Oval chainrings any good?

[Ben98]

Having found the same as Ian with the chain coming off the sprocket, http://singletrackworld.com/2014/06/abs ... speed-cog/ these look very interesting?

[Bearbonesnorm]

Having found the same as Ian with the chain coming off the sprocket, http://singletrackworld.com/2014/06/abs ... speed-cog/ these look very interesting?

I'm not having this ... I mentioned the AB NW sprocket when they first came out and everyone told me it was pointless and they couldn't see any need for it

[FLV]

Having found the same as Ian with the chain coming off the sprocket, http://singletrackworld.com/2014/06/abs ... speed-cog/ these look very interesting?

I'm not having this ... I mentioned the AB NW sprocket when they first came out and everyone told me it was pointless and they couldn't see any need for it

Still feel the same way about that little piece of sillyness

[Singlecrack]

I ran a 33 oval with 18 on the back of my Stooge No problem ..never missed a beat ..

[GregMay]

Well I'll add that to the list of things I don't need.

[NewRetroTom]

by Pirahna » Fri Jun 05, 2015 9:18 pm
I'm surprised the chain dropped on the single speed setup. Even though the chainring is oval, the number of teeth in contact with the chain remains constant throughout the revolution of the ring. The only way the chain could go slack then tight is if the chain length altered, and it can't.

I should add that I don't ride SS and the above comment is probably nonsense (but the rear mechs on my bikes don't move back and forward with oval rings).

The above comment seemed intuitively wrong when I first read it, but couldn't immediately get my head around why.

Now I've had a little time to reflect I think I now understand. If the chain ring is an ellipse (special kind of oval) and you split it into 4 segments along the major and minor axes then the length of each segment of the circumference will be the same (so what Pirahna says above about teeth in contact with the chain is correct).

However what isn't taken into account here is the change in distance from the point the chain leaves the chainring to the point it hits the sprocket.



In my diagram the line B is parallel to a line through the centre of the BB and rear hub and doesn't change. When you rotate the cranks with an elliptical chainring you are changing the dimension C, and since A² = B² + C² you've also changed the length of A.

The bigger the difference in size between the chainring and sprocket the bigger the difference in chain tension/slack as your cranks go round. If you had a sprocket whose diameter was the average of the major and minor axes of your oval chainring you probably wouldn't notice the difference in chain tension (wouldn't be a very useful gear for singlespeeding unless you had huge wheels though).

Feeling better now.

[Pirahna]

NewRetroTom

I don't think splitting the ellipse into segments works. When you look at an oval ring rotating you naturally think the chain will get tighter then looser, my maths isn't anywhere near good enough to explain why the chain length stays constant but it does. As I said, I think it's because the number of teeth on the oval ring in contact with the chain is constant the chain length is constant. I had a look on YouTube and found this which might help:

https://www.youtube.com/watch?v=k3qr2Er-EdA

[Mrhoppy]

I've just fitted one to my mtb and it's good, not lifechanging but it does make life a little easier on the hills and doesn't affect chain retention. And no, it doesn't impact on the mech, chain angle varies but the is always the same number of teeth engaged so the tension is constant.

[jay91]

I've done about 60miles on 32x16 all is good no problem with tension