Chain slipping/falling off

[Jo25]

Hi,

I've got a Trek 7200 hybrid bike and am having problems with the chain slipping/falling off when I try to change down into the lowest gear (small cog at front, big cog at back). It only does this in the lowest gear but sometimes doesn't shift cogs when I try to change between higher gears. The bike is about a year old but has had very little use until vey recently. Any ideas what the problem may be? Is it something I could fix myself? (Not much experience but willing to give it a go!)

Cheers

Jo

 

[John the Monkey]

Hi Jo, which end (front cogs or back cogs) does the chain come off?

 

[Jo25]

It comes off the back.

 

[PrettyboyTim]

There are two little screws (called L and H) on the derailleur arm at the back which control how far it is able to move in each direction. From the sounds of it, the L screw allows the arm to be moved too close to the wheel, which is pulling it off the cog. There is a guide to adjusting derailleur gears on the Park Tools website, but I think the long and the short of it is that if you turn the L screw clockwise a little, it should prevent the chain from coming off.

Edit: If you search on YouTube, you'll find a bunch of videos that take you through the process.

 

[John the Monkey]

Do you know what model the rear derailleur is? (2008 Trek 7200s come with Alivio (according to Evans ) so you could look up the techdocs on Shimano's site here.

More generally, rear derailleur adjustment is covered here.

From what you say, it sounds as though the low limit screw needs adjusting...

 

[Jo25]

It is a Alivio derailleur. I have tried adjusting the low limit screw, but it continues to have the same problem unless I tighten it all the way in which case I can't change down at all.

When changing down the chain shifts over but as it gets to the top of the cog it falls back down to the second cog, if this helps with any diagnosis?

 

[walker]

sounds like a bent mech. getting it straightened properly by a shop

 

[Jo25]

Erm... slightly embarrassing this........ but I was turning the screw the wrong way - now seems to be fixed. Cheers for all the advice!

 

[John the Monkey]

Erm... slightly embarrassing this........ but I was turning the screw the wrong way - now seems to be fixed. Cheers for all the advice!

I love problems like that - although it's a bit of a "D'oh!" moment, at least it's fixed!