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Sprocket Ratios

Started by teeds, November 06, 2018, 06:58:30 pm

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teeds

November 06, 2018, 06:58:30 pm Last Edit: November 08, 2018, 09:41:58 am by teeds
Can someone clarify if the GPB gear ratio settings represent the number of sprocket teeth?

Manu


teeds

Thanks Manu.

So based on that is there a bug where, I think with all bikes, 1 tooth change on the rear incorrectly makes more difference in speed than 1 tooth change on the front?

My understanding is that 1 tooth change on the front equates to a 3 tooth change on the rear.

Manu

November 06, 2018, 11:08:06 pm #3 Last Edit: November 07, 2018, 12:00:35 am by Manu
Supposedly the front sprocket (driver) has a shorter diameter (less teeth) therefore it rotates faster than the rear one (more teeth) when they are connected by a chain. Therefore the change of one teeth in the front sprocket makes more difference in speed than changing one in the rear.

If front sprocket is changed with less teeth = Less bike speed
If front sprocket is changed with more teeth = More bike speed

If rear sprocket is changed with less teeth = More bike speed
If rear sprocket is changed with more teeth = Less bike speed

HornetMaX

Quote from: teeds on November 06, 2018, 09:46:19 pm
My understanding is that 1 tooth change on the front equates to a 3 tooth change on the rear.

Depends on the number of teeth.

Starting from 2 front and 4 rear, ratio is 1/2. If you have -1 front you get 1/4. To get the same 1/4 adding to the rear, you have to do +4 (2/8).

There's no absolutely correct rule like "-1 front = +3 rear" (even if it may be a decent approximation in some usual ranges).

teeds

Fair comment Max, I'm going by my MX experience so probably not ideal for GPB comparison. But regardless of exact ratios I think it still remains that 1 tooth change on the rear will never affect speed more than 1 tooth change on the front as I've never seen a bike with more teeth on the front sprocket than the rear.


HornetMaX

Give a concrete example (numbers) and we can sort this out in no time.

I remember a long time ago I checked GPB calculation of top speed (shown in the garage) and I got the same exact results.

teeds

My logic doesn't seem to have been shot down so i'll get some examples  :)

Only seen this in non stock bikes, stock bikes seem fine.