Do oval chainrings affect measured wattage on a trainer?

Equipment
1

I’m curious if the oval chainring on my mtb is lowering my power numbers. I have it on an elite suito.
It could just be that I lost 50 watts, and maybe there’s other losses happening on my mountain bike?
Not the biggest deal, the workouts feel appropriate mostly so the measurement (accurate or not) is working for training just fine.
If the oval chainring is having an effect, I’d love to know the details on how that works too.

2

Possibly, because of the change in crank velocity, oval rings can influence the accuracy of power meters. The more oval/non-round your chainring is, the greater the impact on the accuracy of your data. However, I believe there are PM specifically designed to work with oval chainrings.

3

Pulled off the internets:

Using an Oval Chainring with 4iiii Powermeters

Like with most power meters, oval chainrings will result in a measurement error. Due to the changes in velocity oval chain rings produce through a pedal cycle, the power will be skewed higher than with a round ring, which has a constant velocity throughout each rotation.

The error induced by some oval chainrings is approximately a 2% over-estimation of power. The amount of error depends on the ovality of the chainring you choose, and you can roughly expect a 1-4% over-estimation of power.

The more important aspect is the accuracy and consistency of the powermeter and it is less important If the powermeter is reading 255 W when you are actually putting out 250 W. If you are more comfortable riding with an oval chainring there will be no issues in terms of power measurement, just keep in mind that your power numbers may be slightly skewed.

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4

I have an oval on my bike at home on my KICKR Core and a round one on my other bike on a Neo that I use about 1/2 the time. I FTP on both and at this point do not “feel” a major difference. I intend to match them at some point, but am not able to at this time. I am just n=1 so YMMV. I feel like they are really close. The bikes are in different states so a direct comparison is a bit of a challenge in my case.

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5

I’m measuring with the trainer, so not sure how that changes things, but I appreciate the info.

6

So you’re saying its just I’ve lost 50 watts :joy:

7

Not sure anybody really knows. I searched to see if I could find info on oval rings used with a PowerTap hub, since it’s effectively as close as we can get to regular power measurement comparison. But I came up dry with nothing worthwhile.

It seems to be unknown whether it will or won’t be good data.

8

Thank you for searching!

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9

I believe that any power readings on the rider side of the chainring needs to take into account the non-constant angular velocity to make power readings correct with an oval ring.

Anything wheel side of the chainring does not see the non-constant angular velocity so simply averaging force over a single revolution is fine.

I think you just got weaker!

Mike

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10

Since you’re effectively measuring at the hub, the only impact of using oval chainrings would be a different pattern of power variation vs time compared to round rings. The difference would not be more noticeable to the trainer’s power measurement than that between standing and seating, for example.

So short answer: no, the trainer power reading is not affected by the shape of the chainrings.

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11

I also doubt that oval rings would effect power measured by a direct drive trainer.

12

From now on no further replies please unless they are providing me a believable excuse for my power loss. My ego can’t handle this :grin:!

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13

There’s an adaptation period for oval rings and/or they may not be in the right place for you.

Alternatively, calibrate the trainer. Hopefully you will find out it was under reading, or that might make the problem worse :wink:

14

So sorry!! That’s not what I meant! I’m just not sure I can totally feel the difference. Rumors are that the Neo has a higher RPE than the KICKR Core. Since the ovals are on my KICKR it is possible there is a slight difference, but they seem to be pretty close from my perception.

15

Haha! It’s all good, no worries!

16

I used to run an SRM with Rotor QXL chainrings with an Fluid2 trainer. Dedicated trainer wheel I used had an first gen Powertap Hub (wired, steel hub body) I threw together as my first (and only) endeavor at wheelbuilding. Note that this hub I bought on eBay on impulse 5 years ago.

Never did download the data from the Powertap computer but my eyeball checks with an SRM PCV and the Cycleops wired head unit side by side were pretty comparable. The SRM read slightly higher due to being on the cranks vs the rear wheel hub. But both moved in the same directions proportionally.

Also found an Power2Max Type S that I’m currently using to be pretty consistent with the SRM numbers compared to that Powertap hub as a reference (swapped due to going shorter crank arm lengths). Used the same brand Rotor QXL rings with both cranks.

My not so methodical comparison. Be interesting to do a DC rainmaker and also record on pedal based PMs at the same time too along with the smart trainer. But I don’t own that powertap hub anymore nor any other type of PM I can run concurrently on the same bike.