I'm trying to find out what happened to my drive train that would cause me to crash and break my shoulder. During the ride when I accelerated I felt the rear wheel stuttering; like spinning on loose gravel (I should have stopped right then). When I got in the drops and put all my force into passing someone, it sounded like my chain snapped/popped (was loud) and my foot went straight down and the bike flung me over the handle bars. When I came back home from the ED I inspected my bike and found no slack in the chain nor was it broken. The lock ring however was only finger tight, my chain line was a little off as well. This also occurred after installing a new chain the previous night, and I noticed a little play in the cranks afterwards. Now, would this loose lock ring cause such a noise and drive train failure? This is on a fixed gear Leader 725 by the way. It's going to cost a little over 150$ for the LBS to check out/tear it down and inspect it, I'm trying to diagnose the problem before it comes to that. This spill has me worried about when I heal up and try to ride again.
Thanks.

too bad about your shoulder.


which lockring? are we talking about a FG bike? or is it the BB lockring on the adjustable cup?

if your refering to the lockring on the cog, how did you tighten it? did you have a chainwhip and a lockring tool?

What is the lock ring? could you provide a picture or two? If you mean the nut that holds on the rear wheel, that could have been contributing to the problem.

It's on Origin 8 lock ring for a fixed gear. Reverse thread to hold down the cog.
Best I can do atm.

does the wheel spin smoohtly? perhaps you bulloxed up your bearing adjustment on the axle when you removed and replaced the wheel.

Yeah, it spins smoothly. It's just always been a little noisy, I passed the noise off as the chain being 1/8. I spun the wheel a few times before the ride to see that the new chain didn't have stiff links etc, and it was spinning great. It is slammed in the drop outs too, but the chain still has decent play.

Did you by chance hit a slick spot on the road? that could cause the rear wheel to stutter like that. The foward pedaling motion would act to tighten a fixed gear so unless your cog was crossthreaded onto the rear hub then that is most likely not the problem. If the chain stayed on the both the crankset and the cog on your rear hub then the loose bottom bracket was probably not what cause this either. It sounds like you hit a slick spot in the road while you were trying to accelerate (possibly with your weight forward) and when the wheel slipped and your foot went straight down as you described you stopped pedaling for an instant....this is what I believe most likely caused you to be thrown over the handlebars. If you have even a modicum of speed and you unconciously stop pedaling the bike will buck you over the bars quite fast. You may still need to trouble shoot your bottom bracket and your cog/lockring but I don't think this is what directly caused the problem.

I used to commute between my apartment and university by fixed gear. The route I took one evening had me ascend a hill with a plateau on top for about 100meters and then descended a hill that I lived halfway down. I sprinted up the hill and as I was crossing the flat plateau I found myself bodyslammed onto the ground with the bike on top of me feet still strapped into the toe clips. I had no idea what had happened at first. I dragged myself to the side of the road and sat there to catch my breath and make sure I was ok. While sitting there the light bulb came on and I realized that at the top of the hill when I sat back down there must have been an instant when I tried to coast...no can do on a fixed gear...and over the bars to the ground I went.

-j

Nah, no slick spots on the trial that I use. It is very dry here, and there was a loud ass pop that sounded like the chain snapped. It was loud enough of a pop that the guy that I tried to pass and ended up helping me said he thought my chain broke. I was thinking the loose lock ring caused the cog to actually walk loose and the acceleration combined with my full weight on each down stroke when in the drops caused both the lock ring and cog to bind and somehow caused the chain to jump or slip.

The lock ring being only finger tight wouldn't matter when your pedaling forward because the cog itself is tightening on the hub. My guess is you might have some kind of chain line problem and your chain is trying to walk up and off the chainring or cog and this is causing some binding.

"play in the cranks"? I'd start there.

Damn chain line. Yeah, the cranks have about 1-2mm of play in them and the drive train is loud. The new chain probably didn't flush right or something. Hopefully its the cranks. It's going to be awhile before I am comfortable sprinting on the bike again. I just hope the fear of crashing goes away quick. I was going downhill at about 25mph, broke my acromion and surgery consultation next week. Thanks for replies.

Fixies really belong on the velodrome.

This isn't so rare of fixed wheel bikes. While fixed bikes have gotten trendy, education about how to properly set a sprocket and lock ring hasn't kept up.

Many tighten the sprocket to what they think is tight enough, then set the lock ring. The problem is that the high torque of sprinting or climbing can tighten the sprocket a bit more, which would be fine for a freewheel. But if the sprocket moves tighter, that takes pressure of the lockring which is now loose. The sprocket can work back and forth, and with the lockring becoming looser eventaully the free play is enough to cause what happened to you.

In the future, mount the sprocket & and set the lockring as you normally do, except maybe don't set the lockring too tight. Ride the bike doing a few interval sprints and/or climb a steep grade, being careful slow down via brake or coasting without applying any reverse torque. This will fully tighten the sprocket so that future sprints can't pull it any tighter. Now remove the wheel and properly tighten the lockring.

BTW- if you were riding a fixed sprocket on a single thread hub made for freewheels you learned the hard way (sorry about the shoulder) that this arrangement is highly unreliable. As someone who road a fixed wheel bike for decades, stopping only when it got trendy, I've posted on dangers of mounting fixed sprockets on freewheel hubs, and get roundly poo-pooed bu folks who think that the magic of loctite can cure all ills.

Francis, would you be kind enough to elaborate on this a little
(i.e. is it a threading difference btwn a regular freewheel hub
vs a genuine track hub?) Is it just that the cog tends to loosen
up when you use your legs to brake?

I get these kids in all the time at the coop, and all I can tell
them is basically I've always been a roadie/commuter and all
I only know about fixed gears that going around a corner is
very exciting.

Seems like everything i see around here at the coop is a modified
standard road hub.....there are some flip flop fixed/free wheels
that i presume are designed for this use.

Was thinking that's what happened FBinNY. When I started my ride I felt the rear wheel stutter/slip a few times, and when I'm on the trail I end up releasing the tension of my legs to slow down a bit before I use the hand brake and see if the left hand lane is clear before passing; I think this was causing the rear cog to loosen up a bit and when I stood up in the drops and started to sprint past someone the cog tightened up again caused a skip; or jumped off and back on making the loud popping noise, and caused me to eat pavement. The play in the cranks probably worsened the problem as well. I ride the fixed only on trails when I want to do sprinting intervals. The hub is stepped threaded as well.

step thread 1 RH the other LH is the normal track hub.

Curious, I, following a few links offered here in the past ..
saw, now there are alternatives to the more common reverse threaded pairs
some bolt the chainring to the hub with 4 bolts
and the heavy duty spline, [not the cassette conversion spline,]
I forget specifics, .. but both are significantlyhigher priced kit.

I think the new sturmey S3X uses the spline from a Shimano BMX pattern,
they thread the spline tips so a common freewheel will go on too
then you get 3/5ths of their 5 speed hub.

Starting with what you're familiar with, a standard freewheel hub (single or multiple) has a simple RH thread. The freewheel threads on and riding torque makes it tighter. You can't loosen it with the chain, or a chain whip because if freewheels when spun backward. That's why you need a freewheel remover to turn the inner ring off the hub.

A fixed sprocket on the same thread would also tighten as you ride. But if you reverse the torque, it'll unscrew. To eliminate the risk of a sprocket unscrewing track hubs (fixed wheel hubs) have a second thread outside of the sprocket for lockring. This has a left hand thread, and is smaller, some the sprocket slide past it. The left hand lockring ensures that the arrangement won't unscrew because as the the same torque that would loosen the sprocket would tighten the lockring, making both turning together impossible. The system has served well for over 100 years and continues to do so.

Simple rule to remember is that if you can remove the sprocket with a chain whip, you can also do so with the pedals, therefore it needs a reverse thread lockring.

As Fietsbob pointed out, there are newer arrangements which are equally sound. Either bolting the sprocket to the hub the way brake discs are done, or usinga splined hub and sprocket with a lockring. (common on vintage Strumey Archer hubs).

The thing fixed wheel riders need to remember is that any time they feel play in the sprocket, it needs to be attended to immediately. And tightening the lockring before fully tightening the sprocket is pointless.

It sounds like you stripped the cog threads on your hub. I'd pull the cog and lockring and take a look. Your hub might be toast now.

A loose lock ring would not cause this type of problem while accelerating, as this would tend to tighten the cog not loosen it. Are the cog threads perhaps damaged?

Nah. People have been riding fixed gear bikes on the road for over a century. Heck, they didn't even allow derailleurs in the Tour de France until 1937.

ONLY if everything was adequately tight in the first place. As FB pointed out, if the cog was not initially tight enough then it would screw on further with enough force, causing the lockring to loosen, and later allowing the cog to loosen from back-pressure to its original position.

At best the pattern of tightening and loosening would strip the RH threads. At worst what happened to the OP could occur: he sudden tightening of the cog upon hard acceleration could cause the chain to jump and jam or just throw the rider off-balance.

Plus back pedaling, and track standing a few skid stops here and there with a loose lock ring would cause the rear cog to loosen up as well. It's my fault for not checking the torque before that mornings ride and I feel so dumb. The rear hub is stripped as well, caused by the cog not the lock ring; the lock ring came off fine. Luckily its a flip flop track hub and I can just flip it and use the other side. I still think I'm going to be a little uneasy when I heal up and jump back on.

No reason to be uneasy, just alert. With a tight sprocket, the pedals have a few degrees of backlash an the chain tension transfers from the upper to lower loop. Once you're familiar with how far that is, be aware if it increases. Any change from normal means trouble. It could be that your wheel shifted or the sprocket is loose.

BTW- riding a fixed wheel for decades I long ago developed my system for tightening the sprocket. Since there was no steep hill near my home, I'd mount the bike, roll until my front wheel touched the wall, set the pedals horizontal, then hop on and try to ride through the wall. Afterward, I'd set the lockring.

Using that method I've never had a sprocket come loose.

Would you place it on a trainer, or just get on and try to pedal through?

A trainer would defeat my purpose. The objective is to lock the rear wheel so I can torque the sprockets up to abd beyond anything I'd do in the real world.

I suspected a chainline issue as a derailed chain on a fixed gear is very dangerous and can cause lockups like that. But if you found your hub threads stipped, you've found your problem.

+1 Get the cog really tight then tighten the lockring.

Rotafixing works as well, but if you go too far you can break your chain or strip the hub. I've found JB Weld effective on a freewheel hub but then of course you can never remove the cog..