Quick Sense check on Final Drive Ratio

[LeeJackson]

Hi all

Been trawling through lots of threads on chain ratios etc and wanted to get some input before ordering parts..

Z650 with z750 Engine - Final Drive stock is 2.63
Running a 17” rear wheel and 530 Chain
Thinking to stop the ratio to about 2.5 to adjust for the 17” wheel

Would this make sense or should I drop it a bit further?

Use will be just short runs etc.

Thanks in advance

Lee

[zed1015]

To duplicate the existing ratio.
Measure the diameter of the 18" wheel/tyre in cm/mm and divide by the number of teeth on the current rear sprocket.
This will give the distance travelled in cm/mm per tooth.
Now measure the 17" wheel/tyre diameter and divide that figure by the cm/mm distance travelled measurement which will give you the number of teeth needed for the new rear sprocket.
Unless you are very lucky the divided 17" diameter figure will include a fraction of a tooth and you will have to decide whether to round up or down to the nearest full tooth.

[DeadZedDave]

What ZED1015 said. You already know you need a higher ratio (numerically lower) With a 530 chain you might try a 38 Tooth rear sprocket assuming you can get one to fit your wheel / sprocket carrier and then for z650/750 there are I believe front sprockets in the range 14 to 17 teeth as standard which should offer you around 2.23:1 up to 2.7:1 with easy change over for testing. Being able to do this however will depend on how you manage your chain alignment. Those standard front sprockets are flat / no offset so ALL of the adjustment would need to be machined off the rear wheel sprocket carrier. If your rear tyre is more than say 160 (at a guess) then this probably is not possible. It may be possible to fit a slim spacer at the front sprocket - but not much. Just as a stab in the dark with gear ratio, back in the 80's I briefly ran a standard z650C with 18" rear and the motor was a Gpz750 (early twin shock type) This bike ran well with gearing as high as 2.23 (38/17) - thats Z1000 ish territory. With your 17 inch wheel it'll also make some difference what profile tyre you have, 60 or 70 ? but maybe 38/16 or 38/17 would be a good place to start ?

[LeeJackson]

Hi Guys

Thanks for the advise, I’m running an offset from sprocket with 18 teeth just to give me some more clearance. So I’m going to go for a standard ZRX rear which I have in good order, giving me around 2.5 and take it from there. Easy to go smaller later and remove a link if needed etc.

Will keep you posted! But of time to go yet till it’s at test run stage!

Thanks
Lee