[eric]

Man, and I thought SS'ing I'd be free of this! Two bikes, similar issues:

My bike:
ENO hub, Shimano freewheel, and an FSA 32t ring meant for shifting. Chain tension seems to fluctuate a bit, so I guess the ring is oval. Yesterday I killed about 10 of the 32t teeth on it. How is this possible? The chain keeps getting very loose, although the ENO really is staying put now. Think I should get a good, round 4-arm ring?

Her bike:
I built up a 1x1 for a friend of mine, using het 4-arm XT cranks, an XT freehub, and a loose 17t cog. The XT chainring was oval enough to cause wild chain tension variations, so I replaced it with a BlackSpire. It seemed to be perfectly round.

Turn out it isn't, or it looks like it isn't. She's dropping the chain on very hard climbs, and the chain tension is going all over the place. I'm thiniking: I don't overtighten the chains, so is this a problem with a) XT cranksets? Why does a seemingly round ring act like it's oval or b) the loose 17t cog on a Shimano freehub? Could the 17t be oval?

Thanks very much for any input.

 

[nogearshere]

The rings are oval...the best you can do is tension it for the tightest section.
I am not a fan of ramped rings b/c it does encourage chain jumping (if the chainline isn't just right).

I would suggest you take a look at the tensioner installation one more time. Here is a link for the instructions I include.

http://www.misfitpsycles.com/PDF/Tensioner.pdf

Try that and let me know if you are still stuck.

 

[eric]

No tensioner though

One bike's ENO eccentric'd, the other is an SS frameset.

Anyway: with a bit of help it seems that part of the problem may be the use of a regular 8-speed cog on the Surly. A DX - or better still a Surly or King Kog - cog might solve part of it.

Another thing I'm thinking is getting the chainline 100% perfect......

 

[nogearshere]

ahh. yes on both counts, chainline is very important and specific cog/ring/chain setup will help too.

 

[itsdoable]

Ramped cogs and chainrings are desiged to allow the chain to derail - use a cheap BMX cog (if you compare the teeth to a cassette cog, you'll see a big differeance). You can 'usually' get away with a ramped chainring, but a ramped rear derailleur cog is usually trouble.

The other thing that most people don't do (because you can often get away with it) is to NOT use a derailleur chain. For a chain to shift smoothly on a derailleur, it needs to have some lateral flex, and the side plates are necked to facilitate shifting. Usually the more expensive the chain, the easier it derails. SS or coaster brake chains have minimal latteral flex and wide plates, which prevents them from derailing. I use a cheap 15-speed KMC chain, which works well.

 

[eric]

Well

ahh. yes on both counts, chainline is very important and specific cog/ring/chain setup will help too.

We've got a pre-HG Shimano cog to fit the 1x1, so that may be part of the solution.

Since I killed the ring on my bike yesterday, I've ordered a 36t BBB ring (bling CNC, looks as if its round), and an 18t freewheel. Maybe the larger total diameter will reduce chain tension variation a bit, too.

 

[pacman]

Her bike:
I built up a 1x1 for a friend of mine, using het 4-arm XT cranks, an XT freehub, and a loose 17t cog. The XT chainring was oval enough to cause wild chain tension variations, so I replaced it with a BlackSpire. It seemed to be perfectly round.

Turn out it isn't, or it looks like it isn't. She's dropping the chain on very hard climbs, and the chain tension is going all over the place. I'm thiniking: I don't overtighten the chains, so is this a problem with a) XT cranksets? Why does a seemingly round ring act like it's oval or b) the loose 17t cog on a Shimano freehub? Could the 17t be oval?

Thanks very much for any input.

Check that the bearings don't have any sideways slop. They entire assembly could be moving when loaded.

I hope the loose 17T cog has a wide base and is locked down, you don't need a tilting cog.

 

[eric]

well

The 17t is locked down with a bunch of plastic cassette spacers, as well as alloy ones. Maybe I should just machine some pipe down to size.

The cog isn't wide-base, but it is on a steel carrier. Cloxxki had a wide-base cog for me, so I'll try that if the old non-HG cog doesn't solve it.