Flatballer

Hey all,

I have a 34-50 compact SRAM Red BB30 crankset with an 11-25 Force cassette. When in the 34 and either 11 or 12, I get chain rub. It's not rubbing the FD, it's rubbing against the 50 chainring. I think it might be because the Red chainrings are so huge.

They supply 5 spacers. 3 go on the non-drive side normally, the other 2 are extras in case your BB shell is faced wrong and you can't get the crank arm tight enough (run out of threads). Is there any reason I couldn't put a spacer or two on the drive side to fix the chainline?

Thanks,
Kai

johnknappcc

Hello, you shouldn't be in that combo. It's called cross-chaining, don't do it. No matter what, you will get chain rub with a compact and that combo.

Flatballer

I'm aware it's cross-chaining, but it seems that it's been established that new chains can handle the angle, as long as the chainline is correct.

Why do compacts do it worse?

FBinNY 

Whenever you have a large difference in chainring diameter, it limits the angle the chain can take coming from the outside. It isn't only a question of compact vs. standard but the number of teeth difference. It's function of the tangent distance from the smaller ring to the larger one. (that distance is proportional to the square root of the difference of the squares of the number of teeth in each chainring). In theory, with the same difference, compact drive's smaller chainrings would accommodate a larger angle.

As John said, you shouldn't be ride those crossed combinations in the first place, but if it's an issue for you, first check your chain-line, then consider moving the entire crankset outboard a few millimeters if possible. Note, doing so will worsen problems with the chain coming from inboard cassette positions, so it's something you want to think long and hard about doing.

johnknappcc

Two things, Wha? and Huh?

Just because a chain "can" do something doesn't mean it "should". And why would you be in a 34/11 combo when you can achieve the same ratio/gear inches in the 50/middle combo? My compact/shifters/everything in the Shimano documentation still advises against certain combos (namely 34/smallest 3 cogs), and unless SRAM has magical chain bending properties (like bending the chain around a 16t larger ring), I'm assuming somewhere in their documentation, it's pretty much the same.

Flatballer

Yeah, I guess I'd rather be able to ride big/big if I need, than small/small. I kinda live in the 50 ring most of the time anyway. Front shifting is so crisp that it's not a big deal to switch.

Flatballer

Because when you're in a race you don't always have a chance to shift the front, so it's good to be able to ride in as many combos as possible when you have to.