should bicycle crankrings be flat ?
Looked at my front derailleur to see about a scraping while on the large chain ring and saw that there was only one spot during a rotation that seemed to be scraping.
Is this damage (bent) or a feature to allow shifting ?
thanks
It’s bent (or the crankarm or crankshaft is bent, but it’s much more likely to be the chainring). Chainrings and sprockets should be flat.
I concur. Features to improve shifting include notches, ramps, pins, and teeth that are shorter than others. There are also oval chainrings out there. But I’ve never seen a chainring that’s intentionally bent to the side.
Just bend it back. Or replace it.
They are usually aluminum these days, and even T6 6061 bends pretty easy. Curbs, bike racks, crashing, etc can all do it. Mountain bikers have to be careful of rocks.
But first verify that your stack bolts are all tight, and that the arm is tight on the bottom bracket taper. You can just tighten the stack bolts, but if the crank is loose on the taper, it is ruined, and will need replacing.
If not bent too bad, you can probably straighten it enough to work OK. You can either remove it and check it on a flat surface, or try to straighten it in situ. For the latter, a zip tie around the seat tube might be useful as a reference pointer. Take care not to pry against another chain ring, or part of the bicycle and end up bending that. Pushing, pressing, levering is the better way to go rather than banging on it with a hammer or other blunt object. If it were me, I would take it off, put it in a break, and nudge it gently back to truth. You could improvise that by clamping it between a table top and a wood block, and then pry on it with a crescent wrench. Try not to overshoot, as every bend fatigues the metal.
You can choose to replace it if it is bolted on. Beware that there are several bolt ring standards. If it is swaged to the crank, then you will have to buy a new crankset. There are multiple standards for that as well. (various chainline spacings, bottom bracket tapers, etc. etc.)
Google “Sheldon Brown” for details on options.
Replacing the chainring requires a chainring nut wrench. Don’t do it yourself if you don’t have access to one of these; if the bolts aren’t properly tightened, the chainring will break under use. I’ve learned this the hard way.
I dunno, I’ve changed them without special tools, nothing broke for me.
I agree, I’ve changed *many *chainrings with just a wide-blade screwdriver. In fact, I never even knew that chainring nut wrenches were an existing tool. And I’ve got lots of other bicycle specific tools (crank extractor, bottom bracket tool, pedal wrenches, etc. . .) Some of those special tools are truly necessary, but you can definitely change a chainring without one.
One more. I went for years without a chainring nut wrench. Then I got one and misplaced it, so I refused to buy another one. It turned up about a year ago, and I think I may have used it twice. IME it is a very easy tool to get along without.
The only time I have had stack bolts come loose was on new bikes or cranksets when I did not check them…happened to me twice IIRC. The first symptom was the chain rubbing the FD cage, per the OP…well besides the creaking/clicking I ignored for a while.
I have one and rarely use it. It’s a pretty useless tool, since the tool is awkward to position and hold on the nut when your other hand is turning the hex wrench. Usually the friction between the nut and the chainring is enough to allow tightening.
I banged up the alloy chainring on my cross bike. I replaced it with a steel Surly ring and have had no problems since then.