[Closed] Singlespeed with oval ring, size up 2 teeth?

Why? There's the same number of teeth on the oval as on a round chainring it's just that the torque varies throughout the pedal stroke.

Absolute Black do one for the 6800 - https://absoluteblack.cc/oval-road-110-4-bcd-shimano-11s-winter-oval-chainring/

If you keep the same number of teeth but go oval (for the sake of argument lets say the ring has 10% ovality) then during the downstroke you will be pushing 10% bigger gear, due to the ovality*

During the 'dead' phase of the pedal stroke, you will be turning a 10% lower gear which allows your legs to push through the dead phase easier and/or faster.

If you size up 2 teeth, you're increasing your gear even more, so now during the downstroke you're pushing a 10% bigger gear PLUS the extra 2 teeth's worth of gear. Better hope your knees can cope with the extra gearing!

*I believe that the tooth count is irrelevant, it's just a proxy for diameter which in turn dictates the torque which (for a given rear gear) dictates the 'gear' size.

@13thfloormonk - if you think about the number of teeth that pass a given point as you pedal you'll see that it doesn't matter what shape the chainring is, it could be square for all intents and purposes though that would be weird to actually use. If you start with the DS crank at 12 o'clock and go through one revolution then, in the OP's case, 32 teeth go past. That's just the same for an oval ring.

As you say the ovality affects the effort at a given point of the stroke but not the gear ratio.

Thanks both - will stick to same number of teeth and give it a go.

I think that "size up two teeth" thing dates back to when people were promising the world from oval chainrings. I think the claims being made for them now tend to be a bit less dramatic, they're more like "easier on the knees" and/or "a bit more traction"

On a 32t oval you're pushing 34t on the stronger part of your pedal stroke and 30 t on the weak part. If you sized up from 32t round to 34t oval, you'd be pushing a 36t at the stronger part of the stroke, but you'd still on a 32t for the weaker part. Which sounds like a lot harder going to me.

if you think about the number of teeth that pass a given point as you pedal you’ll see that it doesn’t matter what shape the chainring is, it could be square for all intents and purposes though that would be weird to actually use. If you start with the DS crank at 12 o’clock and go through one revolution then, in the OP’s case, 32 teeth go past. That’s just the same for an oval ring.

I've given this a lot of thought as quite frankly it confused the hell out of me, but I have to say I respectfully disagree. Gear ratios are dictated by torque ratios, and torque is dictated by the distance between the chain and the pivot point (e.g. the bottom bracket axle) When you are applying power, it is completely irrelevant how many teeth the chain is wrapped around, all that matters is the distance from where the chain disengages with the chainring, and the bottom bracket axle.

Imagine replacing the chainring with a stick with only one chainring tooth on it. The length of the stick is the same as your chainring diameter. Pushing the cranks would require exactly the same amount of force and would apply exactly the same amount of force at the cassette, ergo you have the same gear ratio. Only difference is as soon as the stick moves past perpendicular with the chain, your diameter changes and the chain will probably fall off!

On a 32t oval you’re pushing 34t on the stronger part of your pedal stroke and 30 t on the weak part. If you sized up from 32t round to 34t oval, you’d be pushing a 36t at the stronger part of the stroke, but you’d still on a 32t for the weaker part. Which sounds like a lot harder going to me.

This is a much neater way of saying what I was trying to say with my 10% ovality nonsense!

you’d be pushing a 36t at the stronger part of the stroke, but you’d still on a 32t for the weaker part. Which sounds like a lot harder going to me.

But isn't that sort of the point. If you can push a 32T at the weaker part of the stroke then maybe you can push a 36T at the strongest part. Whereas with a round ring you have to push a 32T all the way round as that is all you can push at the weakest point. So with the oval you are closer to your max all the way round. Either way it is a cheap and easy thing swap and try for yourself.

From a real world perspective, an oval chainring will help a bit in the dead zones of a singlespeed pedal stroke, which is the main benefit I'd say. Have made the switch on road and off road bikes and have always added a couple of teeth and it has felt about right. If you just want to make life a bit easier for yourself go with the same number of teeth.

...and apologies if this is stating the obvious, but I assume you have some sort of chain tensioner to deal with the differing chain length?

I know from the past how quite small movements of the dropouts changed the chain tension by quite a lot, so imagine trying to get something that works for an oval chainring would be a nightmare.

so imagine trying to get something that works for an oval chainring would be a nightmare.

Its surprisingly small. There are always around 50% of the teeth engaged so the only difference is the very slight change in chain angle, which with a bit of vector maths makes very little difference on the chain length

…and apologies if this is stating the obvious, but I assume you have some sort of chain tensioner to deal with the differing chain length?

It's been discussed on here before, apparently there is very little difference in chain tension as the number of teeth doesn't actually change. The only difference is due to the slight change in angle of the chain between the 'low' point and the 'high' point.

I've set mine up and although you can see the difference in chain tension, it isn't enough to make the chain come off or anything. Wouldn't recommend for track racing I guess...

Edit: beaten to it by Nickjb

Thanks all, i thought i'd ask as want the Absolute Black rings arent that cheap really!