Hi all,

After an unfortunate stick incident last Saturday I mangled my rear mech into some sort of modern art and sadly beyond fixable.

The usual approach from here is to tidy up the mech bits, remove them and Singlespeed the bike into “get home” mode.

I couldn’t get this to work well and ran out of patience and heat in fingers so walked/scooted home.

I suspect this is due to the Oval chainring not keeping a constant chain tension, an unexpected feature, can anyone confirm this?

Cheers.

SSers use oval rings, so it’s not that the rings oval, maybe chainline or too much slack or chain damage. Unless the mech was partially usable to stop the chain falling into lower gears you should have been on the smallest cassette sprocket.

I’ve had to rig up a get-me-home singlespeed when a rear mech has self-destructed. I didn’t use the smallest cassette sprocket but rather one that gave me a decent chainline combined with reasonable chain tension. It worked out at 32:20, I seem to remember.
Worked well enough for me to continue with the ride anyway, rather than just baling out for home. Chain tension doesn’t change much with elliptical chainrings either.

chain tension* change with elliptical rings is very small – I worked it out once, in this thread

* actually not tension as that is noticeably different at the extremes as you adopt the squeeze method of testing, what i mean is that the actual chain length difference is very small

http://singletrackworld.com/forum/topic/psa-works-components

However, may have been enough on ramped sprockets with suboptimal tensioning and imperfect chainline and….that the slight loosening and tightening allows for the chain to start to try to move up or down

What theotherjonv said. They’re mathematically correct if the chain was somehow parallel top and bottom, but it’s not as it wraps round more than 180deg and converges towards the back of the bike (on a singlespeed, with a mech it probably wraps less than 180 and diverges).

Also, even ’round’ chainrings seldom are, or they are but it’s hard to mount them perfectly concentric on the cranks, so there’s always a high spot, which will probably compound an oval rings problems.

You don’t need it bar tight to singlespeed, infact it should be as slack as possible before it falls off.

Assuming it’s a hardtail (SS’ing a FS is going to be a non starter) then stick it in whateve gear gives the best chainline and see if that gives a good place to break+join the chain, if not, move up/down one so that the tension is about right. It should work as I’ve run old ramped sprockets in SS setups when I’ve not had the right ratio to hand, although that’s with a tensioner (or EBB etc), but the ramping won’t help as modern sprockets really are designed to throw the chain off to facilitate shifting.

(SS’ing a FS is going to be a non starter)

You’re incorrect.

SS’ing a FS is going to be a non starter without some form of chain tensioner in place – is closer to correct. But not always correct. It’s dependant on the pivot positions and chain growth – if any.

Not my image. But the Kona A is a fine example.

Note – I’m currently riding a Salsa Spearfish as a singlespeed, with a Rholoff tensioner – it works fine. Also oval ring.

OP – Oval rings on SS work fine. Your chain line may have been off causing the issue.

[url=https://flic.kr/p/D2PSAB]IMG_0084.jpg[/url] by Greg.May, on Flickr

I had to turn my FS into a singlespeed at the Dyfi Enduro last year and that had an oval ring and I just locked out the shock and it was fine.

Well, it wasn’t fine, it was hard work, but it functioned perfectly.

I also run a singlespeed with an oval ring and everything said above is true.

You’re incorrect.

I was actually thinking of the Kona A, DMR Bolt, Cove G-spot as I typed that. And yes you can use something like the rolhoff tensioner (although they’re as big a smost mech’s) on designs with limited chain growth.

But………..

Emergency Singlespeed

A DMR Bolt is a little heavy to carry in a backpack for the eventuality of losing a mech. So I’m still correct, emergency singlespeeding a FS bike is pretty much impossible (unless the OP happens to ride one of the aforementioned designs, in which case he probably wouldn’t be asking).

Trying to ss with a regular cassette can be problematic, even with horizontal dropouts and a well tensioned chain.

Your rear mech shifts the chain from the slack bottom run, and there are ramps in the cassette to help. If you are grunting over something singlespeed, you create a bit of slack in the bottom run of chain. If that then flaps about and coincides with a shift ramp, it can try to shift up a sprocket and jam everything solid.

The bc trail leader repair option looks good – shorten chain to suit best gear and chainline, then take up any slack with a slidy bit of toothpaste tube and loop of duct tape pulling chain up towards chainstay.