which rear mech?

fred_dillon

Hi, I have a shimano set-up (unknown name), with 52/42 on front and 8 speed cassette 11 - 30 on the back, the existing mech is a long cage, but it rests almost horizontal to road. and does jerk and snag in highest gear.

as the mech is a bit tired, I thought change it for a tiagra, which on an 8 speed double should be a short cage, but the Shimano web site says maximum cog on rear is 27.

Any advise welcome

thanks

alfablue

If by "almost horizontal to road" you mean it looks like this:



Then the chain is too short. (Maybe someone fitted the larger cassette without swapping the chain).

The mech may be fine, jerking and snagging might be due to gear setup (indexing), chain length, chain wear, cassette wear, cable worn or gunked. A new mech may not solve these issues if these things are unresolved.

To be honest, rear mechs seem to last almost forever, apart from jockey wheel wear or springs losing their strength, but even this takes an age!

Much more likely to wear out are the chain and cassette, if you haven't changed them for many miles, your money might be better spent in this area.

fred_dillon

no it's pointing back-wards, suppose that means the chain's too long?

alfablue

double post

alfablue

Oh yes, too long. When the chain is on the big ring and on the smallest sprocket the jockey wheels should be vertical (see Shimano tech doc for illustration).

The mech will point backwards or forwards depending on gear used, but it should not point so far back that the lower chain run colides with the top jockey wheel or cage, just set it as above, this seems to be a very accurate approach.

The long chain could account for poor performance, adjust the chain to the correct length first. If there is then skipping (assuming your indexing is set up correctly) then I suggest your chain and probably cassette need replacing.

fred_dillon

thanks for the advise, bought the bike second hand, so I guess the previous owner got the chain length wrong :?

alfablue

Yep, probably fitted it without cutting to length.

When you change the chain length I recommend a KMC joining link rather than the Shimano pins, they are re-usable. You need one to suit the type of chain you have (8, 9 or 10 speed). Like these.

Monty Dog

For a used bike, I'd check the alignment of the mech hanger - is the mech cage vertical and aligned correctly front-to-back? If your bike has been crashed, dropped or suffered a knock to the rear mech, then it could have bent the bracket on the frame. Your LBS should be able to sort you out - they have a tool to check alignment and bend it back into the right position.