Long story started off with 42:16 (commuting bike), worked fine. Decide I needed a smaller rear cog, made the chain too short (must remember to measure twice) and chucked it away.

Put the 16T back on the bike and brought a new chain (8sp sram). Put that on and it now slips under heavy load, never did this on the old chain. I am using a sprung push down tensioner (the same one that worked initially) and I cannot take anymore links out.

The old chain was a single speed specific (I think) from Charlie the bike monger, could this make a difference? The rear cog is ss specific and the front is a betd/goldtech unramped downhill/ss ring. Frame has vertical dropouts. Chainline is good, I also moved it around just I case my eye was off and I still get the same problem.

Given that it worked fine before I don’t understand what the problem is. Are there some chain, chain ring and cog combinations that just don’t work? Any other ideas?

new chain on worn cog = jump

yup what he said

new chain on worn cog = jump

+1
applies to both sprocket and chainring

Either as above OR with the new setup, it's possible that there's not enough chain wrapped around it – this happened recently with my SS when I fitted new chain, chainring and sprocket – the new chain was considerably shorter than the old (very worn) one and as a result, the push down tensioner wasn't wrapping enough chain around the sprocket. My solution was to remove two links of chain (inner and outer) and fit a half-link in it's place – that increased the chain wrap on the sprocket enough and it doesn't slip.

I also found that SS specific chains are not ideal on many cogs. the SS/BMX chains sold are of lesser quality than 7/8 speed chains and from my experiance are prone to jumping with high quality sprockets and chainrings. I'm guessing that the extra width of SS/BMX chains on thinner sprockets causes the chain to twist and jump since it's not being held straight by wide cogs.

from my trials most quality SS cogs seem to be best suited to 7/8 speed chains.

1/8th chains on 3/32 cogs are fine, trackies do it all the time and put out a lot of torque. I thinks the problem is using poor quality chains Mister T, not the size. Plenty of good quality fat chains about.

Should have said the chainring and cog have probbaly done 100km max. I will look at how much chain ia wrapped round. Still hard to explain since it has worked once already.

Still hard to explain since it has worked once already.

The new chain will probably be shorter than the old one (assuming it had worn/stretched a bit) so you may actually have an extra couple of links in there than the old one, hence less wrap around.

Did you sort things out?

I've got a push down tensioner from the bike monger, on an Orange E8, running a new SS specific front ring and chain and it's slipping under heavy load.

Interesting that it might be due to a wider chain twisting and slipping.

Might see if i can take some more links out, if that doesn't help then a 7/8 speed chain.

how much chain to you need around the rear cog?

As much as possible. Skipping on ss is pretty much always due to wear.

I'm guessing that the extra width of SS/BMX chains on thinner sprockets causes the chain to twist and jump since it's not being held straight by wide cogs.

hat doesn't make sense. 1/8 chains don't twist.

Well I pushed the tensioner up a bit and took a couple of links out(with difficulty)

seems fixed

I gues he fact that I could get a couple of links out point to the cause being the chain was too long..

"I've got a push down tensioner"

that, plus a smallish rear cog, means v few teeth properly engaged so more chance of slip – could you convert it to push up instead ?

hmm, will give it a test to see if it slips…