Gearing

[ozktm151]

Standard gearing on an old KTM is 13:52 ie 1:4 on the final drive. If I change the gearing to 14:56 (still 1:4) would that have a different effect on the engine as it is trying to spin a 14 tooth C/S instead of a 13 tooth one. Any thoughts from the physicists out there? Cheers Bill

[Montynut]

should make no difference except a much longer chain

[Lozza]

Easier on chain/sprocket and slightly more torque applied to the rear wheel

[Montynut]

I'd be very interested to hear how it would make more torque?

Torque is only affected if the ratio changes.

Yes chain & sprocket life should increase but the down side is that unsprung weight increases as the larger rear sprocket, longer chain are heavier. Much more than 50% of the chain is considered unsprung. Also the angle change of the driven chain length will affect suspension action by a small amount.

Effectively? It will make no noticable difference I would think unless you a racing MX1 or MX2 at world level.

[GD66]

Trying to turn a chain around a sprocket with less than 14 teeth causes the chain to not seat properly on the sprocket, wasting power and causing wear. Not that it would make any appreciable difference on a dirt bike, but it is verboten on a road racer.

[Lozza]

I'd be very interested to hear how it would make more torque?

Torque is only affected if the ratio changes.

The distance from the centre of the shaft/axle to the PCD of the chain is greater for a larger sprocket=greater drive force on the tyre. Top speed and acceleration change with gearing, that is unless John Bradley from "The Racing Motorcycle Vol 1&2" has it all wrong   

Larger sprockets have a lower chain speed as well.

[Big Bird]

The torque from the gearbox countershaft is your input, and is independent of what sprockets you have chosen.  Your torque at the rear wheel is your countershaft torque multiplied by your sprocket ratio - in both cases,  4.  The difference is in the chain tension

The 14:56 combination has greater "moment arms" than the 13:52 combo, as they are bigger sprockets.  Therefore to transmit the same engine torque, less tension is required in the chain => longer chain and sprocket life.

The larger combination will add to rotational inertia - but that is almost negligible.

[Michael Moore]

Actually, the rotational inertia in the final drive can be significant.  Roadrace EX250 twin Kawasakis will show about 3/4-1 hp increase on an inertial dyno when switching from the stock 5/8" pitch chain and sprockets to 415/428.

The change of sprocket sizes will make a small change in the squat/anti-squat geometry.  Tony Foale's kinematics software has a section that lets you play with that along with pivot height, swing arm length, etc etc.

cheers,
Michael

[Montynut]

I'd be very interested to hear how it would make more torque?

Torque is only affected if the ratio changes.

The distance from the centre of the shaft/axle to the PCD of the chain is greater for a larger sprocket=greater drive force on the tyre. Top speed and acceleration change with gearing, that is unless John Bradley from "The Racing Motorcycle Vol 1&2" has it all wrong   

Larger sprockets have a lower chain speed as well.

I think you should read Mr Bradley again (you can not consider one sprocket in isolation) unless you have invented some sort of machine that gets something for nothing ie "perpetual motion".

Agree with all the other comments as they relate to physical change of masses and geometry (re the suspension squat) of the different setups.

[mustanggrahame]

The larger sprocket set (with same overall ratio) has a higher chain speed for any given wheel speed. This and the fact that there is more chain length and mass must have a significant effect on inertia. This would be much more critical on 125's particularly 70's ones with little bottom end power.
Cheers, Grahame