rear sprockett size

[captaincondom]

hi all i am putting a 17" rear wheel in my old fz rather than a 18".but do go smaller on the sprockett or up to keep the gearing the same.

[dracken1]

if you use the same percentage profile tyre as you used on the larger wheel (eg, 150/70/18 to 150/70/17) then you would need a slightly smaller wheel sprocket as you would not be putting a 1" smaller wheel on but only 1/2" smaller as it works based on the radius not the diameter. so one tooth smaller maybe all that is needed.
the way to see the difference is to mark the tyre at the bottom and the ground then rotate it one turn, mark again and measure between the marks on the ground. you'll see theres not a lot of difference.

[Pigpen]

If you're asking about the maths of it, to keep the gearing the same would give you a better acceleration at the cost of a lower top speed, theoretically. However you don't mention the radius of the tyre so it might be negligible and not worth the hassle.

[Pigpen]

opps too late. What he says. ^^^

[captaincondom]

old wheel /tyre 150/60/x18 with 45 teeth on the sprocket
new 170/60x17 with 47 teeth on the sprocket.as yet i have not workd out the radius but the 47 teeth i thought was a bit big.since i do a lot of miles i am after keeping the revs down.and i don't wont to keep takeing the wheel out so i was after getting all the bits ready so its a once only job

[Pigpen]

Assuming those tyre radii to be roughly the same ,then the number of teeth you require is 17/18 x 45 = 42.5. So you should be looking at a 42 or 43 tooth cog. 42 for mpg, 43 for performance.

[johnr]

put a chalk mark on the tyre and the ground, then roll it one complete revolution, mark it and measure it, then do the same with the other. i bet its so neligible that it really wont matter that much, and the new sprocket will probably cost more that the minute difference in fuel consumption if any at all.

[MSVA Tech]

The design diameter of the 150/60 18 is 637mm, for the 170/60 17 it's 636mm meaning a circumference dif of 3 and a bit mm (pi if you're pedantic) so the gearing won't be affected with the original ratio.

Cheers

Chris

[captaincondom]

thanks people .i have bin looking and i can get a 46t for the new back wheel but i can not get a 45.so the other option is to go up on the front. if i go up 1t on the front how many will it take of the rear any idea's i seam to remember reading something about it would take two of the back .
i knew i should stick to a shafty

[MSVA Tech]

Work out the ratios of the various options you have for final drive. It's driven over driver.

For example my Trident has 17 tooth drive and 46 tooth rear wheel as standard, this gives a ratio of 2.706. I have upped the front to an 18 tooth so the new ratio is 2.556.

The lower the ratio the taller the gearing (less acceleration and in theory a higher top speed if the engine can pull it and better fuel consumption on longer runs).

Working backwards 17 x 2.556 = 43.45 so I have the equivalent of losing about 2 and a half teeth off the rear.

Here endeth the lesson

Cheers

Chris

[captaincondom]

thanks for the info