Cannondale SI non-flat Crank-Spider interface

[Kurets]

I have an issue where it has become apparent that the mounting surface on the crank arm on my Hollowgram SI chainset is not flat but instead quite cupped making the big ring wobble U-shaped. Initially I thought this was an issue of a bent chainring, but measured it to come from the spider. I replaced the spider with a flat one which I bought used, replaced everything (even chainrings) and came back to the same result, a few mm of U-shaped wobble when turning the crank.

I'm stuck at what to do, I can purchase a new right crankarm SISL(saving a few grams?) or a complete new chainset for about the same amount of money. It's quite a lot of money though, and I really don't want to spend a lot if I can't be sure that I solve the problem. Has anyone else run into this problem?

[dogsnadgers]

I think you will need to post some pics and measurements to really get yor issue understood. If there is a manufacturing defect then you may have some recourse that way, if it is worn then I'm not sure how it could have worn that way if my interpretation of your issue is correct.
Dave

[Kurets]

Measurements are tricky as it would require indicators to get a true relative measurement. Right now I just know that the teeth of the big ring deviate about 1,5 mm from running true (positive offset only as I managed to shim the chainring to straighten the negative going arms.
Given that the radius of a 52T ring is about 210mm this would mean an angle of 0.4 degrees and something like 0.1mm out of flat on the crank-spider interface (assuming about 40mm diameter for the lock nut).
Obviously this is not easy to measure without access to a mechanical workshop.
I do think this is a manufacturing defect, but I am worried Canondale will not want to help me now that I have been replacing the spider, chainrings, etc at home to try and resolve the issue.

[topflightpro]

Are you sure it isn't the chainring pin pressing against the crank arm?

I had that happen for a replacement ring I bought, also a 52 on Hollowgrams, and your issues sounds similar to what I was experiencing. I remember thinking the chainring or spider must be bent, but then I realized the pin was too long given the angle of the crank arm. I used a metal file to file it down, and it worked fine.

My guess is that on the 52, the pin is higher up on the arm, where it is still angled, and the pin is too long. If I remember correctly, I only had to file half the pin at an angle to make it fit. The far end was not too long.

[evan326]

^ This
I had the same issue that the FSA rings had a longer chain stopper pin than the one piece spidering. I used a file also, taking it down about 3mm was enough.

[Kurets]

It's not a too long drop pin. It would be pretty obvious if that was the issue (and very odd seeing as the problem has persisted with tha factory spec chainrings and spider onto a new spider and new rings).
The issue really seems to be that the spider interface slightly U shaped or just slanted. The part of the chainring which is 180 degrees from the crank is 1.5 or so mm too far outboard. The parts at 4 and 8 o'clock are inboard by 1mm and the ones at 10 and 2 o'clock seem decently in-line.
Shimming the 4 and 8 o'clock positions I could get the ring semi straight. But, without bending the chainrings I can of course not offset the 6 o'clock part inboard (maybe I could file the spider a bit, but that feels uncomfortable).
Right now my course of action will be to check if I can get a new crank arm through warranty, don'r know how picky Cannondale are with a home mechanic doing things, but they shouldn't be able to explain away a non worn crank not being flat.