Sturmey resprocketing

[jimboalee]

When I take 1 tooth off the size of the sprocket, how far back will the rear axle move?

Reduction in sprocket circumference is 1/2" less. Diameter is 0.16" less, Radius is 0.08" less. Axle moves rearward by 0.08"

I haven't got enough dropout, so I take 1 link out of the chain, and the axle comes forward by 0.32", 0.24" from its original position,

If I put a half link in the chain, the axle shifts back 0.16" to be 0.08" forward of its original position.

Result. Now waiting for my 21 tooth sprocket to arrive from Wiggle.

 

[Gerry Attrick]

One tooth less is the same as adding one link of chain.

 

[jimboalee]

One tooth less is the same as adding one link of chain.

Can't be. 1 link of chain is 1", 1/2" extra on the upper and lower stretches of chain.

So, taking off 1 tooth shifts the axle back 1/12". Taking out 1/2 link shifts the axle forward 1/4". So in combination, the axle moves forward 1/6".

That still does nicely.

 

[Gerry Attrick]

For more years than I care to remember, I was involved in the restoration of vintage motorcycles. It was fairly usual to muck about with the sprockets to either alter the gearing to suit racing conditions or because an original size sprocket was not available. The "formula" I used is exactly as i gave you. Never caused me any problems.

 

[RecordAceFromNew]

Hi Jimbo perhaps we are using different terminology, I think I can follow what you said except I don't understand why you said taking one link out of the chain the axle would come forward 0.32"?

I appreciate it is approximate because these calcs can't be accurate without accounting for stay length and respective sprocket sizes (since the chain slope makes the calc nonlinear, but the error is really quite small for normal stay length/sprockets), but I would have thought taking one link (0.5") out the axle should come forward by roughly 0.25"?

You might want to take a look at this if you have not seen it? It seems to me to be quite an interesting way to choose options from what is possible.

 

[jimboalee]

I've been over to a friend's in Bournville this morning who has a 21 tooth Sturmey sprocket. It's done and dusted.

 

[MacB]

Hi Jimbo perhaps we are using different terminology, I think I can follow what you said except I don't understand why you said taking one link out of the chain the axle would come forward 0.32"?

I appreciate it is approximate because these calcs can't be accurate without accounting for stay length and respective sprocket sizes (since the chain slope makes the calc nonlinear, but the error is really quite small for normal stay length/sprockets), but I would have thought taking one link (0.5") out the axle should come forward by roughly 0.25"?

You might want to take a look at this if you have not seen it? It seems to me to be quite an interesting way to choose options from what is possible.

I though a link was 1" and removal/addition would alter the axle by 0.5", or you can use a half link which would be 0.5" and 0.25" respectively, I'll have a nose at the link in your post and Sheldon

 

[jimboalee]

You know when bicycles have taken over your life when you have nightmares about getting the rear axle position correct...