anyone tried oval chainrings with SRAM-Etap

[greenmark]

For SRAM eTap an oval chainring won't make pedalling easier. In fact it will make it more difficult.

@roger44, I'm assuming that when you say you have a "32 ring", you mean you have a 32 sprocket at the back.

A SRAM crankset with 50/34 has a 110mm bolt circle diameter (110 BCD). The very smallest chainring I know that fits a 110 BCD is 34 teeth (apart from some specially designed 33T designs). You can't get a SRAM crankset with a ring of fewer than 34T.

If you use an oval chainring, then the narrowest part of the oval will still need to be wider than the 110BCD. So the narrowest part will have at least the same diameter as a chainring of 34T. All that the oval chainring does is increase the number of teeth at the downstroke. So instead of a round 34T chainring, you effectively have an oval one that varies between 34T and 36T. You can't reduce the easiest part of the rotation to fewer than 34T.

Normally on SRAM I would suggest also using a 11-36T cassette - I use one with my 50/34 SRAM Force set up. This works well with the Force WiFly rear derailleurs, but I am not sure whether it will work with the eTap derailleurs.

If you're using SRAM eTap it may require downgrading from your SRAM Red Crankset to a supercompact chainest. You will likely need a new BB too. You also need to check on whether there is room to drop your front derailleur a few millimetres.
https://www.bike24.com/p2273557.html?q=fsa+crank+

 

[Yellow Saddle]

I dunno, I've used oval vs round chain rings on MTBs and my impression is that the oval ring does somewhat even out the delivery of torque to the rear wheel throughout the pedal stroke, particularly when standing and mashing. I'd happily use oval chain rings again off-road for that reason. On road where rear wheel traction isn't an issue I can't see much point in them.

@Yellow Saddle, while you're clearly right from a physics point of view, it's surely possible that a non-round chainring of some shape could conceivably yield better (i.e. more efficient) bio-mechanics throughout the pedal stroke than a round ring? After all, crank length doesn't affect the physics you cite, but I wouldn't want to ride on 100mm or 250mm cranks!

I'm not sure there is a need to even out torque. Pedaling a "two-cylinder bicycle" with two huge torque pulses in every revolution of the crank seems to work OK for just about every situation I can think of. I think the "smoothing" argument is a solution based on a non-existent problem. If you say (not that you do) that pedaling up a slimy incline causes momentary slip of the rear wheel, I'll just say that it is easy to modulate the pulses with your mind and muscles and technique.

Efficiency is a ratio of energy input vs energy output or, minimising energy loss in the system. The biggest energy loss at low speed is through chain friction and that stands at about 3%. Then another one percent is lost through hysteresis or tyre friction and suspension losses through jiggling your fat bits around as the bike shakes and you bob. To suggest that there are energy losses in your joints which can be minimised through lower gearing is just marketing BS. A lower/variable gear isn't gonna somehow make any losses in the joints smaller and the ratio of input vs output higher. It is just fake news science for the gullible.

I've come across people who think the oval ring somehow changes angles in the leg joints as the crank rotates. It doesn't. Remember, the crank remains the same length throughout the rotation, it is only the required force that changes with position.

Isn't the reasoning behind an oval chainwheel that mechanical advantage changes with rotation and the position of your legs ?

You could argue that MA is irrelevant, but if you followed that argument to its logical conclusion, you would only need one gear on your bike.

Yes, mechanical advantage does change with crank position but so what? If you need the lower advantage, just change gear. It is not that having mechanical advantage to spare in certain parts of the crank position (i.e. use a lower overall gear) is detrimental or a waste of energy.

At the end of the day, climbing is about aerobic capacity (once you have an appropriate gear selected) and aerobic capacity is a function of total work done. That means, climbing a given hill requires a given energy output (aerobic capacity). No gear can give you more of that.

 

[Sharky]

To improve on hills, need to consider technique. When I am tired, I can start climbing and go slower, so change down and go slower and change down, until I run out of gears. But when fresh, I will at the foot of the hill keep a "bounce" in my peddling and speed is maintained for longer and often reach the summit with gears still in hand. There is a balance between leg pain and lung pain. Get it right and hills become easier.

Also look at the bike. Make as light as possible with no unnecessary luggage and it goes without saying that you need to be clipped in for a firm shoe/pedal grip and the occasional pulling up on the steeper stretches of a climb.

And it's not that youngster in my avatar talking. He is more than 50 years younger than me now.

 

[ChrisEyles]

Thanks for the response @Yellow Saddle - I've been thinking about oval chain rings for a while in my head now but you have a knack for putting things nice and clear and simple.

One situation I can still see they might be useful in (maybe!): climbing a steep loose/slippy slope off-road single speed, where your gear is always higher than optimal for climbing, you're often honking out of the saddle and fine modulation of torque to the rear wheel is not so easy to do.

I remember seeing on this site a while ago some kickstarter invention to drive the rear wheel via a stairmaster type motion of the legs (i.e. straight up and down on the pedals rather than going in a circle). Obviously that's more mechanically convoluted, with all the associated weight and efficiency penalties, but I wonder how that compares from a bio-mechanics point of view to the conventional drive system...

 

[roger44]

thanks guys, I think im looking for the 'magic bullet' where there isnt one.
My bike is a light as I can get it, under 8 kg, im using a 32 rear and a 34 front ring, and I DO run out of lung capacity before my legs give out, but its touch and go which gives up first.
Im currently using the 'shut up legs' mental approach and it seems to work. If I switch off the pain in my legs and just concentrate on turning the crank I can get up the main hill in my area, just.
I am also build up muscle endurance in my thighs by star jumps and sqats and it is showing benefits. I can at least stand up and pedal for about 1 minute now before collapsing, lol.
Im 74,so perhaps I should lighten up on myself a little.
Many thanks for all the tips

 

[fossyant]

You are doing blooming great for 74, so don't beat yourself up.

 

[TheDoctor]

Plus, at 74 you'll have seen the oval chainrings fad come and go a few times...

 

[Nigelnightmare]

Can you not go to a 34 or 36 on the rear to make it easier on yourself?
Or is that outside the limits of the Sram system you've got?

P.S.
Your doing better than me and I'm only 54 and the older you are the longer it takes to build the muscles.
So don't knock yourself too much and enjoy your rides.
Remember there's no shame in getting off and pushing up the last bits.

 

[si_c]

You could consider sub compact gearing but you'd need to change the whole chainset most likely. I'm thinking something like a 46/30 front would make things a tad easier. Bit spendier perhaps though.

 

[roger44]

Can you not go to a 34 or 36 on the rear to make it easier on yourself?
Or is that outside the limits of the Sram system you've got?

P.S.
Your doing better than me and I'm only 54 and the older you are the longer it takes to build the muscles.
So don't knock yourself too much and enjoy your rides.
Remember there's no shame in getting off and pushing up the last bits.

believe me, there is a great deal of shame in getting off