smurf hunter

When I have the chain in the middle ring AND the either of the 2 lowest (biggest) gears in back, sometimes with enough force the chain pulls off of the middle ring. If I'm lucky it'll catch on the inside granny gear and I can recover, but more often it's off altogether.

This mostly happens during a climb when I stand up. In my opinion my chain line seems reasonable, and I never use those 2 big gears in back aside from climbing. On flat ground it shifts smoothly, no rubbing. It doesn't matter if I spin or mash, UNLESS I'm in that bizarre combo of middle ring and the 2 biggest rear gears.

This is an 8 speed bike. The middle ring has 38t, the biggest in the rear is a 32. It came stock with a shimano 32t cassette, but when that wore out I got an SRAM chain and cassette.

Ideas?
Thanks
-S

Retro Grouch 

Was it ever right?

If the answer is "yes" I'd think you can safely assume that chainline isn't the issue.

You're talking about a bike that's obviously got some miles on it. I'm wondering if the middle ring has worn enough to not quite match your new chain and the less-than-perfectly matching chain pulls off under climbing pressure with a greater chain angle.

If it was my bike I'd try a new chainring or even replace the whole crankset.

Al1943

Have you measured the chain for stretch?
I agree that you may need a new middle ring.

Al

smurf hunter

Retro,

That's a really sensible suggestion and replacing the ring. This is a year round bike with lots of wet winter riding miles.

You just reminded me of another problem. When climbing, the whole crank bends slightly. I can watch the rings move a bit as my leg comes around. This has been happening for a while. Last summer I pulled off the cranks and rings, clean it all and torqued it all to spec hoping it would help.

This has the old school square taper BB if that matters.

Retro Grouch 

What kind of frame? That sounds like bottom bracket flex to me.

back in the days when men were men and bike frames were steel we used to get that all of the time. If you were riding with a group on any steep climb you could hear chainrings scraping on front derailleurs in rhythm with the pedaling. The big, fat tubed, aluminum frames went a long way toward eliminating that sound and I guess that steel frames have now improved in that regard too.

smurf hunter

This particular bike (Kona Dew) is rather heavy duty (7006?) aluminum. I am a bigger rider though (6'3" 200lbs), and standing during slow climbs is probably torquing the frame just a bit.

Maybe I should stop being so macho and just stay seated and use my triple gear on the bigger climbs
Thanks for all the help.