BikeErg Power Meter Accuracy

[flatbread]

I've been training with powermeters on the bike since 1998.

Want to see what's possible, based on actual collected data?

These are in w/kg. World class pursuit is 7 w/kg.

https://d3laewezlz9ul2.cloudfront.net/w ... rofile.png

[johnlvs2run]

Ganna's WR was about 600w.

The video shows 607 watts for Ganna's 2019 record, which he broke in 2020 so it would be higher. I used C2's watt calculator and 1000m instead of 500m, which would have been much higher. This shows again that C2 greatly overestimates the BE watts, and that the BE power meter accuracy is way off. All that needs to be done is to compare the times and paces, because BE (and road/track) watts have a direct correlation to the pace.

Ganna --> 4:01.934 ... Metcalfe at age 55 --> 4:53.1 pace for an hour!
C2 bike fastest effort over 3 years and many more attempts than are done on a track --> 5:47.5.

That can't be accurate. The BE power meter accuracy is way off.

[Ernits]

Ah, there's the problem. You used C2 pace calculator and set Ganna's pace to the same pace a PM5 on a BikeErg would show. However, the speed and distance on the BikeErg is arbitrary and doesn't really reflect real world. Only watts are comparable.

[johnlvs2run]

the speed and distance on the BikeErg is arbitrary and doesn't really reflect real world. Only watts are comparable.

BE watts are calculated from the pace, and visa versa. So you can't have one be comparable and not the other.
Look at C2's watts calculator and you should be able to see the formula that is used.

Formulas Used
2.80 is a constant used in both formulas. The formulas used are:
watts = 2.80/pace³
pace = ³√(2.80/watts)
where pace is time in seconds over distance in meters.
For example: a 2:05/500m split = 125 seconds/500 meters or a 0.25 pace.
Watts are then calculated as (2.80/0.25 ^ 3) or (2.80/0.015625), which equals 179.2.

Thus because the BE pace is not accurate, the watts are not accurate either.

[Ernits]

But ... it can't be. Watts are recorded, pace is derived from watts. Concept2 set the BikeErg speed to be twice that of RowErg.
If pace is calculated from watts and vice versa, it's a closed circle. You have to record something somewhere, no? And the erg part clearly means it measures work done, in watts.

C2 pace calculator is meant to show you either watts, distance or pace with one of the variables missing and to be calculated. On BikeErg. This doesn't translate to outside PM5 numbers.

Thus because the BE pace is not accurate, the watts are not accurate either.

That's a bold claim. So all the numbers are made up and mean nothing on the PM5 is what you essentially say?

[Nomath]

@johnlvs2run : You are wrong in applying the Concept2 formula's for Ganna's effort.

As the name ergometer suggests, the C2 flywheel unit primarily measures work and power. How it does this is explained in the internet paper The Physics of Ergometers . Recently, in a topic Fan Blade Physics and a Peek inside C2's Black Box , I gave more insights on how the physics is applied in the C2 rowing ergometer. The BikeErg and the SkiErg use the same flywheel unit. Work and power are measured accurately because the measurement is based on first principles. Speed and distance are artificial constructs.

The accuracy of the rowing ergometer has also been validated several times using sensors for force and speed of the handlebar (power = force * speed). The figure from Boyas that I copied in an earlier post shows that the discrepancy is small, about 20W. The discrepancy can be explained because power was measured at different points. For the RowErg: handlebar and chain are moved back and forth, hence inertial losses + friction losses for chain and sprocket wheel + inelastic losses for the bunjee. So the power measured at the handlebar will be higher than the power measured at the flywheel. I am not fully informed about the BikeErg drive, but the drive-train looks simpler. The discrepancy between the power measured at the pedals and the power measured at the flywheel is likely to be smaller.

From all I know, the BikeErg will measure work and power accurately if it is calibrated properly. The RowErg is self-calibrating during the recovery of each stroke. The BikeErg has to be calibrated in separate mode.

[flatbread]

The Bike Erg watts are measured off the known mass of the flywheel. If you go back to the OP, the erg watts were within the margin of error of commercially available power meter units. The watts are accurate if the erg is calibrated.

The speed number was just thrown out there for the PM5 and has no relation to real-world cycling.

Just set the PM5 to watts and train by that. Forget about the silly speed number.

[D666hudd]

I use two bike ergs one at my home which is brand new and one at my gym which is 8 months old the one bike reads 381 watts for 30 min ride and the my new bike was well short of this the damper is exactly the same ! Any ideas why this is happening

[johnlvs2run]

Did you calibrate both of them and then set the drag factor the same before riding?
If not, do that the next time, and see if it makes any difference.

[CaseyClarke]

Filippo Ganna's 4k world record is 4:01.934, which is 1582 watts.
The 4k record on the BE is Joshua Bugajski at 5:47.5, which is 535 watts.

Kevin Metcalfe recently broke the age 55 world hour record on the track at 49.121 kilometers,
which is a 4k pace of (3600 / 49.121 x 4) 4:53.1, which is 889 watts, way higher than the 4k best on a BE.

I would accept BE record watts being 80 to 90 percent of the watts for a world class cyclist.
But 34 percent?? And only 60% of the watts that an age 55 cyclist can do for an hour on the track??
I'm highly sceptical that the BE watts calculation is anywhere close to the watts produced on road and track bikes.

John, this is all 100% incorrect. You’re totally misunderstanding watts. Firstly, C2 BikeErg wattage is pretty accurate. They read a bit low on sprints, but that’s another discussion.

Nobody is doing 1500+ watts for any more than a few seconds, and nobody is doing 889w for much more than about a minute.

Ganna’s 4:01 4k track WR isn’t comparable in any way to doing 4:01 on for 4k on a BikeErg, which would be impossible. The best 4k we’ve seen on the bikeerg so far is around 5:47 (~537 watts) and it’s been out 4 years. Top times are loosely comparable to top 2k RowErg scores.

Speed & watts are independent of one another on a real bike. But related on a static bike, like the C2. On an outdoor ride I could be riding a heavy mountain bike up a hill and pushing 500w to go 8mph, or I could be cruising along on a road bike in a pack doing 20mph whilst only putting out 250w.

The watts on a C2 bike are just a function of pace. Double the pace (ie /1000m instead of /500m) of the row/ski to achieve the same power output.

2:00 = 202w
1:50 = 263w
1:40 = 350w
Etc etc

[CaseyClarke]

I use two bike ergs one at my home which is brand new and one at my gym which is 8 months old the one bike reads 381 watts for 30 min ride and the my new bike was well short of this the damper is exactly the same ! Any ideas why this is happening

The 381 watts is totally miscalibrated. Probably something disrupting the airflow, ie too close to a wall or not been calibrated properly. What’s your 30 minute best on your home bike?

[D666hudd]

So the bike in the gym has been calibrated and isn’t too close to a wall or anything it’s in open space so my 30 minute ride on my bike at home is 331 watts , the bike is in my conservatory about a meter from any walks etc , the bike in the gym is in open space

[johnlvs2run]

So the bike in the gym has been calibrated and isn’t too close to a wall or anything it’s in open space so my 30 minute ride on my bike at home is 331 watts , the bike is in my conservatory about a meter from any walks etc , the bike in the gym is in open space.

Due to varying conditions, I calibrate every time before time trials, and often before workouts as well. For the rhythm to be the same between the machines, the drag factors should also be checked each time after doing the calibrations. As long as you do the calibrations, being a meter away from a wall shouldn't matter that much.

[Nomath]

So the bike in the gym has been calibrated and isn’t too close to a wall or anything it’s in open space so my 30 minute ride on my bike at home is 331 watts , the bike is in my conservatory about a meter from any walks etc , the bike in the gym is in open space

In my view it doesn't matter whether a BikeErg is positioned close to a wall or in open space, as long as it calibrated in the same spot. The nice thing about the calibration is that all specific air flow conditions are taken into account, including wall effects. However, changes in barometric pressure are not automatically compensated. So the calibration should be repeated roughly every day.

Regarding the conversion of pace to power, I have read somewhere that C2 simply assumed that you go twice as fast on a BikeErg as on the RowErg for the same power input. If this is true, the pace-to-power conversion formula for a BikeErg is : Power[W] = 1/8 * 2.8/(t1000/1000)³ = 0.35/(t1000/1000)³.
This is roughly borne out if you compare the "average-Joe" rankings for 10K cycling and 5K rowing. The 50th percentile time for 5K rowing is 21:19.6, or 167W. The 50th percentile time for 10K cycling is 20:38.9, or 184W (using the above formula). I guess the 10% difference is due to the higher metabolic efficiency of cycling.

The actual pace-to-power formula used by C2 for the BikeErg is easily calculated from the time and average power on the PM5 results page.