapter, and tie the top of the engine very securely to the top frame tube. This is for an all-out race bike. There are other modifications that can be made, some will result in vibration, such as that tying the starter area to the top frame. Side braces will help SOME.
XI. Carburetion and intake system:
see: Intake and Exhaust tuning
The intake system can be modified, depending on your desired performance level and usage. For an all-out motor for racing; individually supported gauze intakes are desirable, but these do NOT work well for street/touring, where the clamshell or square air-cleaners are far better. The original paper filter elements are BETTER.
Slide carburetors will give vastly improved throttle feel, and often better acceleration...sometimes dramatically so, than CV carburetors.
If you use the Bing CV carbs, you can modify them, even going so far as to thin the butterflies, and many parts are available for the Bings for modifications, including various slides. Experience is needed...lots of it.
Definitely give a lot of thought to additional air, COLDER air, to the intake system. A very significant power boost can be had.
NOTE: Questions often arise as to whether or not RAM AIR would be helpful. This is not overly easy to accomplish on our airheads, but CAN be done. It seems relatively effective, as it gives COLD air at any speed, by modifying the intake snorkles. But, for a RAM AIR improvement, it is UNLikely to do anything, except at warp 10 speeds.
Keep in mind
Dynamic air pressure on a flat plate moving object (motorcycles are like that!) is relatively small at slow speeds, and has no real effect of any consequence. At 68 mph, for instance...let us say 100 feet per second...there is 12 pounds of pressure on a SQUARE FOOT. A square foot is 144 square inches, thus the pressure is 12 divided by 144, or under a tenth of a pound. Pressure is proportional to the SQUARE of speed. So, at 136 mph, one has 48 pounds per square inch...or about 1/3 of a pound of pressure per square inch. This gives only SOME barely noticeable 'boost'. Another thing to keep in mind is that the horsepower to attain some higher speed, goes up ROUGHLY as the cube. What all this means is that if you are not doing over maybe 120 mph, forget trying to get RAM pressure, but it is always worthwhile to go for cold air...which has a MUCH bigger effect, until speeds generally unattainable by a motorcycle are reached. Try to AVOID increasing the intake path enclosed length in order to get your colder air.
XII. Exhaust system:
Pulse air system
XIII. Swapping parts such as heads, cylinders, etc:
various articles on this website dealing with engine internals.
Luftmeister side tanks
Install the later reed breather valve.
You may want to install a deeper oil pan with extended pickup, and for racing or VERY sporty riding, suitably install oil surge baffles.
Depending on how far you want to go, you can modify for
lightness all sorts of things. Like drilling and polishing those
holes in the cam chain
sprocket (yes, race folks go to that extent).
Use a later driveshaft housing, but install the earlier non-cush shaft. This will be controversial.
****Many a time someone wants to swap wheels from one year to another, one style to another. This can get VERY complicated. BMW used several sizes of front and rear rims in snowflake, regular cast alloy, wire, and GS type external wire. I cannot list every combination or swap here. You may have to put #1 eyeball on your proposed modification.
But, here are a few ideas:
a. A front wire wheel can be put into an early 80's airhead (that came with snowflake
wheels). Use an R80ST front wheel; or use a front hub from a R65 or R80ST and lace
to a 19" rim. You could also machine a hub from a 1975-1980 spoke wheel bike, which
is narrower, as the 1975 to 1980 hub is too wide and spokes will contact the Brembo
calipers. Use R65, not R65LS, discs, or make spacers.
b. As above, but for the rear wheel, use any 1970-1980 spoke wheel.
c. A snowflake front wheel can be put on a very early bike, let us say a 1974 /6. You need
a 1974 R90S front end, with dual disc. This is a hassle, but can be done. The tube
spacing is different, you need the triple clamps. You need a 1975-1980 dual disk ATE
lowers, with 17 mm axle holes, and you need the 17 mm axle of course. You CAN use
the ATE calipers.
d. Early airheads had a narrower fork brace (fender mount). If needed for your tire size,
you can use a 1977-1980 brace from a /7. Or, buy one: 46-61-1-234-907.
e. If converting single disc ATE to dual-disc, ATE, let us say a 1978, use the 1979 parts:
new right slider, caliper, disc.
d. Watch out for cracking rear rims on 1974-1975 bikes, these cracks develop at the
spoke hole area; the fix is to use later heat-treated rims. TAP the spokes on the
proposed 74-75 rim, and if any are quite dull sounding, there may be a hidden problem at
e. Early snowflake rear disc brake wheels on later airheads: see #26, below. see
also #27, below
f. In 1975, BMW changed to 17 mm axles on the R90S. BMW then continued with 17 mm
axles. Swapping wheels from one year to another, can be a tad complicated. The 14
mm axle wheels can be modified to fit a 17 mm axle bike. This can get more
complicated if you try to swap into a much later bike, but for the /6, for example, it is not
all that bad. For the front wheel, remove the size reducing sleeve in the wheel and install
the larger ID wedding band spacer (that sets the preload) used on any later model up to
and including 1984. This spacer is used on both front and rear wheels of later
models. There is a cast-in web on the old wheel that held the smaller spacer captive.
Drill it out, to a bit larger diameter than the later wedding band spacer's O.D. To install
the wheel you will need the top-hat spacers . You may have to fiddle with things a wee
bit. Do the usual preload work before installing the wheel.
02/23/2005: update and release to web
01/17/2009: expand section VIII on brakes
04/21/2012: Update article
08/06/2012: Update article again
10/10/2012: Minor cleanup. Also add QR code, language button, update Google Ad-Sense code
04/25/2013: Minor updating and add link to cams article.
08/29/2013: Add photo of Krauser 4 valve head (link)
© Copyright, 2013, R. Fleischer
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