it is a 6 speed mtb the chain saggs and falls when trying to backpedal on the highest gears.thanks!

So why does it do that?

You have the bike, you are not sharing any pictures with us, nor telling us anything about the condition of the bike, chain, freewheel, chain rings, etc. Thus we have no clue as to why you are experiencing the issue.

Possible but all causes are- Stiff rotation of der pulley, stiff rotation of freewheel/freehub body, gears not well adjusted, chain twisted or otherwise problematic, spoke guard contacting the outer portion of the freewheel/freehub/cogset.


Try a quick test. Situate the bike with the rear wheel off the ground. Turn the cranks a bit to get the rear wheel spinning. Watch the chain running through the rear der/pulleys and geat teeth for any catching or bad spots. Lift the chain off the rear cogs with the rear wheel spinning so the wheel and cog set still spin. While the chain is disengaged from the cogs and the cogs and wheel are spinning lightly touch the cogs to see if you can stop their spinning. If you can stop the cog from spinning easily then the freewheel/freehub/spoke guard isn't the problem. Andy.