Bottom bracket size / chain line

[jnbrown]

I am upgarding the crankset on my Santana Sovereign.
The rear crankset requires 73x127 BB.
I would like to use 68x109 on the front.
Will the chain line from front to back be OK?
I could use 68x118.5 but they are harder to find and more expensive.

Thanks

[Tandems]

You can use a shorter bottom bracket on the front with no functional chainline issues.

However, you do need to use a bottom bracket made for a 73mm shell on the Santana.

So, a 73x109 or a 73 by 118.5 (using your two examples)

BTW, before going to the 109, I'd eyeballt the current clearance between the crank and the eccentric to make sure there is over 1cm of clearance on each side, or else the crank will tighten touching the frame:(

[mrfish]

Difference will be (127-109)/2 = 8.5mm. Most likely this would work fine since the chain angle will be small in comparison to the angle which the rear chain routinely makes. Disadvantage is that the chain is slightly more likely to come off since there is no guide. You could shim the front chainring out a few mm to minimise the difference. Best to avoid over doing it and catching the crank in the chain. This normally leads to a crash. Unless the tandem is something you are building from scratch on a very tight budget I would recommend buying parts to give the correct chainline from the outset since the cost difference is minor compared with the niggly feeling that the bike isn't quite right wears off.

Second question is why you are thinking about using b/bs designed for 73mm and 68mm shells on the same bike? This seems a bit odd to me - I would have expected the bike to have either one or the other.

...beaten by Tandems!

[ElRey]

If yer trying to get a better "Q" I suggest you reserve that fine distinction for your racing bikes, not a tandem. Stick with the factory standard BB. Try pedals with adjustable Q if you must.

[TandemGeek]

I am upgarding the crankset on my Santana Sovereign. The rear crankset requires 73x127 BB. I would like to use 68x109 on the front.

Why the new cranks and why the narrow bottom bracket? What are you trying to accomplish?

[TandemGeek]

I would have expected the bike to have either one or the other.

It is one... 73mm front & back.

The only two tandems I know of that use bottom bracket shells with different front and rear widths are the Ventana El Conquistador de Montanas and perhaps the Ventana ECdM-derived Ellsworth Witness full suspension off-road tandems, which use a 73mm shell in the back to accommodate the more robust bottom brackets developed for their rear-suspension systems. The front end of these bikes use a 68mm bottom bracket for compatibility with the Bushnell eccentric and associated front bottom bracket eccentric shell.

[jnbrown]

Well I don't why I thought it was 68 in front. I talked with somebody at Santana and they said 73x130 for both front and rear at $80 each compared to $30 for a standard 68X109. No such thing as a 73x118 that I can find. That is one thing I don't like about tandems and Santana in particular, high priced non-standard parts. I do like my Santana though.

Thanks - Joel

[vosyer]

Joel,

I agree with you about non-standard parts $$ for tandems, but a similar frustration exists for finding bottom brackets for older single bikes with what I consider superior older cransets. For this reason I have rebuildable bottom brackets on most of my bikes. Just new bearings in most cases and you are back in business. Yes there is sticker shock to begin with. I have also found that with tandem it pays to have better bracket installed in the first place.