Lowering Gearing by changing Belt Sprockets

This text appeared as an article in "American Thunder" and was written by member Brian Marshall.

The UK/European Buell’s come with different gearing to the US Buell’s. The US Buell’s are geared for faster acceleration - the European models are geared for a higher top speed.

This is done by using different size final drive belt sprockets. US models have a 27 tooth engine sprocket (A) and a 61 tooth rear wheel sprocket (B). European models have a 29 tooth engine sprocket (A) and a 55 tooth rear wheel sprocket (B3).

PART NUMBERS:
A 29 TOOTH = 4020595
A 27 TOOTH = 40288-95
B 55 TOOTH = 37727-97Y
B 61 TOOTH = 34927-94Y

You can change the gearing on European Buell’s to be either full US spec gearing by changing both the engine and the rear wheel sprockets or half US spec gearing by just changing one of them.
There was an article in an issue of Battle 2Win magazine (Vol. 3, Issue 1) which gives a lot of data about different gear ratios so I’ve used some of that here and added a bit more information relevant to UK Buell owners.

The different combinations are:

A = 29 TOOTH B = 55 TOOTH
This is the "normal" UK spec and gives an overall gear ratio of 3.034.
Top gear - 24.02 mph/l000 rpm giving a theoretical top speed of 163 mph at 6800 rpm.
Fourth gear - 20.36 mph/1000 rpm - maximum speed 138 mph.
Third gear - 16.80 mph/1000 rpm - maximum speed 114 mph.
Second gear - 12.19 mph/l000 rpm - maximum speed 83 mph.
First gear - 8.93 mph/1000 rpm - maximum speed 61 mph.
At 80 mph in top gear the engine is running at 3300 rpm.

A = 27 TOOTH B = 61 TOOTH
This is the "full" US spec and gives an overall gear ratio of 3.615.
Top gear - 20.16 mph/l000 rpm giving a theoretical top speed of 137 mph at 6800 rpm.
Fourth gear - 17.08 mph/l000 rpm - maximum speed 116 mph.
Third gear - 14.10 mph/1000 rpm - maximum speed 96 mph.
Second gear - 10.23 mph/l000 rpm - maximum speed 70 mph.
First gear - 7.49 mph/l000 rpm - maximum speed 51 mph.
At 80 mph in top gear the engine is running at 4000 rpm.

A = 27 TOOTH B = 55 TOOTH
This is the US engine sprocket with the UK rear wheel sprocket to give an overall gear ratio of 3.259.
Top gear - 22.36 mph/l000 rpm giving a theoretical top speed of 152 mph at 6800 rpm.
Fourth gear - 18.95 mph/l000 rpm - maximum speed 129 mph.
Third gear - 15.64 mph/1000 rpm - maximum speed 106 mph.
Second gear - 11.35 mph/1000 rpm - maximum speed 77 mph.
First gear - 8.31 mph/l000 rpm - maximum speed 57 mph.
At 80 mph in top gear the engine is running at 3600 rpm.

A = 29 TOOTH B = 61 TOOTH
This is the UK engine sprocket with the US rear wheel sprocket to give an overall gear ratio of 3.366.
Top gear - 21.66 mph/l000 rpm giving a theoretical top speed of 147 mph at 6800 rpm.
Fourth gear - 18.36 mph/1000 rpm - maximum speed 125 mph.
Third gear - 15.15 mph/l000 rpm - maximum speed 103 mph.
Second gear - 11.00 mph/l000 rpm - maximum speed 75 mph.
First gear - 8.05 mph/l000 rpm - maximum speed 55 mph.
At 80 mph in top gear the engine is running at 3700 rpm.

I have a ‘99 M2 and I found the standard UK spec gearing too high. At motorway speeds of 70 - 80 mph the engine was running at 2900-3300 rpm in top and I often found I had to change down a gear to overtake briskly, especially if on an uphill section. It’s acceleration that gives me my kicks on a bike rather than the top speed. I need my licence for my job so I rarely go over 100 mph and have only pushed it towards the top speed on a very few occasions.

I decided to change the gearing to give me better acceleration so I changed the engine sprocket to a 27 tooth version, leaving the rear wheel sprocket at 55 tooth. This is a low cost option as the 27 tooth sprocket costs about £50 and is fairly easy to change. You don’t need to replace the belt because you are only changing from a total of 84 teeth (29+55) to 82 teeth (27+55) - 2 teeth less - but you do lose a lot of the adjustment on the belt. The rear axle is now almost at the end of the slots in the swinging arm with about l/2 inch left to go. This shouldn’t be a problem because these belts only stretch once during the first 500 miles and then stay the same length for the rest of their lives, so hopefully I won’t need to adjust it again. I don’t know if the original belt will work if you go to a full US 27-61 spec (4 teeth more) or the 29-61 option (6 teeth more). You’ll need to try it and see for yourself.

There are two minor problems with 1999 (and later) models when making the change to 27-55. Firstly the speedometer doesn’t read correctly. On post 1999 models the speedometer is electric and has a sensor in the gearbox. This sensor doesn’t know you’ve changed the overall gearing so the speedometer now reads about 7% fast. You can calculate how the speedometer will be affected by different sprockets - it changes by the same percentage as the overall gearing. In my case I changed the gearing from 3.034 to 3.259 - an increase of 7.4%. This isn’t too bad because at 30 mph the speedometer reads 32 mph, at 40 mph it reads 43 mph, at 70 mph it’s reading 75 mph, at 100 mph it’s reading 107 mph etc.

The second problem is that the belt is slightly closer to the swinging arm with the 27 tooth engine sprocket, so there is a very small mark where the belt occasionally rubs on the swinging arm when the rear suspension is at one extreme. This is not serious and I’ve done over 2000 miles with this gearing and no sign of any damage to the belt. I guess with full US gearing the larger rear sprocket keeps the belt away from the swinging arm so this doesn’t happen.

These two problems will not affect earlier models because the speedometer is mechanically driven from the front wheel and the swinging arm is not so thick. The result gives me just what I wanted. Acceleration is much better and the pickup from 80 mph in top is fantastic. I never have to drop a gear to overtake anymore because the motor is just at the ‘sweet spot’ at 3600 rpm and picks up a treat. It’s just like being in 4th gear instead of 5th. You don’t notice any loss in top speed because the bike is still geared to pull over 150 mph and it can easily run to speeds over 120 mph just by winding it on in top gear. The only downside is when cruising on the motorway at a steady speed, the engine is now revving a little higher which must adversely affect fuel consumption on long runs.

I’d definitely recommend this change - it’s low-ish cost and is a very easy way to gain a big increase in all round performance.

Many thanks for that Steve

Thats the very article I was looking for, I have full US gearing on the M2 but it's got the electronic speedo so the speeds way off, I was just wanting to know by how much off so I can put the relevant marks on the speedo dial.

cheers Steve that archive.org is a handy wee site I'll need to remember it's there