Author Topic: What size transfer rings?  (Read 1252 times)

What size transfer rings?
« on: July 11, 2012, 08:51:44 pm »
Trying to fettle a tandem trike which hasn't come with a transmission. It has a conventional eccentric bottom bracket in the front. If I measure the bottom brackets centre-to-centre with the eccentric in the "slack" position, how do I calculate the size of transfer rings that best suit? Is there a formula for this, or is it just trial and error?

LittleWheelsandBig

  • Whimsy Rider
Re: What size transfer rings?
« Reply #1 on: July 11, 2012, 11:21:41 pm »
It doesn't matter. As long as the chainrings are the same size, the required chain length increases by 1" for every extra tooth.

A properly designed tandem has the length of the bottom tube (and reartop tube) chosen to let the eccentric be nearly all the way back with a new chain and nearly all the way forward with a severely worn chain. At that point, you remove a link (1" of chain) and rotate the eccentric back and keep going.

Bigger timing chainrings reduce wear on the chain, chainrings and BBs. Smaller rings reduce the weight slightly and flex the frame more. Pick something bigger than 40t.
Wheel meet again, don't know where, don't know when...

Re: What size transfer rings?
« Reply #2 on: July 12, 2012, 08:42:22 am »
Most tandems are built so that you get full adjustment from the eccentrc using rings with an even number of teeth. I had one that wasn't.

Re: What size transfer rings?
« Reply #3 on: July 12, 2012, 10:19:55 am »
It doesn't matter. As long as the chainrings are the same size, the required chain length increases by 1" for every extra tooth.
A properly designed tandem has the length of the bottom tube (and reartop tube) chosen to let the eccentric be nearly all the way back with a new chain and nearly all the way forward with a severely worn chain. At that point, you remove a link (1" of chain) and rotate the eccentric back and keep going.
To be honest, that's what I'd thought - it seems logical. But to fill out the info in my first post, the frame came to me with a new transmission set (not "original" to the frame) with a pair of 39t transfer rings. With these 39t rings, I couldn't get the chain taut until nearly all the range available from the eccentric BB was used up. If I removed a link, the chain was too short. So I swapped out the 39t rings and fitted some 40T ones I luckily had lying around and this fixed things. Hence I started to think about calculating the the correct length and wondered if I needed more education on the matter!

Most tandems are built so that you get full adjustment from the eccentrc using rings with an even number of teeth. I had one that wasn't.
I might possibly have an example of the opposite way round!

But I wonder, does the framebuilder have a "formula", or is this just one of those "trade secrets" bits of knowledge that those in the business just acquire along the way?

LittleWheelsandBig

  • Whimsy Rider
Re: What size transfer rings?
« Reply #4 on: July 12, 2012, 10:48:42 am »
It is incredibly easy to calculate given unworn 1/2" pitch chains and the range of the eccentric. Some stupid eccentrics don't allow in excess of 1/2" movement but they are rare.
Wheel meet again, don't know where, don't know when...