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I'm new to fixed gear riding, and my bike only has a front brake. I have noticed that when I use resistance at the cranks to slow down, rather than the front brake, eventually the cranks will begin to loosen. I've tried thread-lock and torquing them down really tight, but they still eventually loosen up again.

(edit) It's a brand new bottom bracket and the cups are not slipping - it's the crank arm bolts that are coming loose. I will take a look at the crank arm interface to see if they are rounded off at all. It doesn't seem like it would be though, since once I re-tighten the bolts, the cranks are stable. It's only through the repeated "engine braking" that they get loose.

Is this just a thing that fixed gear riders deal with, is it a mechanical defect, or am I doing something wrong?

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    I've never had loose crank arms on my fixie. I also don't see how using the pedals to slow you down would have any effect that would cause the crank arms to loosen. Are you sure it's the crank arms that are loosening, or is something else happening such as the rear sprocket becoming loose (because you don't have a lock ring) that's causing the crankset to appear loose? I could see how your crank arms could become loose over time. I don't see how it could be linked to using your pedals to brake.
    – Kibbee
    Nov 24, 2015 at 19:09
  • Did you clean the threads on the bolt and bottom bracket before applying the thread locker?
    – Benzo
    Nov 24, 2015 at 21:55
  • yes, the cranks wiggle after a long ride using the pedals to slow down - if i don't back pedal and just rely on the front brake, the cranks stay tight. and i cleaned the bolts before applying thread lock, but not the axle (which is new, and hasn't been ridden in the rain). I suspect that the cranks are worn and no longer sitting properly on the square axle as suggested below, but will definitely check to make sure the axle threads are clean - I suppose it's possible i inadvertently got some lubricant on the threads when i was installing the bb, which might prevent a good tight seal.
    – Doug
    Nov 25, 2015 at 16:36

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I guess your bottom bracket is a standard square taper? And that your bike is getting a little old in years? The fix is to replace the cranks.

The cause is that as you tighten the nut, the square taper hole on the crank mashes up close to the axle. But as you pedal, and brake in your case, the cranks will minutely flex in the hole which creates a tiny space so fretting can occur. This is when minor amounts of metal are rubbed off the inside of the crank's square hole - the bottom bracket axle is normally tougher than the crank metal.

Once there's a bit of play in the square hole of the crank it will never tighten up right again. If you're handy with tools you might be able to take some metal out of the middle of each side of the square hole, but in reality its only a delay.

Worst case is your cranks crack through while emergency-braking.

Check out your local bike co-op. Cranks are mostly okay on scrapped donor bikes, so they should have some in spares for cheap.

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    Once you've tightened the bolt and the crank-arm sits tight, unscrew the bolt and check that the axle is below the edge of the square hole. If the axle is flush with the edge then the hole has widened and the crank has reached the end of its life.
    – Carel
    Nov 24, 2015 at 19:25
  • Great information. I will check the axle/crank relation relationship after the holiday. Much appreciated.
    – Doug
    Nov 25, 2015 at 13:55
  • @Criggie yes, standard square bb axle and also yes, not the newest bike. I picked it up for a song on CL this summer. needed a thorough cleaning, wheel truing and relubrication, but other than this crank issue, it rides like a dream now.
    – Doug
    Nov 25, 2015 at 14:00
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Maybe some of the information in this link is helpful. http://www.bikeforums.net/singlespeed-fixed-gear/690261-fixie-my-pedal-crank-keeps-getting-loose.html

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  • It's a brand new bottom bracket and the cups are not slipping - it's the crank arm bolts that are coming loose. I will take a look at the crank arm interface to see if they are rounded off at all. It doesn't seem like it would be though, since once I re-tighten the bolts, the cranks are stable. It's only through the repeated "engine braking" that they get loose.
    – Doug
    Nov 24, 2015 at 17:59
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    @doug Put that detail in the question. Answers are only as good as the question.
    – paparazzo
    Nov 24, 2015 at 18:11
  • @frisbee you assume that i had considered the bottom bracket before posting the question. if i were smart enough to have known to check that first, i certainly would have included that detail. i added that after reading the information that Mr. Kelley kindly provided.
    – Doug
    Nov 25, 2015 at 13:53
  • @Doug IN the question. Someone else added that information to the question for you.
    – paparazzo
    Nov 25, 2015 at 15:07

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