Rear sprocket question

[craigdiver]

I have a chinese 125 which sits at around 8000rpm at 60mph, I use the bike for commuting and would like to bring the revs down a bit for economy. my rear sprocket is a Crocodil 428-38T (38 teeth). A friend has suggested I can bring the revs down or increase speed by 5mph by changing to a 37 tooth sprocket (or less). I understand that it will reduce acceleration.

Has anyone any experience of going down 2 teeth, will it give better economy or will the engine just working harder at lower revs use the same fuel and wear the engine the same?

Any advice appreciated........

Craig
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600 Km on the clock and rising!

[paddlesat16]

yep that will work, also up 1 tooth on the front will help as well if you want to reduce the revs further and make the gears longer. Tho you go from riding a bike to driving miss daisy.

Regards
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I once saw a bloke shagging a donkey.... in Saudi arabia. Theory Test Passed 26/10/09 Mod 1 Passed 26/4/10, Mod 2 Passed 7/6/10 Current Bikes Suzuki GSX1400 K5, Aprillia Pegaso 650, Suzuki DR350T.

[craigdiver]

[Kickstart]

Hi

Assuming it has the power to pull the higher gearing, then yes a smaller front sprocket will mean less revs for the same speed. However going from 38 to 37 teeth is going to mean doing you are doing ~7800rpm at 60mph rather than 8000rpm.

I wouldn't be certain that it will happily pull much higher gearing. Also using higher gearing to get better fuel consumption is only going to work if you can still sit in top, rather than be forced to change down all the time.

Going down 2 teeth on the back will probably mean that the chain needs to be shortened.

Increasing the size of the front sprocket has a similar effect as reducing the size of the rear sprocket (just that the front has FAR less teeth so a one tooth change is a far bigger percentage change).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[craigdiver]

What concerns me, being a car driver for 20+ years and being very new to motorbikes, is that sitting at 8000rpm for prolonged periods seems like kicking the 4r5e out of the engine. My car would explode at 8000rpm!
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600 Km on the clock and rising!

[DrDonnyBrago]

[Kickstart]

Hi

All comes down to what they are designed for. Rpm is a limiting factor based on how quickly things move up and down (and to a far less extent how quickly things rotate). How far they move up and down and how heavy they are basically determines how many revs you can use. Your cars pistons probably weigh twice as much and move twice as far, putting far more strain on things for the same revs.

Honda sold the CBR250 a while ago as a normal road bike which had a red line at 19500rpm.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing