i figured this was the best place to get a quick answer. where do you measure a frame to get the correct measurement for a bottom bracket? is it the diameter between the inside threads? (my assumption)

i know the easy thing is to just go to the bike shop and ask them to measure for me, but i thirst for knowledge and want to know for myself

well you need to know first the diameter of the BB shell. 68mm or 73m. This can be measured with a metric ruler. Then you need to know the thread guage. Most modern bikes use english threading, though there are some italian

It's not diameter of the shell, it's the width.

Sheldon Brown at www.harriscyclery.com has the specs. Knock yourself out.

oh whatever, he can figure what i mean. But yea thats what you need, sheldon brown probably lists the numerical values for the different threads so you can use a thread pitch guage

You're sure of that?

half way

There are two key measurements that you need to know when ordering a bottom bracket.

The first is the width of your the bottom bracket shell on your bike's frame. You can measure that with an ordinary metric ruler but you have to be pretty precise. If it's 68 or 73mm, you have an English threaded bottom bracket. If it's 70mm, you have an Italian threaded bottom bracket.

The second is spindle length. Spindle length, with a very few exceptions, is determined by your crankset. If you are reusing the same crankset, just measure the old BB spindle. If you are installing a new crankset, it should come with specs. for spindle length. Sometimes a crankset will recommend spindle length as "110/113mm." If that's the case, measure the distance between your frame's rear dropouts. If the distance between the rear dropouts is 130mm, use a 110mm spindle length. If the distance between the rear dropouts is 135mm, use a 113mm spindle length.

One last thing, which should be obvious, is the that the end of the spindle has to match the hole in your crankset. If it's splined, the splines have to match. If it's square, there are two different square tapers that look almost the same. There are undoubtedly exceptions but generally if you match only a Campy crank with a Campy bottom bracket you'll be OK.

Since this type spec is typically seen with mtb cranks,and current mtb rear spacing is 135, I think seattube diameter rules. Use the longer with 11/4' and 13/8" seattubes.

[QUOTE Sometimes a crankset will recommend spindle length as "110/113mm." If that's the case, measure the distance between your frame's rear dropouts. If the distance between the rear dropouts is 130mm, use a 110mm spindle length. If the distance between the rear dropouts is 135mm, use a 113mm spindle length.[/QUOTE]

It's some time since this post and I don't know if the author is still in touch with the forum, but here goes!

Why would you use the wider spindle length to suit the wider dropout please, I don't quite see the connection? The reason I ask is that my dropout width is 135mm, but I fitted a 110mm bb, and front and rear deraiilers don't quite mesh together nicely. Should I try the 113mm?

Chainwheel and cranks fitted - Shimano Deore M510

With thanks,

Frank

The chainwheels and cassette ideally are the same distance from the centerline of the bike, so they line up with each other (distance is called chainline). If the rear is wider presumably that sets the cassette out further, requiring a longer spindle to move the chainwheels out. One thing that should be noted is that all bets are off if we're talking about an older bike. Many variations in distance between cones, threading, right and left extensions and taper of the spindle ends.