BB and Chainline Questions
02-17-07, 09:28 PM
#1
Senior Member
Thread Starter
BB and Chainline Questions
I have an 05 Surly Cross Check complete I'm in the process of converting to a triple up-front. I added a 26t Salsa ring and changed the FD to one that will accomodate a triple (XT FD-M761).
I will now need to replace the bottom bracket with one that is wider. This is because the FD cannot shift down to the bottom ring because it (the ring) is in too tight to the frame. The bike comes stock with a 68 x 113 BB. From Sheldon Brown I have figured out that the Shimano spec. chain line for a road bike is 43.5 mm with the stock double; Shimano recommends 45 mm (measured to the middle chain ring) for a road triple set-up (I believe this assumes the road-standard 130 mm rear spacing). Given my Cross Check has 135mm rear spacing, the same as is the mountain bike standard, would I use Shimano's MTB specs? If so, Shimano specs a MTB triple chain line at 47.5 - 50 mm.
A replacement BB 68 x 118 would give me a chain line of 46 mm, which would nearly match the road specs. Or, should I consider going even wider, say 68 x 122.5 (this would increase the chain line to 48.25 which would better fit MTB specs)? It seems like the narrower 68 x 118 would give me better access to the big ring. I ride it exclusively on the road.
I'd like to space the small ring out far enough to be able to shift down to it, as well as maintain as close to a proper chain line as possible. Given all this, what BB do I get: 68 x 118 or 68 x 122.5?
Thanks in advance!
I will now need to replace the bottom bracket with one that is wider. This is because the FD cannot shift down to the bottom ring because it (the ring) is in too tight to the frame. The bike comes stock with a 68 x 113 BB. From Sheldon Brown I have figured out that the Shimano spec. chain line for a road bike is 43.5 mm with the stock double; Shimano recommends 45 mm (measured to the middle chain ring) for a road triple set-up (I believe this assumes the road-standard 130 mm rear spacing). Given my Cross Check has 135mm rear spacing, the same as is the mountain bike standard, would I use Shimano's MTB specs? If so, Shimano specs a MTB triple chain line at 47.5 - 50 mm.
A replacement BB 68 x 118 would give me a chain line of 46 mm, which would nearly match the road specs. Or, should I consider going even wider, say 68 x 122.5 (this would increase the chain line to 48.25 which would better fit MTB specs)? It seems like the narrower 68 x 118 would give me better access to the big ring. I ride it exclusively on the road.
I'd like to space the small ring out far enough to be able to shift down to it, as well as maintain as close to a proper chain line as possible. Given all this, what BB do I get: 68 x 118 or 68 x 122.5?
Thanks in advance!
02-17-07, 09:41 PM
#2
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Originally Posted by hopperja
I have an 05 Surly Cross Check complete I'm in the process of converting to a triple up-front. I added a 26t Salsa ring and changed the FD to one that will accomodate a triple (XT FD-M761).
I will now need to replace the bottom bracket with one that is wider. This is because the FD cannot shift down to the bottom ring because it (the ring) is in too tight to the frame. The bike comes stock with a 68 x 113 BB. From Sheldon Brown I have figured out that the Shimano spec. chain line for a road bike is 43.5 mm with the stock double; Shimano recommends 45 mm (measured to the middle chain ring) for a road triple set-up (I believe this assumes the road-standard 130 mm rear spacing). Given my Cross Check has 135mm rear spacing, the same as is the mountain bike standard, would I use Shimano's MTB specs? If so, Shimano specs a MTB triple chain line at 47.5 - 50 mm.
A replacement BB 68 x 118 would give me a chain line of 46 mm, which would nearly match the road specs. Or, should I consider going even wider, say 68 x 122.5 (this would increase the chain line to 48.25 which would better fit MTB specs)? It seems like the narrower 68 x 118 would give me better access to the big ring. I ride it exclusively on the road.
I'd like to space the small ring out far enough to be able to shift down to it, as well as maintain as close to a proper chain line as possible. Given all this, what BB do I get: 68 x 118 or 68 x 122.5?
I will now need to replace the bottom bracket with one that is wider. This is because the FD cannot shift down to the bottom ring because it (the ring) is in too tight to the frame. The bike comes stock with a 68 x 113 BB. From Sheldon Brown I have figured out that the Shimano spec. chain line for a road bike is 43.5 mm with the stock double; Shimano recommends 45 mm (measured to the middle chain ring) for a road triple set-up (I believe this assumes the road-standard 130 mm rear spacing). Given my Cross Check has 135mm rear spacing, the same as is the mountain bike standard, would I use Shimano's MTB specs? If so, Shimano specs a MTB triple chain line at 47.5 - 50 mm.
A replacement BB 68 x 118 would give me a chain line of 46 mm, which would nearly match the road specs. Or, should I consider going even wider, say 68 x 122.5 (this would increase the chain line to 48.25 which would better fit MTB specs)? It seems like the narrower 68 x 118 would give me better access to the big ring. I ride it exclusively on the road.
I'd like to space the small ring out far enough to be able to shift down to it, as well as maintain as close to a proper chain line as possible. Given all this, what BB do I get: 68 x 118 or 68 x 122.5?
Sheldon "Depends On What Gears You Use Most" Brown
02-18-07, 01:44 PM
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With the dérailleur adjusted correctly, shifting shouldn't be a problem. The compromise will tend to cost you use of the smaller rear cogs with the small front ring: not a commonly used combination anyway.
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02-23-07, 02:52 PM
#5
Senior Member
Thread Starter
Sheldon and DMF, thanks for your help with this. Instead of replacing the perfectly good 68 x 113 BB I have (with only 800 miles on it), I went ahead and got a 2.5mm spacer. This has the effect of moving the whole thing over 2.5mm, which in terms of shifting is the same as a 68x118 BB. It works! When I end up needing a new BB, I will replace it with a 68 x 118 so I don't need to use the spacer.
I'm not sure the effect this has on the chainline, but I am careful about not cross-gearing, so it should be OK. Thanks again!
I'm not sure the effect this has on the chainline, but I am careful about not cross-gearing, so it should be OK. Thanks again!
