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Old 08-27-07, 08:04 AM   #1
staplemachine
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trek 1000 bb lengh

My lbs swapped out a triple crank for a double crank for me.He changed the 113mm bottom bracket for a Shimano UN54 68x110mm BB. All my research and the bontranger(truvativ) literature that came with the crank tells me different.I should be using a shimano un54 107 or a truvativ 108mm bb.Any one know what length bb the stock 2006 or 2007 trek1000 double comes with?
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Old 08-27-07, 08:35 AM   #2
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Your concern should be whether or not the resulting chainline that you have is acceptable. Shift into the little/big combo and the big/little combo and see how straight the chain is. If the chain is slanting in the same direction (either towards or away from the bike) in both of those combos, then your bottom bracket length is not the right length. Ideally, the chain rings are centered on the cassette allowing you the best compromise of a chainline while using any gear combination.

Trek may have used a 107 BB and that may be a compromise versus a 110 BB or it could be that your LBS made the compromise. Either might have made their choice simply based on what they had in stock. It's very possible that both lengths will give you basically the same chainline compromise just that one length has the compromise slightly shifted towards the bike and the other has it away from the bike. After all, the different is only 1-1.5mm on the crank side.

Basically, if the bike shifts ok using the normal gear combos (avoiding the little/little and the big/big) I wouldn't worry about it.
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Old 08-27-07, 08:46 AM   #3
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Thanks for reply.
I removed spacers that the shop put in the small ring to avoid chain rub and adjusted f derailer a bit.Im getting 5 highest gears on big ring before it rubs derailer and 5 or 6 low gears on the small ring before chain rubs the side of big ring.Im ok with this combo.Chain line is not 43.5 as Sheldon Browns chart shows it is more like 46mm which is why I started researching the issue.
Thanks again
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Old 08-27-07, 08:59 AM   #4
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I just doublechecked and realised 46 is front chainline and the chart shows rear chainline. oops
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Old 08-27-07, 10:26 AM   #5
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They should be the same front and rear.
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Old 08-27-07, 11:31 AM   #6
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Well then its definatly out.
I forgot to mention that its an 8 speed bike.
For all I know 5-6 high gears on big ring and 5-6 low gears on small ring is good.
lowest big ring and highest small ring gears may actually be similar ratios?
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Old 08-27-07, 01:45 PM   #7
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Originally Posted by staplemachine View Post
Well then its definatly out.
I forgot to mention that its an 8 speed bike.
For all I know 5-6 high gears on big ring and 5-6 low gears on small ring is good.
lowest big ring and highest small ring gears may actually be similar ratios?
On an 8 speed bike, I'd be annoyed if I could only get to 5 or 6 of the eight cogs in the rear without rubbing. It should definitely be able to be setup so that you could use the entire cassette without any rubbing should you want to (yes, cross chaining is "bad" but it's also useful quite often depending on where you ride). Unless your bike has very short chain stays, I'd be surprised if you could not dial out the rubbing (with the correct chainline to start of course).
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Old 08-28-07, 01:07 PM   #8
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Unless your bike has very short chain stays, I'd be surprised if you could not dial out the rubbing (with the correct chainline to start of course).
The LBS did this by adding washers between spider and small ring.I took them out because I think that thats a band aid fix.The problem IMO is the BB that was installed was too long
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