Final Drive

EMSimon · Jun 19, 2012

I am a little confused and could use some help:

I want to buy a spare final drive for my rear drum brake 1978 R100S.
The ratio of the unit on the bike is a 32:11
I have a line on a great condition 32:10.
What will the effect be?

Will the bike be faster or slower at the same rpm??

mneblett · Jun 19, 2012

The 32/11 FD is a 2.91:1 ratio. The 32/10 is a 3.20:1 ratio. That is a 10% difference in gearing

If you were at 4000 rpm with the 32/11, you'll be at 4400 rpm with the 32/10.

20774 · Jun 19, 2012

Anton has a nice little chart that shows some of this visually:

http://www.largiader.com/articles/gearing/

dduelin · Jun 19, 2012

Acceleration will be a little quicker and gas mileage might be little lower with the 32/10. That is the trade off for running slightly higher rpm per given road speed.

boxermaf · Jun 19, 2012

And, unless you swap out your speedometer for one with the proper ratio (or get your existing one converted to the new ratio) then your odometer will also be 10% in error (too high) and your speedometer reading will be in error by similar amount.

ISAMEMON · Jun 19, 2012

boxermaf said:
And, unless you swap out your speedometer for one with the proper ratio (or get your existing one converted to the new ratio) then your odometer will also be 10% in error (too high) and your speedometer reading will be in error by similar amount.

does that mean 10% more than the error that it seems airhead speedos already have

Darryl Cainey · Jun 20, 2012

that could work in your favor, or not.

EMSimon · Jun 20, 2012

Thank you all. I am familiar with the ratio calculation and all. I just wasn't sure which one would result in higher vehicle speed at the same rpm or lower rpm at the same vehicle speed. I am totally amazed about the number of different ratios that BMW offered on the bikes.

dduelin · Jun 20, 2012

Isamemon said:
does that mean 10% more than the error that it seems airhead speedos already have

My R100 has the most accurate speedo of any motorcycle I have owned. It is less than 1 mph off at an indicated 60 mph.

20774 · Jun 20, 2012

dduelin said:
My R100 has the most accurate speedo of any motorcycle I have owned. It is less than 1 mph off at an indicated 60 mph.

Is that the '81 R100? Doesn't that come with the 85mph speedo? If so, those were touted as being very accurate.

bob_m · Jun 20, 2012

20774 said:
Is that the '81 R100? Doesn't that come with the 85mph speedo? If so, those were touted as being very accurate up to 85 MPH .

There fixed

20774 · Jun 20, 2012

Yup...no disputing that! Thanks, Bob!

EMSimon · Jun 20, 2012

It really must be an exception! There was a study a few years ago about the deviation of speedos and BMW was one of the worst. Ducati and Harley Davidson instruments were the most accurate.
One of the reasons is, in Germany it is governed by law that a speedo can not be showing less than the actual speed. That's why all manufacturers calibrate to the safe side.
An officially verified and calibrated speedo by a qualified calibration service will have a max of 4% deviation. +4% that is.

20774 · Jun 20, 2012

BMW released a bulletin showing that the maximum permissable speedo error should be 10% of the actual speed +2.4 mph. The reasons given were to compensate for the negative tolerances in tires sizes between manufacturers, electric speedometer indications, and other factors. In the end, as you say, the displayed speed must never be less than the actual speed.

stanley83 · Jun 21, 2012

EMSimon said:
There was a study a few years ago about the deviation of speedos ....
One of the reasons is, in Germany it is governed by law that a speedo can not be showing less than the actual speed. That's why all manufacturers calibrate to the safe side.

I wonder if mine is a real anomaly? I recently purchased a GPS and discovered my speedo read quite low, 55 indicated is 61 on the GPS and it gets worse as you go faster. Can these be re-calibrated?

20774 · Jun 21, 2012

stanley83 said:
Can these be re-calibrated?

Are the ratios on the speedo and final drive compatible? It's unusual for the speed to be wrong so much in the direction you described.

Yes, they can be recalibrated. There is a link in the Resources and Links speedometer section that discussion this. Also, any of the speedometer/gage repair places also listed in that section can do recalibration.

stanley83 · Jun 21, 2012

20774 said:
Are the ratios on the speedo and final drive compatible? It's unusual for the speed to be wrong so much in the direction you described.

How does one check the speedo ratio?

20774 · Jun 21, 2012

The speedo ratio is a small number printed on the bottom of the dial face. Something like 1.112. That number means that the cable will turn 1112 times in 1 mile. Certain ratios went with certain final drives. Snowbum lists some common ratios on this page:

http://bmwmotorcycletech.info/ringgears.htm

Palo Alto Speedometer has a test to verify the ratio:

http://www.paspeedo.com/ratio.htm

stanley83 · Jun 21, 2012

20774 said:
The speedo ratio is a small number printed on the bottom of the dial face.

Thanks, according to the 'Bum, I have the right combination, now I need to recalibrate.

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