Adjusting rear derailleur...

[lump_a_charcoal]

Hey all,

After having my bike serviced by a shop last week, I noticed that when back pedalling, the rear derailleur starts to derail the chain - not handy when trying to trackstand, something I am practising. I didn't do it before the service.

What is this a symptom of?
It is Ultegra, by the way.

Thanks.

[Reman]

Sounds like a symptom of bad work by the LBS. take it back and get the to fix it.

The problem is probably the index alignment is slightly out. change gears to the middle and see if the cog and closest jockey wheel line up using a screw driver.

[Stuey]

You might be able to fix this with a tweak to the cable adjuster on the rear der. Pedal backwards using your hand while turning the cable adjuster on the der one way or the other until the jockey wheels run smooth (in 1st gear), then try that.

You could firstly try lining the top jockey wheel with the large rear sprocket by eye from behind the bike, while tweaking the adjuster.

[twizzle]

What is this a symptom of?

Pedalling backwards. Don't do it. Seriously, it's not designed to work that way.


Sent from my iThingy...

[Stuey]

No, but it shouldn't cause a problem if you do. I'd suggest if it's tweaked, it'll be quieter going forwards as well.

[DJIntegr8]

Did the LBS replace the chain? Are you cross-chaining while track standing? A newer chain won't flex as much while cross-chaining, and tend to try to derail itself when pedalled backwards. A tweak of the RD might help though, try 1/4 turns of the barrel either way.

Brendan

[rkelsen]

How does back pedalling on a non-fixed gear help you "trackstand" anyway?

[twizzle]

How does adjusting a derailleur which is *behind* the cassette affect anything when you back pedal.

Hey lump_a_charcoal - don't forget to insist that people who suggest fiddling with the derailleur offer to come and fix it personally if you bugger it up. If the limit screws aren't set properly, dropping the chain into the spokes or the frame is always a good look.

[lump_a_charcoal]

Thanks for all the replies guys.

Let me clear one thing up - when I say back pedalling, I don't sit there clicking away like an idiot, I am talking about a brief 1/4 to 1/3 reverse rotation, if that much.
When taking off, I tend to do it with the pedal in the same spot. I get it there by back pedalling, because the bike tends to move if I do it the other way.

rkelsen, I find it helps. maybe a 1/10th forwards against the brakes, then back a bit. Hard to explain, but it helps me.

DJI, no, it is the same chain.

Thanks again.

[twizzle]

Cross-chained? I've never had this problem with any of my bikes, but my chain line is always relatively straight when stopped.

[Stuey]

How does adjusting a derailleur which is *behind* the cassette affect anything when you back pedal.

Hey lump_a_charcoal - don't forget to insist that people who suggest fiddling with the derailleur offer to come and fix it personally if you bugger it up. If the limit screws aren't set properly, dropping the chain into the spokes or the frame is always a good look.

The chain bunches up against the top jockey wheel and causes the problem, because there's limited tension on the bottom chain run - it's not usually that the chain is guided off the top/side of rear (cassette) sprocket from the front. If the jockey wheel is in the right spot laterally, it runs smoothly. I didn't suggest adjusting the limit screws at all - I referred to fine tuning with the cable adjuster.

[twizzle]

How does adjusting a derailleur which is *behind* the cassette affect anything when you back pedal.

Hey lump_a_charcoal - don't forget to insist that people who suggest fiddling with the derailleur offer to come and fix it personally if you bugger it up. If the limit screws aren't set properly, dropping the chain into the spokes or the frame is always a good look.

The chain bunches up against the top jockey wheel and causes the problem, because there's limited tension on the bottom chain run - it's not usually that the chain is guided off the top/side of rear (cassette) sprocket from the front. If the jockey wheel is in the right spot laterally, it runs smoothly. I didn't suggest adjusting the limit screws at all - I referred to fine tuning with the cable adjuster.

Re. "not usually"... keep dreaming. The chain has to derail from the leading side, not the trailing. Otherwise the teeth engaged in the links keep it on.

And re. limit screws, some lazy people rely on the indexed shifting to keep the chain on and don't adjust the limits. Especially if they have different wheel sets with different offsets on the free hub that require the RD to be adjusted when swapping wheels. Unless you have checked them, you should assume they are not set properly.


Sent from my iThingy...

[Stuey]

I'm not speculating this could happen; I've seen it happen a few times. I'll explain it as carefully as I can for you, and that'll be the end, for the others' sake. I use the back pedalling routine by hand as a fine tuning set-up for my RD's. I like mine to run smoothly backwards as well. Maybe it's a hangover from meticulously setting up my old Suntours as a 10 year old.

The chain travelling backwards passes over the cassette, contacts the top jockey wheel slightly too far off centre so the chain binds and piles up on the top jockey wheel. This is partly because of the lack of full tension in the bottom run due to the der. spring - if it was under full tension, the chain would just be pulled through. Usually this is accompanied by the jockey wheels 'bouncing' and clacking. The top chain run can then lift off the sprocket of the cassette, like a train derailment type pile up, to use a recent Melburnian metaphor to explain how it works.

FYI, the reason I wrote 'usually' is because in cases of extreme cross-chaining when pedalling backwards, it could be that the top run of chain is guided off the chosen rear sprocket of the cassette, like in a typical chain drop situation. It's not because I was having a half-hearted punt at an explanation. But 'usually' it's what I said.

Take it or leave it, Twiz, me old mate!

[twizzle]

Ah, O.K., I get what you mean, You have to fairly race the chain though, and it's obviously dependent on how much drag there is in the free hub body and the B screw adjustment. And, given that the guide jockey wheel has built-in lateral float (if still genuine Shimano, which is why mine aren't) you would have to have it a fair way out of adjustment for it to be the main cause. I think the current generation cassettes also give the appearance of some lateral "wobble" due to the profiling of the teeth, once again to make them shift faster but this would also make it more likely to jam the chain when pedalling backwards.

However - I still say "don't do it"!

lump_a_charcoal - derail or jam? If it's jamming, check that the guide jockey wheel isn't running against the cassette, it could be the B screw has been adjusted to make it too close.

Sent from my iThingy...

Edit: Smiley added.

[lump_a_charcoal]

lump_a_charcoal - derail or jam?

It starts to derail, towards the smaller cogs.

[twizzle]

lump_a_charcoal - derail or jam?

It starts to derail, towards the smaller cogs.

Big chain ring?

If not... maybe start looking at the freehub body, if it's not turning freely the chain will go slack which gives it a lot more room to drop.

And.... there's always Ui2. Upgraditis solves many problems.

[ironhanglider]

It sounds to me that when l-a-c says 'derail' he is talking about the chain staying on the cogs but coming off the top jockey wheel. (If it is derailling off the cogs then this is not influenced by the derailler when pedalling backwards.)

First call would be to check the hanger alignment, the jockey wheels should be pointing in the same direction and parallel to the bike wheels.

If the derailler is pointing in the right direction (ie the hanger is not bent) then it could be that the jockey wheel is sitting too close to the inboard side.
If this is true then he'd also find changing gears on the stand would be really good when going to a lower gear (bigger cog) and less good when going to a higher gear (smaller cog). This is a characteristic of there being a little too much cable housing to the rear derailler (the tension on the cable doesn't change). Try winding the cable housing adjusting screw in a notch or two to shorten the cable housing and see if it improves.

The adjusting screws can not cause the problem as described (or at least as I understand it).

Cheers,

Cameron