Did it just stop the pedal from turning or did it cause the rear wheel to stop? If it's the pedal then I've had this happen a few times, but it shouldn't lead to a crash. I guess if you were pedalling really aggressively then the force of your legs could cause a crash and/or the shock of your pedals seizing up could cause you to tense up and lose control. But in my experience this tends to happen when I'm on a very bumpy surface and freewheeling - it's the lack of tension on the chain that enables it to jump off. If I'm pedalling then unless I'm also trying to shift then the chain should be under tension and can't jump off. I guess maybe you got very unlucky with the timing - e.g. stopped pedalling for a rough section, then hit the pedals hard again without realising the chain had jumped.

If it's the wheel seizing up that's a totally different matter and at 19+mph a crash is pretty much inevitable unless you're an experienced fixie rider who can pull off a hell of a rear wheel skid! But if that was the case I'd have expected the chain to jump left into the spokes/hub, not right. And at that speed I'd also expect your spokes/hub to be seriously damaged.

Is it a SRAM rear derailleur?

Here are a bunch more diagnostics questions to help everyone figure out what might have happened. When it occurred...

• Were you pedaling hard or average? Not hard. It was a flat very rough road. Road had pebbles in asphalt but not loose.
• Were you on the seat or standing? seated
• Did it happen when you shifted or just pedaling along? pedaling
• Did you shift the front or read derailleur? no shifting
• Did you shift to a harder or easier gear?
• If rear shift, were you on the hardest gear, middle, or easiest gear?
• If rear shift, did you shift one gear or multiple gears?
• Is there any evidence of spoke damage by the largest rear cog? not sure - I am taking it to the shop this eve.
• Does your rear derailleur look straight and aligned vertically? Yes
• See earlier questions about age of equipment and most recent maintenance. Just had my chain looked at and they suggested a replacement before my August race. All other components looked good they said.

I am wondering if I could have coasted to a stop. It happened so fast and stopped me in my tracks. I keep second guessing what happened and if I could have stopped the wreck. A friend was behind me and fell on me then over me. He only injures is hand and elbow.

Pretty sure edit: her (sorry didn't see the name....) pedals locked up, not the wheels, and she crashed because she was surprised by the unexpected situation and probably panicked a little.

As to what happened, buy a chain gauge and see just how stretched that chain is. ;) A chain link probably bound on a chainring tooth

One way this happens is if the front derailleur is a touch too high.
Flexy frame hangers make it worse.
The chain rides up and towards the outside of the ring and jams between ring and cage and then the rider simply pedals themselves off the bike in a highside.
Usually this is during an upshift at the front but I can envisage a bouncing chain achieving the same thing.

This is what I believe too. I had a crash where I jumped out of the saddle (road bike) to accelerate, the chain fell off the smallest cog on the rear, jammed, causing me to lock up the crank. So imagine your crank locking up while out of the saddle and putting out 800+ watts...instantly unclipped and down I went.

I tried to figure out what happened, and then realized that my chain was HEAVILY worn...like way beyond what a chain checker could read. I quickly replaced my rings and cassette, and also replace my chain on a regular basis going forward.

Hey Tara, if the chain fell to the outside of the crank, that would instantly cause a ton of slack. If your rear derailleur was near the spokes, the chain could have gotten sucked into the spokes, and caused a 19-to-zero miles per hour instant stop. Have you ever dropped your chain to the outside on this bike before?

Nick, Never but that is what happened. So frustrated but I have to stop obsessing over the reason and a possible different outcome. It is what it is.

This is my gut feeling as well- the chain slipped off the front chainring into the BB area, and your cranks got locked up at a bad moment- while out of the saddle, etc.. I believe the chain was outside the rings later simply because someone was carrying your bike around and it fell to that side.

Glad you are ok. Maybe time to get a new chain professionally installed, and have your front and rear der. adjusted.

How to prevent this in the future? Concentrate on getting a smooth, even pedal stroke- practice one-leg drills (ride with one foot unclipped for 30 seconds, then the other), think about scraping mud off the bottom of your shoe as it goes through the downstroke, and learn to anticipate things like this when something doesn't feel right with your front derailleur, chain, etc.

Shit happens.

There are 600 or so fairly small parts that make up a chain and all your effort goes through said 600 odd parts, shit's bound to go wrong occasionally. Pro Tour riders drop and break chains on a semi regular basis and if it happens to them with the best mechanics and equipment then you can be sure as hell it will happen to us mere mortals.

Only a couple of weeks ago I was sprinting and had my chain fail at 65kph. Pretty new chain, probably about 700km on it. Bike just had a service with the drivetrain cleaned and freshly lubed so everything was running sweet. Shit happens.
Scared the crap out of me when the chain whipped around and locked the rear wheel up. Skidded down the road a good 40-50 meters before I managed to pull up. 10/10 would do again.

Yes derailed and jammed back wheel