Big vs Small Chainring - Same Power

power-meter

#21

I was going to do exactly the same thing the other night only to find it was gone. Bummer


#22

It’s be interesting to see how the pedal peanut differs between a kickr and something like a vortex, both in the small ring etc.

I believe Shane has described some cheaper trainers before like pedalling through sand. This could also have a lot to do with things feeling harder with the lower flywheels speed - just the way the trainer applied resistance.


#23

I’ve got a Kurt Kinetic Rock & Roll and earlier this year fitted the larger flywheel. All the workouts after that felt a lot easier for the same power outputs. I guess it’s the same effect as gearing in erg mode.


#24

@GPLama would expensive and budget smart trainers make noticeably different pedalling peanut shapes? Just wondering if it would make for a good video to highlight the differences in what you get for your money?


#25

Yes…ish. What it represents is inertia. And the goal (for me) is/was always to replicate the inertia indoors as close as possible to my outdoor events. That means I’m usually back on the Lemond Revolution for TT intervals.

There’s some really interesting work happening with high frequency cadence monitoring that does a similar thing… Rather than just a Spinscan peanut (which has been around for… 15-20 years?) that I haven’t seen any real-world applications for. Mostly because it has been limited to indoor bikes.


#26

I have a big question, is it related to chainring x power?
I was not very deep in the subject, but someone here probably already knows how to answer

time back when I was training in indoor zwift, when I had to make LT/vo2 intervals for me it was easier to do on climbs, if I made those even intervals on the flat I would suffer brutally, for me to do intervals in small chainrings and high cadence was less suffered than the opposite big chainring.

this is psychological or it really is easier to take intervals in climbs.


#27

Thanks for clarifying!


#28

I don’t think this was answered in this thread, but will the outcome of my ramp test be affected depending on whether i use the big or small chainring? I’m using a Tacx Flux if that has any bearing on answers.


#29

Very likely the answer is ‘yes’.

The inertia effect is real and will play into the test, IMHO.

I think it is probably best to use the same gearing for Ramp Tests that you plan to use for most of your training. I think mixing gearing is worthwhile depending on the particular workout and your specific needs. Sticking to one gearing can work, but leaves out some of the advantages that are offered with the use of the different nature of each setting.


#30

If you do the ramp test in ERG mode you will (or should!) get a higher FTP in the big ring compared to the small ring.

The faster-spinning roller/flywheel when in the big ring (if cadence is the same in either case) gives you a helping hand via greater intertia.

EDIT - simultaneous post to Chad, and the same answer!


#31

All said well. I tested it and confirm. With small gear get lower FTP but I am MTB rider so it works perfect for me when do majority of workouts on small ring


#32

Then why does it feel like riding downhill when in little chainring on my Kickr?

Look at my ride above @mcneese.chad and @Darkgerbil - the smoother power in little ring clearly shows the helping hand is in little chainring, not big. Have either of you done two ramp tests in a week, one in little and one in big chainring?


#33

@bbarrera, that seems to defy logic, it’s the opposite of what is generally observed! I can’t explain it to be honest.

I’ve done an identical workout in big and small and big is way easier, but not the ramp test (yet!).


#34

I don’t think there is anything special with my setup.


#35

No, I have not done the Ramp both ways, and should do a test.

My comment above is speculation based on my experience in other training and presuming general physics at play.

Those ideas could be incorrect, for sure.


#36

your assumptions seem to solely focus on the inertia of the flywheel, and ignore the braking that Kickr applies. What I’m observing and reporting is real and repeatable. You can clearly see less power variation when in little chainring. From where I sit, It seems a lot of people watched GPLama video, which to my eye shows small differences in muscle recruitment (and was GPLama’s conclusion), and extrapolated that to “prove” things (higher ftp test in big ring) that weren’t proven in the video.


#37

For clarity, I answered ‘yes’ to this specific question above.
Meaning I expect some tangible difference between the two extremes.

However, I made no claim to higher or lower FTP. I simply don’t know if FTP will be affected.
But I do expect there may be a different stress/strain on the body that may manifest in one way or another.

My experience, and that relayed from many others in workouts, seems to indicate different effects on the body from gearing changes. As such, I think it’s likely that there will be some impact on the Ramp test.

But, the shorter overall duration of it and the fact that it is short steps that are effectively increasing constantly, will play into the actual results. I don’t know what that will be, but I think something will be different.


#38

I’ve thought about this for a while to try figure out why there is a difference between high vs. low inertia riding. I believe it comes down to the variation in angular velocity over the course of the pedal stroke, and how that’s different between high vs. low inertia riding.

I’d love to see the angular velocity data that maps to the torque data from the screenshots you took from Shane’s video, but since I don’t have that data, in good scientific practice, I’ll propose a theory, and hopefully somewhere I’ll be able to dig up the data to prove or disprove the theory :grinning: So here goes (warning, this ended up being pretty long):

Riding uphill (low inertia):

When riding uphill, the bike accelerates during the max power portion of the pedal stroke (~3-4pm), and decelerates during the min power portion of the pedal stroke (~noon). This acceleration/deceleration of the bike in turn means the angular velocity of the pedal stroke varies over the course of the revolution, being the slowest at noon, and fastest at ~3-4pm.

The reason the bike accelerates/decelerates (and the angular velocity subsequently varies over the course of the pedal stroke) is due to the nature of the loading on the bike - when riding uphill, the majority of the force is to overcome gravity, which does not vary with speed, hence its possible to accelerate the bike relatively easily during the power stroke of the pedal. Also the baseline speed is pretty low, so a given mph increase in speed will be a higher % increase vs. if speed is higher.

Riding on the flats (high inertia):

When riding on the flats, the bike does not accelerate much during the max power portion of the pedal stroke (~3-4pm), and does not decelerate much during the min power portion of the pedal stroke (~noon). This means the angular velocity of the pedal stroke does not vary much over the course of the revolution.

The reason the bike does not accelerate/decelerate much (and the angular velocity subsequently does not vary much over the course of the pedal stroke) is due to the nature of the loading on the bike - when riding fast on the flats, the majority of the force is to overcome aerodynamic drag, which varies with the square of the speed (power varying to the cube) - hence when riding at an already fast speed, its difficult to further accelerate the bike during the power stroke of the pedal.

Uphill vs. flats riding:

So the difference between the two is that the angular velocity of the chain ring fluctuates over the pedal stroke when riding uphill, but remains pretty constant when riding on the flats (despite the same average cadence in both situations). This means that different muscle groups are being employed for different durations over the course of the pedal stroke - in the case of riding uphill, the pedal takes a longer duration to rotate through the 12-3pm position than when riding on the flats. And a shorter duration to go through the 3-5pm position. And vice versa for riding flats vs. uphill.

Different muscle groups are used in the 12-3pm position vs. the 3-5pm position, so depending on the rider, and which muscle groups are relatively more developed, some riders will put our power better going uphill, and some riders may put out power better on the flats.

For less experience riders, they typically can put out more power going uphill, as the relatively slower rotation of the chain ring at the ~noon point in the cycle gives them more time to activate and deploy the glute muscles to get power out in in the earlier part of the pedal power stroke.

Big ring vs. small ring on the trainer.

The effect is the same here, but instead of gravity vs. aerodynamic drag causing the difference in loading, its the inertia of the flywheel. When riding in the small chain ring, the power stroke of the pedal may accelerate the flywheel by, say, 0.5mph which might be a 5% acceleration, whereas when riding in the big ring, the same power stroke will accelerate the flywheel by the same 0.5mph, but this might be only a 2% acceleration due to the faster baseline speed. This results in the same outcome vs. riding uphill/flats: In the small ring, the angular velocity of the chain ring varies over the course of the pedal stroke; In the big chain ring the angular velocity is relatively constant.

Round vs. oval chain rings

Oval chain rings by their design will have a different angular velocity over the course of the pedal stroke, even if the bike speed is fully fixed. Typically, these are placed on the bike where the angular velocity will be higher at the ~noon position, and slower at the ~3pm position. The goal with this is that it gives the rider more time activate muscles and deploy power at the ~3pm position - which is the max power point during the pedal stroke.

Compared to a round chain ring, using oval chain rings will have the effect of reducing the variation in angular velocity of the chain ring when riding uphill (or small ring on the trainer), and increasing the variation when riding on the flats (or big ring on the trainer).

This is the opposite to what happens when riding with a round chain ring - so riding an oval chain ring uphill (or in the small ring on the trainer) feels more like riding the flats on a round chain ring! i.e. low variation in angular velocity of the chain ring.

Riding the flats in an oval chain ring results in higher variation in the chain ring angular velocity - but this variation is “opposite” to that when riding uphill in a round chain ring. With oval on the flats, the chain ring is rotating faster at the noon position than the 3pm position.

Whether or not you’ll ride faster with oval vs. round is another issue - I know there’s been attempts to evaluate this, but I’ve not seen any study yet that has revealed a conclusive answer.

If anyone knows of any data to support or refute this theory, I’d be interested to see it!


#39

Great stuff, @DaveWh.

Nothing to add to your well thought out and clearly stated ideas. On a first pass read, it all seems logical to me.


#40

Thx Chad.

My next question is how do manufacturers of power meters address this issue of variation in angular velocity over the course of the pedal stroke.

I’ve read reports that oval chain rings give a different power measurement than round. Does this mean the power measurement also varies on flat vs uphill - despite the “true” power output being the same?

This one would be a can of worms…