Matching front and rear sprockets

We are just finishing off a GSXR K7 rear wheel conversion onto a Curvy.
My dad wants to increase top end cruising speed at the expense of acceleration. So would normally use a larger rear sprocket. But we have to remove the front sprocket to flip over to give the aliment needed to the rear sprocket, so while its off he wants to fit a new Renthal one. He has a new rear Renthal 45T, Which he would like to use, but gives the opposite to what he wants.
So the question is, what size front sprocket, if any, will give the required higher cruising speed with the 45T rear.
Thanks
Mike

[mikerj]

You would use a smaller rear sprocket to lower RPM at cruising speed (at the expense of acceleration).

15F/45R is the standard gearing for a naked SV650 giving 3:1, 15R/44R is standard for the SV650S giving 2.93:1. If you fit a 16 tooth front sprocket with your 45 rear you get 2.81:1 which seems a reasonable compromise. 6000 RPM with standard 650S gearing would drop to 5754RPM for the same speed.

Originally Posted by mikerj

You would use a smaller rear sprocket to lower RPM at cruising speed (at the expense of acceleration).

15F/45R is the standard gearing for a naked SV650 giving 3:1, 15R/44R is standard for the SV650S giving 2.93:1. If you fit a 16 tooth front sprocket with your 45 rear you get 2.81:1 which seems a reasonable compromise. 6000 RPM with standard 650S gearing would drop to 5754RPM for the same speed.

Thanks Mike, thats just what ineeded. Do you have a formula for the different combinations?
Mike

[maviczap]

One tooth up on my S model front sprocket was a great mod, less vibes and sits in the right rev range for easier motorway cruising

one tooth up on the front would then equate to how many dropped on the rear? one tooth up on the front (16/45) how many revs are you doing in top wheel at 80mph? thanks

Originally Posted by shelleyamy

Thanks Mike, thats just what ineeded. Do you have a formula for the different combinations?
Mike

Divide the rear sprocket teeth by the front sprocket teeth this will give you the ratio,

To compare the standard 3:1 at 6000 rpm with your newly calculated one, :-

new ratio
----------- x 6000 = new rpm. (for the same speed of standard @6000)
3

Obviously substitute any rpm figure for the 6000 to find new revs at any engine speed.

to make it easier you could just Multiply the ratio by 2000 and it will give you the rpm that is equivalent to the standard bike at 6000rpm.

maybe I am wrong? I thought the 2.81:1 ratio would be 5620rpm????

long time since I went to school so hope Im not wrong but willing to be corrected. Dave.

[maviczap]

Originally Posted by AirArmed

one tooth up on the front would then equate to how many dropped on the rear? one tooth up on the front (16/45) how many revs are you doing in top wheel at 80mph? thanks

bout 5500 I think

[mikerj]

Originally Posted by warmshed

Divide the rear sprocket teeth by the front sprocket teeth this will give you the ratio,

To compare the standard 3:1 at 6000 rpm with your newly calculated one, :-

new ratio
----------- x 6000 = new rpm. (for the same speed of standard @6000)
3

Obviously substitute any rpm figure for the 6000 to find new revs at any engine speed.

to make it easier you could just Multiply the ratio by 2000 and it will give you the rpm that is equivalent to the standard bike at 6000rpm.

maybe I am wrong? I thought the 2.81:1 ratio would be 5620rpm????

The RPM example I gave was going from standard SV650S gearing to 16F/45R. Going from naked SV you would get 5620RPM as you correctly calculated

Ah! then if you need to compare on the 650S rather than the naked, you will need to change the equation to:-

new ratio
----------- x 6000 = new rpm. (for the same speed of standard @6000)
2.93

Should have re-read your post. thanks for putting me right.

[mikerj]

If it helps I have put the SV gearing into a spreadsheet so you can fiddle with sprocket teeth, tyre size etc. and see the effect on speed in each gear.

N.B. Primary and gear ratios were found on a random web site, they look right but I can't guarantee their accuracy!