Road chainring rubbing on frame

Question

Problem

My chainring is rubbing on the right, rear chainstay when I attempted to install my crankset.

• Frame: Trek 520 Disc (54 cm)
• Crankset: Shimano 105 FC-R7000 (52/36, 170mm)
• Bottom Bracket: Shimano Ultegra BBr60 (68mm English)

I have built up a few bikes now but hadn't yet encountered this issue. I was thinking a touring frame would accommodate road cranks, but I guess I was wrong. Trek doesn't provide really any documentation on this frame besides the geometry measurements given on the link above.

Potential Workarounds

• Spacers between BB and frame. Based on other forums and Shimano's "Dealer's Manual," this does not seem safe or even plausible. If I add spacers, the spindle will be too short and the inner cover will not sit snugly as it's designed to.
• Buy smaller chainrings. I may upload a picture if needed, because the rubbing is on both the large (52t) and small (36t) chainrings. I wouldn't even know what size to buy or how to measure to ensure it wouldn't rub.
• Replace crankset and bottom bracket with long spindle and smaller crankset (Octalink, square taper whatever). I'd prefer not to do this, because for some reason I decided to throw the packaging away and now I can't return it.

Questions

1. What kind of compatibility between cranks, BB, and frames do I need to be aware of to prevent this?
2. What might be the most efficient/cheapest workaround?
3. Is there something fundamental I am missing that should come with a bottom bracket?

Answer 1

I don't think you are missing anything. I think these frames were intended to be used with mountain bike triple chainrings. The fully built up version that Trek offers comes with Alivio/Sora 3x9 with a 48 tooth big ring.

If the 52 an 36 tooth rings are just hitting the stay you may find a 50/34 pair provides enough clearance (A 50t ring is 4% smaller diameter, 34t is 5% smaller). The 105 crank has a BCD of 110mm and 50/34 rings are available for it. This would be the cheapest option I think. You should be able to sell unused 105 52/36 rings and largely offset the cost of the new ones.

If a 50/34 does not look like it would provide enough clearance then there are sub-compact 48/32 cranks available (8 and 10% smaller diameter) and I thikg you should be able to find one that is compatible with the Shimano BB. Again, you can sell the unused crank to offset the cost.

Update to add info on chainring compatibility:

Chainrings with a difference of less then 16 teeth (52/36, 50/35, 49/32 etc) will work well with a Shimano road derailleur. A potential problem with small chainring pairs is lowering the derailleur enough. This can be a problem with braze-on mountings but is generally not a problem with clamp on. The limiting factor for chainring size on a crank is the bolt circle diameter. For compact cranks with a BCD of 110mm the limit is about 34. This is for the smaller ring obviously. You can mount a 48/34 pair but then you might have a problem of the high and low gear ranges being too close together.

With Shimano cranks you have to be aware of the differences between the groupsets: bolt pattern and the shape of the ring where it meets the crank body makes inter-groupset compatibility poor.

I think you easiest route would be to sell the 105 crank and get a sub-compact one. I note there is a Tiagra 48/34 option which will fit in your BB.

Answer 2

You should measure the chainline of the crankset. This is generally the distance from the middle of the seat tube to the middle ring of a triple or halfway between the inner/outer in a double crankset.

It's more than likely that the bike is setup to use the MTB chainline rather than a road one. This puts the cranks about 5-8mm further from the seat tube than a road double. Another thing that is easier to check than the chainline is the width of the rear axle, MTB wheelsets have 135mm locknut to locknut, whereas road wheels are 130mm. Another flag is the width of the BB shell, MTB's are 73mm, road is 68mm. All this info and much more is available on Sheldon Brown's website and that is an invaluable resource for building up your own bikes.

Since a hollowtech crank has the axle built into the crankset, there is not much you can do to fix the situation without either getting a new crankset or running much smaller chainrings.

You can use any 24mm crank with your current hollowtech BB, if I am understanding the links correctly. Deore XT's are very nice cranks that can be found relatively cheap, but getting 52/36 chainrings will add some cost.