Question I have a bmw r100, Iwant to change the rear drive, Mine is bad. I want a highway gear , I do mostly highway . I have found many differan ratios. I need to know how to tell witch one would do what one would work best?
Ratios I have found, How are they rated? 34/11=3.09 .37/11-3.36 ,32/11-2.90 ,33/12=2.75 ,32/10=3.20 ,33/11=3.00 ,32/12=2.66 , 31/11=2.80 . 32/9=3.50 I was told was a side car rear,IS this true. Thanks,Jerome
AnswerThe first number of the ratio is how many teeth are on the final drive ring gear, the second number is how many teeth are on the drive pinion. Dividing the ring gear teeth by the pinion teeth gives you the final drive ratio, expressed as how many revolutions of the pinion gear is needed to rotate the ring gear one full revolution. In simpler terms, it means how many times the driveshaft turns in order to rotate the rear tire one full revolution.
So a 34/11=3.09 ratio means that the ring gear has 34 teeth and the pinion has 11, so the pinion rotates 3.09 turns for every one rotation of the ring gear (or the driveshaft rotates 3.09 turns to rotate the rear wheel one complete revolution).
Generally speaking, the more turns the driveshaft has to make in order to rotate the rear tire, the higher the motor will rev at cruising speed. With that in mind, if you choose a higher than stock ratio, you will get better acceleration from a standing start, but the engine will also be revving higher at highway speeds. A lower than stock gear ratio will result in slower acceleration, but will allow the motor to rev lower at highway speeds. So if your riding is mostly highway, you will want a higher gear ratio. Throwing a sidecar into the mix can get confusing, but basically you want a higher than stock gear ratio because it allows for more torque to be used in the lower rpm range, which is helpful when the motor has to move the added weight of the sidecar. But it also means that the motor will run out of power sooner and will be revving higher at highway speed. So whether you are looking for more pulling power for a sidecar, or better acceleration in town, or lower revs on the highway, you need to know that there will be tradeoffs for every gear ratio that you consider.
What you need to determine now is what your stock final drive ratio is on your R100 and then pick a new gear ratio that is higher. But exactly how much higher is hard to determine, because it all comes down to your personal preference... which can only be determined by experimenting with different ratios. Obviously, this is an expensive and time consuming project, so you might want to ask your local BMW dealer for their recommendation and/or pose the question on a BMW owners online message forum.
