SF Cyclotouring

Ride reports and other ramblings from a San Francisco cyclist.


I Hate Chainsuck!

Several months ago, I was riding my Bontrager and got some random nasty chainsuck. I winced when it happened, and I cried later on when inspecting the aftermath. Woe is my powdercoat! It was bad enough that the middle 34-tooth chainring got kinda warped, so I replaced it with another new one I had on hand, better to be safe than sorry I figured.

I rode that bike again yesterday. I scored a cheap set of long-sought XTR M-900 cranks at the recent Trips for Kids warehouse sale and I was eager to try them out. I wanted those cranks to fix a nagging chainring clearance issue that the Bonty's had since I first built it up using some cheap no-name cranks sourced from Craigslist. The M900s went right on using an absurdly-short 107mm UN-71 BB and everything looked good with improved 'ring clearance, the proper chainline, and less-but-still-enough crankarm/chainstay clearance (low Q definitely!).

So I went on a ride and had a good time, except at one point the front shifting got weird: Downshifting under any sort of load just wasn't happening. I stopped at one point to have a look-see and discovered that the front derailleur on this bike (a top-pull XTR unit) has an adjustment screw for the take-up spring tension. During a strange moment of deja-vu where I could've sworn that I'd dreamt about just this incident a few weeks ago (?!?!?), I tweaked that a bit and it seemed to set things right again.

As I was riding home from the GGB through the Presidio, I got a minor bit of chainsuck. Fortunately the Bontrager-specific chainsuck plate on my bike did its job protecting the paint this time, but after I re-'ringed the chain I was still having problems. I limped home and then discovered that a single tooth on the middle ring was bent outward, which served to throw the chain off the ring whilst pedalling. A-ha and rats, that was a new chainring.

So, two bouts of chain suck in only 3 rides, and this last one with a new chainring and a just-cleaned/lubed chain. Not a good record.

I measured the chain's wear and it's near the end of its service life, so I'll probably replace it. I popped on over to Sheldon's bike-oracle to do some reading up on the subject, and he links over to Jonathan Levy's extensive chainsuck thesis, which now has me asking further questions...

First, Levy's info suggests that a new chain will only exacerbate the problem, and that a slightly worn chain may actually be a short-term solution for suckage. He also posits that "2-ring suck" (chainsuck that happens when downshifting on the front and the chain spans adjacent rings) may be caused by a weird happenstance between the tooth-arrangements of the two rings...and in this case various solutions include: using ramped/pinned/profiled 'rings (e.g., Shimano SG), changing the tooth-difference 'tween the two 'rings (e.g., instead of 24-34-46 use 24-36-46) as the chain may be happier moving between a larger jump between middle and granny, or even simply rotating one of the rings to re-orient the adjacent teeth. Finally, replacing the granny 'ring seems like a safe thing to do.

There may be some truth to this: my C'dale with 24-36-46 rings has only ever once gotten chainsuck. It also has profiled Shimano Superglide (SG) 'rings (7-speed era). My Bonty has 24-34-46 standard unshaped 'rings. However, I've been riding that bike with those same 'rings for years (and before that, those same rings on a different crank on a different frame) and only recently has this problem cropped up.



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