B-screw

dinyull · July 2014

I noticed the other day that when in the smallest (rear) cog and in the inner ring my chain was close to the rear derailier cage - so close it's started to cut a groove in the cage! It wasn't touching when not moving but can see when everything's in motion how it would make contact.

With the help of my Zinn book and youtube I started to adjust the b-screw and the long and short of it is: to get any kind of gap between chain and cage makes the chain loose to the point of it slipping when in that gear. It's fine in small cog, big ring though.

I know I shouldn't ride in that combo because of crossing, and I don't, but something isn't right as it never used to be like that.

I've recently changed chain - end of may - but replaced it with the same chain and made sure it was the same length before fitting.

Any ideas? Could it be that the spring is gone in the RD?

jgsi · July 2014

Just a couple of things which crop up in your post
the b screw just acts as a pivot adjuster on a small cam to make more clearance or not as case may be

if chain is becoming 'loose' then perhaps not the correct chain length in 1st place

using this gearing combination is ok for storage of the bike as it releases cable tension, but not for riding in

probably not a lot wrong just needs tweaking the right way. I'm afraid.

dinyull · July 2014

Cheers for the quick reply.

Almost positive the chain is correct length, I checked it 2 or 3 times before I replaced and it has been working fine (in that gear) till Thurs. Well, apart from cutting a groove!

I spent a good hour trying to tweak (quarter turn here and there) but just couldn't find a sweet spot, was either too close to cage or too loose.

jgsi · July 2014

Just another thought.. try the Park Tool tutorial instead, which is pretty easy to follow..
http://www.parktool.com/blog/repair-hel ... derailleur

Ben6899 · July 2014

When you checked the chain length, I guess you measured it against the old chain?

Did you?

1. Measure old chain - in inches/cm/mm and then cut the new one to the same measured length?
2. Count the number of full links (1") and also allow for any quick [half] links which you used in the new chain?

1. A stretched old chain will give a spurious measurement.
2. This method works as long as you allow for half links.

I think your chain is too long.

dinyull · July 2014

1. Kind of.

I offered the chains up against each other on the bench. I still have the old one knocking about in the garage, I'll count links and have a check tonight.

How much does a chain stretch over time though? I would have thought that if the old chain had stretched that much I would have suffered the same problem with that one?

jgsi · July 2014

A ten speed chain (and hate to think how quickly an 11 speed) lasts me about 3 months during the racing season... I dont put that much power out, I hasten to add, but racing miles are always on the rivet.
I can get 3/4 of a link stretch easily... but this is using a KMC SL.
I use a heavier duty chain over winter to last out training usage.

gloomyandy · July 2014

Sounds like your chain is too long. There seem to be several ways to set chain length. The one that works best fro me is to put the bike in small, small and adjust chain length so that the chain does not cut a groove in the rear mech. The other way of doing it resulted in the same problem you have just described fro me (SRAM Red 10 speed setup). So maybe your old chain was also too long?

dinyull · August 2014

Thanks for all of your help.

Counted links on old and new last night and turns out I had 1 too many links in the new chain. Can honestly say that I would never have checked that without your suggestions, but you live and learn. Extra link has been removed and all is working well.

Cheers

B-screw

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