Any gearing experts?

[250Renvy]

I accidentally bought a 39 tooth rear sprocket, so my chain won't fit, but I know a 12 tooth front with a 38 will gear down, so if I have a 12/39 combo will it be like stock?

And if I go to 22" rears and 23" fronts will that gear it up or down?

[86 Quad R]

a 12/39 gearing is mighty low, even with a 22" tire :ermm:

[spent21]

if I have a 12/39 combo will it be like stock?

no.

divide the number of teeth on the front sprocket into the number of teeth on the rear. this will give you a drive ratio, i.e.:

38/13 = 2.923

38/12 = 3.167

39/13 = 3.000

39/12 = 3.250 and so on.


roughly speaking 1 tooth on the front ~ 3 teeth on the rear:

41/14 = 2.928 (here i added 3 to a 38 and 1 to the 13)

remember up a tooth on the front makes for a taller gear (more top end, less 'grunt') while a tooth up on the rear makes for a shorter gear (more 'grunt', less top end).

essientially, if you drop to a 12 (shorter gear) just because your chain won't fit on a 39 (shorter gear), you're going to have a bike that will idle up a hill in 3rd gear, but you could run beside it flat out.
ok, not really, but you could probably start off in 3rd.

taller tires make for a taller final drive ratio. if you like the way it pulls, and you get rear tires that are taller or shorter than what you have, you have to change sprockets to keep your final ratio.

there are some ratio calculators out there. try a search for one of those, but if you want stock gearing, buy stock sprockets.

p.s.
you never said what size tires are on it now. if you have 25'' boggers on the rear, then a 22'' will slow it down. swap that for 18" tires.
the fronts have absolutely nothing to do with gearing, unless of course you're talking about the inability to keep them on the ground with that 12/39 torque monster that you're talking about~!:eek2:

[spent21]

if I have a 12/39 combo will it be like stock?

no.

divide the number of teeth on the front sprocket into the number of teeth on the rear. this will give you a drive ratio, i.e.:

38/13 = 2.923

38/12 = 3.167

39/13 = 3.000

39/12 = 3.250 and so on.


roughly speaking 1 tooth on the front ~ 3 teeth on the rear:

41/14 = 2.928 (here i added 3 to a 38 and 1 to the 13)

remember up a tooth on the front makes for a taller gear (more top end, less 'grunt') while a tooth up on the rear makes for a shorter gear (more 'grunt', less top end).

essientially, if you drop to a 12 (shorter gear) just because your chain won't fit on a 39 (shorter gear), you're going to have a bike that will idle up a hill in 3rd gear, but you could run beside it flat out.
ok, not really, but you could probably start off in 3rd.

taller tires make for a taller final drive ratio. if you like the way it pulls, and you get rear tires that are taller or shorter than what you have, you have to change sprockets to keep your final ratio.

there are some ratio calculators out there. try a search for one of those, but if you want stock gearing, buy stock sprockets.

[250Renvy]

Spent 21 thanks for the info.

As crazy as it sounds, I think I'm going to try the 12/39 just to see how it is. I ride a lot of gully trails that are almost 70degree short hill climbs.

Torque is good. I'll post how it is, but likely I'll go to a 12/38.

Just to clarify - if I go from 20" tires and 13/38 gearing to 22" tires and 12/38 gearing, it should ballance out?

Thanks.

[86250rrider]

yes it will. it will be close to stock effective ratio with 22's and a 12 tooth. i switch between a 13 and 14 on mine with 20 or 22 tires.