False readings - clever guys please help :)

1

Hi guys,

I hope that you can help me solve my basic physics puzzle :slight_smile: I just ran into a problem with my settings on the Favero Assioma pedals - I found out that after switching to a different bike some 6 months ago, I forgot to change the crank lenght in the app setup, so the value of 172,5mm was there for quite some time. But my cranks are in real 165mm on this bike. So I know that my reading were false, but the big question is - how much and in which direction :thinking:
I did a bit of math and came to the conclusion that the difference should be around 4,76% and that the real output was higher than the readings, but I am really not sure. And the difference of 4,76% would mean, that if I saw 280W at the cadence of 80rpm, the difference in real values would be in this case 13,3W. And should it have been 293w or 266w really? :face_with_raised_eyebrow:
That is quite some difference and a lot of anxiety for me now :cold_face: :rofl:

I would greatly appreciate if you could clarify me that…

2

Not expecting to be amongst the “clever guys”, so bear with me :stuck_out_tongue:

Looking at Assioma | Frequently Asked Questions (FAQs) | Favero Electronics, looks like the crank-length is picked up from the bike computer if present, so looks like a lesser issue from a pure power-reading perspective.

And in my mental model of how a power-meter works, I don’t really see the crank length coming much into play anyway? Like: force (measured by the strength gauge) * rpm, right? Where would crank length come into play? Measuring the torque? :thinking:

1 Like
3

I can be wrong here so please correct me if so, but like @Giacomo_Rizzo I think crank length has no influence on Power measurement. It does influence torque calculation from power/cadence however.

4

Thanks for your reply!

Well I have found 2 rules:
Torque = Force * Vector (in this case cank lenght)
Power = Torque * Angular velocity (RPM)

So my understanding is that for the powermeter to give a power number, it has to calculate the Torque from Force applied * crank lenght. And only when it gets Torque, can multiply it by RPM to get a Power number in watts…

5

So my math was for an example, when I got 280W at 80rpm

Torque = F * 165
280 = Torque * 80

280 = (F * 165) * 80
F = 280/165/80
F = 0,021 for 165 mm and 0,020 for 172,5

And that is how I got this difference of cca 4,5%

6

But now my brain struggles to invert this exercise…

If the power applied would be the same, eg 0,021212121 in this case, cadence is the same, but the torque is different - 3,5 for 165mm and 3,659 for 172,5
So far so good, but what does that mean in our case? If the force and rpm are the same, but I confuse powermeter by “artificialy” (in the settings) make the lever (crank leght) longer, am I really pushing 3,5 or 3,659 of the “real” Torque value? :thinking:

7

Overthinking, @Jan_Eichler!

The difference is the ratio of the crank arm lengths. If you set your cranks long and run on short cranks, you will overstate the power, and vice-versa. 172.5mm is 4.54% longer than 165mm.

torque = force x crank length, so power = rpm x force x crank length.
If the power meter thinks that the crank is 172.5, then it calculates power as X, say. If the actual crank length is 4.54% shorter than the power meter thinks, then the calculated power is 4.54% over the actual power.

1 Like
8

And I am affraid I know the reply…

If I make a Force equal to 0,0212121212121212 on the crank of 165m, it makes a real Torque of 3,5
But if I tell my powermeter that the crank is 172,5, it will take this force * 172,5mm and give me a higher (artificialy) Torque of 3,659 and give me a number of 292,7W … but I really was pushing only 280W in this case :cry:

9

Yeah, I came to the same solution, my power readings have been artificaly inflated :sob:
Guys, I was really hoping for the opposite, this sucks…

But thanks for your help!

10

Happens to us all, @Jan_Eichler. I regularly forget to adjust my crank lengths between my road bike [172.5] and my steel touring bike [175].

11

As long as you did your power tests with the same wrong crank length, you are Gucci. I mean your power maybe is incorrect but still you were training in correct power zones. That’s something :wink:

Not sure if the difference is so easy to calculate. Probably the easiest way of discovering the real difference is just doing some tests with your pedals and a trainer. Just ride at 100w, 200w, 300w (power @ your trainer) for couple of minutes with both crank length settings and you will see exactly how much you lose or get.

I will check it tomorrow :wink: