Chain scraping issues

togg

my chain is scraping on the hardest gears to pedal, it starts on the 3rd but last chain ring and the next hardest gear to that, but it's fine on the very hardest gear to pedal, which if I'm correct in thinking you should never really use.

It's a boardman team carbon with shimano 105 and it's really hampering my enjoyment of this new bike, I've had it back to Halfords 3 times about this and they seem to fix it for a while then it just gradually starts scraping again.

I hope this makes sense and someone can offer me some hints or suggestions on why this keeps happening, I've barely done 20-30 miles on this bike yet.

rc856

Has it been like that from the start?
Is it scraping on the front mech?
Sounds like it isn't set up properly.
Find an LBS rather than Halfords.

You could also get Campag so you can trim the front mech

togg

Yes it's been like that from the start, I presume it's scraping on the from mech, not scraping near the back, Halfords top guy has looked at it, he boasted he was "boardman gold" it's just so annoying having to take it back all the time, maybe it's the way I'm changing gears, I've got a habit of braking stuff.

Arghyrou

Definitely front mech, my mate has it with his, like above worth taking it to a decent LBS.
He has sora, 105 should definitely be better.
He just loosed the mech a little from the frame and it went away and shifted smoother. If your not sure what your doing definitely LBS, Halfords are near useless.
Good luck

togg

I can't understand how Halfords put it right then it just starts scraping again, can it be anything I'm doing wrong or a more serious problem with the gears in general, it just seems to get worse the more you ride the bike

Arghyrou

Thinking about it now the chain could be loosening the mech and causing it to rub more and more. Definately worth a try tightening everything up? (Still problem but...)

OR Halfords idiots just stick some grease on it and it wears away wouldnt be suprised

jermas

Have you tried "trimming" on the front derailleur shifter? When on the small front chainring and heavy gears at the rear, move the shifter slightly (not enough to change up). This will move the derailleur away from the chain thus stopping the noise. You can also trim whilst in the large front chainring by lightly operating the change down lever.

Flamez

I guess you ate aware that your front mech has a trimming function.

In lo chainring you can trim across fro the higher gears by moving the left brake lever across half a shift

In hi chainring you can trim for the lower gears. By moving the left gear lever half a click...

Fyi shimano state the the when in the small chainring you shouldnt use the three top gears and vice versa for the big chainring.

togg

Very interesting, it's only slightly scraping so this "trimming" may work, I'll give it a try, so basically you lose 6 gears? It's the last 3 gears on the large ring that scrape.

Also on big hills I always end up in the small cog, on the easiest gear to pedal, is this wrong? I don't fully understand which gears I shouldn't use to be honest.

Flamez

Google "cross chaining" for a full explanation...

nakita222

unscrew the high limit screw on the front derailleur if it is scraping on the front derailler.
If it is just being a b1tch adjust the derailler with the barrel adjuster.
If it is only scraping in two gears though, the mech hanger may be bent.

merak

OK - no need to go elsewhere for an explanation of "cross-chaining". You just want to avoid the chain running at too acute an angle from the front rings to the rear sprockets. There are just two gears you should avoid: 1: the innermost sprocket (ie the biggest one) at the back when you're in the outer bigger one at the front and
2: the outermost sprocket at the back when you're in the inner small chain ring at the front.

As for trimming, the problem it solves is this. The chain runs off the front ring to the sprockets at the back - as you move up and down the gears at the back the chain line shifts from side to side - a lot at the sprockets at the back, but also slightly where the chain passes through the front derailleur cage. If the front derailleur cage is not set up right then that causes the chain to scrape on the side of the cage in some gears - it should be possible rto set it up so it doesn't do this even without trimming.

Which ring at the front are you in when the problem occurs? It's easy to adjust the front derailleur if you know exactly under what condition the problem occurs.

Also just check the height of the front derailleur. The outer plate should just pass 2 - 3mm above the top of the teeth of the big chain ring - if it's set too high it can cause the chain to scrape somewhere else on the cage (ie on the bolt that holds the cage plates together.

Finally check that the allen bolt that grips the front derailleur cable is snugly tightened up.

woodywmb

Thank goodness for Merak's advice. Pity he hadn't been in first with a reply. Keep working the transmission and you eventually learn how to do make the gears work silently. It's like riding a bike. Practice make perfect. Happy fettling.