Swapping cranksets - What did I miss?

[partyofone]

I recently picked up a new crankset - 50/34. I'm planning on swapping the 46/36 I have now. I am not changing the BB.

Is this the procedure, assuming the chain is long enough?

1. Unbolt crank arm and remove.
2. Pull old crank
3. Install new crank
4. Move FD up and adjust.

Is that about it?

Thanks.

[JCavilia]

Assuming the new crank uses the same BB attachment as the old (e.g., both are square taper), that's sort of it. You do have 2 cranks to remove and replace, of course. And pulling them requires a special tool, depending on the type of mechanism.

You could have replaced just the rings, though it might not have saved much money.

So what kind of bb/crank interface do you have?

Edit: I see from your earlier post that you have an external-bearing BB. This might be helpful reading:
Park Tool Co. » ParkTool Blog » External Bearing Crank Systems (Hollowtech II, MegaeXo, Giga X Pipe, X-type, Campagnolo Ultra Torque)

[partyofone]

They are both FSA Gossamer MegaExo cranks. I got the crank as a take-off cheaper than I could get the rings.

[JCavilia]

They are both FSA Gossamer MegaExo cranks. I got the crank as a take-off cheaper than I could get the rings.

Cool. the Park Tool page should have all you need to know. Especially note the admonition to be gentle when tightening the left cap, as it is used only to adjust the bearing play. The arm is secured to the spindle with the pinch bolts.

[seppo17]

They are both FSA Gossamer MegaExo cranks. I got the crank as a take-off cheaper than I could get the rings.

46/36 and 50/34 are the same BCD. unbolt rings, bolt on "new" rings. Adjust FD as needed.

Unless you really want to swap the cranks... Then the only thing you left out was the pedals.

[partyofone]

The cranks right now are 175mm. The new crankset is 172.5. I think I want to try the shorter arm just to see how much difference it makes.

[demonrider]

Be careful when pushing the new crank into the old BB, I was a bit too heavy handed and ended up ruining the plastic seal ring on the NDS; damaged it and it popped right out of the other side. Had to get an entirely new BB as no one around here sells just the plastic washer.

[pmt]

And be sure to use a torque wrench, following the manufacturer specifications.

[DrSmile]

Since you are moving the derailleur up, you'll have to undo the cable and move the derailleur before you install the new crank. Do the big/big combo for chain length, and I doubt that the current chain will work perfectly for a 4 tooth difference unless the original small/small combo was too slack to begin with (yes I know it's only half the number of teeth difference on the chain).

[JCavilia]

Since you are moving the derailleur up, you'll have to undo the cable and move the derailleur before you install the new crank. Do the big/big combo for chain length, and I doubt that the current chain will work perfectly for a 4 tooth difference unless the original small/small combo was too slack to begin with (yes I know it's only half the number of teeth difference on the chain).

Actually, it's only (roughly) one fourth the number of teeth. One link is two teeth, and the chain is only wrapped a little more than halfway around the ring. So the 46 to 50 change would require one more link, if it was close before. Chances are good it will work with the existing chain length, but he should check carefully before shifting into the big-big combo.

[DrSmile]

Actually, it's only (roughly) one fourth the number of teeth. One link is two teeth, and the chain is only wrapped a little more than halfway around the ring. So the 46 to 50 change would require one more link, if it was close before. Chances are good it will work with the existing chain length, but he should check carefully before shifting into the big-big combo.

How is two 1/4 of four?

[gumbafish]

I also had to remove the shifter cable (to reposition my front derailleur) and readjust the stops on my deraileur when I swapped to a compact crankset.

[JCavilia]

How is two 1/4 of four?

One is one-fourth of four. The ring has four more teeth. The chain contacts half the ring, so that difference is two teeth. One chain link is two teeth.

[partyofone]

I got it swapped last night. Before I made the change, I checked the chain length using the Park Tool site's recommendations. There was some slack in the chain. Very little.
I loosened the cable and the FD and moved it up, pulled the old crank and installed the new one. Got the FD back in position and connected the cable. Everything seems to be running fine. I'll know more tonight when I ride it on the trainer for a little while.

The chain length seems to be fine. It was long enough that in the big/big combo, there is still an S bend in the chain at the RD and it does not rub in the small/small combo.

I'm sure there will need to be some fine tuning in the FD, but it is shifting fine and not rubbing anywhere.

Thanks for the help along the way.

Erik

[DrSmile]

One is one-fourth of four. The ring has four more teeth. The chain contacts half the ring, so that difference is two teeth. One chain link is two teeth.

Actually, it's only (roughly) one fourth the number of teeth.

Please be consistent. I'll leave it at that. I can start to see why CXWrench replies so rudely...

[JCavilia]

Please be consistent. I'll leave it at that. I can start to see why CXWrench replies so rudely...

What's inconsistent? The difference between the rings is 4 teeth, and I demonstrated that that requires a chain-length difference of one chain link.

[partyofone]

Either way, the old chain worked and it is shifting better than the old crankset. And to think this was my first try at something like this.

[JCavilia]

Either way, the old chain worked and it is shifting better than the old crankset. And to think this was my first try at something like this.

Well done.