Bicycle Mechanics - Bottom Bracket differences

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Banzai
03-29-07, 08:43 PM
What, fundamentally, is the difference between a "Road" and a "Mountain" bottom bracket? I've been researching my parts list for my upcoming bike build, to include crank/bb compatibility. I find "mountain" bottom brackets that appear to be compatible with "road" cranks, and vice-versa.

So...what designates a bottom bracket as one or the other?

fiver
03-29-07, 08:50 PM
Nothing really.

All BB's will have three distinguishing features:

1) Shell width (68, 70, 73, etc)
2) Spindle Length (108, 113, 118, etc.)
3) Spindle/crank interface (JIS square, ISIS, V1, etc.)

As long as the BB fits the crank and frame, you can run anything you want.
TO11MTM
03-29-07, 08:52 PM
It depends who you talk to, but lots of people claim that the Road BBs are smoother, while the Mountain BBs are more geared towards mud and whatnot.

... Let's face it, they all use the same dang bearing covers.

Now, mind you, there are some very heavy duty Mountain BBs with Quad Bearings and fancy things like that... but that is out of the scope of this discussion.

One thing to keep in mind, however, is that Mountain BBs from some manufacturers nowadays are a single '68/73mm' unit... They use a shim on the drive side to fit in the smaller size. If you guessed that this totally messes with your chainline, you get a gold star.

Course, Two Piece Extrenal Bearing setups for mountain bikes have spacers on both sides, and you can mess with the chainline at your leisure...

z415
03-29-07, 09:09 PM
I have the same BB in both my bikes (road and mtb)
astrodaimler
03-29-07, 09:45 PM
What's great about the mountain bike, is the evolution of parts that made its way to road bikes. Campy BBs, had no dust cover. You put grease on the ball bearing ring and screw on the cups. Early mountain bikes BB were same technology. Until they started using sealed bearings. I thought "wow!, i don't have to overhaul them every month!". So I bought Phil Wood hubs (sealed bearings) so I never have to worry.
Now, everything is sealed which is great. I haven't taken apart a hub or a BB or a headset for repacking in ages.
So I think most parts are quite interchangeable.
I love the MTB stem technology made its way to road bikes. Its a darn pain to change those quill stems without taking apart the brakes and tape.
fiver
03-30-07, 06:14 AM
Now, everything is sealed which is great. I haven't taken apart a hub or a BB or a headset for repacking in ages.
So I think most parts are quite interchangeable.
I love the MTB stem technology made its way to road bikes. Its a darn pain to change those quill stems without taking apart the brakes and tape.

word