Bicycle Mechanics - New botttom bracket for a crankset
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03-10-11, 04:56 PM
Might be a bit of a noob question, but I am looking at a crankset that does not include a BB. So how do i find what kind of BB I need? I checked the specs on the bike but does not give any info on the BB. Any ideas?
03-10-11, 05:04 PM
little more info trying to replace a sram s300 gxp with a sram rival gxp on a 2010 tarmac comp
03-10-11, 05:32 PM
gxp crank means you need a gxp bb
03-10-11, 06:17 PM
right, but is there a certain size or any other considerations?
No, just GXP. You can use your existing bottom bracket.
Well, I guess if SRAM makes a mountain version for 73mm bottom brackets you would need to remove the plastic sleeve and run just the cups, but otherwise yeah, just use your current or any other GXP spec bottom bracket.
You need a GXP, English threaded bottom bracket.
SRAM cranks come in two versions, GXP and BB30. You have and are replacing with GXP.
You do need to know what kind of threading. Since it's not an italian bike, it will be a GXP with English threading not Italian threading. My guess is that "default" is English and you'd actually have to ask for an Italian BB, but just make sure.
So, buy any GXP bottom bracket with English threading. Spend as much as you want, but the cheaper ones are only about 20 grams heavier than the most expensive and objectively function the same.
A question I have: Isn't your current crank installed in the bike? Why not just use the existing bottom bracket? Both of those SRAM GXP cranks use the same BB and you don't need to replace the one you have unless it is not working right.
My primer on BB:
There are two categories of bottom brackets: threaded and press-fit.
With threaded BB's you need to know two things:
First, threading, Italian and English. A rule of thumb is unless it's actually an Italian bike, it's probably English threading and I don't even know how many so-called Italian bikes are Italian threads any more. Vintage bikes could have even other threading patterns, but modern bikes have one or the other of the above.
Second, which "standard" does your crank require: All of the following generally are offered in either Italian or English: GXP (sram), MegaExo (fsa), ISIS (~generic), square taper (~generic), Octalink or Hollowtech, or Hollowtech II (Shimano), probably some I'm not remembering. These all differ in how the bearings/spindles/crank arms fit together and there is little or no cross-compatibility.
Press-fit BB's such as "BB30" are a newer development. The frame's BB shell doesn't have threads, so the bearings are pressed in. Supposedly better in terms of weight and stiffness, I don't know. THese shells are somewhat back-compatible with threaded BB's and cranks in that you can press in adapters. Not the other way around though (you can't adapt a threaded BB to, say, a BB30.
BB30 is maybe the most common but there are others. I thought Specialized had their own standard for pressed in BB's and corresponding cranks that is different than BB30, but don't really know about that. Whether or not, yours isn't, it's GXP.
Hope that helps.
11-08-11, 10:14 PM
Camilo... this is fantastic... great quick overview that "hits" well on a keyword search... you answered my questions!
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