Rear mech problem

[Big John]

Here's one that should have you all scratching your heads. The following relates to a good quality road bike belonging to my clubmate.

When on the small chain ring and the largest sprocket at the back the cage of the rear mech catches the spokes of the back wheel. We've adjusted the LOW screw on the rear mech and this had some success but didn't completely cure the problem. We've checked the cage to make sure the cage isn't bent, which it isn't or doesn't appear to be.

We've re-indexed it and it changes gear sweetly across all sprockets. It's an 8 speed cassette, has an eight speed rear mech, chain shows no sign of wear and the gear cable is new. The chain length has been checked and appears to stack up with recommendations in the Bike Book.

To add to the mystery this problem has only arisen since we put a new gear cable on the bike. For the life of me I can't think why a new cable should have this effect.

The back wheel runs true and to the naked eye appears dished correctly. The wheel rim sits pretty damn central in the frame.

Any ideas/suggestions? We're toying with the idea of replacing the 8 speed freehub with a 9 speed and fitting a spacer to give a decent gap between cage and spokes when in the biggest sprocket but this may present us with further problems.

Help!

 

[TheDoctor]

Well, assuming we're talking Shimano here, an 8-speed freehub is also 9 speed, so that won't help!! You could add a thin spacer, probably.
Has the rear mech hanger taken a knock - maybe the bike fell over around the time the gear cable was changed?

 

[Big John]

Thanks for the quick reply, Doc.

That's exactly what we thought so we actually put another mech on which is known to be good i.e. not bent. The cage still brushed against the spokes so we put the old mech back on.

Thanks for the info about the freehubs. I wasn't aware that an 8 speed and 9 speed were the same size. As you say, that blows that plan out of the window.

 

[TheDoctor]

By mech hanger I'm talking about the part of the frame that the mech bolts to - the bit extending below the drop out. Not part of the mech itself, so swapping the mech doesn't rule that out.

 

[gbb]

By mech hanger I'm talking about the part of the frame that the mech bolts to - the bit extending below the drop out. Not part of the mech itself, so swapping the mech doesn't rule that out.

+1...the hangers the first thing i thought as a possibility.

 

[mickle]

The hanger.

 

[MartinC]

The hanger.

+1

Or a badly dished wheel which worked with a combination of bad B tension adjustment to keep the mech cage away from the spokes.

 

[Big John]

All good stuff, fellas, and thanks for taking the time to reply. We checked the hanger and it looked OK but I guess it doesn't have to be much out to screw things up. We'll take a look again.

Agreed about the badly dished wheel - that's a possibility. If the hanger checks out then what we could do is re-dish it a touch to add a little space between the cage and the spokes.

For those interested I'll keep you posted. Thanks again.

 

[normgow]

Check out the hanger with the appropriate tool which screws into the hanger where the rear changer would normaly be. Aluminium hangers easily go out of true, even a muffed gear change will sometimes cause problems but they are easily remedied with this gadget.

 

[02GF74]

does it do this when riding or when spinning with no rider?

if only when riding, then the spoke tension at lower part of wheel is quite low - axle hangs by spokes ffrom the wheel rim - so the spokes can bow out and catch.

if it does it when not ridden then it is the hanger.

the derailleur is a parallelogram and designed so the jockey wheels are always in line and move same distance from the wheel. you have verified it is not the mech hence it is the hanger.

cannot explain why a new cable does it but it probably was adjsuted slightly differently not to do it - probbly a red herring since it is impossible to install a new cableto be exactly like the old one - hence the need for adjuters.

 

[Alf]

It can't be a badly dished wheel because the spokes are always going to be inboard of the biggest sprocket even if the dishing is way out. (Unless, as 02GF74 suggests the spokes are all loose and bowing out).

If the derailleur is correctly set (low limit screw set at the right point) the two jockey wheels should be directly below the big sprocket. If the top one is OK but the bottom one isn't (and hitting the spokes) then the hanger is bent inwards. Take an adjustable spanner to it and it will either bend out (if it's steel) or break (if it's badly bent aluminium) in which case it is likely to be a replaceable - bolted on hanger and you will have to buy a new hanger.

If the jockey wheels are not vertically in line with the sprocket, adjust the cable tension until they are and then set the low limit screw (the bottom one on shimano derailleurs) so it won't go any further inwards. You may have to do a bit of final fiddling when you do a complete run through all the gears to check they all work properly. That is, you may have to unscrew the limit screw slightly to let it over change a little bit to engage the biggest sprocket properly. In any case there is no way it should interfere with the spokes unless either the derailleur or the hanger is bent.

Good luck!
Alf

 

[Big John]

Thank god for cycling forums. Where would we be without them, eh? Many thanks for the input everyone. Looks like we should crack it now.

 

[MartinC]

It can't be a badly dished wheel because the spokes are always going to be inboard of the biggest sprocket even if the dishing is way out. (Unless, as 02GF74 suggests the spokes are all loose and bowing out).

Ah, but the inside edge of the jockey wheel cage is always inboard of the largest sprocket too. Some wheels that can take either a Campag or Shimano pattern freehub body can have very little clearance between the big sprocket and the spokes when a Campag freehub is fitted.

Remember too that it's often a combination of factors that causes a hard to track down problem - it could be loose spokes on a heavily dished wheel with a slightly bent mech hanger and cage. You have to eliminate the potential causes one by one 'til the problem goes away.