A couple days ago, I was riding in downtown DC and my left crank arm fell off...I was going down a hill at about 20 mph and I almost crashed in the middle of traffic. Luckily, I recovered. I like to think that a small amount of skill prevented something worse, since I have no brakes. I think I'm gonna put on a front brake again. The crank arm bolt backed out. I ended up riding about 2 miles with only one crank to a local bike shop to get another bolt.

Has this ever happened to anyone?? The mech at the LBS said you need to check your crank arm bolts frequently on fixies because the frequent back pedaling can cause the crank arm bolts to come loose...I guess that sorta made sense to me, but I wondered because it seems to me that forward pedalling would tend to cause the left crank arm bolt to loosen, whereas backward pedaling force would tend to tighten the left crank bolt. Anyway, my lesson learned is to check my crank bolts periodically. BTW never had a crank arm bolt loosen on any of my geared bikes...knock on wood.

It happened to me way back when. Neither pedaling direction will cause your crank bolt to tighten. I think the issue with fixies and backpedaling is not so much the direction, but the amount of force it takes to do so combined with the chance that it will cause the cranks to flex and play to develop between the crank, spindle and crank bolt.

At least is is easier to pedal a fixie with one crank. . .;)

I had something along the same lines happen to me today. I was cranking up a hill and my right crank arm snapped in half.... down I went. I am just glad it happened while I was going slow or it would have been nasty. I guess it's the same idea... a lot of torque. They were road cranks converted to fixed gear use. Looking at new cranks now... thinking maybe track cranks would be better. I realize this was a freak accident but still... it scared me enough to think about shelling out the extra dough for track cranks.

. . . were you riding clipless pedals - and the crankarm was dragging the road still attached to your shoe?

A couple days ago, I was riding in downtown DC and my left crank arm fell off...

. . . were you riding clipless pedals - and the crankarm was dragging the road still attached to your shoe?

yup, i felt my left foot wiggling around and by the time i looked down, the crank had fell off. since i was in traffic, i was trying to slow down with my right leg and i knocked the crankarm with the pedal off my foot...i'm sure i looked like an idiot...but i didn't crash :D

Glad you didn't go down.

To a lot of onlookers, a bike crash about to happen always seems to be a huge source of amusement. I think some people are actually disappointed if you stay up.

Anyway, my lesson learned is to check my crank bolts periodically. BTW never had a crank arm bolt loosen on any of my geared bikes...knock on wood.
I have, but never enough to come off. I check mine periodically.

Make sure you torque them, using a torque wrench, to spec - they need pretty high amounts of torque to keep them tight.