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BMW Airhead GS 

(Frankenbeemer)

 

BMW Airhead Frankenbeemer Project Plan          BMW Airhead Frankenbeemer Project Log

 

  Frankenbeemer - Project Plan

 

The intent here is to build a BMW Airhead fire road touring bike. 

Priority Goals: 

Reliability

Simplicity

Serviceability

Light Weight

Range

Fuel Mileage

Trailability

 

Points of Concern

Charging System – Reliability & Capacity

U-joint

Clutch

Air Filter Seal

Valve Recession

Detonation ??

Front Brake

Suspension

Chassis

Either a Monolever or twin shock with modified rear suspension. Suspension travel in the eight inch range should be adequate. 

Frame – Main frame is probably adequate for bee-bopping along. Probably need to investigate reinforcing the subframe if doing saddlebags 

Footpegs & Foot Controls – Need complete revision 

Skidplate (s) 

Front Suspension - The forx from either stock twin shock or Monolever should be adequate as a starting point, but with spring and dampening mods. Twin shock forks should get a pinch bolt upper clamp.  

Rear Suspension - Mono chassis probably only needs a good quality aftermarket shock, perhaps a bit longer than stock to reach the 8” travel mark. Twin shock swingarm and driveshaft probably needs a 2” extension to get acceptable travel with lay down shocks. Need to do scale drawing. Need to research u-joint angularity limits. 

Front brake – a single 300mm floating rotor and 4 piston caliper. The under tank master cylinder is an excellent location, immune from crash damage that would damage a bar mounted item. Can optimizing the other components compensate for the cable actuation of the master ? Can a G/S be retrofitted with an undertank ? Braided stainless line from master cylinder to caliper. 

Rear brake – Stock drum format is probably adequate, perhaps optimize shoe material and reroute actuating arm to above swingarm. 

Rear rim – 2.50 x 18 would be first choice, 2.15 x 18 would be OK 

Front rim – 1.85 x 19 would be first choice, 1.85 x 21 would be OK 

Low front fender in plastic, rear some enduro bit 

Fuel tank - /7 6.3 gallon (23.8L ) and/or 43L (11.4 gal ) with quick disconnect lines 

Saddle  - Solo ? - probably depends on discovering a superior use for passenger space. Dual /7 saddle really works quite well for lashing storage. Need better seat attachment/release method. 

Side covers  - of what value ? 

Hand Controls – Crash tolerant dirt bike stuff would be highly desirable. Leave the choke by the carbs where it belongs. Simple starter button on right, off-on-off on the left. Turn signal, headlight on/off high/low high flash. 

Instrumentation – Analog speedo or inductive computer ? Digital tach, digital voltmeter, oil temp 

Lighting – 7” Bosch 100/55 H4 or something wiz bang which is considerably more compact, lighter and certainly horrendously expensive. Is the wiz bang stuff serviceable in the outback or are the component bits so small as to be inconsequential in storage. All other lighting should be L.E.D., mini turn signals, tail and brake. Perhaps separate but identical tail and brake modules with one spare. 

Electrical Misc. – Custom wiring harness, Mil grade connectors

 

Engine

An R80 seems best choice at first thought. There seems to be anecdotal evidence about superior longevity and the smaller engines 750 & 800cc generally have a better reputation for general overall civility. The iron barreled 980 is probably the least desirable. Intrinsic fuel efficiency of 800 vs 1,000 ? Is the 800 always more efficient at ”typical” operating load conditions. Can you get have the added displacement without compromising fuel mileage. Some magic combo of displacement, heads, compression etc.

Stock cam seems best all round. Not aware of any intrinsic problems with stock lifters and pushrods.

Nikasil highly desirable, especially in any engine near or over 1,000cc. 

Twin row cam chain.

Twin plug heads.

Electronic ignition with points system in situ backup. Can alternate advance curves compensate for crap fuel while still generally accommodating high compression for best fuel mileage.

Starter - ?? Bosch serviceable world wide ? Modified Toyota ?

Eurolast alternator & reg/rec.,  Need to calculate lighting & heating loads.

AGM battery.

Air filter – /7 setup is a pain to service and looks pretty sketchy for seal. Could you take stock covers, section them horizontally above garb inlets, weld lower halves together and box them to make a still air chamber, cylindrical filter attaches to top of chamber, old upper air box halves are filter covers – how to attach. G/S is probably a flat filter  

 

Driveline

Clutch – Is stock hopeless or is maintenance the key to perfectly acceptable longevity. What is the truth about Touratech’s ceramic friction plate

Trans – mods to improve reliability. Is synthetic fluid a magic elixir. Aftermarket low 1st and high 5th ratios are certainly appealing – how much. Kickstarter has rap for unreliability – truth ? Would certainly be a handy emergency bit. “transmission output shaft circlip” – seen this more than once, what is this about

Drive Shaft – Can /7 and/or monolever  shafts be rebuilt with serviceable u-joints ?

Final drive – Ratio ?

 

Electrical System

Fabricate a custom electrical harness or rebuild a stocker? Certainly relocate fuse panel.

 

Frankenbeemer  - Project Log

 

Sorry, we haven't started on this one yet.