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Originally Posted by autotravis
I see a lot posted about sprocket changes (like theycallmebryan's 250r) and I was wondering if someone could explain how they work.
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Take the rear sprocket's number of teeth, divide by the front sprocket's teeth.
That's the final drive ratio.
[let's say you have a 15 front, 45 rear setup, drive ratio is 3 - 1 revolution of the wheel takes 3 revolutions of the output shaft of the engine]
The higher that ratio is, the more rpm the engine will need to do to turn the rear wheel for a given speed.
The lower the ratio is, the less rpm needed for a given speed.
Lower rpm means less fuel is going to the engine over a same distance.
That's what your car's overdrive or top gear does.
Say you could do 60mph @ 6000 rpm using the common standard ratio of 3 (45:15)
An extreme 34:17 setup would give a ratio of 2
3:2 = 1.5
6000:1.5 = 4000 rpm @ 60 with the 34:17 setup
A more moderate 36:16 gives 2.25
3 : 2.25 = 1,33 and 4500 rpm @ 60
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Switching the back sprocket out for a lower number of teeth can increase the fuel mileage by lowering rpms?
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Yes.
You can also add teeth to the front.
1 more tooth at the front equals about 3 teeth on the rear
(as most bikes are set up around 1:3)
When adding teeth to the front sprocket, make sure the chain still clears the chain cover, and any possible pushrod to the clutch in front of it
(Suzuki aircooled 4-in-lines like the bandit come to mind).
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At what point do the benefits cease?
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Well, the engine must be able to - literally - pull it off.
Something with plenty of torque at low rpm would do well.
And the rider must be OK with the reduced acceleration - others who are less concerned about mileage would likely call it sluggish
As Old Mech already pointed out, the lower rpms also load up the engine more during acceleration as you make it work harder at low rpm, hence it produces those HP more efficiently.
That's a second benefit.
It also reduces vibrations, most of the time, though you should avoid bringing your most frequently used engine speed into the rpm range where the engine's natural vibrations are - on a 4 in line, this is often around 4000 rpm.
See where your bike passes off more vibrations than usual to you, and avoid getting in that rpm range by regearing it.