Airburst

I pulled my old Caloi MTB out of the shed over Christmas in an effort to get the drivetrain running, but I've still had no luck stopping the chain skipping. It's a singlespeed drivetrain, with a fixed-arm Shimano Alfine tensioner. I thought at first it was the cheap sprocket I had on the rear, which I've just replaced with a brand new Halo Fat Foot one. I've measured the chain wear across 12 inches with a decent ruler, and there's literally no elongation (which makes sense, as I've never ridden the bike any distance because of the skipping drivetrain) but the chain skips on the rear sprocket as soon as I apply virtually any amount pedalling torque. The chain and rear sprocket are both 1/8", although I think the chainring is 3/32". I've ensured that the chainring and sprocket line up, and that the tensioner pulley is right underneath the sprocket, but I still get the skipping.

It's not the kind of skipping where the side plates of the chain ride up on the sprocket teeth, it's the rollers which ride up, which is weird, because that generally happens when the sprocket's worn out and the chain isn't.

Any ideas what's happening?

hueyhoolihan

a pic would help a lot.

Airburst

I'll take a few tomorrow once it's light.

FBinNY 

For all practical purposes (related to chain drive) you have a derailleur bike, so what makes your type of chain skip impossible for single speed doesn't apply.

As you point out, the skip is happening because as the chain winds onto the bottom of the sprocket, something is delaying the rollers from fully bottoming into the troughs and it's climbing a bit. That means the pitch is no longer matched so it rides the tips until the roller lines up with a trough and it drops back in.

So there are three possibilities.

1- friction. Friction within the chain or between the plates and sides of the rollers causes the chain to delay dropping in, which starts the process. Easy to fix with better lube, or by gently sanding the sides of the sprocket teeth on both sides to eliminate any burrs or surface defects. BTW- chainline also matters, so make sure the tensioner pulley is feeding the chain straight on.

2- chain. Friction within the chain, or a twisted or stiff link can cause what you describe. Test by slackening the tensioner (push on it) so the lower loop sags an inch or so, and back pedal. Watch for any link that doesn't straighten immediately, of "hiccups" as it emerges from the pulley. Also watch for any link in the upper loop that looks like it wants to climb up as it engages the rear sprocket. When you find a bad link, tie a thread around the outer plate, and continue testing. If the chain is the issue, the same area will skip each time, if something else is going on, you'll have threads on a bunch of links here and there.

3- sprocket. There's not much you can do, but an incorrectly cut sprocket can cause your problems. Sprockets require a certain amount of precision to ensure that the pins ride at a diameter so the distance pin to pin while on the sprocket is precisely 1/2". If the troughs are cut too deep or shallow, the chain rides at a different radius and the theoretical pin to pin distance will be shorter or longer. You can test for this off the bike by winding a chain onto the sprocket and eyeballing to see if the pins seem to advance or ****** from one end to the other.

Poor sprockets used to be rare, but there are many players making them these days, and the issue is more common. Perversely, you're more likely to see a poor sprocket on an expensive CNC machined one than a cheap stamped one. Stamped sprockets are all good, or bad, but on a machined one, a tiny change in position will cause errors.

---------------------------

If you cannot isolate the cause, consider that break-in can solve it. Shorten the chain until it's the shortest length that can loop the sprockets (by passing the tensioner). With any luck at all, the slack will be too little for the chain to climb off, so it will not be able to skip. Now run the drive train, first on the stand, then by riding it, until it smooths out. If things don't improve, buy a new sprocket from another manufacturer.

Airburst

Well, I get skipping on the old stamped sprocket I took off tonight, and the new CNCed Halo sprocket I replaced it with, so I'm fairly sure I can eliminate the sprocket as the issue. I've oiled the chain with decent lube, and I can't seem to find any stiff links on the chain. I get skipping pretty regularly (I put SPDs on and managed to get about four skips on one revolution of the cranks by keeping the pedal force on all the way round) so if it was a stiff link there'd have to be a lot of them...

The other issue with this is that the tensioner isn't sprung - the arm adjusts back manually to remove chain slack, and is held in place by tightening a bolt. That means that the first time I get a chain skip, the chain tries to find slack that isn't there and ends up pulling the tensioner arm forwards and loosening off the chain, at which point the skipping gets a lot worse. It's almost like the chain tension (or more accurately the lack of slack in the chain, there's no "tension" per se) *is* stopping the skipping, but I can't understand why that would happen with a new sprocket and a virtually unworn chain.

I'm inclined to suspect the first option you proposed, but lubing didn't cure it at all, so I don't know whether that's the case,

hueyhoolihan

if it were me, i'd double check the chain width. it's a bit suspicious that the chainring exhibits no problems but the cog does. and the chainring is possibly 3/32.

FBinNY 

Remove the sprocket (or wheel) and wind the chain around it, then test for side play. These days 3/32" chain inside width is calibrated around cassette and freewheel sprockets which may be thinner than yours. If when you wind the chain, you don't find significant free side play, that is your problem. A chain will force itself onto a tight sprocket but that will spread the inner links (which aren't attached to each other) and cause binding against the outer ones. Buy a vintage 5s or 6s chain which will be slightly wider between the inner plates than yours and solve the problem.

Or thin/chamfer the sprocket with sandpaper wrapped around a paint stirrer. Spin the wheel and "lathe" the sprocket, improving the bevel and "gothic arch" point of the teeth, then carry that down the sides to thin it very slightly.

Or if you've confirmed that the chain inside width and sprocket match, do as I suggested earlier, and ride it a while as a no tensioner single speed, even if the chain sags. With no place to give, the chain will be forced to engage, and break itself and the sprocket in until the skipping problem resolves.

BTW- I know your post history, and you're better than this. Stop pulling your hair out, go get a beer, than come back and observe and think with an open mind, and I'm sure you'll see what you've been missing.

1saxman

Just curious, did you manipulate the whole chain before installing it? You know, put light bending stress both ways on every link?

fietsbob

Still have a converted Cassette hub & cog on the Back or did you build a Freewheel hub single speed Wheel with a Proper single speed freewheel?

Tooth profiles on cassettes are about shifting , constantly taller teeth on freewheels help chain stay Put.

Airburst

I went out again armed with a headtorch and checked it, and the chain has plenty of side-to-side play in it when it's on the sprocket. It's a 1/8" chain and sprocket. My next idea is to just go and buy another singlespeed chain and see if that helps, because I can always make use of that on another bike if it doesn't do the job here. If that doesn't work I'll punch a few links out of the chain and just run it without the tensioner like you suggested.

Interestingly, if I pull the crank down to apply tension to the top run of the chain, I can lift the chain off all but about the top three teeth of the rear sprocket without any effort at all, even when it looks like it's engaged the teeth properly. I have no idea if that's normal or not, but it happens even when I push the crank down hard.

It is still a freehub, but I've got a proper singlespeed sprocket on there.

FBinNY 

I really want to be wrong here, but is it possible that your spacers are coming up close to the troughs of the sprocket teeth. Chain side plates extend below the troughs, and there must be clearance for that. This is a common problem when people use 11t cassettes with a lockring intended for 12t, and I wonder (in mental desperation) whether a similar issue is involved here.

Airburst

I wish it were that simple, and no, it's a 18-tooth sprocket. The spacers are out of a couple of junk cassettes I pulled apart.

I do appreciate the compliment there, thank you! Hopefully the new chain will fix things, if not I'll break the whole lot in one way or another.

fietsbob

perhaps cure is put together a non vertical drop out frame bike, so you can get rid of the spring chain tensioner , and just slide the wheel back in the dropout.


I have seen classic British made frames , made with holes for mudguard mounts in rear open dropouts , & the fork, and mounts for rim brakes .. 2..


'winter training' .. can be fixed or freewheel ..

But old Hzntl dropout road frame would do..

FBinNY 

OK, so now I'm really scraping the bottom of the barrel.

So if I have this straight, you have a 1/8" chain on a 3/32 cassette sprocket. New - desperate - theory to consider.

Derailleur chains have chamfer or bellmouth on the inside plates to help funnel the sprocket tooth in. Meanwhile cassette sprockets have square profiles so they can grab and lift the chain. -------OTOH many 1/8" chains have flat inner plates, or minimal bellmouth, and depend on a pointed (sideways) profile on the sprocket to slide in like a boat's bow.

So, it's possible that you have square plates meeting a square profile tooth, and it's like trying to guide a barge into tight boat slip. The corners meet and snag lifting the chain.

The fix is easy -- wrap sandpaper on a paint stirrer, and spin the wheel while sanding a bevel onto the sides of the teeth to convert the barge into a speedboat.

I hope this is it, because the barrel is pretty damm empty now, and I don't think I could come up with anything without having it in my hands.