Drive Ratios for the RS.

[Sunbeem]

When my '78 RS has 3,000 rpm on the clock, it's doing 60 mph in top gear.
Does anyone have any info relating to which final drive I've got, and what other options are available please ?
Occasionally my foot looks for a higher gear, and knowing little of the bike's history, or actual mileage, I sometimes wonder if that's the original F/D. It's also a bit sloppy, so I'd like to know what to look out for.
Ta.

Sunbeem.

[Kurt in S.A.]

Listing of ratios here:

http://bmwmotorcycletech.info/ringgears.htm

Looks like the FD should be 33/11 or 32/11:

http://www.bmbikes.co.uk/specpages/R100RS.htm

Kurt in S.A.

[SteveD]

The ratio should be imprinted onto the FD just near the shaft filler plug. At 3000rpm at 60mph I'm guessing you have the 32:11.

[Ken in Oklahoma]

. . . Occasionally my foot looks for a higher gear, and knowing little of the bike's history, or actual mileage, I sometimes wonder if that's the original F/D. It's also a bit sloppy, so I'd like to know what to look out for.

Sunbeem, can you amplify what you mean by "a bit sloppy", or rather correct me if I've got it wrong? My presumption would be that you're talking about "slop" in the driveline which manifests itself when you roll the throttle on and off, especially if you do it back and forth.

If so the slop can accrue from several places including wear between the clutch splines and transmission input shaft splines. Then there could be an accumulation of wear among the gears and dogs in the transmission. Then the U-joint coupling the transmission output to the drive shaft could be bad. Then there is the spline connection between the drive shaft and the final drive input shaft. Then the ring and pinion gears in the final drive. And finally there's the spline connection between the final drive and the rear wheel.

Of all those possibilities the most likely place would be spline wear between the final drive and the wheel, followed by the spline connection between the clutch disk and transmission input shaft.

You could measure the drive train slop by putting your RS on it's center stand with the rear wheel off the ground. Then, with the transmission in 5th gear, lay down or squat by the rear wheel and slowly rotate the rear wheel back and forth. The "slop", and there will be some, can readily be felt, though the limits will feel a bit ambiguous. Mark the tire where it would touch the ground at the two limits and you will have quantified your drive line slop.

I can't remember what "good" or "bad" is, but there will be some people here who have it freshly in their minds and can give us their opinions.

Do you remember how the wheel and final drive splines looked when you changed a tire? Have you pulled the transmission to grease the splines, and if so do you recall what the clutch/input shaft splines looked like?

Note to self: I need to start looking at those two things and writing them down in my logbook for future reference. Up until just now I hadn't yet thought about it as a good thing to do.

Ken

[mattcfish]

When my '78 RS has 3,000 rpm on the clock, it's doing 60 mph in top gear.
Does anyone have any info relating to which final drive I've got, and what other options are available please ?
Occasionally my foot looks for a higher gear, and knowing little of the bike's history, or actual mileage, I sometimes wonder if that's the original F/D. It's also a bit sloppy, so I'd like to know what to look out for.
Ta.

Sunbeem.

If you're looking fo a higher gear a 32:11 is the highest. The number is stamped where the drive shaft attaches.
Slop is most likely spline related.

[chasbmw]

Don't depend on the speedo they can be up to 10% optimistic, use a gps to check what the revs are at 60 mph, rev counters can be accurate, some multimeters can have a tacho function...........

Mind you don't depend on the numbers stamped on the final drive, as a PO could have changed the ratios! Maybe best to count the number of turns of the rear wheel compared to the number of turns of the driveshaft to be 100% sure?

Charles

[Sunbeem]

Thanks for all the feedback, I'll check the markings and compare with a turn-count at the wheel.
I find it hard to see thirty-odd years of use in the bike, but I can't assume we're still on the first time round either.
Most fasteners are in really good shape, which is very cheering.

I can feel slop all the way up from the wheel forwards Ken, but on the stand, not while riding so much. I marked the limits of movement twixt wheel and casing when I got the bike, if only I can find them again.
There is an occasional jerkiness as the clutch engages, which surprises me on occasion.
I've not met my drive input splines as yet, the final drive splines were only reasonable about 5,000 miles back.
I've wondered about the clutch splines, but the gearchange, (when I'm awake) is usually fine, so I associate the occasional sudden grab with a slipping bearing ?

It must be very useful to have a spare engine and tranny to explore, that makes more sense to me than a diagram.
Thanks again for the tables, info, comments etc.

Sunbeem.

[Major Softie]

There is an occasional jerkiness as the clutch engages, which surprises me on occasion.
I've not met my drive input splines as yet, the final drive splines were only reasonable about 5,000 miles back.
I've wondered about the clutch splines, but the gearchange, (when I'm awake) is usually fine, so I associate the occasional sudden grab with a slipping bearing ?
Sunbeem.

Uh oh. That can be splines that are just about completely gone. I hope it's something else.

[2valve]

hi to all, im no expert , but 60mph at 3000rpm doesnt sound right , the rpm should be alot more than that at 60mph around 3500 rpm + , is the speedo reading correct ? the R90S has a 33/11 and the rpm is below 4000rpm , is that right she'llbe ?

[SteveD]

Yep...the 33:11 on my R100RS allows close to 3750rpm at 60mph/100kph.
The 32:11 should be less rpm at the same speeds, but 3000 seems low. That's why I thought 'beem's FD might be a 32:11.