BB problem - chainline?

[mad_mark]

I have a carbon bike with ISIS BB, 177.5mm FSA Team carbon cranks and Record 10s elsewhere. When on the inner chainring, I'm finding that the FD can't swing in far enough to allow the chain to sit in the 25t cog (of a 12-25t) without slight grinding. Due to the design of the ISIS system, everything sits against stops and is non adjustable unless I put a spacer in behind the fixed cup to move everything over say 2mm. Doing this will cause the cranks to sit off to one side slightly, something I don't want to do. So I got to thinking that it could be possible to fit a 113mm BB in place of the 108mm, which would fix another problem I have, my heels rubbing slightly on the chainstays when I'm sitting back in the saddle climbing steep grades. But, does anyone know if the increase in axle length on the ISIS system is equidistant? If it is, I'm home free, if not, then I'm maybe finding a fix, but not as good as I'd like.

Comments or suggestions?

Mark.

[Dave]

I checked a spare 118mm ISIS BB that I have and found the right side spindle is about 3mm further to the right. Even a 108mm double BB might not be symmetrical. It would be wise to check.

I assume you've turned the low limit screw all the way out and excessive cable tension is not keeping the FD from moving all the way to the left? If the derailleur cage is not actually touching the seat tube, some careful filing or dremel grinding might get the FD to move a bit further to the left.

If you do change to a 113mm spindle, you may also want to avoid the big ring and next to largest cog combo, or at least use it minimally. With cog spacing a bit over 4mm, a 2.5mm shift of the crank isn't quite as much, but it's over half the distance. Also remember that this change will increase the "Q-factor" (distance between the crankarms). A wider Q doesn't bother me (I use a triple with a 160mm Q) but some folks have problems with wider spaced crank arms.

Ultimately, you should check your chainline. You're wrong in assuming that your crank is perfectly centered now. If it was, you probably wouldn't have the problem that you do. The big ring should be 47-48mm from the center of the seat tube to the center of a tooth. Use a machinists scale and measure from the edge of the seat tube, subtracting 1/2 the diameter of the ST. If the ST is 32mm, then the meaurement will be 31-32mm. If the chainline is off, you can get a spacer to move the righthand cup 1 or 2mm.

http://www.biketoolsetc.com/index.cgi?id=669940958548&c=Repair%20Parts&sc=Bottom%20Bracket

[Too Tall]

Couple observations. First it is OK to space the ISIS with 1-2mm that's legit. Second, if you want an adjustable BB they are available...you must be using an FSA which I believe to be the best. Last, my coach actually likes to have the derailleur rub a bit in this gear as it reminds your TO GET OUT OF IT SOON!

[Jeff N.]

I had a similar problem with my ISIS/FSA crankset combo. What I did was:
1. Sold the crank/bb as a set on EBAY.
2. Went back to Campy Record BB/aluminum crankset.

Problem solved. I just find the need to use spacers to cure such things unacceptable. Jeff N.

[Matt Barkley]

doppleganger. Or at least Jeff N makes like minded or even more gifted decisions than myself. :beer:

I have seen the FSAs with a washer (spacer) on drive side. - Matt

[terry]

you could be on the right track. i went the other way on my mtb, from a 113 to a 108. everything was fine before except it was lousy getting into the granny (not that i ever used it!!!), it would rub loudly and take it's sweet time before it dropped in. when i put the 108 on it shifted into the granny perfectly, could still get into the 42 x 12 without rub and i still had plenty of crank arm clearance. sounds like the 113 would cure your problem.

[mad_mark]

The FD is touching the side of the seat tube, I don't think I'd like to grind it, and I'm not sure doing so will help as I need more than 1mm from what I can see. If I use a spacer, then my LH heel will be more likely to contact the chainstay as it'll shift the LH crank closer to the stay.

I checked the ISIS DRIVE website. It states dimensions of 38-38.5mm for a 108mm spindle, and 40.5-41mm for 113mm from center of BB shell to outer face of the RH male spacer (what the crank butts up against). So I guess that answered my question somewhat (the difference is 2.5mm, half of the axle length change). I'll buy a BB and try it, I hope I'm able to post positive news and join the ranks of the dancing bananas.

Thanks for the responses, it got me thinking along paths I hadn't.

Mark.