Bicycle Mechanics - "Bouncy" chain and slipping cranks on fixie

I bought a entry level single speed with a flip flop hub for commuting. It only came with a fixed cog (non-free freewheel). The first time I rode it in traffic, I had to scurry through an intersection so I stood up out of the saddle and pounded on the pedals.

Terrifyingly, the cranks "slipped" and I didn't get any acceleration and had to coast through the intersection. In other word, my lead foot/crank/pedal drops from the 1 o'clock position to the 6 o'clock position at high torque loads.

The pedals are securely screwed into the cranks.
The cranks are securely bolted onto the 46T up front.
The chain didn't jump the front gear.
The chain didn't jump the rear fixed 15T gear.

What's left to "slip" and how do I stop this from ever happening again?

Here's the bouncing chain. I think it's related:
http://www.youtube.com/watch?v=3tRoD8gKegQ

Any ideas?

From your description I can only guess that the chain might have been too slack. The rear wheel has to be pulled back so that the chain has only about 1/8" of vertical play at the center of the span. Another way is to rotate the pedals while watching the chain to find the tightest spot and at that point pulling all but the last vestige of slack out.

It's important that you leave some slack because the sprockets aren't perfectly concentric and you have to provide for changes in tension.

If the chain was already tight it's possible that you were sold a bike with a stripped rear hub, and when you nailed it the sprocket spun. Given that you don't know the pedigree, I'd remove the lockring and sprocket to make sure.


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PS. I hadn't watched the video before posting, and now add that the chain tension seems OK, though the eccentricity is more than I like to see. The possibility of a stripped hub is now increased in likelihood.

BTW- if you loosen all the chainring bolts, and bring the crank to the tightest chain position, you might be able to tap the chainring back a bit then to reduce the eccentricity. Do this a bit trial and error to find the position of minimum "bounce" then tighten the chainring bolts to keep it there.

maybe I just missed it in your reply FBinNY because I saw you mention "stripped hub" so I assume you were saying his cog and lockring are most likely loose, I would bet money thats what the OP is experiencing.

As for the "bounce" in the chain, don't worry, that is because the chain ring is not perfectly round and so the chain gets tighter and loser as it rotates. You can reseat the chain ring and try to get it in a better position (loosen bolts and tighten again). But it's not causing the slippage.

I think most likely cause of the slippage is:

1> loose lock ring (cog was able to unwind a little, then when you apply forward pressure it rotates forward, giving the impression of slippage). You can test by pedalling back pressure and see if the pedals move, and then forwards again.

2> stripped hub due to improper cog installation.

The bouncing chain is caused by non-concentric machining on the chainring or cog or both. High-end track equipment is machined to higher tolerances than road equipment or entry level single-speed equipment.

Sometimes you can reduce the bouncing by shifting the chainring a bolt or two one way or the other on the crank spider.

You say you 'coasted' across the intersection... is it possible you have a freewheel that does not freewheel properly? If so then it is likely that your freewheel was the cause of the slippage.

You say you 'coasted' across the intersection... is it possible you have a freewheel that does not freewheel properly? If so then it is likely that your freewheel was the cause of the slippage.

Fixed gear.

Fixed gear.

I understand what a fixed gear is, and that the OP says he has one, but he also said he 'coasted' across the intersection... I'm not sure what your idea of a fixed gear is, but I have never ridden one that coasts.

That's the thing. Without knowing the OP's ability to correctly identify what he's looking at or seeing a good close up picture of his hub area on the drive side it's possible that it's a screwed up freewheel that was locked up with a jammed pawl or something and then suddenly busted free now. Or if it truly a fixie then the sprocket must have stripped the threads and allowed the cog to slip.

To CollarBones, can you get a picture of your hub on the drive side? Preferabley unmounted so we can see what is there more clearly? I'm sorry if it seems like I'm slagging your ability to correctly identify the components but I/we do not know your abilities in this area and have to assume the worst until proven otherwise. It's happened too often before when dealing at distances and with inadequite or incorrect descriptions. So please don't take it personally.

coasted by taking his feet off the pedals?. loose crankarm can feel awfully weird too ,
may he have torqued the cog on tighter with the first really strong pedal stroke?

I would loosen all the chainring bolts and slowly go back and forth tightening the bolt
on the opposite side of the spider,
and in the process see if the chainring can be centered as well as practical..
so reduce the tight spots in the chain.

You say you 'coasted' across the intersection... is it possible you have a freewheel that does not freewheel properly? If so then it is likely that your freewheel was the cause of the slippage.

Obviously you don't understand what a fixed gear is or you wouldn't have even brought up his freewheel.

Obviously you don't understand what a fixed gear is or you wouldn't have even brought up his freewheel.

No need to be a jerk about it.

The OP DIDN'T say he had a "fixed gear," he said he had a "single speed." He did say that it came with a flip/flop hub with only a fixed gear, but he didn't provide any pictures and went on to describe a situation that could be explained by a faulty freewheel. Go back and read the original post.

I had no trouble reading and understanding the original post. You did.

"The chain didn't jump the rear fixed 15T gear."

I had no trouble reading and understanding the original post. You did.

"The chain didn't jump the rear fixed 15T gear."
This still begs the question of how he managed to "coast" across the intersection.

I think we need the OP to clarify things further.

I bought a entry level single speed with a flip flop hub for commuting. It only came with a fixed cog (non-free freewheel). ......

With that little bit in bold it opens up the door quite wide to a lot of doubt about what is actually on the bike under the chain. There's a good chance that it was a jammed freewheel that he THOUGHT was locked and that was only broken and decided to pop when he stood on it hard.

In any event it's sad to see someone here slagging another member over this stuff. I think Grand Bois needs a group hug since he's obviously disraught over something and is lashing out at any and everyone.

Grand Bois - I love you, even though you are mean sometimes!!!:love:

My wife says the same thing.

My wife says the same thing.

I know. She says it to me, too. :D

This still begs the question of how he managed to "coast" across the intersection.

I think we need the OP to clarify things further.

Yeah, it's helpful when posters give enough information for us to help them right?

I know. She says it to me, too. :D

Doh!