hi ive been running my cassette and rings for a while now i think im on my 2nd chain its measuring less than 0.75 but recently when trying to drop in the granny ring its now rubbing on my bottom of the front mech and its as tho there is slack in the chain

i fitted a new chain yesterday and its the same prob but not as much its when im in the lower gears its almost like the front mech has moved but i know it hasnt (from the mud lol)

could it be that the cassette and chainrings have now died and its causing the extra slack (even on a new chain)

or any other thoughts ?

cheers

Spring in the rear mech has died, the 'B-spring' is the obvious one, assuming it's a Shimano mech, as SRAM ones don't have one in the same place, failing that the other spring could cause the same problems.

bugger thats an expense i can do without is there a way of testing it out as its fine in the middle ring/ smaller cogs and on the big ring the whole cassette the problem just seems to occur if im in the middle ring on the bigger back cogs and try drop to the granny the chain seems to get all caught up and drag rear mech up. if im in the middle ring and smaller rings on the cassette and drop to granny it doesnt happen ?? if that makes sense

Have you got the correct chain length?
Are your jockey wheels worn out?
Have you had the rear mech off and fiddled with the return spring and/or the position on the hanger?

Did you fit the chain out of the box or did you shorten it first?

sticky freehub maybe?

chain length is right jocky wheels look ok ive not had the mech off to fiddle yet as i presumed it was just worn drivetrain. will have to have a fiddle with it looking at price of new mechs 😉

would worn jockey wheels cause this ? im thinking its not the spring as it works fine on the parts where the rear mech would need to actually take up more tension as the chain was slacker ie middle ring and smaller cogs

most puzzled lol

Sounds like you may have a stiff jockey wheel. I'd start by cleaning the rear mech thoroughly and making sure the jockey wheels turn and are well oiled.

Just twiddle the mech on the bike. The spring should be smooth, if you push the lower jockey wheel forward it should spring back easily, if it's stiff, in either direction that's your problem.

It won't happen when there's more tension in the chain, only when you drop into the granny as you're relying on the mech to take up the slack, which it's not doing.

cheers fellas i'll have a clean and fiddle

Seems you have two seperate problems…

In the first post the chain rubs on the front mech due to lack of tension.

In the second post, your chain is getting caught up when changing from middle to small ring, this is chainsuck.

Have you only been getting chainsuck since you put the new chain on? If yes, thats due to the drivetrain being too worn for a new chain, two solutions: Replace everything (£££) or put the old chain back on and ride it for another year or two until it gives up. Adjust the front mech down slightly to stop the chain rubbing.

If you had chainsuck before as well, then it could been the same problem as the chain rubbing on the mech – lack of tension. Lift the back wheel off the ground and spin it. If the pedals spin as well, the freehub is probably the cause. If not, its going to be either stiff jockey wheels or the main spring in the mech. Or it could even just be that the mech/spring are caked in mud and just needs a clean 🙂

The chain is more likely to get 'sucked' because it's going very slack with the mech, I'll wager that's the only real problem going on. Stiff jockeys wouldn't do it, and the fact you've changed the chain with no difference suggests it's not chain/cassette related.

i think its one problem just im rubbish at explaining lol the chain swap eased matters slightly. Ive fitted chain in the big ring big cog plus a link measurement which seems to work but this is a full sus could that change owt. Its only a fairly recent thing tho that said its only recently ive started using the granny ring lol

On my commencal I cannot get rid of all the slack in my chain when on small front and about 6 at the back due to the slack needed for sus movement.
You should NOT be able to lower your front mech to avoid chain contact if currently set correctly or it will crash into your big front ring(should be 2mm gap).

Has the rear mech jockey cage rotated backwards so much that the lower wheel is higher than the top wheel before a usable gear at the back is reached(in conjunction with your granny front)
You might have a short or medium cage mech where you need a long one.

will check it out when home cheers

stripped cleaned and swapped drive train and hats off njee20 its the b spring . bugger lol are they replaceable ? the mech is a shimano xtr m971 cheers mick (or maybe a 970 but they look the same to me)

it appears my hat has to be replaced lol

ive stripped my rear b spring bit and it was intact and in place correctly. so i cleaned and rebuilt. still having the same problem

so far ive cleaned and moved and adjusted the front mech, cleaned stripped and rebuilt and refit the rear mech

cleaned and lubed the cables

and fitted new chainrings, cassette and chain

now im really lost ???

could it be that the b-spring altho still in place has lost some of its spring ? i dont really want to have to buy a new mech just to find the same problem still ??

ta

Springs don't lose their springiness unless they've been stretched beyond their elastic limit. This is unlikely for a B-Spring so I'd look elsewhere.

im at a total loss. ive just looked at my other bike and the front mech when in the granny still has about 1cm clearance from the chain in the 4th smallest cog where as on this bike its hitting the front mech. but ive got it so it only just clears the front outer ring when shifting so cant have it any lower. under drive tension it lifts slightly off the mech but something somewhere has gone amiss ;-(

well after much faffing its all working , swapped the drivetrain as i had spare set for when my stuff wears out

ended up striping and cleaning both mechs and it now works . dont know what the problem was. i nearly lost my hand with the screwdriver removing the bspring lol but its all working spot on now 😉

still cant decide if i should put the old drive train back on the lol 😉