I have just finished working on my 1973 R75/6 - purchased by it's former owner in Europe, then shipped to the USA in '73. It was stored in a garage for many years thereafter - purchased by a friend at the original owner's garage sale four years ago. 2900 miles on the odometer. Regretfully, it was not covered, abused by his kids and exposed to salt air for all those years of storage. I've finally got it running and am working on cosmetics (see attached picture). Question: It has parts from /5's including a "kick start" five speed transmission and /5 hand controls - I presume because it's an early, European /6 model. Anyway, at 55 actual mph, it's turning approximately 4M rpm; at
60 mph, it's turning approximately 5M RPM - is this normal?
I will take my 750 (R75/5) out for a spin later today and report my observations later. My wild ass guess would be that the gauges are playing tricks on you. The gear ratio in high gear is the same for a 4 speed and a 5 speed transmission b.t.w., the only difference would be the final drive gear count.
former Airmarshal, IL.
Here is a link to Snobum's info on speedo ratios for various rear end ratios. Since this bike was a European model it may have had a KPH speedo, maybe when a US speedo was installed the incorrect ratio was installed for the rear end ratio on the bike.
If you have a rear end ratio of 10/32 your speedometer should be 1.186, if you have 11/37 you should have a 1.244.
http://bmwmotorcycletech.info/ringgears.htm
O.K. the final drive on my 750 is 32/10. In high gear, @ 4k rpm, my indicated speed was 65 mph. @ 5k rpm my indicated speed was 83 mph. Make of that what you will. I still think it is funny stuff with the gauges.
former Airmarshal, IL.
What speedometer ratio do you have, it's on the bottom of the face.
Clutch slippage? jagarra is 10 mph slower at 4K RPM. The difference is up to 23 mph at 5K. Increasing engine RPM by 20% I would expect to see a 20% increase of that 10 mph gap to 12 mph, not 23 mph.
Ordinarily clutch slip is easy to identify at low speeds or hard accel. If the clutch is able to handle the load up to 50 mph it will start slipping slightly above that speed and increase slippage as speed increases. A gradual increase in slippage may not be easily perceptible on a bike new to the rider.
4th gear on a 5 speed is between 3rd and 4th on a 4speed with respect to ratios. On my 750, 4k rpm prodeces 55mph in 3rd gear. I would think that any drive train slip that would present itself as 1k rpm = 5 mph would be obvious to any observer. When a clutch slips in high gear, you get a rise in rpm with little or no acceleratoin.
former Airmarshal, IL.
It might be a good idea to check the clutch adjustment for free play.
former Airmarshal, IL.
Clutch slippage? Ordinarily clutch slip is easy to identify at low speeds or hard accel. If the clutch is able to handle the load up to 50 mph it will start slipping slightly above that speed and increase slippage as speed increases. A gradual increase in slippage may not be easily perceptible on a bike new to the rider.
Clutch slippage will be most apparent with Load. This appears in 2 ways: heavy acceleration in second or third gear, and/or high speeds where the wind load becomes significant.
Typically, in high gear, on a stock system, both tach and speedo gauge needles move together. For instance, if the speedo is pointing to the 11 o'clock position, then the tach will also be pointing at 11 o'clock.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
I checked my speedometer face - it reads 1.186. At one point, the needle was swinging wildly around and I, consequently, had it repaired and calibrated to true MPH. Checking the speed against my Garmin Zumo 550, the indicated speed matches the GPS speed. I have a new clutch cable that is properly adjusted. My clutch is not slipping. However, I can not find the ratio numbers on my final drive. I have looked underneath, on the sides, etc., but can not find any markings. Am I looking in the right place?
The ratio is typically metal stamped into the aluminum of the final drive case, on the top side, near the final drive and drive shaft case interface. It will be 5 digits in the form of a fraction, such as "32/11". You may have to use a bright light to see it.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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