SS conversion - why does my chain keep popping off?
 

First time SS conversion and I've got a major chain jump issue. Smallest bump and my chain is jumping off my 40x17 REAR cog. Is it simply that I have too little chain tension? Thanks!


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That is what you call chain tension? That thing is slacker than some pot smoking hackey sack player at one of those big jam band festivals.

I would remove some links and tighten up that chain and give yourself some room for actual tensioning.

Like veganbikes says, shorten that chain. I'd size it so the hub sits halfway in the dropout. Then adjust the axle location so you can lift the chain about 1/2" to tight. (Spin the pedal slowly several turns and watch the chain go tight and loose. Make sure it never goes fully tight at the tightest. With luck, the loosest will still be tight enough so the chain never comes off. (This can be a balancing act with cheap components. I see you have what appears to be a high quality track crankset so I am guessing you will be blessed with little tight-slack action.) Don't let the chain ever go fully tight. That means you are trying to crush steel ball bearing with a steel band. Good way to make BB and hub bearings old quite quickly.

Halfway in the dropout means you will be able to add or subtract probably 2 teeth with different cogs to change to different gears as you get stronger or wish for lower in view of your hills. If you are running the wide 1/8" chain, you can further fine tune length using 1/2 links that are male on one end and female on the other. (Bike shops that handle fix gears and hardware stores. 1/2 x 1/8" chains are a common size for machinery. The 1/2 links will look big and clumsy and use super secure cotter and clevis pins but will work just fine.)

And the trick to adjusting the slack. Push the wheel back. Tighten the left nut. Center the wheel. Tighten the right and check chain slack. Adjust the slack by loosening the right nut and push/pulling the tire side to side between the chainstays. Tighten right nut, loosen left and now center the wheel and tighten. (Left axle location has a big effect on slack. Right far less. Right can also be used as a fine ture to the slack. And contrary to what some say (and OCD brains allow) the tire does not have to be exactly centered. Trust me, The bike doesn't care at all and if you do not know or simply "forget" and keep your eyes where they belong, you will feel the difference. Not even no-hands. I've been riding fix gears for close to 50 years and coming up on my 5th lap around the planet. Took me a long time to accept that the ride was no different when my OCD was in full force or it was on vacation (as long as the nuts got properly tightened!)

Ah yes this is exactly what I stumbled upon: take the axle almost out of the dropouts and size the chain so there's some slack then push the axle back into the drops until the tension is perfect. Looks like prob is fixed - thanks all!

Looks like you're missing a couple of chainring bolts, too. That might not be causing your problem, but it could cause others, in time.

Oh yeah ThermionicScott is right get those chainring bolts on there, they are not optional. Nothing good will ever come from not using chainring bolts and there is no weight savings to be gained from that. If you need a set of single chainring bolts in colors Wolf Tooth Components makes them and they are light and seem to work quite nicely (I haven't had issues yet on mine)

Yeah I've got a new set on order somehow I lost 2 of them from this set. Thanks-

It's fine...

The problem isn't too little chain tension; it's too long a chain. If your chain is tight, hub and crank bearings will wear quickly. You want no tension, but also no slack. You should be able to vertically deflect the upper run of the chain by a centimeter or so. If you can't, it's too tight.

Thanks John - got the chain sized and axle tightened so I have about 1 cm play as you suggested. Also went with a wolf's tooth rear cog - much nice teeth and cut that the ones I had tried.