Chain keeps popping off?

[yashicamat]

Finally put my SS together and gave it a ride out today. Fantastic! Very rapid, easy to cruise at about 20mph where my tourer would be doing about 16mph tops.

Anyway, as I was about 400 yards away from a junction, I started to ease off on the pedal pressure at about 24mph ish, next thing I know, the chain has jumped off! I pull over, examine it and latch it back on again. The tension seems quite slack now although I was sure it was tighter before. So I gently limped back home and adjusted the chain tension so it was a fair bit tighter (I had to pull the axle into place while I tightened the QR as the chain would pull the axle out a bit if left "at rest").

I then went out for another ride, the chain then popped off again at about 29mph this time. The odd thing is, it's coming off the (new) rear 14T sprocket (a campag one), but it's coming off on the side I'd least expect it to as thanks to the spacers, there is a very very very slight angle on the chain, but it is coming off away from that direction!

Is this a failing in the drivetrain components, or just a case of the chain not being tight enough?

The drivetrain bits are:

Campag double (36/52 with the 36 in use)
Campag 14T rear sprocket
Shimano 9spd chain

No chain tensioners, the bike has angled dropouts so I am using those to adjust chain tension.

Cheers.

 

[Theseus]

Make sure you do the nuts up as tight as you can without stripping them. If you have chain tugs, use them.

 

[Big John]

I've not got a SS bike but I've got a fixed and I presume certain rules still apply. Like Touche says make sure you get those nuts as tight as possible and the chain as tight as it will go. Chainline, I guess, is as important on a SS as it is on a fixed i.e. it should be cock on. Use spacers if necessary. If these two essentials are in place then the chain should stay where it is. Are you using a 3/32 or a 1/8 chain?

 

[Radius]

I reckon your chainline is out and you should be using a slightly thicker chain...

 

[rustychisel]

I reckon wot he said. Your chainline is slightly off, your rear sprocket is a Campagnolo one designed for gear changes with ramps and tooth profiling (from a cassette) and isn't really suitable for single speed riding without perfect chainline and perfect tension.

A wider chain 'can' help. Sometimes.

Address the chainline issue. Address the tension issue. Don't use a QR if it moves forward or slips. Consider using a 'proper' SS or fixed cog which are manufactured with longer teeth (deeper profiles) so a little chain slap shouldn't affect them.

 

[Ian H]

I think you say you're using a quick-release rather than track-nuts. I think the wheel is slipping slightly. You might solve the problem by doing the QR up really tight, but track-nuts are best.

 

[ColinJ]

I had exactly the same problem when my mate built up a singlespeed bike for me.

We used the spacers from two old cassettes and a 15-tooth sprocket from one of them. Here's a picture of the back end of the bike:

We got a near-perfect chainline but on my test ride the chain came off twice. I came to the conclusion that this was because the cutouts and ramps on cassette sprockets are designed to make it easy for the chain to move. It was only doing what comes naturally!

After a bit of thought, I decided that what I needed was something either side of the 15-tooth sprocket to keep the chain place. I ended up installing 17-tooth sprockets either side of the 15 (suitably spaced by spacers). There isn't enough slack in the chain for it to actually get up onto either of the 17s so it stays in place.

I've done hundreds of miles on that bike since then and the chain has never played up again.

Oh, and wheels don't pull out if you have decent quick releases which are done up properly - if they did, they'd do it on geared bikes too!

 

[Ian H]

Oh, and wheels don't pull out if you have decent quick releases which are done up properly - if they did, they'd do it on geared bikes too!

True, with nice new components properly used. But cyclists used, once upon a time, to pull wheels over, for whatever reason. Vertical drop-outs put paid to that for the derailleur fraternity, but it still happens occasionally to fixed-wheelers.
Considering how cheap a decent single-speed hub can be, I think I'd rather rebuild the wheel (no dish, so much stronger) rather than spend time and effort on a freehub bodge.

 

[yashicamat]

Cheers ColinJ, that's an identical setup to me really. I'd rather not shell out for two brand new Campagnolo sprockets though, so I'll root around and see if I can get some large washers and use them instead!

 

[yashicamat]

Well, I took ColinJ's idea and expanded on it:

Two 2.5" OD steel washers, less than £2 for the pair. The chain hasn't budged since! I sprayed the washers with PFTE spray too so they should retain a degree of rust-proofing until I can be bothered to remove them and paint them. Result!

 

[ColinJ]

Considering how cheap a decent single-speed hub can be, I think I'd rather rebuild the wheel (no dish, so much stronger) rather than spend time and effort on a freehub bodge.

I'm skint! I've borrowed the frame/forks/wheels and most of the other bits came from my junk box. It only cost me £25 to get the bike on the road and so far it has been 100% reliable.

Well, I took ColinJ's idea and expanded on it:

Two 2.5" OD steel washers, less than £2 for the pair. The chain hasn't budged since! I sprayed the washers with PFTE spray too so they should retain a degree of rust-proofing until I can be bothered to remove them and paint them. Result!

Another satisfied customer!

 

[jimboalee]

When you adjust the chain tension on a SS, the tightest it should be is 1/4" vertical movement midway between ring and sprocket.

There will be some loosening because of the eccentricities in the ring and sprocket. It will be a very lucky man who gets both smack on.
If the loosest is over an inch movement, there's something wrong with the concentricities of ring or sprocket or both.
If this is the case, the other check is....
When the chain is rolling round, push it sideways with a finger midway between ring and sprocket ( as the chain is GOING to the sprocket ). If it derails off the sprocket, it's too loose.

It is helpful if ring and sprocket are in alignment, but 1/8" either way won't hurt.