Modifications For Performance.
BMW Airhead Boxer Engines, Electricals, Suspension, ETC.
Suggestions & information for improving performance ...
but keeping reliability and likely longevity.
© 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.
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 this 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 more of the time. Fuel economy will likely decrease, even substantially in some instances; from many causes, including that the camshaft will likely allow considerably more intake mixture to go through to the exhaust system, UNburnt. However, raising the compression ratio may IMPROVE mileage, as can dual-plugging, if you have not modified other engine components, very specifically the camshaft. http://bmwmotorcycletech.info/cams.htm
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 higher 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 low 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 are in this article: http://bmwmotorcycletech.info/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'. It is common for that with cylinders where they fit to the engine casting. 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. Keep in mind that suspension and brake improvements can be VERY helpful.
I. Electrical System (these items in blue, underlined, just below, are hyperlinks):
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 low end acceleration by using a faster advance. In the earliest /5 bikes, BMW used an automatic advance that started advancing at 800 rpm, which was ~ idle RPM then....and 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 from 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 the Alpha....to several crankshaft driven types. For top notch performance, a crankshaft triggered electronic ignition is primo. But, the Alpha ignition, which uses a canister replacement and has a built-in advance curve in the special separate ignition module, and they also have a special model just for dual-plugged Airheads, is a nice choice. ALL have advantages and possible 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. One of the better solutions is the Alpha ignition; or, the exact parts used in the very last Airheads, which have a changed module to match low primary ohms coils. You can use TWO high performance low primary resistance coils with that module.
If you decide to stay with points ignition: 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 http://bmwmotorcycletech.info/clutch.htm 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. Southland Clutch in National City, California, should be able to fix you up with good disc material, machine your clutch parts, etc. Southland Clutch 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. They can also modify your worn clutch, so you do not have to purchase all new parts from BMW.
The 1981 and later clutch and flywheel are quite a bit lighter than earlier models.
NOTE that PROPER lightening WILL help acceleration considerably. See: http://bmwmotorcycletech.info/lightened-flywheel.htm
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 http://bmwmotorcycletech.info/clutch.htm article.
HINT!....BMW switched to non-asbestos friction disc material. Even after break-in, I do not think it grips as well.
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. 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.
For the more technically inclined DIY types, here is some old information on how things were done long ago:
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, pistons and camshaft:
http://bmwmotorcycletech.info/cams.htm NOTE that companies like Motoren Israel, etc., have a wide selection of camshafts for your Airhead. Selection of a moderately hotter camshaft can be beneficial, without excessive drawbacks.
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 http://bmwmotorcycletech.info/cylinders.htm and http://bmwmotorcycletech.info/break-in.htm articles.
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 quite 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.
You can also change 5th gear, and in combination with a changed rear drive ratio, get exactly what you might want for performance.
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 low 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 good 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. For street use, use somewhat bigger intake valves that you can fit PROPERLY....probably 45 mm is the limit on the R100, and that may be too big, and grind the inside of the seat. Remove the unused unneeded area of threads on the valve adjusters. Clean up & polish the rocker arms, or leave them stock. Use the later needle bearing rocker arms, etc. Use the narrower 1985+ type parts, and modify them slightly. 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. Note that there are aftermarket camshaft makers that have a wide variety of cams available, and they have cams that don't have the problems with fitting to the oil pump, and front of cam fitment problems (the "336" may not be available in the style you want, from BMW).
If you modify an airhead engine, particularly if with an aftermarket 'hotter' camshaft, modifying the oil breather to the latest reed type IS A MUST! See this article: http://bmwmotorcycletech.info/oilsketch.htm
The 4 valve Krauser head is tricky to get working correctly, but will perform very well, with a large increase in power.
I have the original Krauser installation guide. http://bmwmotorcycletech.info/Krauser-heads.htm
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). Add the larger exhaust system, preferably the early eighties dual crossover 40 mm setup, and stock or slightly modified BMW mufflers. You can get higher compression ratio pistons for some models from BMW, or get them aftermarket. If you decide to add or modify an existing squish area, I suggest you do not go less than about 0.085" clearance.
NOTE: The R100 engines that have 32 mm carburetors can gain a considerable amount of power by using the Euro intake stubs and 40 mm carburetors. You could even go to the larger valves. These changes, together with increased compression ratio, will deliver a nice smooth engine, with very considerable power increase, yet be very tractable over the entire rpm range. Reliability is not really affected. With dual plugging added, you may be able to use mid-grade fuel (or even Regular at higher altitudes). If you add a moderately hotter camshaft, and use near 9.8:1 compression, you can come up with a bike that still can idle OK, and has a LARGE increase in power.
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 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, not easy; does work. 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. http://bmwmotorcycletech.info/fuelfltrs&petcocks.htm
You will want to perform a fuel tank flap-ectomy, and modify the fumes and overflow system if you have a later model.
***If you have a later eighties and nineties R100 engined 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 and strong wrist pins, actually, unless all-out-racing.
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. Modifications for racing can include using the later stronger transmission case, ensuring you have the shift kit parts (see http://bmwmotorcycletech.info/transmission.htm 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.
The so-called Shift Kit is fully explained in a section of my above transmission article.
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.
From 1979, BMW installed a cushioned driveshaft. This amounted to spring loaded male and female cams. The 1979 and 1980 swing arm is smaller on the inside, so a 1981-1984 driveshaft MIGHT not fit into a 1979-1980 driveshaft housing. The first two years (1979-1980) of the 'cush shaft' have two flats machined into the spring base unit, that allow the shaft to fit into the smaller housing. The 1981-1984 shafts don't have those flats.....but they could be machined. There WAS a retrofit cush drive kit, that fits the smaller swingarms used between 1973-1/2 and 1980: 26-11-9-055-059; but it is NLA. NOTE that you COULD install a NON-cush driveshaft from a 1973-1/2 and later (to 1978), into any 1979-1984 model. The R80ST and R80G/S use the same cush drive driveshaft as the 1981-1984 models. The use of the cush drive probably improves shifting some, but adds some lash in the drive system. None of these comments apply to the R65 and R45.
VII. Front and rear suspension:
The first of the Airheads, the early /5 models, had a shorter wheelbase than the later /5 and later models. The /5 was modified for a longer driveshaft housing in mid-1973. Thus, it may be known as a LWB, or a 1973-1/2. The swap is simple. There are reasons to change, or not. The SWB handles a bit 'quicker'. It won't accommodate a larger physical size battery; but you may not need one; and, if making a Café bike, you might even want a smaller physical size lithium battery. The SWB has a more prominent torque feeling if the throttle is suddenly snapped off, and this is more noticeable in turns, some of this is due to the rear drive trying to climb, a property of no-chain bikes like these. The SWB is also more susceptible to high speed wobbles from installation of FORKS MOUNTED windscreens or fairings; and, a slight bit more susceptible than the LWB if the rear tire is worn to squared-off condition. The installation of windscreens on the LWB is less noticeable, and hardly noticeable as far as instabilities if they are FRAME mounted.
Note that quite a few shocks from other brands of bikes will likely fit and be operational. I have a modest list of these.
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; like Toaster Tan's. Modify the front forks for improved valving. If you use the original old used piston rings in the forks they will work better than new ones. 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 modify 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.
Go to a REAL 10 weight suspension or fork fluid. Select from my
http://bmwmotorcycletech.info/viscosity.htm article. Be SURE you spend a LOT of time on stiction and alignment, see Randy Glass' article.
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; OR, going to a Monoshock, DO invest in QUALITY shock(s).
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: http://bmwmotorcycletech.info/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 pads!
(6) Some unexpected 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 size 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. see: XV: WHEELS:
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 only SOME. Beefing the swing arm does little. Earliest bikes need beefing at the steering head, with added sideplates.
XI. Carburetion and intake system:
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. Even the original paper filter elements are BETTER than those racy-looking intakes! Keeping the intake air cool is very helpful, but difficult. Individual filters (those racy cone shaped ones in particular) at the intakes should NOT BE DONE except for an ALL-OUT race bike. LOTS of problems, including frothing of fuel from vibration ....plus LOUSY air filtration that causes wear on your engine in all sorts of places, such as cylinder walls, rings, valve guides, lower end bearings.....etc.
Slide carburetors will give improved throttle feel, and sometimes actual over-all better acceleration ... than CV carburetors. Be very careful about going overboard on throat size. As a general rule, increased throat size tends to DEcrease lower RPM 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. Insulating the intake system can be very helpful, if you are using, more or less, the standard rectangular airbox and snorkels. Help for the clam shell type is also had. This type has far too much heat given to the intake air.
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, hardly attainable by an Airhead bike. Cold air GOOD; ram air not much, unless HIGH speed....read onwards:
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 modestly modified Airhead motorcycle are reached. Generally try to AVOID increasing the intake path enclosed length in order to get your colder air.
XII. Exhaust system:
XIII. Swapping parts such as heads, cylinders, etc:
Various articles on this website dealing with engine internals.
Install the later reed breather valve. Consider modifying the engine for better breathing.
Install a deeper oil pan with extended pickup, and be sure the oil pan has 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). Looks racy, if not much improvement.
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.
f. 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 the nipple/rim.
g. Early snowflake rear disc brake wheels on later airheads: see #26, below. see also #27, below.
h. 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. 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.
i. You can use various combinations of wheels/rims, eliminate tubes, etc.
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.
05/02/2015: updated entire article
05/22/2015: evise VI
05/31/2015: Add link for Krauser heads
03/07/2016: Update meta-code, layout, etc.
09/04/2016: Update metacode, script, layout, again!
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Last check/edit: Thursday, December 08, 2016