Modifications For Performance.
BMW Airhead Boxer Engines, Electricals, Suspension, ETC.
Suggestions & information for improving performance but keeping reliability.
© Copyright, 2013, R. Fleischer
NOTE!!...this is NOT the only place on this website that information on modifications are listed! Find those things by topic.
NOTE!!: The author is not "promoting" anything in this article. 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.
I am not interested in having long multiple E-mails, nor telephone conversations, etc., with those intending to modify their bikes. If you want to make inquiries on the Airheads LIST I may respond; & others might also. What is in the article that follows is strictly information for which YOU are responsible. 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, ruining a good Airhead", or any other arguments here. I am simply listing information.
In the below article 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.
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 can possibly reduce
reliability or create a more peaky engine. A quite peaky engine is fun in aggressive riding, but constant shifting and throttle work is NOT what BMW is about for cruising, touring, etc.
Faster acceleration CAN mean that the moving parts, in particular the flywheel and clutch assembly, have been made lighter. That leads to more vibration from a variety of other effects that are masked by heavier components. This is noticeable with the STOCK 1981+ bikes, just from the factory changes in 1981. That can make carburetion synchronization more critical for smooth balance.
If you increase compression ratio, you may have problems with the octane rating of gasoline. If you change camshafts, to produce more higher rpm power, you will find that the lower rpm areas are reduced in power, and must do something else to bring that power back up. Typically the idle is no longer smooth, and may be otherwise irregular. You will find you need to use higher rpm most of the time. Fuel economy will likely decrease, even substantially, in some instances. However, raising the compression ratio may IMPROVE mileage, as can dual-plugging, if you have not modified other engine components. Fuel mileage can improve with a flatter rear drive ratio, and the reverse is also true. Generally, if you install a 'hotter' camshaft, that terminology means that you have moved some of the lower power 'curve' up into higher rpm, which means that peak horsepower has increased. This can also mean peak torque is increased at some rpm on the way to peak horsepower. What almost always happens with a hot camshaft is that power is substantially DEcreased down in the normal idle THROUGH normal cruising rpm area. This CAN be made up, somewhat to substantially, with modifications to such as compression ratio; the limit on that is the octane of the fuel that you have available.
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 exhaust valves, and top end in general. 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 be close to disaster. The last thing you would ever want to see, besides the stealing of your bike, is a valve failure.
Two photos of what can happen is in this article: valves.htm
As a general rule in 'hot-rodding' 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, even such things as the engine casting parts moving in relationship to each other, will occur, causing problems. This phenomena, which may seem impossible to you, is called 'walking'.
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, and very careful attention to basic tuning almost always will be excellent, 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.
ALSO, keep in mind that suspension and brake improvements, not too wild, can be VERY helpful.
I. Electrical System (these items in blue, underlined, 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: 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 slightly 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; or, simply remove some metal from both automatic advance weights, until you obtain the advance you want. You could even tailor your own advance curve by re-shaping the advance weights and even changing the mechanical stops for those weights. 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 several crankshaft driven types. ALL 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 your bike has a modestly hopped-up engine. If you are replacing an often troublesome early grey-bodied single coil (dual output type), you can use the later type, OR, those two mentioned 6v coils. For use with pump grades of gasoline, on a somewhat hotted-up engine, it will be best, especially if the compression ratio is 8.8 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. Dual-plugging has almost no drawbacks, and many advantages.
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. Accel and 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.
See http://www.qkits.com/ www.apogeekits.com and maybe others...
I think any of these KITS can have the 'output transistor' changed, or heat-sinking changed, so the units can handle ANY coil. Velleman is probably the actual maker of a number of these kits sold by others, using the model number K2543. 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.
II. Clutch and Flywheel (called Clutch Carrier from 1981):
The stock clutches before 1981 require a heavy amount
of hand pressure, that is sometimes complained about by those with less muscular
hands. There were two types, one had a stiffer diaphragm spring, and was called the Sport Clutch in the literature.
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.
The 1981 and later clutch and flywheel are quite a bit lighter than earlier models.
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.
HINT!....BMW switched to non-asbestos friction disc material. Even after break-in, I do not think it grips as well.
NOTE that PROPER lightening WILL help acceleration 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,
the better the shifting of the gearbox. As the assembly gets lighter, the carburetors become more critical to adjust for engine smoothness. 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 OK.
Other sources can be such as Southland Clutch, that advertises in Airmail....101 E. 18th St., National City, CA 91950 (619) 477-2105. email@example.com They can machine your plate, etc., and provide a stronger clutch if you need it. They can also modify your worn clutch, so you do not have to purchase all new parts from BMW.
For the more technically inclined:
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 Jim here.
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. NO, I do not have recommendations on what part from where for the shims.
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. You may want to just contact
Southland Clutch, see link above.
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.
III. Carburetion, valve gear, cylinder heads, and camshaft:
KRAUSER 4 VALVE HEAD
Earliest engines do not have counter-boring at the top studs for the small O-rings, because these O-rings were not used. Next came the counter-boring in the CASES, for those O-rings. Slightly later the cylinders themselves got the machined groove for the large O-rings. Machining is possible, for cylinders and if warranted, the cases, for whatever combination of cylinders and cases you want to use. The later cylinders not only have that machined groove, and a slightly different shape near that groove which affects sealing if the case does not match, but the LENGTH is also different, which causes a difference in Compression Ratio, by about half a point. You can install ANY cylinder that fits the case hole, with OR without the large O-ring, with minor cylinder machining if and as required.
If the cylinder spigot is too large, the case needs to be machined too...or, you can machine the cylinder. In EVERY combination of cases and cylinders, you SHOULD use a sealant, even on those cases and cylinders that use the small and large O-rings.
See BOTH my cylinders and break-in article in particular for further information.
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 much. It can 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 5 mpg, and in some instances nearly 10 mpg.
If you had an R80 engine, you might consider changing to a lower ratio.
Some R80 bikes had a high high numerical ratio of 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.
Seldom does one talk about changes to a R65, but those with 3.44 might consider 3.2 or 3.36; and with 3.36, might consider 3.2.
If you go to dual-plugging (no matter the method), 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.
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.
Going further: You can start by using the '336' type camshaft if you are going to be using rpm consistently above 5500 or so. This is a GOOD camshaft for sporty to even 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 is the limit on the R100, 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 narrower 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. I recommend you do NOT grind the valve stem keepers to keep them from rotating. My valves article has my recommendations for seats, valves, and guides.
The 4 valve Krauser head is tricky to get working correctly, but will perform very well, with a large increase in power.
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 stock heavy and even modified, 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 if/as desired. For
the larger engines like the 800, 900, or 1000 cc engines, you need appropriate carburetors and the proper INternal size of intake adapter (that's
the screw-in metal adapter, often called The Stub, that screws into the intake port of the head).
In the petcocks, the actual length of the stock petcock 'straws' themselves is not the same between all models. The dimensions I show here are PROUD OF THE SURFACE THE STRAW IS PRESSED INTO! The straws can be metal or plastic. The diameter of the straws is approximately 0.215" (5.46 mm), but that varies a bit. You can usually use 7/32" copper tubing available at hobby shops. If you need to, sand the end area diameter of the metal straw you purchased, and SLIGHTLY chamfer the down side very end ...both for ease in assembly and proper fit. The stock length (proud of the surface) of the SHORT straw, is 0.935" (23.75 mm); and 3.27" (83 mm) for the TALL straw. Yes, you CAN shorten the short straw and gain a SMALL amount of usable fuel. My advice is to NOT eliminate that short straw, certainly not below 3/8" proud of the metal. You will need to add a small amount to these various lengths for the straw itself, as they are pressed-into the petcock.
For a modified engine, if you have a single outlet port fuel tank, you probably should change to a dual-port type, OR, modify the petcock on the single outlet tank. The stock single petcock will not flow enough fuel at extended W.O.T. Careful drilling and modification of the petcock will work well. While you CAN retain the in-tank thin tubes leading upwards from the petcock, by drilling them out a wee bit, that is not easy, and, the best thing is to remove them, drill the petcock a fair amount larger, all the way through, and fit larger inside and outside diameter tubes to the petcock.
**** Article 1A: will have above
plus more detailed information on modifying the fuel system for better fuel
***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's. 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 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. I've already mentioned a lighter clutch and flywheel assembly. 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 (cheaper, just modify the shaft length). 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 these routes.
V. Rear drive:
Select the ratio to match your engine and your expected speeds and rpm. I've already mentioned changing ratios if your goal is fuel mileage.
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 few problems. It is possible to adapt much later BMW forks, even K bike forks. You get the added advantage of much better brakes.
Doing a carefully done fork brace can help, but not all that much.... but installing a billet top
triple clamp is usually much more effective. Be SURE you get a billet top
triple clamp that is properly made. Modify the front forks for improved valving. You can go much further than just using aftermarket springs. NOTE that TRUE progressive springs, which are nice to have, do not necessarily come from the company by that name. The old method of installing anti-dive lower small springs does work reasonably well.
Most of you will not want to modifying the BMW heavy duty ones,...some ideas though:
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 enough. I'd do 25% or 30% unless you had lots of springs.
Most will not experiment in that way, which is tricky, so I would suggest you 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 for race, one for street.
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 to the swing arm fitment in the frame. 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.
The below photo shows a fairly simply conversion. I did them this way early-on, but I prefer to make the top mounting more sturdy, and, as mentioned, with an adjustment: a curved slotted plate. I have a photo of one of those conversions I did someplace, and will post it here if I find it. A QUALITY shock absorber unit should be installed if the budget allows, and careful attention to springing in particular...do NOT get an overly stiff spring, some shelf-stock shock units, like many Ohlins', are too stiff.
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.
There is an entire article on brakes on this website: brakes.htm. Use it WITH the below information.
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. You CAN make a drum brake work fine.
upgrades to discs
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
(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 rear added frame section, and suitable beefups above the battery are a good idea and VERY easy. 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; but may result in a lot of vibration. Side braces will help SOME. Beefing the swing arm does little. Earliest bike need beefing at the steering head, with added sideplates.
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 than those racy-looking intakes! Keeping the intake air cool is very helpful, but difficult.
Slide carburetors will give vastly improved throttle feel, and often better acceleration...sometimes dramatically so, than CV carburetors. Be very careful about going overboard on throat size. As a general rule, increased throat size tends to DEcrease lower torque and response.
If you use the Bing CV carbs, you can modify them, even going so insanely 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, as 'free' boost, or free supercharging. 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 TRUE ""RAM"" AIR improvement, it is UNLikely to do anything, except at VERY high speeds. Cold air GOOD; ram air not much, unless HIGH speed.
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 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 required 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. Generally try to AVOID increasing the intake path enclosed length in order to get your colder air.
XII. Exhaust system:
Pulse air system
Intake and Exhaust tuning
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,
install oil surge baffles.
Depending on how far you want to go, you can modify all sorts of things for lightness. You can go crazy
and even drill and polishing the 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.
ULTIMATE (???) RACE-TYPE PERFORMANCE:
I am putting this link here, so you can get an idea what type of effort goes into out and out racing performance when you are "a fanatic" (pick YOUR choice of words here). Never finished as far as it could be, this BMW racebike is well-described in this web article, and is for your reading pleasure. This is some of the story of BMW airhead, plate #36.....
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. R/language button.
08/29/2013: Add photo of Krauser 4 valve head (link)
08/24/2014: Add photo of mono rear conversion to a R100S
10/03/2014: Re-arrange article, make a few clarifications, fix coding, add links
12/09/2014: Add note to III.
© Copyright, 2013, R. Fleischer
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