Final drive ratios with 17" wheels.

[zed1015]

There's been a lot of discussion just lately about what size sprockets to use with 17" wheels on the zeds so i thought i'd post some calcs i used which should solve everyones problems. Lets say you run std 15-33 on 18" wheels and want to go 17" but keep the same 2.2 -1 ratio equivalent and associated rpm per mph ratio. Taken from where the tyre bead sits an 18" rim has a circumference very close to 144cm . So divide this by 33 (std amount of rear sprocket teeth) and you get 4.3636 cm per tooth. Now measure the 17's circumference and you get as near as 136cm. And when divided by the 4.3636cm you get 31.16 (Rear sprocket teeth) which will have to be rounded down to 31 . Now 31 divided by 15 (front sprocket teeth) = 2.06 which is the new ratio to work from for the 17" rims. So depending on which front sprocket and pitch you are running you will end up with one of these closest equivalents - 15/31 - 16/33 - 17/35 - 18/37 or 19/39 . OK ?

[Rich]

The rim size doesn't matter, it's the circumference of the tyre that does. The only time that the wheel size plays a part is in the calculation of the circumference which also includes the width of the tyre and aspect ratio.

[zed1015]

The rim size doesn't matter, it's the circumference of the tyre that does. The only time that the wheel size plays a part is in the calculation of the circumference which also includes the width of the tyre and aspect ratio.

I've tried to keep this simple and use a constant measurement which should not change from wheel to wheel and not confuse anyone with aspect ratios giving differing measurments (which would only make a difference of a fraction of a tooth anyway) . I have also done the calcs with the measurements taken from tyre circumferences and it works out the same give or take point 000.0 whatever depending on what profile tyres fitted which makes no difference in the real world.

[Rich]

You do it your way, I'll do it mine

[zed1015]

As i stated , My results give the closest to the std 2.2-1 equivalent give or take a fraction of a tooth. Feel free to post your findings. Sure you won't be more than half a tooth away from mine on the rear cog depending on tyre profile.

[Pigford]

Even better way is to measure the rolling circumference. Mark a point on the tyre & ground. Push bike 1 x revolution of wheel & mark ground to show where it stops. Measure the distance between the 2 x marks on the ground. Most accurate circumference measurement out there coz it allows for tyre pressures too

For all us normal chaps..... try the bike with whatever you have laying about, fitted on the bike or what you have to hand... and go from there

[zed1015]

Measuring the rims is a direct comparison between 17" and 18" with no variables. Doesn't matter what profile tyre or air pressure is used, gonna work out the same like for like. The whole point of this thread was to give the closest std equivalent gearing as a starting point for anyone fitting 17"s so that they are not miles out and not to get into annaly retentive detail nit picking. I don't mind going into a more detailed technical explanation of the calculus and all possible variables but don't see why i should when i know i'm right.

[Pigford]

Zed1015, a wise man always explores all the alternatives

All the input is useful, and I, like any other reader of these thread will pluck out the relevant info as required. Your response was very good, but I ust couldn't resist adding my bit of cack

[zed1015]

18" wheel with avon roadrunner 400H/18. 38 psi cold , slightly worn tread but loads of wear left. Day is wednesday ,Time is 8.15 am, a slight breeze coming from the east and the postmans just been. With these important variables ? 18" circumference is 208.1 cm . Divide this by 33 rear sprocket teeth (New batterys in calculator of course) and you get 6.303030 cm distance travelled per tooth. 17" wheel with a 180/55/17 dunlop sportsmax slightly worn tread ( but not as much as the avon) 38psi (again cold for accurate comparison ) Still wednesday, 8.25 am, a slight breeze coming from the east but the postman has now gone although the sun has come out to compensate. With this taken into account 17"s circumference is 195.6cm which divided by 6.303030 gives 31.032693rear sprocket teeth, and when divided by 15 ( front sprocket teeth) you get "lo and behold" 2.0688462. Which even if rounded up to 2.07-1 only gives 31.05 rear sprocket teeth for a closest std ratio equivalent of 15/31. DISCLAIMER - Please Note , Results may differ slightly to those shown due to - Time of the month , Birds flying south, Colour of underpants or whether your gran's just popped in for a visit.

[Robw]

[Ginger Bear]

Well my head is 'cabbaged' after all this talk. Cogswallop comes to mind!

I agree with Zed1015 re: 17" wheels. It's something that people didn't really want to take into account when advising me! Problem I have is my postman doesn't arrive till nearly lunchtime....... Should I add or deduct teeth from the rear sprocket to compensate for this

On a more positive note, I came accross this on the Wemoto web site yesterday! http://shop.wemoto.com/info.dyn?title=S ... ear_Ratios

[zed1015]

I agree with Zed1015 re: 17" wheels. It's something that people didn't really want to take into account when advising me! Problem I have is my postman doesn't arrive till nearly lunchtime....... Should I add or deduct teeth from the rear sprocket to compensate for this

Just depends on whether it's bills / junk mail or parcels he's delivering . P.S. Again use 15/31 - 16/33 - 17/35 - 18/37 - 19/39 for the closest to std gearing as a starting point when using 17"s. Although when using 530 pitch i'd recommend 19/39 at least to try and overcome chain to s/arm clearance issues associated with fatter ally swing arms and also a 39/40 tooth 530 rear cog has approximately the same core diameter as the std 630 33 tooth which is probably the smallest you would want to go.

[debben1]

I have some sums for this which I used for gearing my own bike as tachos, speedos and riders judgement are all subject to error

[debben1]

Primary reduction ratio is 56/97 = 1.7321428
5th gear reduction ratio is 23/28 =1.2173913
Final reduction ratio is 15/33 = 2.2
Overall rreductio ratio = 4.64
So the cranks spinning at 1000 rpm then the clutch and input shaft are spinning at 577.3196
Input shaft at 577.3196 then in top gear the output shaft and front sprocket is spinning at 474.22681
With the front sprocket turning at 474.22681 the rear wheel would be turning at 215.55764
Therefore the crank at 1000 rpm and the wheel at 215.55764 rpm an overall reduction ratio of 4.6391303

Circumference of a 17 "rim (431.8mm) with a 180/55 (55% of 180mm = 99mm x 2) = total diameter of 629.8mm
629.8 x 3.14 = 1977.572 (circumference in milimeters)

Wheel speed of 215.55764 per minute travels 426'280.75 milimetres a minute, or 25'576'845 an hour, there are
1609344 milimeters in a mile so divide 25'576'845 by 1609344 and you have 15.892714 miles an hour for every 1000rpm

8000rpm = 127.14171mph
10000rpm = 158.92714mph

Change to a Final reduction ratio of 2.02526315 19/39
and the Overall reduction ratio is 4.2706635

Wheel speed of 234.15565 per minute travels 463'059.65 milimetres a minute, or 27'783'579 an hour, there are 1609344 milimeters in a mile so divide 27'783'579 by 1609344 and you have 17.263915 miles an hour for every 1000rpm

8'000rpm = 138.11132 mph
10'000 rpm = 172.63915 mph

17/38 = Final reduction ratio of 2.2352941
wheel speed at 1000 rpm= 212.1541

Wheel speed of 212.1541 per minute travels 419'550 milimetres a minute, or 25'173'000 an hour, there are
1609344 milimeters in a mile so divide 25'173'000 by 1609344 and you have 15.641777 miles an hour for every 1000rpm

8'000rpm = 125.13421 mph
10'000rpm = 156.41777 mph
I hope this helps, Regards, Steve

[zed1015]

Was trying to keep this VERY simple as rough guide as to where to start when fitting 17"s and that's it. especially with 530 pitch. Could have gone into how a 33 tooth on an 18" is worth APPROX 63mm distance travelled per tooth and only 59mm Approx on a 17" and will create a discrepancy of a tooth here or there but that wasn't the point.