So how long do you reckon it will be before the big boys (Shimano / SRAM) start releasing oval chainrings? Or have they been burned too badly by the BioPace debacle?

They are probably not that bothered since you don’t have to buy a whole new groupset to fit them…so they wont gain much in terms of revenue. If enough people adopt them they will eventually but I doubt it will be anytime soon

The new Shimano bolt spacing looks a bit like it was designed with oval rings in mind. Can’t see it as a retro fit, though

Road, maybe.

I can’t see shimano offering it off-road as they only ever like to release stuff that’s 100% perfectly compatible with a groupset. And oval chainrings can hasten the demise of mech clutches.

And oval chainrings can hasten the demise of mech clutches.

Really , have you any evidence of that ?

Really , have you any evidence of that ?

Mine does move the mech cage very slightly every revolution, which means it must be working the clutch very slightly…. which means it will wear out quicker.

What quicker amounts to I don’t know….Within the lifetime of the mech???

Whats a little more concerning is that since the clutch is a friction device, this movement must be taking energy out of my legs each revolution too….albeit a very tiny amount.

My oval rings only move the mech by a tiny amount, not as much as the movement caused by rear suspension.

Do Sram mechs even have a ‘clutch’ – isn’t it just a tight spring?

ndthornton – Member
Mine does move the mech cage very slightly every revolution, which means it must be working the clutch very slightly….

Put a round ring back on there and you’ll likely notice it doing the same – it’s more likely an installation issue than an out of round issue.

it’s more likely an installation issue

How do you install a chain ring incorrectly?

Both Absolute black oval chainrings I have create a VERY slight amount of chain growth.

I wasn’t suggesting that it was incorrect installation, simply that there is a tolerance built in to the manufacturing so that the parts (cranks spider and ring) can go together. The ring will naturally bias itself one way or the other with a gap on one side vs hard metal contact the other and then you have an out of round situation. It doesn’t have to be much.

Do Sram mechs even have a ‘clutch’ – isn’t it just a tight spring?

There is a bush inside it that you can adjust the tension on.

compared to an oval ring how much chain growth does the suspension cause?

I thought we’d already established that there was no chain growth with an oval ring (the amount of chain wrapped around the ring doesn’t change with the orientation of the ring.) They work just fine on singles speeds.

I thought we’d already established that there was no chain growth with an oval ring. They work just fine on singles speeds.

There is a very tiny amount of chain growth (i.e. more than a round ring). Its visible as a slight back and forth movement of the rear mech but you are right… its not enough to throw a single speed chain if set up correctly. But the chain will slacken slightly as you turn the pedals so you need to keep an eye on chain stretch more so than normal. I run an alfine with an oval and no tensioner with no issues

I wonder why though? If you cut an oval in half, regardless of where you draw the line the distance around the outside of the segment will always be half the circumference of the oval. So there shouldn’t be any chain growth.

mrblobby – Member

I wonder why though? If you cut an oval in half, regardless of where you draw the line the distance around the outside of the segment will always be half the circumference of the oval. So there shouldn’t be any chain growth.

because the chain then goes to a smaller circle (sprocket) at distance, so with the 2 orientations of the oval you have 2 different angles to the sprocket, but at a fixed distance at the centres, so there is a minor increase in the tangent distance, i.e. chain length.

Basic maffs init.

oval sprocket to compensate 😉

Bet you the tangential difference is more than wiped out by the difference in pitch along the length of the chain…

Bet you

how much?

That is a good point STATO. On a 32t AB ring it’s about 15mm difference in diameter. Taking, for example, a distance from BB to rear axle of 440m and a rear chainring of 100m diameter, that gives a difference in chain length on the top run of chain of about 0.14mm. Assume it’d be about the same on the bottom, though obviously a bit different with rear mech, but I think you’re still looking at a chain growth of less than 0.5mm.

Or have I completely messed that up? My basic maths is admittedly a bit rusty 😳

Or have I completely messed that up? My basic maths is admittedly a bit rusty

My brain hurts after trying to work it out. Its a lot harder to type up in excel when you learnt it all on basic calculator.

Anyway for a 32t (129.8mm diameter) ring with 16t (64.9mm diameter) sprocket, presuming the 32t oval is equivalent of a 30/34 in its ‘ovalness’ (~16mm diameter diff 30-34) gives the following…

Top run chain length
30t 440.9161828
32t 441.1962665
34t 441.51348

So about 0.6mm chain growth over the oval. Of course this is calculated at the angle on a small sprocket so this growth would not be the same to a lower jockey wheel, it might be equivalent/less/more depending how much lower it is relative to the chainring (possibly a lot!?)

Growth would also be less in bigger sprockets and more in smaller sprockets.

Right, just amusing myself now.

assuming the top jockey wheel meets the chain level with the 16t, and a shimano medium cage is 74mm centres, with 11t jockey wheels. That puts the chain ~151mm from the QR. So with the same oval ring you get a chain length variation of 449.15 to 447.59mm, -1.56mm, i.e. the other way as the angle (to the jockey) is less at the peak oval! So add the growth on the top means the total chain growth fluctuates about 1mm. This is probably about right for the amount of movement you can see at the derailleur.