Chain comes off largest rear sprocket

[bristol-dave]

hi all. recently whilst climbing a steepish hill on my commute to work, gear wise, middle sprocket at the front and largest at the rear, my chain slips off, which im finding a little scary as i have cleets, so when the chain does come off, and not engaged , i come to an abrupt halt and nearly fall off the bike . im guessing i need to make a minor adjustment, but not sure where, any ideas please. Hope the above makes sense lol.

 

[uphillstruggler]

Have you tried adjusting the limiter screw on the rear mech?

That's where I would start.

 

[bristol-dave]

Hi, uphill, could you explain that one in a bit more detail possibly ? cheers

 

[fossyant]

There are two screws on the mech body - one stops you shifting off the small gears, the other off the big sprocket.

Dead easy to adjust. Just take a look at the mech and you'll see them marked - you'll also see which one does what - it's worth learning this.

 

[Scilly Suffolk]

If the chain is thrown off into the back wheel when you change into bottom gear (the largest sprocket), then you need to adjust the "L" limit screw ("L" means "low", as in "low gear") to stop the derailleur moving too far towards the back wheel.

If your limit screws aren't marked then put the bike, carefully, into bottom gear and have a look at the screws: the bottom of one of the screws will be closer to touching the arm of the mech. than the other (when correctly adjusted, it will actually touch it).

To double-check, put the bike into top gear (the smallest sprocket) and look again: the other screw will now be touching the mech. (assuming it is correctly adjusted).

On some derailleurs you can't actually see where the screws touch, in which case you'll need to turn the screws back & forth to observe the derailleur for movement.

Personally, I find the Park Tools website useful for instruction on such things, but there are plenty of alternatives.

If you've been dumping the chain into the spokes, I suggest you inspect the spokes near where they enter the hub for damage; if you find anything significant, or are in doubt, take it to a bike shop for inspection.

 

[yumpy]

If the rear mech. limit screws have gone out of adjustment, then its worth checking your front ones as well. They work in the same way stopping the chain going further than the small or big cog.

 

[jay clock]

as above, here is a pic of the two screws. http://assets.seattlepub.com:8020/SuperContainer/RawData/BicyclePaper/Photos/2445?width=300 If you manually flex the rear mech (with the two pulleys on) in and out, you will see which screw needs adjusting