Mountain Biking - crank and bottom bracket size ?

how do you know what size bottom bracket to get do you have to measure bottom bracket shell. I am thinking of get new xt cranks 752 but with the bottom bracket what is the differents between 113mm,68mm 113 mm, 73mm, 118mm,68mm, 118mm ,73mm thanks Or maybe I should try truvativ I have race face prodigy cranks and bottom bracket and I want to beef up the bottom bracket any suggestion on what to go with that would be compatible thanks

the best bet is to remove your BB and check the numbers on it. the 113mm and 118mm refer to the over all length of the BB while the 68mm and 73mm deals with the length(i think) of the area that it sits in on the frame

superjoe is pretty much correct. The big number is the length of the shaft while the smaller number is the width of your bottom bracket shell. Most common for triple ring mtn bikes is 68 X 113 but there are a few 73's. A couple brands make one that fits both. As far as I know, all the mtn bike bb shell diameter's are the same, commonly know as "American". So you don't need to worry about that measurement...

im a 73x121.........figure that one out....

bah ill tell you i have LX cranks lol and THEY"RE THE CRAZIEST! :eek:

Either the truvativ ISIS spline or the Shimano spline is good but your cranks have to match the spline, they are different.

121 wow I've seen them in the cat's but never had one that wide on any of my bikes. I have one of the ISIS drive bb's that works for either 68 or 73, I've had the same bb in 2 different bikes and it works for both but works better for 73mm shells. I think it's a gigapipe...

thanks guys I think that all means I would need 68/113
and I need to find something compatible with my raceface prodigy crankset. thanks again

thanks guys I think that all means I would need 68/113
and I need to find something compatible with my raceface prodigy crankset. thanks again

A particular model crank will require a specific bottom bracket. The manufacture (of the crank) should provide the details...although is usually not obvious, as I found out on a recent crank swap.

There was a thread about the Deore LX 872 over on Cycling Forum that is pertinent

http://www.cyclingforums.com/showthread.php?threadid=78034&highlight=deore

The smaller # (68 or 73) is the width of the bottom bracket shell. Check the website of your frame manufacturer to see if they have it listed. (Usually do!)

However, you can run a 73 with a spacer on a 68. A lot of the ISIS bottom brackets are coming this way now.

L8R

Choosing the right bottom bracket

When choosing a new bottom bracket or crankset, you must take note of the varying standards used to attach the crankset to the bottom bracket, as well as shell width. There are currently 3 competing standards used to attach the crank to the bottom bracket:

I.S.I.S.

The I.S.I.S. international standard is used on bottom brackets from Race Face, Truvativ, and others. I.S.I.S. uses a splined system similar to (but not compatible with) Shimano. I.S.I.S cranks and bottom brackets are available from a variety of manufacturers, including Race Face and Truvativ.

Shimano

The Shimano system also uses a splined interface. It is not compatible with the I.S.I.S. standard. Within the Shimano system, two incompatible standards are used:

Shimano V1 spline - Used on the XTR line of components. Only XTR cranks will fit on XTR bottom brackets.

Shimano V2 spline - Used on the Deore XT and LX line of components. XTR cranks will not fit on the XT and LX bottom brackets.
Because of the incompatibilities between various Shimano components, it is best to choose the same model of bottom bracket and crankset. For example, if you plan to use XT cranks, you should also purchase an XT bottom bracket.

Square Taper

Conventional square taper bottom brackets are becoming less common on both mountain and road bikes. This equipment can be identified by the square mounting hole in the cranks and square shape of the bottom bracket spindle.

Bottom Bracket Shell Width

Bottom bracket shells come in varying sizes. The most common are 68mm and 73mm. Many bottom brackets (Race Face, some Shimano, Truvativ) can accomodate either size shell. Others (some Shimano) need to be ordered in a seperate size depending on the size of the bottom bracket shell. Measure your frame to be certain prior to ordering.

When installing or removing a bottom bracket, you must be aware that most frames are REVERSE threaded on the drive side of the bike. This is the opposite of most fittings on your bike.

A particular model crank will require a specific bottom bracket.

Second that. The manufacturer for your particular crankset will require a specific spindle length to maintain a proper chainline. For example, if you're upgrading from an old Square Taper RaceFace Forged that used a 110mm spindle to a 2003 LX M-572 crank, you'll find that the spindle length needs to be 121mm Octalink even though the chainline is the same.

Check what the manufacturer recommends.

ya my old bb was a 73x113 square i now have the LX M572 cranks with the 73x121. ive only come across LX being kinda screwey. so if you go with LX and had a 113 get a 121 or if you had 118 get the 126