chain crunching

i was out with my mate today and his newish bike was crunching the chain and skipping gears like mad,the top jockey wheel was just touching the big cog on the cassette too.he missed his 6week free service but think its more serious than just a stretched cable,im not a 100% certain but his rear mech looks out of line or even bent.would this be a contributing factor to the horrible crunching noise.

Try the cable first. Next thing I’d try is the B-screw.
The B-screw controls the derailleur body angle, hence the name, B-screw. Shift to the innermost rear cog, which is the largest cog. View the upper pulley relative to the largest cog. If the pulley is rubbing against this cog, tighten the B-screw to increase upper pivot spring tension, which pulls the pulley back and away from the cog.

If either of the above don’t solve your problem, it could be that the mech hanger is bent?

cheers skoolshoes,i had a wee fettle yesterday when we were out,i screwed the B screw all the way in but it didnt move the mech out so i assume it must be bent.the top jockey wheel was rubbing like mad on the mech.he’ll need to take it to the lbs to get them to fix it..

First things first I would inspect the mech/hanger for any signs of bending.
Does the mech hang straight and parallel to the cogs in the cassette? Is the mech hanger cage straight as well?

In all likelihood, unless the rear mech and associated gubbins had taken a whack the rear mech probably just needs setting up correctly.
How new is the bike? Did the gears work fine to begin with and only now have started to play up?
If you or he are confident fiddling with bikes, I’d have a look at the Park Tools tech help pages & follow their rear mech set-up guide.

Here’s a link: http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

cheers stumpy.in all fairness its only maybe 2-3 months old.the mech does look abit of centre.he thinks he gave it a dunt out riding last week so most probably ****.the gears did work ok so all in all think the dunt has bent the mech or hanger off line.looks like a wee job for the lbs or a new mech.if its bent is it fixable or is it for the bin??

I’ve straightened 2 mech hangers before now and they’re both holding up ok. It depends on how badly it’s been bent.
Probably best to try and source a spare one, sooner rather than later…