My Mrs has a new bike, a Marin Wildcat WFG5. It's got a 1x10 drive train and the other day I noticed the rear mech as an on off lever on it. Any theories ?

Shimano? It's a 'clutch mechanism'. Turn off to make it easier to spring the derailleur out of the way to remove the wheel, turn on in use to reduce chain slap, etc.

psling wrote:Turn off to make it easier to spring the derailleur out of the way to remove the wheel

It's pretty much essential to turn it off when putting the wheel back in. Within minutes of my wife getting her new bike home she'd snapped the rear mech hanger off trying to force the wheel in.

On a 1x setup it stops the chain jumping off the front ring.

Fab. So default is "on" for general riding. "Off" only when fiddling with rear wheel. Cheers

Pretty much that for the on-off lever on the mech. I've not noticed much (if any difference) between on and off regarding chain retention, a narrow-wide chainring when using a 1x system is far more use. As for chain slap - if I'm going down terrain that's likely to cause chain slap there's so much other noise I can't hear it!

Fab. So default is "on" for general riding. "Off" only when fiddling with rear wheel. Cheers

I find the settings are very similar to the lockout on forks & are:

Off - to remove the rear wheel
&
Off - because I've forgotton to turn it back on.