FurryOnTheInside
In Memoriam
I thought it'd be good to have a thread discussing the effects of changing the stock S.B.P. Shift Kit 17T sprocket for the new 21T sprocket that Sick Bike Parts just released. Perhaps we can collectively come up with suggestions or answers to the question of what sprockets/chainrings are needed to make best use of the new 21T.
Or just enthuse about it nerdily.
I am very excited about it and can hardly contain myself. It seems it will (well it IS intended to) solve the issue of matching optimum or at least reasonable engine RPMs with optimum or at least reasonable pedalling RPMs, for those riders who like to pedal while being motor-assisted by their engine, both staying happy and working together as one!
It will (and is intended to) also slow the RPMs of the jackshaft itself, which will (if the RHS jackshaft sprocket remains unchanged) slow the speed of the RHS/secondary/intermediate chain (to approx 80% of what it was with the 17T); that should help enormously to stabilise it and make the RHS chain tensioner's job easier (less chance of chain coming off the tensioner(idler) wheels), and extend the life of the tensioner(idler) wheels.
All in all, I think the 21T sprocket from Sick Bike Parts is a GIANT leap forward.
Now, correct me if I'm wrong....
An increase in tooth count from 17T to 21T is 123.5%
If the outer/input chainring at the pedal cranks is left the same, it means the final/combined drive sprocket at the cranks would need to be enlarged also, by 123.5%, assuming you want to keep the road speed unchanged at the same engine RPMs.
If you were previously using a 36T for the final/combined drive chainring, then a 44T would give (near enough) the same road speed, because 36 x 123.5% = 44.46
If you were previously using a 44T for the final/combined drive chainring, then a 54T would give (near enough) the same road speed, because 44 x 123.5% = 54.34
If the final/combined drive chainring is left the same, it means the outer/input chainring would need to be reduced in size/tooth count, assuming you want to keep the road speed unchanged at the same engine RPMs.
If you were previously using a 48T for the outer/input chainring, then a 39T would give (near enough) the same road speed, because (48 / 123.5) x 100 = 38.86 or 48 x 80.97% = 38.86
If you were previously using a 44T for the outer/input chainring, then a 36T would give (near enough) the same road speed, because (44 / 123.5) x 100 = 35.62 or 44 x 80.97% = 35.62
I'd like to get a 54T final/combined drive chainring personally. Only road bike frames will accept such a large chainring in the normal position (distance from bike's centre line) but that is fine for me. 54T chainrings are included on road bike triple chainsets, sometimes on doubles if intended to run in the outer position. To use a road bike chainset with a S.B.P. Shift Kit requires the optional S.B.P. spider, probably the 5-arm 130mm BCD version. I'll have to find out what issues there are with combining the road bike chainset with the S.B.P. spider and whichever outer/input chainring I end up using. I think it won't be a problem, but I know I'm going to have a lot of fun though.
Which chainrings/sprockets will you use when you make the leap from 17T to 21T?
Or just enthuse about it nerdily.
I am very excited about it and can hardly contain myself. It seems it will (well it IS intended to) solve the issue of matching optimum or at least reasonable engine RPMs with optimum or at least reasonable pedalling RPMs, for those riders who like to pedal while being motor-assisted by their engine, both staying happy and working together as one!
It will (and is intended to) also slow the RPMs of the jackshaft itself, which will (if the RHS jackshaft sprocket remains unchanged) slow the speed of the RHS/secondary/intermediate chain (to approx 80% of what it was with the 17T); that should help enormously to stabilise it and make the RHS chain tensioner's job easier (less chance of chain coming off the tensioner(idler) wheels), and extend the life of the tensioner(idler) wheels.
All in all, I think the 21T sprocket from Sick Bike Parts is a GIANT leap forward.
Now, correct me if I'm wrong....
An increase in tooth count from 17T to 21T is 123.5%
If the outer/input chainring at the pedal cranks is left the same, it means the final/combined drive sprocket at the cranks would need to be enlarged also, by 123.5%, assuming you want to keep the road speed unchanged at the same engine RPMs.
If you were previously using a 36T for the final/combined drive chainring, then a 44T would give (near enough) the same road speed, because 36 x 123.5% = 44.46
If you were previously using a 44T for the final/combined drive chainring, then a 54T would give (near enough) the same road speed, because 44 x 123.5% = 54.34
If the final/combined drive chainring is left the same, it means the outer/input chainring would need to be reduced in size/tooth count, assuming you want to keep the road speed unchanged at the same engine RPMs.
If you were previously using a 48T for the outer/input chainring, then a 39T would give (near enough) the same road speed, because (48 / 123.5) x 100 = 38.86 or 48 x 80.97% = 38.86
If you were previously using a 44T for the outer/input chainring, then a 36T would give (near enough) the same road speed, because (44 / 123.5) x 100 = 35.62 or 44 x 80.97% = 35.62
I'd like to get a 54T final/combined drive chainring personally. Only road bike frames will accept such a large chainring in the normal position (distance from bike's centre line) but that is fine for me. 54T chainrings are included on road bike triple chainsets, sometimes on doubles if intended to run in the outer position. To use a road bike chainset with a S.B.P. Shift Kit requires the optional S.B.P. spider, probably the 5-arm 130mm BCD version. I'll have to find out what issues there are with combining the road bike chainset with the S.B.P. spider and whichever outer/input chainring I end up using. I think it won't be a problem, but I know I'm going to have a lot of fun though.
Which chainrings/sprockets will you use when you make the leap from 17T to 21T?