Right size chainring?
 

Fixing my dad's bike up for father's day. He currently has a triple crankset on his road bike, size 172.5. We're putting a double crankset in its place. Do I need to use a 172.5 mm or do I need to change the size since I'm changing to a double?

Crank size has no relationship to number of chainwheels. If you can't find a 172.5 changing by 2.5 mm is not going to wreck his legs, but Google crank arm length if you want more info.

Good to know. Thanks. So if that's the crank arm length, what am I looking for size wise when I swap the crankset? Just the BB measurement?

172.5 refers to the crank arm length in mm, you can buy cranks at various lengths regardless of how many chainrings are fitted, without wanting to start a crank length debate I would say that 172.5 are deemed to be pretty common "standard length" with 170 and 175 being the other common variations.

Assuming the current triple crankset is a standard road triple (52-42-30), I think you probably want to look for a compact double (50-34).

RE: Crank LENGTH
Depending on his age and knee condition, error on the short side.
I ride 165's because my knees simply can't handle 170's.

That's what I found. I'm just having a hard time finding what fits the bb.

you have a wider selection of cranks when you accept 170 or 175. then figure out gearing

34t-50 needs a 110 bolt circle, road uses 130 , 39-53 stock combo then.

Italian Campagnolo has a 110-112 mixed bolt circle for the 34-50 so you buy spare chainrings only from them

their road 135 is a little better for after market spares..

Normally you choose a crankset and then obtain the proper bottom bracket for it. Most bottom bracket types can be found at reasonable prices, taking some of the pain out of replacing a perfectly functional part.

Also... something that's not directly related to your question, but I'll throw it out there just in case: Have you checked to ensure your shifter and derailleur will be compatible with your new crankset?

The BB could be a few different "types." Most likely one of these.
http://bikeforums.net/attachment.php...hmentid=525007

If the bike HAS a triple, it will most likely fit with a double. Note, I'm saying most likely, not certainly. The only thing that would make it NOT fit is if the "chainline" was different. The "chainline" is the distance from the center of the BB to the center of the chainring(s). Or, if the double had significantly different chainring sizes (unlikely), the rear derailleur wouldn't be able to keep up. But that more often happens when going from a double to a triple, not the other way around.

Since I'm Clueless to Precisely what crank set you have, you have to find out and report that ,

then report how long and what kind of BB spindle type and length you have .

If you are similarly un able to say as I'm unable to guess, take the bike to a Bike Shop where they can see it and tell you.

Buy a new BB. Whatever the type, you'll likely need a shorter spindle length. Plus, you'll get fresh bearings.