chain jumping

[stump ss]

hey, my old '92 stumpjumper with converted track dropouts is having a problem - when i really torque on a climb or sprinting from a stop, sometimes the chain drops (sometimes with disastrous outcomes)...my chain line is good, using eyeball, ruler, good friend #1, bike shop X, all say so. however it looks like my chainring isn't exactly in round, and the bike shop said that maybe i needed to loosen the bolts then retighten (i didn't really buy this explanation)...i decided to pull the cranks and i don't know if it is real or just the beer i was drinking but i decided my b.b. was a bit off whack so just installed a new one, we'll see if that works. but has anybody else had similar problems? it is an old flexy steel frame so i can imagine that the right circumstances of cranking and the chain getting a bit slack could throw it off. anyway maybe the new bb will fix all.

[wooglin]

Yeah, sometimes chainrings aren't square (round?) in relation to the bb spindle axis. Usual solution is to loosen the bolts to a bit more than finger tight, spin the crank till you've got max tension, then either whack the chain with your hand or the ring with a rubber mallet. The object is to move the chainring towards the dropouts. Its an imprecise science. You'll probably have to do it several times until your satisfied, and maybe even remove the ring and rotate it a quarter turn or whatever and try again. Don't forget to tighten the bolts back up.

[Two Circles Tim]

What kind of chainring and cog/freehweel are you using?

I find that with a SS specific chainring and cog (I am using a Blackspire ring with a King Kog) my chain can stretch quite a bit and get very slack before I need to adjust it. The SS specific stuff generally has taller teeth and no ramps so they drop the chain less.

If you already are using SS specific stuff, how worn is your chain? I'm assuming the shop would have mentioned this if it was the problem though.

[stump ss]

I'm intentionally trying to keep the old girl a salvage job, as parts wear out I'm happy to replace them with SS-specific stuff but for instance I've just got an LX cassette with spacers and an old 18T non-hyperglide shimano cog on there, in good shape, no ramps, not cut. So it isn't the problem. The chainring is in pretty good shape too, it isn't SSpecific though so maybe I should look into that. What is a good source?

The history on this beauty is she is the bike I raced back in the early 90s when I thought I was hot shit and getting lots of podiums. As she aged, it became harder to put parts on (want V-brakes? need new levers. new suspension wouldn't fit, etc.) so I bought a Cannondale and kept racing it and used her as a commuter bike. When I moved to New Mexico, I gave her to a charity that trains kids to work on bikes and ride them off road, she did a year of service with them before I learned they needed money more than bikes, bought her back for $100. now she has cute girly stickers on her and the drivetrain was shit (derailleurs, shifters etc.) so I just turned her piece by piece into a dedicated SS. now I never ride the Cannondale! Or much much less.

Damn, what am I doing at work today? Now all I want to do is ride........

[wooglin]

When I first converted my 91 Stumpy to SS I got a cheap 36T chain ring from harriscyclery.com. I don't see it listed anymore, but you might give Sheldon Brown a call.

[stump ss]

guys and gals, thanks for the advice. turns out (i think) i had a slightly bent bb spindle, now i've gone a week with no chain drops. tomorrow will be the test with a good fast hard climb - will the old stumpy flex and drop a chain? or will the more-round chainring hold everything in place? stay tuned....

[stump ss]

okay, not that it is the most important thing in the world but somebody might benefit from my little exploration of chain jumping. i realized (through a careful process of experimentation - which involved a few face plants) that the chain always jumped to the outside. the chain line looks really good, but i think the old steel frame is just flexy enough (and i'm OH so powerful!) that when i torque on the drive side it is twisting the chain off. so i took an old beater 34T ring and put it on the outside of my 32T (which is in the middle ring position). so far all is fixed. we'll see how it survives on a longer "real" ride.