Mountain Bike Reviews Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
23 Posts
Discussion Starter #1
Hello, I was hoping to tap into the collective knowledge of the forum. I am building up a bike for my son to take to college. I have an older Trek 4900 mtn bike frame with a threaded, 73mm bottom bracket shell. I am putting on a Specialized Strongarm II 3 ring crankset on it. It takes a square taper bottom bracket. How do I know what size bottom bracket I need? I tried a Shimano 73mmx110mm BB and after torqueing everything to spec, there is 4mm of clearance between the drive side crankarm and the chainstay, and a 13mm clearance between the non-drive side crankarm and chainstay. Besides trial and error, does anyone know how to figure out what BB I need so that the chainstay to crankarm clearances are the same or close to it?

Thanks in advance.
 

·
Registered
Joined
·
6,285 Posts
Since you put the 110mm one in, make your measurment based on that. Figure out how much farther out you want to move the middle chainring to get good chainline. Double that value and add it to the 110mm axle length.

I don't know if the bottom bracket you have is symetrical, but worth measuring if it is. 4mm on one side and 13mm on the other does seem a bit off. I'd rather them both be the same/similar.
 

·
Registered
Joined
·
6,285 Posts
Are you sure it's a 73 mm BB? Almost sounds like a 68 mm BB in a 73 mm BB shell.
I thought about that too, but the larger offset on the non-drive side threw me. Memory is failing me, but doesn't the loose BB end cap go on the non-drive side? With the difference in offsets, it would seem that it's a 73mm BB in a 68mm frame, but if the OP did that, the non-attached BB end would be screwed so far onto the bottom bracket it would look really wierd and wouldn't engage the bearing right. I don't even think it would screw on past the bearing, which would leave a whole lot of exposed threads on the non-drive side.
 

·
Registered
Joined
·
3,781 Posts
True, although not all BB's go in from the drive side - not sure I've ever seen one that didn't but Parks shows both.
 

·
Registered
Joined
·
23 Posts
Discussion Starter #6
Thanks for the replies, guys. The BB I tried is definitely a 73mm by 110mm. After posting this, it occurred to me to call Trek and ask them what BB they put in that frame. I was expecting to get vmail but lo and behold, a real person picked up the phone. He looked it up and said I need a 73mmx118mm. So I guess the extra 8mm over the 100mm would all go on the drive side, giving me about 12-13mm chainstay clearance on each side.

Bottom bracket sizing is more complicated than I thought. I had no idea they made asymmetrical ones!

Thanks again.
 
1 - 6 of 6 Posts
Top