bb help for Cyclecross build.

blablablacksheep

Hi all, got crankset and bb for Cyclecross bike build but I'm concerned about the clearance between chainset and frame.

Pictures attached, the frame is 73mm depth apox so no spaces, using a ultegra Road bb with Cyclecross chainset.

http://tinypic.com/view.php?pic=2wmncrl ... BraE8twYm8

http://tinypic.com/view.php?pic=2e17j3k&s=8

earthvspace4XTuUWcX

Looks a bit close for comfort, particularly the small chain ring.

paul_onabike

If you have got a 73mm bottom bracket you probably need a MTB type chainset with a wider 'Q' factor dimension in order to get correct ring and crank clearance. What frame is it?

k-dog

That's way too close - any out if the saddle riding will have something rubbing the outside of the

You need a 73mm BB - but the only real difference between the road and BB is the sealing and the length of the plastic tube. Then you need the matching chainset - is that the CX one or just a compact?

I'm not sure if the CX chainsets are spaced wider but that would be the obvious solution. It's either that or spacers and if you added enough for clearance I don't think your BB would be long enough.

nicklouse

So. What are the cranks? And the BB cups are?

Looks like XT cups.

But the cranks look like Shimano you can often get away with adding about 5mm of spacer even on a 73mm shell.

So 2.5mm spacer each side should be ok.

But crank info first.

blablablacksheep

Hi guys its a frame i got off here a while ago that i finally been trying to build on it

Its a cyclocross frame and believe BB is 73mm shell.

The BB is a ultegra road one that i had lying around so assumed it would work.

The chainset is this one.
http://www.wiggle.co.uk/shimano-ultegra ... 5360453675

I just thought that id need 1 spacer for the BB +chainset combo.
If im wrong here please advise, atm im awaiting the spacer but should i change BB for a different one?

thanks

k-dog

That's the perfect chainset.

Your BB isn't perfect as the plastic tube is too short - so water will get in. That doesn't affect fitting though - just longevity. The cups are basically the same and the shell defines how far they go in.

With that chainset you want a 2.5mm spacer on the drive side so that will make the clearance more reasonable. You could probably add another mm if necessary too.

I would be tempted to wrap some tape around the chainstay and watch for it being rubbed - better that then having the paint wear away.

k-dog

http://www.chainreactioncycles.com/mobi ... -prod34674 or http://www.chainreactioncycles.com/mobi ... -prod41445 would be good bbs - suit 73 and the drive side thread is a little longer so is better with the spacer.

blablablacksheep

Cheers K dog, very helpful.

Question, why do the MTB bbs have longer threads than Road ones? Is this by design to give that extra clearance between crank arms and frame?


Il buy a xt one and assume that it's compatible with my chainset.

k-dog

It's because they need a spacer - so there is still a good amount of thread in the frame with the bearing further out.