bboy314

I have a 2010 CoMotion with FSA Gossamer Megaexo crankset, 130 BCD. I’d like to switch the drivetrain to a 110 BCD crank to run a compact double, rather than the current triple.

Anyone know if it’s possible to take a regular FSA single crankset, and switch the drive side arm/spindle with my stoker arm/spindle, keeping the left (timing side) as-is? Seems like it would work, but I don’t have one in hand to try out.



Alternatively, does anyone know a good source for a 74bcd 34 tooth chainring?

Kontact

Should work, as long as you have a 68mm bottom bracket width.

TobyGadd

You can get a 32T 74bcd chainring here: peterwhitecycles.com/tach.php#alize (TA74-32B TA Zelito 74mm inner 32 teeth). Not 34, but maybe close enough?

Artmo 

I have 34T 74BCD Vuelta USA chainring, never used, for free. Contact me privately if interested. Also 30T 74 BCD barely used.

bwebel

If it works, then I would think the chainline would be off for the 145mm spaced Comotion. But, if the Gossamer is like the FSA carbon cranks of that vintage, I don't think it will work. I think the tandem cranks have a longer spindle than the single cranks to allow for the wider spacing/chainline given how the bottom brackets work. I have some single cranks I could measure, but it will be a few days before I get back home and would be able to do so.

Probably better off going with the 50t ring in the middle position and the 34t 74mm BCD ring you are looking for.

bboy314

Appreciate the offer, PM sent!

bboy314

Interesting, I did wonder about the spindle length, and also hadn’t really considered the tandem crank having different offset to match the chain line. I wonder how much longer the spindle might be, and if one could accommodate for that and the chainline with spacers on the drive side. Seems like a long shot though. If you’re able to measure a single crank and report back, I’d greatly appreciate it. No rush here.

Kontact

You can't make a spindle longer by adding spacers. Spacers make the spindle effectively shorter.

bboy314

probably should’ve thought about that one for a few more seconds, thanks for pointing that out! Currently in the throes of newborn-induced sleep deprivation, so I’ll go ahead and blame that.

bwebel

As I recall, and it has been a while, the FSA bottom bracket for the tandem cranks had a least two spacers. It was designed so that for 145mm spacing, you put one spacer on each side; for 160 spacing, both went on the drive side, moving the chainline further out; for 130/135mm spacing, you put both spacers on the non-drive side, moving the chainline in.

it might be possible to get a single crank chainline closer to 145mm by using a spacer, but it would depend on the non-drive side crank/spindle interface being able to handle less engagement. I've done this with Shimano cranks, but their attachment method is different than the FSA and I don't know that it would work with the FSA's.

I'd also note that having the chainline off, even by 7.5mm that would result from putting a 130mm chainline crank on a 145mm spaced tandem, is not the end of the world. The chain would probably rub in the 34t ring and the lower 3 cogs, but those aren't combinations that are typically necessary, you can get the same ratios using the 50t ring and larger cogs.

mikemelbrooks

Hi, I did this conversation on our Cannondale tandem. I purchased two 110bcd cranksets from eBay and cut the spindles so when joined they are 10mm longer( Our bike is fitted with 4 2.5mm spacers). I made a sleeve that fits inside the two spindle halves to align them and took it to a friendly metal fabrication company who welded the two spindles together. We have gone from a 52 42 32 triple to a 46 33 double, but more importantly has allowed us to fit hydraulic drop bar brakes.