In a recent post, a gentleman described a bad accident that happened after his chain derailed. Apparently he was pedaling hard uphill when the chain came off the ring and he was hurled over the bars. Did this happen because all the force that he was applying to the drive train suddenly stop and it had to go somewhere else - in this case into forward motion to propel him over the bars? I'm trying to understand why this derailment resulted in such a bad accident.
I've only had one chain break and I was on a flat surface going only 8-10 mph so I experienced no loss of control - just couldn't pedal anymore.

I haven’t read the post you mentioned. But most likely if he was pedaling hard uphill he may have been out of the saddle and in a position to go for a tumble when the wheel locked from the chain jamming.

Yes, if the chain comes off under high power.

Something similar will happen if you come unclipped from the pedal at speed. Once, when we were all starting a sprint, my friend's cleat broke on an upstroke at about 30mph. That was not pretty.

I'm not saying that the other poster did anything wrong...sometimes things just happen. But this is a good reason for maintaining your bike and replacing things before they are completely worn out.

I think I can answer this as I’m the gentleman you’re referring to. I believe your assessment is correct, I was leaning far over my bars and in fact pulling myself forward with each rotation of the pedals by pulling on the brake hoods for leverage. I had so much forward force in motion that I essentially catapulted myself over the bars and (I think?) I maintained my grip on the bars throughout the somersault thus pulling the bike on top of me once I hit the pavement.
But here’s where I’m still not sure what happened....... my chain is not broken!!!
I was in the garage moments ago as I’ve been laid out on pain meds unable to move a great deal so didn’t see the bike until now. I will eventually take it to my LBS for repair and I suppose only then will I know what the heck happened. All I can describe is that it was like throwing a transmission into neutral while going full speed.

Chuck - you are right - he did say he was out of the saddle. And you connected the dots - chain gets caught & the wheel locks. Okay....

Koyote this makes total sense - speed is a huge factor. Thanks for mentioning the pedal interface too. I've heard of sprinters breaking straps or in your friend's case, coming unclipped at 30mph - Yikes!

I wish I had a video tape of the accident (or maybe it’s better I don’t) but I’m quite certain the wheel didn’t lock up, instead the pedals lost all resistance as though the chain fell off. Now IF that is what happened it makes me curious how that can occur if I’m applying a steady load and not shifting ?
Is there another possibility of how the drivetrain can suddenly spin freely if it’s not a chain related issue?
Broke my heart looking at that beautiful Peugeot with the mangled wheel, I’m hoping that’s all I did to it.

Great to hear from you Tomm! I am glad that you are recovering and will survive to endure another ride or 10000. I was sad to learn of your accident but it's been a valuable lesson for me in bike physics. You're off the saddle, in position to bounce, pulling back on the hoods and -
your chain didn't break...Did it derail and get caught?
May you have a speedy recovery. Your bike looks okay - hey it's steel. Probably just need to build up a new wheel.

Except his chain didn't break. My guess is his freewheel slipped because I've had that happen to me. In my case was out of the saddle mashing the pedals when the freewheel let loose. I dropped chest first onto the bars, fishtailed the bike a bit with one knee dragging on the ground before regaining enough control to steer for a ditch. I had no idea why I crashed at the time. Didn't figure out it was the freewheel until two weeks later when it happened again.

I've broken one chain in my life. It wrapped around both the freewheel and crank arm locking up both. fortunately I didn't fall but did have to carry the bike home, bike didn't even roll with the wheel locked up tight. Seems like a broken chain is more likely to lock the rear wheel than to let the rider free fall into the bars. Having a chain break while in the saddle than should be much less dramatic.

My opinion of what is happening is not “pulling yourself over the bars”, but rather pulling the bike backwards as soon as the chain snaps.

If you are standing up pedaling, you only need to pull the (relatively light) bike back a few inches before the front axle is behind your center of gravity.

Best wishes for a speedy and complete recovery!

Is it possible your left knee bumped the front derailleur downtube shifter as you were standing and peddling? That would disengage the chain quickly from the chainring. I’ve knocked my top tube mounted Silca frame pump off the bike with my knee before while standing and rocking the bike.

Ghost shift?

Freehub/wheel worn and momentarily lost lock-up?

Tomm, have you checked the freewheel pawls? I wonder if either a pawl broke or didn't engage (old grease maybe?). Try spinning the freewheel in both directions.

I've been wondering if perhaps foot retention like your life depends on it might not be a bad thing when the drivetrain breaks. I have done much of my riding with toeclips, slotted aluminum cleats and quality leather straps (Binda and the like) pulled tight. I often have to reach down and release after crashes. Not ideal. I've damaged the same corners on my body many times because creative rolls don't happen. (Not all bad - with my clumsiness, creative rolls = assorted broke bones instead of lots of road rash.)

Leaning way forward and pulling hard - I've been doing that for many decades. All that has changed is the "hard" part. (Aging sucks.) Applying learning from these two threads, I am going to step up my awareness. I have long considered the chain of parts from the front tire/road to the handlebars including the front of the bike frame "sacred". That failure anywhere on that line could be fatal. (I test this 43 years ago. It was decided that I would be allowed to live - at great cost.) I'll now add the line from rear tire patch to shoe. (I already do this with my fix gears because a chain screw-up can be so ugly. Also a foot pulled off the pedal could be lifetime injury if it happens going downhill.)

I haven’t been able to do much more than look at the bike, I’m in too much pain to make any attempt at repairing it.

No problem. It will wait patiently. Sent you a PM. I know full well how painful that trip to the garage is. (I started a beard that stayed on for 20 years because lifting my arm to face high was way too painful.)

Since Tomm chimed in to report that his chain is intact, I've been wondering about the freewheel, too. Seems quite a few posters have the same idea.

And sometimes weird stuff happens. A few weeks ago, I was riding down a long hill and gently shifted the RD to a smaller cog -- and the whole drivetrain locked up. FD had somehow slipped downward and the cage caught on the big ring. If it had happened under higher torque, I would've gone ass over teacups. As it was, I just coasted to a stop.

OT: Tomm, how are you holding up? Ribs still hurting a lot?

The posts here have been very instructive with some fascinating explanations. My take aways:

1) A broken chain is not in itself a big problem but when a chain gets caught and causes the rear wheel to lock up - always bad.
2) Sudden loss of connection between foot and pedal is very bad especially at high speeds

I mostly ride fixed gear bikes on flat surfaces. I use HKK chains which are very strong and Shimano SPD pedals which provide a secure footing. This has been a great reminder of how important and interconnected the entire drive train is.

Tomm let us know how your recovery goes. We're wishing you the best!

The pain is impressive..... I’m taking Norco and still barely able to move around.

Bummer, man. Sorry.

The two times I dropped a chain on my MTB I was using a SPD clipless setup and climbing out of saddle. Both times it happened so quickly, i could not clip-out and tilted right and had minor no injury crashes. Since then, I am fanatical about replacing chains if I cannot fine tune the shifts to my liking and I have any slow shifts or skips.

I have low enough gears so I can pedal seated on my steepest 12% climbs. Leaning way over and pulling on the bars is a very poor climbing technique.

I do a lot of out of the saddle pedaling, but usually I do it on lesser slopes, at a cadence of up to 80 rpm. I do have a short 10% grade that I often ride standing, but with a 30t chainring, I only need a 21 or 24 sprocket.

I have stripped the freewheel threads on a hub. The freewheel just spins forward. I came down hard on the top tube. Also was out of the saddle putting all my weight into the pedals at very low cadence. I was young and durable at the time so no harm to me.

A worn out chain, a worn out chainring and worn out cogs can make the chain skip and slip when pushing hard on the pedals.

Yes - a faulty freewheel has been cited here a lot. Stripped threads, problem with the pawls, it somehow slips. Are cassettes less prone to these issues? Seems like taking the ratcheting mechanism off the freewheel and transferring it to the hub has some advantages - or not necessarily?

do you have brifters?

If you do, you might have clicked the front der into small & that sent you for the loop.

Downtube shifters