Chain Issue

[Nomadski]

So I've had an issue that crops up quite frequently.

Ill be pulling up to some red lights and shift down a few gears to make it easy to pull away. I will hold the brake and push the pedals 3 or 4 rotations to get the chain set in the cog.

I come to a stop and pull my left foot up to get it to 11 o'clock or so ready to push off. The lights go green, I give the left pedal a good push to set off and CRUNCH! All sorts of hell breaks loose on my chain and I have no pedal power and when I look down my chain has come off the rear cassette and is sitting between the cassette and the newbie bit of plastic (that round disc thing) attached to my rear wheel.

I'm assuming this is a derailleur issue, or am I doing something wrong?

 

[gavintc]

So I've had an issue that crops up quite frequently.

Ill be pulling up to some red lights and shift down a few gears to make it easy to pull away. I will hold the brake and push the pedals 3 or 4 rotations to get the chain set in the cog.

I come to a stop and pull my left foot up to get it to 11 o'clock or so ready to push off. The lights go green, I give the left pedal a good push to set off and CRUNCH! All sorts of hell breaks loose on my chain and I have no pedal power and when I look down my chain has come off the rear cassette and is sitting between the cassette and the newbie bit of plastic (that round disc thing) attached to my rear wheel.

I'm assuming this is a derailleur issue, or am I doing something wrong?

The derailleur has high / low limit screws. Google the derailleur, find out which is which and then look for a youtube description for setting the limits. This will ensure that the derailleur cannot go past the point of your top and bottom gears.

 

[paulmad]

could be the limit screws on the DR. look on you tube.

 

[Nomadski]

Suspected it was this. Seemed weird I was consciously setting the chain in the cog right but the pull back on the pedal seems to pull the chain out of place.

 

[wakou]

Holding the brake? Are you trying to change gear by back-pedalling? Don't! Only change going forwards. If I have misunderstood, forgive me, then it might well be the limit screw, they are usually marked H and L, you want the L screw, screw it in a tiny bit at a time until the problem goes away.

 

[themosquitoking]

Holding the brake? Are you trying to change gear by back-pedalling? Don't! Only change going forwards. If I have misunderstood, forgive me, then it might well be the limit screw, they are usually marked H and L, you want the L screw, screw it in a tiny bit at a time until the problem goes away.

No, he's only turning the pedals backward at the lights to get the pedal in the right position to go again.

 

[SpokeyDokey]

Maybe better to do your changes before you get to the lights - that's what I do.

Sorry if I have read you wrong.

 

[themosquitoking]

He is it's just the backward motion of the pedals when stopped which cause the problem.

 

[Nomadski]

Holding the brake? Are you trying to change gear by back-pedalling? Don't! Only change going forwards. If I have misunderstood, forgive me, then it might well be the limit screw, they are usually marked H and L, you want the L screw, screw it in a tiny bit at a time until the problem goes away.

In only holding the brake to slow myself down to give myself time to pedal forward a few times without moving much.

 

[Nomadski]

Maybe better to do your changes before you get to the lights - that's what I do.

Sorry if I have read you wrong.

You have spokey . I'm doing all the changes prior to stopping. Even giving myself a few rpms in the new gear before I stop.

 

[Nomadski]

No, he's only turning the pedals backward at the lights to get the pedal in the right position to go again.

Exactly.

 

[Nomadski]

Just got in from work, so will have a look at this limit screw on the bike stand. Is there a good technique for getting the limit just right? The chain wont just slip off when I move gears and move pedal forward, only under the circumstances I stated above.

 

[SWSteve]

Holding the brake? Are you trying to change gear by back-pedalling? Don't! Only change going forwards. If I have misunderstood, forgive me, then it might well be the limit screw, they are usually marked H and L, you want the L screw, screw it in a tiny bit at a time until the problem goes away.

You used to have to do this on old bikes, is that right? I can remember that from riding my grandparent's ancient bikes when younger and was never able to change gear until they told me

 

[MaxInc]

The chain wont just slip off when I move gears and move pedal forward, only under the circumstances I stated above.

Can you replicate the scenario with the bike on the stand or does it happen only under load as you pull away? When you are stopped at the traffic light, are you on the last rear cog, what about the ring? My impression is that the rear derailleur is not recovering the chain as you pedal backwards to get the pedal into the 11 o'clock position and I'm guessing that something is out of alignment or chain too stretched?

 

[Nomadski]

Can you replicate the scenario with the bike on the stand or does it happen only under load as you pull away? When you are stopped at the traffic light, are you on the last rear cog, what about the ring? My impression is that the rear derailleur is not recovering the chain as you pedal backwards to get the pedal into the 11 o'clock position and I'm guessing that something is out of alignment or chain too stretched?

Im not sure I understood everything you said, but I think your very close to the truth, in particular the rear derailleur bit.

Got it in the bike stand and adjusted the L screw until the gear changing became a little erratic at that end, then turned it back in a little so it was changing smoothly through all the gears.

Then when I moved the pedals backwards the chain jumped 2 cogs at the rear. I moved the pedals forwards again and it resat back in the outer cog. I could replicate this everytime. It only happens when my chainring is the big cog, and the cassette is the largest cog. When it is the smallest chainring and the largest cog in the casssette it freewheels backwards as normal.

So Im guessing when Ive been coming up to traffic lights in the big chainring and moved the gears down to the largest cog, while stationary I have moved pedal back ready to push off which has unsat the chain and when Ive pushed away with a little force it has jumped off the cassette...

Seem to have spotted the issue, but I hate messing with derailleurs and it looks like everything needs to be realigned or something.