[Sshields32]

Can some one please explain Bottom brackets to me? i cant understand their measurements. I know what my BB shells width is but i dont understand what the measurements are when buying one. the differents options like 100x66 or 68/73x51 or 83x56.

 

[binary visions]

Missed this thread before so hopefully you're still reading.

There are two measurements, there's the BB shell width and the length of the spindle. The length of the spindle is going to be the longer number, obviously, since the spindle has to extend past the edges of the bottom bracket.

So, if you have a 73x110 bottom bracket, it means it fits in a bike that has a 73mm bottom bracket shell and the spindle is 110mm long.

 

[davep]

to add to this, as it looks to me like the OP is looking at truvative bbs....

Truvative now lists bb shell width (68, 73, 83 etc) and then the second number is the chainline that the bb will give you with truvative cranks.

a frame with a 135 rear end will generally require a 50mm (actually 47.5 is the correct chainline..) chainline. A frame with a 150 rear will generally require a 55 (ish) mm chainline. (Additionally, the design of extarnal bearing bbs requires a wider than ideal chainline. THat is why you see 50 or 51 rather than the proper 47.5 and 56 or 57 rather than the correct 55mm...)

So 73/51 is for a frame with a 73mm bb shell and a 135 rear end
a 83/56 is for a frame with an 83mm bb and a 150 rear.

Of course, there are some frames that take wierd combos....like some of the Demos that use a 135 rear....with a 55mm cahinline as they have an asymetrical rear end. The older Giant DH was similar but with a 100mm bb IIRC.

 

[Sshields32]

Thanks a lot. i got everything figured out. this website is so freakin great.