pretty scary actually, since it tends to only happen when i am speeding down a hill towards a yellow light.

i've read that chain tension and chainline are the two big things that can cause this, and I was just wondering if people have strong feelings about which is more important, or if it is both in combination. My chainline isn't perfect, but it's pretty close. i had a lot of trouble finding a spindle which would fit weird swiss bottom bracket cup sizes, i ended up pretty close but not perfect. i have horizontal dropouts but in order to get the chain pretty tight i have it all the way back. i am going to try taking a link out to get it shorter and tighter, but if anyone has other advice or ideas about what could be causing this I'd sure love to hear it before i smash into the back of a stopped bus.

btw i am using the SRAM (Sachs/Sedis) PC58 chain pretty new shouldn't be stretched out

Breaking or braking, which is it? That would change your story.

Total Flashback!

Both are very important, although you can make due with a slightly-off chainline. Most likely either the chainline is off too much and it's falling off while being angled oddly, or you're spinning so much downhill that the chain is binding and falling off (too high tension).

Your axle could also be slipping forward.

Well, you could put a front brake on until you remedy the problem. This way you don't smash into that bus.

Breaking without a break and your chain brakes can be a scary thing.

Your chainring may be out of round, making the tension drop significantly at certain points in the rotation. I have also seen variable tension problems come from a bent axle.

Without any details I'd let a bike shop look at it before you keep riding. As mentioned, there could be many things wrong with the drivetrain.

Ja ne!
$peabody

Shorten the chain by a link and fix your chainline by adding a washer or two to the rear axel to push the hub over one way or the other. Both worked for me.

take out a link, and add a half link.

or maybe put a derailleur back on it

I took out a link as you and others suggested, which seems to have improved things. I am intrigued by your suggestion to add a washer or two to the rear axel, how would I go about doing this? Remove the lockring and the cog and then ... ? I've never done any work on hubs before, but maybe now's the time to start, especially if this is a relatively easy area to work on?
Thanks for the help,
noah

I'm sure there's got to be a bike repair co-op type place in PDX - you should try and find one. There, you can get some help learning how to fix your bike, and at the same time, you can get someone to take a look at your chainline.

Go to Bike Central downtown and ask them to check it out. This isn't normal. You can throw a chain, but only to do it on downhills suggests something isn't quite right. Frankly, even with a fairly loose chain, you shouldn't be throwing chains a lot.

I'd expect the chain is coming off the cog in the rear initially, not off the chainring in front?

If your wheel isn't aligned with the frame (i.e., pulled to the left in front because it wasn't tightened sufficiently, but not quite touching the frame and rubbing), and your frame lets you have a decent amount of misalignment, you might be having a problem that the rear cog will feed the chain off the top while pedaling and applying forward pressure, but when tension is on the lower chain traverse while you're trying to slow down, tension isn't sufficient to feed the chain properly with tension underneath. Tension on the bottom would tend to keep the chain on the cog and chainring, but above the chainstay any misalignment might show up as a derailment. Too much chain might cause this, but not that likely (on the track we ride chain looser than fixie riders tend to, and chain drops are almost unheard of). I'd guess at an alignment issue.

The only other possibility is that you have a bent link, which might be bent so it doesn't give problems accelerating forward but doesn't like to go onto the rear cog under tension from backpedaling.

maybe it's just karma.

haha, just kidding. get someone that knows what they are doing to look at your bike.

oh yeah, my chain dropped once on my bike...it was weird. tension wasn't that loose but the teeth on my chainring were a bit worn, it really came out of nowhere, i wasn't backpedaling hard or anything, just cruising down the street and all the sudden i can move my pedals with no feedback from the rear wheel. i pulled a foot out of the strap and put it in between the rear tire and the seat tube, skidded to a stop. my chain jammed in between the hub flange and the cog and i had to pull it out. it sucked.

The washer needs to go on the side opposite the direction the hub needs to move to give a proper chainline. Just take the wheel off, then the appropiate nut and slide the washer onto the axle. Just make sure the washer is inside the drop outs touching the hub assembly. If you need to put more than two or three on, then you really ought to look into a different bottom bracket.

Ive been having a similar problem, it only happens when I am skip-stopping. Ive gone through two different pairs of cranks and 3 chainrings and it still happens. I have a good chainline and everything, so now I am curious of this bent axle idea, is that common? Its only developed in the last couple months and has me totally perplexed.

It would really take a significantly bent axle to create this problem, and at that point you would either have major problems at the hub or it wouldn't even tighten up properly in the track ends. What frame are you riding and did the frame work previously? This sounds like a problem in the frame itself, such as a misaligned bottom bracket shell -- something from a bad crash or more likely, defective manufacturing. If the bottom bracket is tilted so the top of the chainring is in line but the bottom is too far out or in, it will tend to run OK when pedaling forward but will flip the chain when backpedaling. I repeat, I'd suggest you take it to a good shop (I'd recommend Bike Central) and have them check the frame alignment itself and scope for another problem. Doing it without photos or getting to try out the bike is pretty difficult and mostly a waste of your time.

11.4, do you live in Portland?

Community Cycling Center on Alberta and 16th / 17th would also be a good place to have it looked at.

(from portland)

In case anyone is still interested
I took a link out but made sure that overall the chain tension was looser. I can feel some slack in the chain when I pedal, whereas before it was pretty uniformly tight. I haven't had problems with the chain coming off, but I had a new creaking sound from the drivetrain I didn't have before. Tightening the cranks and pedals didn't help, and I ended up taking the crank off and greasing the pressed services which the Park blue book suggested. That didn't help at all, but as I was putting the crank back on I noticed that the chainring was extremely loose - basically only 1 screw was still tight, the others were all loose. I tightened those up which took care of the creaking, and now I am wondering if maybe the loose chainring was exacerbating the problem with the chainline, resulting in thrown chains.

As a side question, does anyone think that braking on a fixed gear, i.e. by resisiting the pedals and especially skid stops, tends to loosen up cranks more quickly than road bikes? And maybe chainrings as well?

Yes.


My suggestions:

1) Use a single-speed, strong BMX chain. Not an 8-speed chain that is actually DESIGNED to move left and right. A BMX or track chain is not designed to move laterally.
2) Use "track" chain ring bolts. They are made of steel as opposed to the standard aluminum
3) When you turn an apple into an orange (read: convert a road bike to a fixed gear) it's normal to have "issues". If you can, get a frame that's designed to be fixed with proper dropouts and spacings.
4) Don't stop the Body Rock!

Don't worry I would never stop the body rock.
Thanks for the suggestions

this type of thing used to happen to me on downhills. i would be coming down the bridge (williamsburg, brooklyn, what) and i'd be going fast and i'd try to just go into a skid and i'd throw my chain - happened like 3 times. now, perfect chainline, tension etc - i'm sure these things have quite a bit to do with this, but you want the most surefire remedy?

DON'T TRY TO THROW YOUR BRAKELESS TRACKBIKE INTO A SKIDDED STOP WHILE SPEEDING DOWN A BIG HILL. RIDE SLOWER. STOP WITH TENSION. OR GET A BRAKE.

seriously, you just shouldn't have to do that if you can avoid it at all. i know it sucks when some chad goes blazing by you on a $15 beater ten speed, but the sad truth is that those things are faster then yr fancy track bike. especially downhill. this is why you gotta school em on the way up. i'm not sure if you're new to fixed riding, but eventually you'll find that skidding is NOT something you should do all the time. its totally a great thing to learn and get good at so you can when you need to, but really it makes for a smoother, prettier, and more efficient ride if you're not skidding around all the time. of course, when the roads are a little wet and your goofing around, its hella fun. so fun i almost f7&king killed myself on delancey last night. so i aint preaching. just saying, be careful.

-pcr

yeah I think you have a pretty good point.
I just wore through my back tire anyhow so maybe that's a sign as well.

Thanks