oversized chain and sprockets...opinions please....

[dixie]

Hi guys, after some advice on buying an oversized chain and rear sprocket....

The bike is having trouble going from 4th to 5th gear, losing speed and power rapdily.

It's been in for a service and mechanic suggests buying a 45 tooth sprocket instead of the standard 42.

Is this actually going to make a lot of difference?

Bike is a CBR125 by the way

Thanks, Dixie

[Guest]

Fozzie is probably ya man, always tinkering with 125s, I have no idea meself Dixie, having never done it...

In the meantime I have put this in pitstop for ya

[Steve1988]

I've no experience with this other than knowing my way around the gears on a bicycle, but from just thinking about it I can't see how putting a bigger sprocket and new chain will help the fact that you struggle to change from 4th to 5th. Surely a bigger rear sprocket will mean you need to be in the higher gears more often. Plus rear sprocket and chain simply drive the rear wheel, not necessarily related to the way the bike changes gear, although i would imagine correct tension would make a difference to gear changes.

This has caught my interest, would like to see what an expert thinks about it!

Edited May 30, 2010 by Steve1988

[2fast2soon]

I think it'll be a torque issue, the engine just won't have enough get up and go past a certain point (the speed its doing when changing from high revs in 4th to lower revs in 5th).

The bigger sprocket would take the top speed of the bike down. I'm guessing the mechanic knows the torque curve and has worked out that the 45 tooth sprocket will put the bike into a better part of its power band when shifting between 4th and 5th. If its a reputable mechanic then I'd trust him/her over the ramblings of a numpty like me though!

[dixie]

That seems to make sense....mechanic is a very good friend, still like to get all your opinions tho

[fullscreenaging]

If you go up on the back sprocket you'll lose top end speed anyway. It will just struggle even more. You can up on the front sprocket one tooth to give you better top speed, but sacrafice low end torque. I wouldn't go any more than one tooth with a 125.