How to measure BCD?
 

First, thanks for any help. I've recently picked up a cheap Univega hybrid to leave at the family cabin for rides. It's drivetrain is in pretty rough shape. Several teeth are missing (or very worn down) on the cassette and large chainring. I have a new cassette and chain ready to go, but I'm unsure how exactly to measure the BCD to replace the large chainring. It is 5 bolt, 42t and the crankset looks just like the one in this ebay posting (circa '96 Shimano Acera):

http://cgi.ebay.com/ws/eBayISAPI.dll...1009&rd=1&rd=1

I'm hoping something like one of these would work, but would someone on here be able to confirm?

http://www.nashbar.com/profile.cfm?c...All%20Products

http://cgi.ebay.com/Shimano-42T-X-94...QQcmdZViewItem

5-bolt cranksets with 42 tooth big rings are generally 94 mm BCD.

Best is to pick up a ruler and measure it, as shown at:

http://sheldonbrown.com/gloss_bo-z.html#bcd

But...are you sure you actually NEED new chainrings?

Back in the old days, every tooth on a chainring was the same as every other tooth on that ring.

Beginning in the 1980s, however, Shimano started experimenting with different shaped teeth in different parts of the chainrings, with the aim of improving shifting.

Newer chainrings typically have some teeth much shorter than others, usually the teeth that are picking up the chain when the cranks are vertical (this is when chain tension is lowest, and is the best time to make the shift.

These special stubby teeth, often coupled with "shift assist" pins and ramps on the side of the chainrings, make a great improvement in shifting.

However, one drawback of this is that folks who aren't aware of this design will sometimes discover the short teeth and will assume that their chainrings are damaged or worn out! They aren't!

It is very rare to actually wear chainrings out, takes many, many thousands of miles with a worn-out chain. When a chainring is worn out, _all_ of the teeth show the wear, usually acquiring a hooked appearance on the sides of the teeth that drive the chain.

For further information on this, see: http://sheldonbrown.com/chains

Don't be embarrassed about this...this is a _very_ common question, so common that I have prepared this generic boilerplate response to save re-typing.

Sheldon "SuperGlide" Brown

Great, thanks Sheldon. I'll take a second look tonight. It was only a few of the teeth (4 or 5 were much shorter than the others) and was unlike anything I'd seen in my short history of attempting my own wrenching. If the cassette and chain is all I need to replace, that'd be great.

If you don't have Sheldon's web page handy, here's another way to determine the BCD for a 5-bolt pattern. Measure from the centre of one bolt (or hole) to the outside of the one almost opposite. It's not absolutely exact, but definitely close enough to identify what you have. That's how you identify car wheel BCD diameters.

If you want to buy a new one at your LBS, bring the old one and have one of the mechanics measure it.

Actually the most common way to measure the BCD of a 5-bolt crank is to measure between the centers of two adjacent bolts. The bcd in mm is the distance between the bolts multiplied by 1.701. For example, a 5-bolt crank with two adjacent bolts 64.7 mm apart would have a bolt circle of 1.701*64.6 = 110 mm.

Thanks, that's pretty much the exact info that Sheldon linked to on his site and it's helpful stuff (gotta remember to bookmark it this time). I found that on this particular crankset, the large and middle rings are riveted together and the BCD is more in the 70-72mm range connecting to the middle ring. The teeth that I thought were badly worn appear to be of the 'shift assist' nature. There are about 4 stubbies right behind the crank arm and 4 more 180 degrees around the ring. I didn't really notice the ones behind the crank arm before and thought the select teeth were damaged. I guess I'll just clean it up a bit and replace the chain and cassette (already did the tires). Should be a decent bike to tool around on.

Thanks again to Sheldon and others here!