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Accidental skid stop this morning (fixie thread)

Hooked the rear brake back to my clown fixie and tightened up the front, to ride it for the first time in too long, and into work for the first time, but with less of the "I am powerless to stop" feeling. That was the plan. I didn't make it far. I'm fine, the bike is fine (well, rear tire and tube are totally dead, but otherwise), and no motorists were behind me so it wasn't even much of a scare. But still, an unplanned high 20s->0 and a walk back home wasn't the way I wanted to start my day. So what on earth happened? So, was trying to go down Panther Hollow deliberately, not pushing it at all, bolt upright, just trying to keep the legs loose and go with it. Worked for a bit, but I'm really not comfortable at high (>120) cadences yet and when I was, I think approaching 130 just before the interchange, I hit the brakes and realized I wasn't just scrubbing speed, I was sliding to a stop and the pedals were locked. Blast. Now, how did that happen? Well, when I inspected after the fact, other than the giant skid mark and coin sized hole in the tire slightly left of center, I saw that the chain had come entirely off the front ring to the inside, and it had bunched up on the top side of the rear crank. My rear tire was angled and rubbing against the left side, but there wasn't any obvious wear so I think that was an effect, not a cause. I don't think the problem was there at the start... I'm not the most observant fellow but tire rub I'd notice. The chain was somewhat slack putting it back on the teeth and I had the thing quite tight last time I rode and it felt fine getting started, so that was weird. I can post pics of this when I get home if it helps. So, basically, I'm trying to figure out the underlying cause here, because if I don't, the next tire isn't going to last either, and I'm back to riding single speed, best case (worst case, the chain jumps off riding single speed, too and the bike is dead). The only difference I can think between when I had been running this thing single speed is if I might've triggered the skid by poor pedaling... the braking by hand may have been a little after, not before my legs were giving up on that cadence. But I thought you were supposed to have had to take your weight off the rear tire for that to happen. I sure didn't expect the chain to jump off, and I sure didn't expect the tire to be TOTALLY dead from one such stop. Possibly the chain was a little slack to start, below the threshold I'd notice, and stronger braking one side vs. the other causes the chain to slide off, bunches around the crank and that wrapping is what pulls the wheel forward especially on the drive side. I say possibly, because while this theory makes sense and ties things together, I was pulling both brakes without consciously favoring one or the other (though I doubt I had the pulls exactly symmetrical, will check when I get home), and moreso, I thought I had things embarrassingly tight before (it'd been a while since I checked, but how often do fixie riders check in on their chain tension?) Last item, I did notice a really, really tiny bit of resistance and hum at certain angles through the cranking motion (this sounds stupid since it's easily explained by the tire being askew from the start, but this was a way more subtle effect I swear, and folks had told me before that chain tension is never 100% through all parts of the crank motion, so I'd lean toward that). And maybe it's unrelated to anything, and I'd ridden with that feeling before fixed without incident, but figured worth a mention. Would like some advice here, and then advice on finding an inexpensive rear tire that's likely to have more than one skid stop in it. Wow, that's poor quality. I'll miss the flaming red though.
2014-08-14 09:00:45
There are several possible causes for what happened. Based on your story, here's my two cents: 1) The rear wheel wasn't secure in the dropouts. This, combined with a lot of force on the rear wheel via the chain could cause the wheel to slide out of the dropouts and be pulled to the left just as you said you found it. It also explains the total lock-up: once the chain tension forced the wheel free, it got jammed against the chainstay bridge where friction caused it to lock up. 2) Loose stack bolts had your chainring misaligned and/or your driveline wasn't straight when you re-assembled the bike in fixie mode. A crooked driveline would cause the chain tension to pull on the rear wheel at an angle, possibly popping it loose from the dropouts. The mis-aligned driveline could also cause you to throw a chain more easily, especially when a lot of force is applied to the pedals. I hope that helps. Good luck with the investigation!
2014-08-14 09:32:00
Try a chain tensioner for your dropouts, that might help keep the wheel from sliding if you're having issues. I always run one on my drive side to prevent slippage (even with bolt on axles).
2014-08-14 10:32:13