Hi all,

I just scored a 2001 GT Aggressor 1.0 frame dirt cheap and need to put a fairly stout (I'm 6'3", 210lbs) crank on it. I checked the GT site for the shell specs for the 2001 frame and it said 73/110mm, which is a very odd size and not widely catered to by bottom bracket manufacturers. Race Face has the TaperLoc which has a 110mm size but is a square taper BB for which they currently manufacture no crank arms. Besides, I'd rather an ISIS or Octalink BB. I notice Shimano has a 112.5 BB length available for XT/XTR but I am wondering is there would be a problem using a BB which is 2.5mm too wide for the frame shell? Or, better still, can anyone recommend a crankset/BB to me which is less than $100 new?

Thanks,

elicious

I ride GT too. And know that they make those brackets at shimano. Don't know any other manufacturer yet.

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The two measurements you have quantify very different things. The first, 73mm, is the BB shell width (the other common option is 68mm.) Pretty much any mountain bike BB is available in both sizes or is some cases (race face for example) one size that fits both 68 and 73mm shells.

The other measurement you have is spindle length. Spindle length is more dependant on the crank than the frame. Different cranks have different amounts of offset for the rings and arms. 110 mm probably produced the best chain line with what ever crank that bike came stock with. If you are buying new cranks then don’t worry about keeping the same spindle length. Match your new spindle length to your new cranks.

You are a bigger person so I agree with your preference for a splined BB of some sort. You can get lighter weight ISIS parts, but they are more expensive and the bearings don’t seem to last as long as the octalink stuff. If you are on a tight budget then get the shimano LX (M572) cranks and ES-51 BB (73x121)

hi,

Thanks for the replies, guys.

Jer, I am clear on the diff between the shell width and spindle length but thanks for spelling it out for me . I am a bit fuzzy about what you said about matching the spindle length to the new cranks; why, then would they offer them with different spindle lengths as well as shell widths? Please help me to understand more about this, thanks .

Best regards,

Erik

There are three reasons for offering different spindle lengths. You probably already guessed the first, but here goes:

Some bottom brackets are used in a multitude of different cranks. Shimano's UN-72 is probably the most commonly used bb in the world and is offered in spindle lengths from 107-127.5mm in 3mm (almost) increments.

The second reason is that some bikes have unique requirements. DH and freeride bikes often need longer spindles because the frames are wider than xc bikes. This is usually because of beefier chainstays and clearance for 2.5-3 in. tires. This yields a less than perfect chainline (too long) but is necessary for the cranks to clear the frame and/or the chainrings to clear the chainstays. (The bad chainline is mostly a factor in the big ring which is often traded for a bash guard anyway.)

The third reason is that some frames with large seat tubes (1 3/8in.) and/or FS bikes with pivots down around the BB shell limit how far the front derailleur can move proximally or to the inside of the crankset. Longer BB spindles mean a less than optimal chainline (increased friction and wear, compromised shifting on the big ring) but are necessary to get the granny ring out far enough (distal) so the front derailleur can pull the chain down on to it.

Chainline btw, is measured from the center of the seat tube to the center of the middle chainring on a triple crankset, and to the point halfway between the two chainrings on a double crankset. The numbers you are shooting for on a mountain bike is 47.5 mm, but 50mm is acceptable too. It is called chainline because you are shooting for a straight line from the middle chainring to the middle cog on the cassette in back. For a much more complete definition of chainline go to: http://sheldonbrown.com/gloss_ca-m.html#chainline

I hope my explination is not too confusing. I can picture it perfectly in my head, I just can't explain it!

Excellent!

So my next (heh, and hopefully last) question to you is whether a 112.5mm bb will suit a frame with a stock 110mm bb? Is this arbitrary? In accordance with the above explanation should I try to keep the difference between the stock (read: rated) bb spindle length and the replacement bb spindle to a minimum in order to minimize the effect it may have on the chainline, even though I could use any size? I've never replaced a crankset/bb before and imagined that if I replaced the crapola 110mm crank/bb with an XTR crank with 112.5mm bb that there would be a 2.5mm side-to-side play between the cranks, which I figure could stress the ends of spindle when the force of the cranks being pedaled would not only torque the spindle axis (as it should) but also cause a stress from 0-2.5mm in distance from where the inside of the crank is mounted on the spindle end to the frame bb shell (where the spindle is housed, and rotates).

Is that pretty much right? can I use the XTR 112.5mm bb as the replacement for the stock 110mm bb with no ill effects, as long as the shell width remains constant (73mm)?

Geez, you thought YOU couldn't explain what was in your head, lol.

Your conclusion was correct, how you got there was a little off.

The fact that the original bb was 110mm is irrelevant. It would be relevant if you were keeping the old crankset, because you spec the spindle length to the crankset, not the frame. (Except when a bicycle has unique requirements that may necessitate a slightly longer spindle--see post above.) The frame doesn't need a 110mm bb, the old crankset did. Perhaps I could have explained it better in the previous post by saying "You try to achieve a good chainline by giving your crankset the proper spindle length."

The chainring location (and thus, chainline) on XTR with a 112.5 should be the same as your old crankset with a 110 spindle (assuming IT was sized correctly. OEM stuff is not always dialed.) The reason they need different spindle lengths is due to design differences in the cranks.

If your new crankset choice is XTR (M952) then you need a 73x112.5mm spindle. Keep in mind that this will ONLY work with XTR M950, M951, and M952 cranks. Those are the ones with the silver aluminum chainrings and the matte grey arms. Shimano has done a pretty good job of making sure that nothing of theirs is compatible with anything else. XTR needs Octalink version 1 (V1), XT needs a 113mm (sometimes 118) Octalink V2, and LX (now) needs 121mm Octalink V2. ISIS is a lot simpler. Any cross country ISIS crank SHOULD work with a 113mm spindle.

I'm sorry if i am over-explaining this. Feel free to tell me to mellow out.

No way, this has been most educational for me! Thanks.

The cranks I will be using are S-Works Strongarm cranks which accomodate the Shimano XTR bb only. I got 'em cheap off of eBay to put on a bike I will be riding/parking in NYC, so the fact that they are reputed to be a bit flexy is not nearly as important to me as the fact that they show no flashy "come steal this bike" logo on them like every other crankset on earth. The idea to replace them made me want to learn more about this upgrade choice, and you've done a fine job doing that, Jer.

Happy Monday!

Erik