Bicycle Mechanics - Are Compack and regular crank arms the same

This may be a dumb question, but are the crank arms on the compact and the regular the same?

What I mean is could one just buy the chain rings for the compact double and replace the rings on a regular chain ring, or does one need to buy a new crank and bottom bracket?

Yes. The "Compact" aspect refers to the large difference in toothcount on the chainrings rather than the dimensions of the arms. A standard road setup is going to be something like 39-53 or maybe 39-50. A compact will be more like 34-50 which give you a gear range more like a triple crank without the extra chainring. Arms will be usually be something like 165, 170, 172.5, 175 just like in a standard road double.

So, can I just buy the the chainrings for the 50 & 34 instead of buying another crank?

The current one I have is an ultegra 6500 with a 53 / 39.

No.

A modern road chainring usually has a Bolt Circle Diameter (BCD) of 130 mm and a compact crank has a BCD of 110.

With a BCD of 130 (which your Ultegra probably has), the smallest possible chainring is 38 teeth.

This site gives you some information on the subject:

http://www.sheldonbrown.com/harris/chainrings.html

OK, Thanks, I will look into getting a compact double crank. With mostly shimano parts, am I limited to purchasing a ultegra crank, or can I buy another brand?

I don't know the 6500, but is it Octalink? You are limited to matching the bottom bracket for sure... or you need to change bottom bracket too.

Of course, if you are changing the crank, the extra cost of a bottom bracket shouldn't be a deal breaker.

You could do a search in this forum to find some recommendations or warnings about specific compact cranks.

Just to add-
"Mountain" type crank sets in the 24/28-xx-44/48 tooth range are also typically 110MM BCD.

I don't know the 6500, but is it Octalink? You are limited to matching the bottom bracket for sure... or you need to change bottom bracket too.
Yes, the FC-6500/6503 cranks are Octalinks. Current Shimano compact cranks are all Hollowtech II, AFAIK and aren't usable with any square taper of Octalink bottom bracket so a new bb is a must.

Just to add-
"Mountain" type crank sets in the 24/28-xx-44/48 tooth range are also typically 110MM BCD.
That was true years ago but current "Mountain" cranks are nearly all 94/58 mm BCD and use even smaller chainrings.

There are still some "Treking" cranks with 110/74 bcd available. Nashbar offers one pretty cheap ($70) and it uses an ISIS bottom bracket. Here is their listing:

http://www.nashbar.com/profile.cfm?category=600081&subcategory=60001034&brand=&sku=11462&storetype=&estoreid=&pagename=Shop%20by%20Subcat%3A%20Road%20Cranks

You may need to look at your front derailleur as well. Some will only handle a difference of 13 teeth between the large and small chainring.

What's your real problem? Is the crank bad and you need to replace it, or do you just want a wider range of gears? The easier and cheaper answer might be to change your cassette. If you have an 11-25, your rear derailleur can easily handle 11-27. It might even go to 28 in a pinch (though probably no higher). 39x27 might be low enough to suit you and it would be a LOT less expensive than changing the crank.

.....but current "Mountain" cranks are nearly all 94/58 mm BCD....

104/64mm BCD is the current, common size for mtb's-

104/64mm BCD is the current, common size for mtb's-
Oh yeah, I was thinking of the 5-arm type that followed the 110/75 5-arm cranks. The new ones are 4-arm and have the bcd you show. Things change fast. :rolleyes:

There isn't anything wrong with the current crank. I was just looking for more capability to get up some the big hills here. The cassette is 12 / 26. So, I will do a search among the different compact cranks to see if there is any to avoid.

Get the best of both worlds and get a triple crank. That conversion is a little more expensive if you have to replace the derailleurs (some report not having to), but the result will be worth it.

Get the best of both worlds and get a triple crank. That conversion is a little more expensive if you have to replace the derailleurs (some report not having to), but the result will be worth it.

He might also have to replace the left STI lever. I believe for Shimano they are designed to be double or triple-specific.

I already replaced the shifter with the 105 ST-5500. So, it may be better to look at the triple crank instead of the compact double.

How do you find out if the derailurers are compatable with a triple?

The part number mostly. The RD should be a long (actually medium) cage version, and the triple FD has a wider and deeper cage.