Longstroke manx gearing

[Brian Riley]

I’m building a new featherbed longstroke manx to run along side my domi next season. I’m dropping down from 19” wheels to 18” wheels due to the new continental tyre supply for classic racing in the UK i understand go up to a 20T gearbox sprocket from a 19T will keep the same 47-49T rear sprockets I use on the domi with a 5 speed laydown TTI gearbox a 2-1 belt primary 34/68 (35 an option) on my domi.
My question is for the longstroke manx using the same 5 speed gearbox and 2-1 primary how can I utilise the smaller manx works rear hub sprockets which run from 36T to 47T on 18” wheels to get the same sort of ratio as 19/48T on 19” wheels Do you go down on the gearbox sprockets or up?

 

[johnm]

First I would check the 18 " tyres do indeed have a smaller rolling radius than the 19". Often you find with different tyre profiles they are very similar.

Put a chalk mark on the tyre and ground. Roll one full circumference and measure the distance. Compare.

Reducing the rear wheel sprocket teeth count decreases acceleration and increases top speed. To compensate for this you need to decrease the gearbox sprocket teeth count.

But you do need to consider the rpm range and power curve of the two motors. If peak power rpm is different then the gearing will need to match that and will not be directly comparable.

 

[Brian Riley]

First I would check the 18 " tyres do indeed have a smaller rolling radius than the 19". Often you find with different tyre profiles they are very similar.

Put a chalk mark on the tyre and ground. Roll one full radius and measure the distance. Compare.

Reducing the rear wheel sprocket teeth count decreases acceleration and increases top speed. To compensate for this you need to decrease the gearbox sprocket teeth count.

But you do need to consider the rpm range and power curve of the two motors. If peak power rpm is different then the gearing will need to match that and will not be directly comparable.

Thanks John always appreciate the help

 

[acotrel]

Fitting 18 inch wheels to a featherbed makes the handling heavier, and even with better rubber the cornering speed might not improve. Gearing always needs to be changed to suit the circuit. With the smaller wheels a large open circuit with fewer tight corners might suit the bike better. I would start by over-gearing it. A long stroke motor usually has more torque. The good effect of more torque is realised by raising the gearing - but close ratios are still needed to keep the revs inside the power band. If you lose too many revs on up-changes, getting them back to where they are needed can be slow.
There was probably a good reason that the long stroke Manx was superseded by the short stroke. In Australia the difference was very significant, but back then any gearbox with more than 4 speeds was extremely expensive. My friends raced Manx Nortons - I think they were insane. Steve Oszko had a 6 speed gearbox in his 350 Manx, and a 5 speed in his 500. My ego has never been big enough - it does not depend on winning races. I am actually surprised that the Commando 850 engine can be made to go fast enough to race. I suggest that the differences in four stroke motors is not a continuum - there are distinct differences between short stroke and long stroke. What applies to a short stroke motor, often does not apply to a long stroke motor. With a long stroke motor, more throttle will probably be used due to the lag in response, so the shape of the needle in the carb is important. For maximum power, the mixture needs to sustained lean enough at the right rate.

 

[Brian Riley]

Fitting 18 inch wheels to a featherbed makes the handling heavier, and even with better rubber the cornering speed might not improve. Gearing always needs to be changed to suit the circuit. With the smaller wheels a large open circuit with fewer tight corners might suit the bike better. I would start by over-gearing it. A long stroke motor usually has more torque. The good effect of more torque is realised by raising the gearing - but close ratios are still needed to keep the revs inside the power band. If you lose too many revs on up-changes, getting them back to where they are needed can be slow.
There was probably a good reason that the long stroke Manx was superseded by the short stroke. In Australia the difference was very significant, but back then any gearbox with more than 4 speeds was extremely expensive. My friends raced Manx Nortons - I think they were insane. Steve Oszko had a 6 speed gearbox in his 350 Manx, and a 5 speed in his 500. My ego has never been big enough - it does not depend on winning races. I am actually surprised that the Commando 850 engine can be made to go fast enough to race. I suggest that the differences in four stroke motors is not a continuum - there are distinct differences between short stroke and long stroke. What applies to a short stroke motor, often does not apply to a long stroke motor. With a long stroke motor, more throttle will probably be used due to the lag in response, so the shape of the needle in the carb is important. For maximum power, the mixture needs to sustained lean enough at the right rate.

The 19” tyre supply for avons is coming to a end soon so now continental are now supplying the classic racing scene and they are excellent tyres and been tested now for a full season with excellent feed back but they are only available in 18” but do get a wider 110 wide tyre rather than the 100 wide of the avons, but I wasn’t sure to what gearbox sprocket to use to make advantage of the smaller works rear hub sprockets that are available but using a new 5 speed TTI gearbox which is a big upgrade to the 4 speed standard manx gearbox for the short circuit racing especially around tight hairpins

 

[acotrel]

If the overall gearing is too high, you can ride faster. If the gearing is too low with a torquey motor, you will always be slow. - The motor can rev without pulling. A short stroke, light-crank motor is different - lowering the gearing can improve acceleration. It is useless having good torque, if you do not make the motor pull. When I first raced my Seeley 850, it felt weird - it was revving and spinning-up without pulling. The rate at which the torquey motor spins-up is not decreased by raising the gearing - the bike just accelerates faster until it doesn't. Short-shifting does not help much.

 

[acotrel]

I suggest that with motorcycle road-racing, there is a tendency to believe things. I always believed short stroke and high revs were the way to go, until I played with the Commando 850 motor. It is surprisingly good. A long stroke Manx 500 engine might be similar.

 

[ES234]

Some surprising answers here. Calculate the rolling diameter of your new and old tyres and work it out mathematically as a percentage difference. Irrespective of the the lower gearing with the 18" tyres (and in consideration of any differences in the aspect ratio), the gearing will need to be adjusted depending on the circuit you are running on.