Bicycle Mechanics - Compact Crankset and Chain Length Question

I'll be installing a new Shimano R700 compact crankset this afternoon on my 105 10 speed equipped bike. Just a quick question to see if anyone knows if I'll have to shorten the chain? It would seem to be necessary since the chainrings are now smaller . . .

Before you start, assuming you have the correct factory set-up right now and shifting is smooth:
1. Put your bike on the work stand.
2. Put chain on highest rear gear, ie smallest cog.
2. Put chain on lowest front gear, ie smallest chainring. This will establish the maximum amount of slack.
3. Rotate pedals forward sloooowly and note the clearance between the chain and the small cog on the cassette. The clearance should be 1.0cm or less, but the chain needs to clear the small cog. On my factory built Trek Top Fuel the clearance is about 2-3.0mm. On my Lemond the clearance is about 7.0mm.
4. Anyway, you know where things are set at the start.
5. Install your new cranks.
6. Then drape your chain over the small cog in the rear, through the deraillier and over small chainring in the front. Pull the chain ends together and check the clearance. You should now know the number of links to remove.

Did you know:
1. You very likely need a compact specific front deraillier for smooth shifting.
2. ...to be careful threading the rear deraillier...keep the chain under both tabs.
3. Don't break the chain where the connecting pin is located. Use a new connecting pin, not the factory one you punch through.
4. Put a plastic tie around the face plate in front of the link you're going to break so that you put the chain back on in the same orientation.
5. The new crankset should be installed using a torque wrench, it's best not to guess. These are alloy parts and the cranks need quite a substantial amount of torque to properly install.
:D Good luck. I just did this; it went well.

The chain-length sizing method that I subscribe to involves wrapping the chain around the largest chainring and around the largest cog. Do not wrap the chain through the deraileur. Pull the chain tight and find the closest spot you could join the chain at. Add two more links to that point, break the chain at the "+2" spot and install it through the derailleur. That should give you a long enough chain to run in the big/big combination and not get "hung up".

As mentioned by bluehair -- a compact specific front derailleur is a lot easier to setup then a "standard" front derailleur. Standard front derailleurs can be made to work with compact cranks, but the margin for error is pretty slim.

Wow, awesome response, thanks! Regarding needing a compact specific front dérailleur, from what I've read, the R700 works great with the standard Shimano front dérailleur. That was the major reason for choosing it over FSA, Truvativ, etc.

And I have my torque wrench at the ready!


Before you start, assuming you have the correct factory set-up right now and shifting is smooth:
1. Put your bike on the work stand.
2. Put chain on highest rear gear, ie smallest cog.
2. Put chain on lowest front gear, ie smallest chainring. This will establish the maximum amount of slack.
3. Rotate pedals forward sloooowly and note the clearance between the chain and the small cog on the cassette. The clearance should be 1.0cm or less, but the chain needs to clear the small cog. On my factory built Trek Top Fuel the clearance is about 2-3.0mm. On my Lemond the clearance is about 7.0mm.
4. Anyway, you know where things are set at the start.
5. Install your new cranks.
6. Then drape your chain over the small cog in the rear, through the deraillier and over small chainring in the front. Pull the chain ends together and check the clearance. You should now know the number of links to remove.

Did you know:
1. You very likely need a compact specific front deraillier for smooth shifting.
2. ...to be careful threading the rear deraillier...keep the chain under both tabs.
3. Don't break the chain where the connecting pin is located. Use a new connecting pin, not the factory one you punch through.
4. Put a plastic tie around the face plate in front of the link you're going to break so that you put the chain back on in the same orientation.
5. The new crankset should be installed using a torque wrench, it's best not to guess. These are alloy parts and the cranks need quite a substantial amount of torque to properly install.
:D Good luck. I just did this; it went well.

If it helps, I recently switched from standard to compact and had to remove 2 full links.

It just occurred to me that if you're going to remove one or two links remove the link with the connecting pin. That way you'll end up with only one connecting pin (and only one break) in your chain.:D

I don't think you can shorten your chain at all unless it's too long with your former chainrings. Here's why:

I assume you are going from a 52 or 53T big ring to a 50T compact so the difference is 2 or 3 teeth. The chain only wraps half the chainring so the difference is 1 or 1-1/2 teeth equivalent to 1 or 1-1/2 "half-links" of the chain. You have to shorten a chain by 2 "half-links" at a time to get the ends to match properly and you probably don't have that much extra.

As mentioned by bluehair -- a compact specific front derailleur is a lot easier to setup then a "standard" front derailleur. Standard front derailleurs can be made to work with compact cranks, but the margin for error is pretty slim.

Maybe for some compacts (even though many still use standard FDs), but Shimano themselves promote the R700 as not requiring a "compact specific" FD. The setup and adjustment process is no different.

If shimano says (and they do) that they don't need a compact derailer, I'd bet on it.

I've used both compact and regular Campy derailers on FSA and Deda compact cranks - no sweat either way.

I tried to make an FSA compact derailer work with Record levers and Deda compact cranks - no go. The spring tension in the derailer was too strong for the detents in the shifter.

Likely an old Simplex Prestige front shifter would also work fine with compact cranks (although maybe not with an indexed shifter).