Replaced 37/11 with 32/11

I recently swapped the 37/11 rear drive on my 83 R80RT for a 32/11. I found the drive on ebay, parted out from a 79 R90. The drive turned out to be in fairly good condition and by the ware on the splines it looks like around 30K on it. Since the R90 of that vintage used a wider wheel and brake shoes, I had to make an 8mm spacer for the rear axle so the wheel wouldn't jam against the drive housing.
A side effect, the speedo is now accurate, using a Bridgestone BT-45 110/80-18 on the rear. I went with the lower profile tire so I could back the bike into the garage with my short legs! With this size tire, and the original gearing, I was spinning much too high in each gear. From published gearing charts, I estimated a drop of 500 RPM in each gear would get me down to a more normal RPM.
I'm a small guy, so I estimated I could get away with the jump to a 32/11, and I was correct. I can easily start in 1st in any situation and cruise at 60 MPH at 3700 RPM. Sustained speeds of 70 (4400 RPM) are now possible without buzzing my brains out.

I did the same on my R80/7, swapping in a 32:10 final. Speedo is now almost perfectlly spot on. Very satisfied.

sbosbach141157 said:

I recently swapped the 37/11 rear drive on my 83 R80RT for a 32/11. I found the drive on ebay, parted out from a 79 R90.

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Maybe a '76 R90? They stopped making those with the '77 models.

There's a service bulletin that BMW put out that indicates how far "off" the speedos can be or actually are. Nominally, they always read high. So, sometimes by making these sorts of changes (final drive, different rear tire), the speeds drop back to being more accurate. As they say, even a blind squirrel finds a nut sometimes... Sometimes, things just work out right...

On my '78 R80/7 I'm turning just above 5000 rpm at 60 mph in 5th gear.I also have the 37/11 final drive.I posted previously on this subject,I'm glad to read your post.I hope to have mine changed after this winter. I wonder why they would come geared like this from the factory,to run at such high rpm's.

georgej said:

On my '78 R80/7 I'm turning just above 5000 rpm at 60 mph in 5th gear.I also have the 37/11 final drive.I posted previously on this subject,I'm glad to read your post.I hope to have mine changed after this winter. I wonder why they would come geared like this from the factory,to run at such high rpm's.

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more power for a given speed and airheads can run at close to redline all day without worry.

I did the same on my R100 and as with others my speedo is now within 2 mph of my GPS. Took a bit to get used to and still wonder if I may go back since I ride the airhead mostly on two lane roads and its fairly hilly/small mountains where I live and the extra power for a given gear is more important for that type of riding for me than reduced rpm on the highway.

RM

sbosbach141157 said:

I can easily start in 1st in any situation and cruise at 60 MPH at 3700 RPM. Sustained speeds of 70 (4400 RPM) are now possible without buzzing my brains out.

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Going from 60 to 70 mph adds 700 RPM? I cruise at 75 at 4200RPM in 5th. Maybe I need to check again what 60 looks like.

I did the same thing. My speedo was accurate before and now is off. By my calculations, math and GPS, my PRM's are dropped by 500 and the speedo is off by 15%. This includes the odometer. I just make the mathematical calculations and am fine. I do so much Interstate riding it is real nice. I recommend doing this if you ride a lot of interstate riding...It is almost like having a 5th gear overdrive. Around town and highway I seldom go into 5th gear...

What is 37:11 in the title actually supposed to be - 33:11 or what.

Since I very seldom ride on the interstate maybe I should leave mine as it is. When I am riding along at 60mph or so I feel I need to shift again while I am already in 5th gear. Also if I go closer to 70 mph I feel the engine is revving too high and I back off the throttle.So you're saying I can ride at 6000 and it's not harming anything?