thermalattorney

I'm building up a steel CX frame with 135 spacing and test-fit the DA 7950 crank arms I was planning on using. Unfortunately the NDS arm just barely contacts the chainstay.

Found this info here: Black Mountain Cycles: Cross frame crank compatibility...

Apparently the 7950's inner arm clearance is on the narrow side. SRAM publishes these measurements but I can't for the life of me find the same from Shimano, FSA or Rotor.

Does anyone know if the 6800-9000 series cranks have more clearance? Rotor 3D+? FSA SL-K? Any other road cranks that sit a bit wider?

FBinNY 

I can't say because I don't track these kind of specifications. But modern cranks are designed balancing 2 considerations. The needed (or expected) clearance for the chainstays, and Q-factor (the distance of the feet (pedals) from frame centerline. Over the years much has been made of Q-factor, so that crank makers feel compelled to offer the minimum possible, especially for upper end road bikes.



Therefore, modern road cranks will tend to be designed around the narrower tires and chainstays of modern road frames, since going wider than necessary increases Q. Triples, and cranks designed for cross or mtb, will have greater separation because more clearance tends to be needed for 3 rings, fatter tires and thicker stays.

The source you found is a handy reference, but I suspect that to get the info that you need for a particular crank, you'll have to track down someone who owns a set and measure. If it's close, be aware that even if there's enough clearance on paper, one side or the other may touch if it doesn't center precisely.

dsaul

The Shimano framebuilders tech doc shows that FC-9000 is 57.4mm and the FC-6800 is 57.7mm, which are not much more than the 57.2mm on the 7950. The best option for an 11 speed CX crank from Shimano seems to be the FC-RS500 with an asymetric 58.6mm DS and 60.2mm NDS.

thermalattorney

This is exactly what I was looking for, thanks! Do you happen to have measurements for the CX-70? I'd love to find something a bit fancier than the RS500.

In the meantime a friend is going to let me borrow a SRAM S900 crankset to get me rolling (hopefully).

dsaul

The tech doc doesn't show the CX-70, but the numbers for the CX-50 are 59.4/60.4mm. They should be similar, since they are both cross specific cranksets.