fitting a rear mech

rollingscone

Hey all,

Am replacing a damaged mech on an 8 spd cassette.

I fit the new derailleur this morning but so far it always pulls the chain up two or three gears from the largest cog.

Is this something I can fix by adjusting the limit screws?

Paul Kiernan

RTFM!

ashleey

The limit screws are used to prevent the chain being thrown off the ends of the cassette.

You need to check your cable tension, as the shifters wind in the cable to move the derailleur in to move the chain up the cassette.

Check also the b screw which adjusts the gap between the top pulley wheel and the cassette. Mess up this spacing and the chain may not clear the teeth of the cassette sprocket correctly.

Park tools website is your friend!

rollingscone

Paul Kiernan wrote: »

RTFM!

This may shock you but I had already consulted Sheldon Brown and the Haynes guide. I couldn't figure out an answer to the specific question.

Namely is it a simple indexing or limiting issue or something else entirely.

Cee-Jay-Cee

rollingscone wrote: »

I fit the new derailleur this morning but so far it always pulls the chain up two or three gears from the largest cog.

Do you mean it pulls the chain off the largest cog and into the spokes/wheel side of the cassette?

There is a very good video on you tube that shows you how to adjust the rear derailleur. Type in 'how to adjust shimano rear derailleurs' and it should be the first one. I used it to adjust mine and index the gears after changing cassettes and now my gears are smoother and crisper than before.

I'd say your problem is a combination of all three, ie limits, indexing and b- tension.

Alek

Also, if the mech was damaged, the hanger may have gone out of adjustment. I was trying to get mine to shift precisely for ages until I realized it is not perfectly parallel to the chainline...

Once you get the range sorted with the limiting screws, check if its shifting evenly up and down all cogs. If you find that you cannot find the optimal cable tension adjustment to achieve this, have your hanger checked by your LBS (using special tool, eyeballing will not do)

rollingscone

Ahoy,

When I went back out to look a it again I realised the chain was very slack.


Taking out a couple of links brought it back down to Sheldon Brown's recommended minimum and seemed to solve the issue along with a bit of tension adjustment.


Thanks everyone who helped