SRAM Force Crankset (36/50) chainring flex?



NattyDread

New Member
Jul 16, 2007
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After six years of service from my old Giant, I built my new "dream ride" this year: A Cervelo R3-SL w/ SRAM Force gruppo ( 36/50 chainrings, 11-26 cassette).

I love the bike - light, stiff, and the psychological boost of the compact cranks (nothing like having 2-3 'spare' cogs on a climb).

Due to a crazy work schedule, i have only managed to log between 400-500km on the bike. All has been ideal until this past Friday on a group ride.

About 20K into the ride we hit a short but stiff climb and I hear a click (actually more like a clunk) coming from the crank on almost every rotation. My first thought was that the crank or BB cups had loosened off (I had been warned that there have been a few instances of this with the Force cranks & GXP BB). On the road I could not detect any play, but stopped at the bike shoppe on my way home.

When we checked the cranks, they were tight, but we decided to pull them, and the BB, to clean and relube anyway. That's when we noticed four cocentric arcs etched into the otherwise perfect black gelcoat on the right chainstay.
The arcs aligned with both the outer & inner edges of the 36-tooth chain ring and the chain ring bolts.

These "etchings" occurred on that Friday ride as they definately were not there when I cleaned the bike the preceeding Wednesday. This was also the first, and only, time I heard noise coming from the crankset. I neither dropped the chain or suffered 'chain suck' (the markings don't fit that scenario either).

As unbelivable as it may be, it was looking like the 36 chainring and the right chainstay had met each other - given that there are 5mm+ space between them, this should not be possible.

We took the chain rings off the cranks and to my surprise (and horror), you can easily deflect & twist each ring more than 1cm ... compared to the FSA and Shimano chainrings at the shop, these rings were many times more flexible.

Obviously, when bolted to the cranks in five spots, the flex will be less, but perhaps not enough less.

I find it hard to believe that the chainrings were designed/intended to be this flexible and am suspecting I may have a defective set [hazards of buying {into} a new groupset].

Has anyone else out there experienced problems with the new SRAM chainrings - either flexing / twisting or even shearing into pieces.
 
Mine have a bit of wobble that shouldn't be there; but it doesn't seem to affect anything else. I have not noticed the chainring flex that you mentioned, but I will keep my eyes open for that as well.
 
I just put together a 'cross bike with the Force crankset (decadent, I know). I haven't noticed flex in the chainrings themselves, but I had more runout in the rotation of the chainrings on the spider than I thought appropriate for brand-new parts. A quick call to SRAM got a replacement crankset en route - should be here next week. I would suggest you approach SRAM for support, they were very helpful when I called.