Difficulty shifting - cassette teeth out of alignment

MediaMan

Would appreciate some opinions on this. I have a SRAM PG-950 cassette (9-speed) on my Trek 1.5 like so:




I've noticed over the past while that shifting from the second smallest (#8 cog) to the third smallest (#7) has become problematic - as in it doesn't want to shift. Sometimes I can make it shift by pushing the shifter a bit further than usual, but some times that just makes the chain skip to the next ring again, skipping one ring entirely. All other shifting combinations work fine, and no amount of adjusting the rear derailleur improves the behaviour without adversely affecting the other combinations.

Having inspected the cassette and taken it apart, it looks to me like the second smallest ring has got some of the teeth bent ouwards (away from the wheel) and this is causing the reluctance to shift off that ring. Either that or the cog isn't sitting level in the next largest cog (as per the picture above, all the larger cogs have spacers between them but the smallest two just sit into the larger ones).

I will probably just replace the cassette, but am curious to see if anyone has seen this type of problem before.

Also, the bike is 2 years old, with about 6,000km on it and the chain is showing wear somewhere between .5 and .75%, so I will also be changing that as well. Then do you think I would need to change the chainrings as well to keep everything in sync? There is no visible wear on the teeth, and as chainrings are quite expensive I want to change them only if not doing so would accelerate the wear on the new chain.

Then I start thinking that if I need to change the chainrings then I may as well get a new chainset (much the same price) and then I start thinking I may as well upgrade the existing one - FSA Vero Compact (Square Taper) - to a Tiagra with a new BB, but if I'm going that far then I should go to 10-speed...

Suddenly a few possibly bent teeth on one cog becomes a whole groupset upgrade...!

Advice and opinions gratefully received!

Eamonnator

Sorry no advice, but with a bit of effort, I reckon you could talk yourself into
a new bike. Go on, you deserve one.

cdaly_

You shouldn't need to change the chainrings. In fact, if your chain's only 0.75% worn, you probably don't need to change the cassette either. I go about 4 chains to one cassette.

The shifting problem could be down to a bent derrailleur hanger. Doesn't need to be that much to introduce subtle shifting problems.

You'll notice that all of the sprockets should have a few 'twisted' teeth. These are part of the shifting mechanism. They catch the chain as it's shifting and help lift it onto the next sprocket. It's quite possible that your 'bent' teeth are meant to be there.

Unregistered.

Have you tried adjusting the barrel on the rear dérailleur? . . .

funnights74

cdaly_ wrote: »

You shouldn't need to change the chainrings. In fact, if your chain's only 0.75% worn, you probably don't need to change the cassette either. I go about 4 chains to one cassette.

The shifting problem could be down to a bent derrailleur hanger. Doesn't need to be that much to introduce subtle shifting problems.

You'll notice that all of the sprockets should have a few 'twisted' teeth. These are part of the shifting mechanism. They catch the chain as it's shifting and help lift it onto the next sprocket. It's quite possible that your 'bent' teeth are meant to be there.

I always thought that the cassette and the chain had to be changed at the same time?

manwithaplan

funnights74 wrote: »

I always thought that the cassette and the chain had to be changed at the same time?

Not necessarily. If you clean and oil your chain regularly and change it when it begins to stretch, you will get two or three chains per cassette. Apparently you can get four chains out of it if you're as anal as cdaly

enas

Unregistered. wrote: »

Have you tried adjusting the barrel on the rear dérailleur? . . .

MediaMan wrote: »

and no amount of adjusting the rear derailleur improves the behaviour without adversely affecting the other combinations.

Otherwise, I'll vote for a bent hanger too. Happened to me in the past when the shift between two of the middle cogs (don't remember which, but it was the same cassette) was problematic. I changed the cassette and the cables to no avail before realising this.

MediaMan

Thanks for the advice folks. I like the 'get a new bike' one best.

cdaly_ wrote: »

You shouldn't need to change the chainrings. In fact, if your chain's only 0.75% worn, you probably don't need to change the cassette either. I go about 4 chains to one cassette.

The shifting problem could be down to a bent derrailleur hanger. Doesn't need to be that much to introduce subtle shifting problems.

You'll notice that all of the sprockets should have a few 'twisted' teeth. These are part of the shifting mechanism. They catch the chain as it's shifting and help lift it onto the next sprocket. It's quite possible that your 'bent' teeth are meant to be there.

Interesting that the teeth might well not be out of alignment after all. I will go look again at the derailleur hanger. Could be tricky to diagnose that though - from what I've heard, it could be a little bit out, not visible to the eye, and still cause problems.

OK - time to get the bike back on the workstand and put thoughts of that new carbon-framed beauty out of my mind...

route66

I had a similar problem once - it happened after I replaced the casette. The problem was caused by irregular spacing between the various cogs. Eh, caused by my breaking some of the rim of the freehub which allowed the cogs to "float". Yes, I overtightened the retaining ring and caused some damage. Woops ...

I would think that if the indexing is ok on most gears, your problem may be that the spacing between the problem cogs is not right. Have a very careful look to check. I'd guess something might not be assembled in quite the correct order or some filler spacers might have gone missing.

Ah, what the hell - forget the above and just buy a new bike :eek:

MediaMan

route66 wrote: »

I had a similar problem once - it happened after I replaced the cassette. The problem was caused by irregular spacing between the various cogs. Eh, caused by my breaking some of the rim of the freehub which allowed the cogs to "float". Yes, I overtightened the retaining ring and caused some damage. Woops ...

I would think that if the indexing is ok on most gears, your problem may be that the spacing between the problem cogs is not right. Have a very careful look to check. I'd guess something might not be assembled in quite the correct order or some filler spacers might have gone missing.

Ah, what the hell - forget the above and just buy a new bike :eek:

Mmm... Interesting. Before this happened, the retaining ring came slightly loose and it took me a while to figure out what was causing the rattle. Got the LBS to tighten it so between all that perhaps the fit between the cogs got damaged.

Will inspect more closely tonight.

I now have a tool for tightening the retaining ring. How tight, on a scale of loosy-goosy to damn-tight should I go with this? Ring says 40nm.

cdaly_

40Nm is not as tight as you'd go with a car wheel nut.

MediaMan

cdaly_ wrote: »

40Nm is not as tight as you'd go with a car wheel nut.

Ok, on the Sheldon Brown torque scale, I will take that as "Real Tight", assuming that a car nut would be "Real Damn Tight"...



http://sheldonbrown.com/tork-grip.html

MediaMan

MediaMan wrote: »

I've noticed over the past while that shifting from the second smallest (#8 cog) to the third smallest (#7) has become problematic - as in it doesn't want to shift. Sometimes I can make it shift by pushing the shifter a bit further than usual, but some times that just makes the chain skip to the next ring again, skipping one ring entirely. All other shifting combinations work fine, and no amount of adjusting the rear derailleur improves the behaviour without adversely affecting the other combinations.

Postscript on this. I bought a new cassette (needed a spare one anyway for a spare rear wheel) and that didn't fix it. Still had shifting problems although not in the exact same area. Took the derailleur and hanger apart, checked everything and reassembled it - no change. Had a feeling that the hanger might be very slightly bent but wasn't sure. So I admitted defeat and took it to the LBS.

Turned out it was a kink in the cable (didn't find out exactly where) that was causing the problem. Presumably that was slowing the movement of the cable and/or causing an expansion/contraction effect that was throwing things off.

Anyway, everything is shifting smoothly now, and I know one more problem area to look for next time round! Thanks to all for the advice.

Mycroft H

MediaMan wrote: »

Ok, on the Sheldon Brown torque scale, I will take that as "Real Tight", assuming that a car nut would be "Real Damn Tight"...



http://sheldonbrown.com/tork-grip.html

Real Tight