How hard is it to bend cogs?

supermurph09

Hi

You'll see my other thread below, but I'm starting to think the mechanic that I took my bike to has made a bit of an error. Apparently cogs 9 and 10 were bent on both cassettes. Now one cassette getting a few cogs bent (if that is possible) I can understand because that is the cassette that was on the bike when the problem occurred. But the mechanic said cogs 9 and 10 were also bent on my other wheel that I didnt have a problem with before. The reason I took that wheel along was because I wanted the mechanic to ensure there was no alignment problems once the problem had been fixed. The chain did have a stiff link, which was causing the problem with the clicking and gears not working, he suggested the cogs getting bent could have been caused by the chain. Now I can half believe this because it was on a hill climb event the problem happened so I guess the load would have been quite high.

The mechanic has actually "straightened" the offending cogs on both cassettes and swapped cog 9 from one cassette to the other to make a "best cassette" and put the bent one on the cassette that wasnt on the bike when the problem happened!.

Advice suggested to me since is that the "bent cogs" could actually have been the design (Shimano Ultegra 12-30) to ensure the cassette picks up the chain effectively. I've not had chance to have a proper look yet but how difficult is it to bend cogs? Bearing in mind I'm 77KG and not exactly a power house.

keef66

I'd have thought it would be quite hard to bend a sprocket even if you were cross-chaining and stomping up a hill climb.

If you take the sprocket off it should lay completely flat. Individual teeth may have different profiles and appear twisted slightly to aid shifting, but you'll have several of those per sprocket.

kajjal

Bending cogs is not common. Check the internet for picture and info on how the cogs on cassettes are designed to aid shifting as this may be what you are seeing.

Cygnus

It would be impossible to bend it with even a heavy bloke riding up a steep hill, you're more likely to break the chain than bend the cog.

desweller

They are bloody strong and well supported. You can check by taking them off and laying them on your kitchen worktop.

The only way they could get bent would be off the wheel IMO.

supermurph09

Thanks guys.

Phoning the mechanic back tomorrow as I cannot see why he would have had to bend cogs. Especially on a cassette that would not have been affected!

Supposed good wheel he left on bike, as you can see its terrible. Much worse than when I took it despite the new chain!

http://youtu.be/j4JSX4xQJqE

Supposed bad wheel he left me with (thankfully this seems fine, just hope the cogs are as they should)

http://youtu.be/J3c7yO0Hh8I

I'll know more tomorrow, thanks again for input.

mitchgixer6

That looks like the rear derailuer just needs adjusted for the first video you linked to. I have a 6700 cassette on 2 different sets of wheels, but slight spacing differences means I need to adjust the cable tension for each set of wheels.

I agree it would be very difficult to bend a sprocket whilst on the bike m

mattv

I work in a bike shop. 2 week's ago, a bike came in with the chain wrapped so well round the hub (between sprockets ans spokes) I had to remove the cassette to get it out. It ripped a couple of spokes out. The cassette was bent out by about 5mm at its outer edge while the chain was snagged. As soon as I removed the lock ring from the cassette, it went totally flat again and set up fine (once a new chain, mech, hanger and spokes were fitted). I have NEVER seen a cassette bent through normal use bad enough to cause shifting issues in 9 years.

onbike 1939

It all depends on the cassette I think. I did bend the 32 tooth sprocket on an XTR cassette and on examining it could see why as it looked like a spider's web. Replaced by something a bit less exotic It never happened again.