Bang on wrong! What is going on? (Assioma vs SB20)

I’m not sure if you’re aware, but you can adjust the Assiomas to be closer to the SB20 in the app. (I’m not saying I don’t think there should be a standard, just that this would keep you from needing 2 FTPs)

I did more digging and testing with another set of pedals. Also talked to some engineer friends and other people having similar problems. Our verdict is:
SB20 cranks have a problem of firmware handling the parasitic loads and also figuring out where exactly the load is coming from.

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Sigma sports currently have the Assima Duo’s available at £554… wondering if i’m likely to see the price drop significantly lower than this or if it is time to purchase them? The other option would be Garmin Vector 3 Dual Sided at £474!

yeah, I’ve been meaning to do the hanging weight test on the Assioma’s and also to put them on the SB20 to do a real comparison, but I just never have the time for that kind of monkeying around.

Yeah, me neither. I just picked one to be “true” (my Neo, since I ride it like 90% of the time) and then adjusted the Assioma to equal the Neo.

A little update on this.

  1. Cranks are calibrated from one side, similar to this:
    https://www.youtube.com/watch?v=3ehqVDuuVBw
    Result is extrapolated to other holes.
  2. Cranks are affected from the Platform Center Offset (PCO). Meaning where you push on the pedal matters. Very easy to test and demonstrate.
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This is something I’ve been complaining about for a while. The SB20 cranks are junk. Something about them is exaggerating a design flaw in the pods themselves that wasn’t evident when bonded to other crankarms. I asked Keith Wakeham about this and he seems to think the sheer mass/stiffness of the crank might be contributing to non-rotational forces not being cancelled out and effectively amplified by the crank design. After all, the stiffer the crank, the higher the resolution needed when measuring ever smaller amounts of strain.

I have tried this for myself with 3 pairs of Garmin Vector 3/Rally dual-sided pedals and 1 pair of Assioma Duos. All of the pedals match up nicely when compared to other power sources (Hammer, KICKR, two DZeros, etc.) If I squeeze my knees/shins inward, I can get the cranks to report lower than my pedals. If I push my knees outboard, the cranks report ever higher numbers. When I get really torquey as my power output increases, the cranks report higher…again because under that much load there are lateral and twisting forces in the system. There also definitely is something to the bear-claw design. It effectively creates an asymmetric crankarm in some positions, and the bores at the ends for 170mm and 165mm cranks will contribute to higher twisting forces (which should be cancelled out, but aren’t.)

On top of all this, the cranks have several more issues:
With fresh batteries installed, voltage tested at 3.3V, the cranks will report higher power values. With partially discharged batteries around 2.8-2.9V, you will notice the reported power is much lower. Either some kind of compensation needs to be done via a calibration table a la temperature compensation or the Stages Gen3 hardware needs better voltage regulation.

Single-leg pedaling also has dumb behavior. I can induce 1800W instant with just my left leg for example. None of my other dual-sided PMs fail this hard.

Also L/R balance is completely wrong if the cranks are allowed to auto-zero while positioned at an off angle.

I really hope Stages fixes this with updated Gen4 cranks or something and sends them out to existing SB20 owners for free.

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Same findings reported on the weight weenies forum :disappointed_relieved:

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I would imagine that all of these effects are because not one of the pedal holes are on the neural axis of the axle.

That is such a bad design.

Bending beam style power meters like this use a Wheatstone bridge arrangement of strain gauges which cancels out all of the strains caused by non-propulsive forces - torsion and non-tangential components of the bending.

When the pedal axle is offset from the neaural axis of the beam the axial load will produce a moment due to the eccentricity which cannot be cancelled out as they are indistinguishable from the moment caused by the tangential force.

In this design the holes ‘ahead’ of the neutral axis will cause a negative moment due to the offset, reducing the power reading and the holes ‘behind’ the neutral will increase the moment.

Mike

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I just don’t think i ever got my head around the reasoning for putting crank based PMs on it instead something else - is it just so they can report L/R balance?

Certainly don’t seem more accurate than the readings obtained from direct drive trainers.

I was thinking about replacing my Gen 1 Wattbike Atom with an SB20 later in the year. This thread has me thinking twice. Until this thread I had only heard good things.

How about getting a crane scale? They can be cheap and accurate for <$100.
Then all you need is a strap and means to wind up tension in a controlled manner and hold it at 20kg.


Hi all,
Are there any updates on this subject? Our SB20 shows a significant difference in power estimation between power pedals and the stages cranks on the SB20, with a constant difference of around 10% (Stages shows a higher power) Crank set at 172.5 mm. I compared it with both Favero Assioma Duo-Shi and Powertap P1, and both showed the same difference, as depicted in the figure. I would appreciate a resolution for this issue.

The figure shows 4 lines, made in two rides, the first ride (r1) compares the stages and the powertap output (‘r1 stages’ vs ‘r1 powertap’) the second ride (r2) compares the stages and the faveros (‘r2 stages’ vs ‘r2 faveros’), what you can see in the figure is that ‘r1 stages’ and ‘r2 stages’ are on top of each other, and ‘r1 powertap’ and ‘r2 favero’ are also on top of each other. Clearly, the favero and powertap measure the same difference compared to stages. which means imo that the stages power measurement is off.

Check with the Facebook users group, there was some talk of a new crank design but I never seen one of those.

I got some Assioma Duo-Shi pedals yesterday for outside riding. I tried them on the SB20 to check accuracy of the bike and it is 10% over the pedals. I found your thread and your solution which is really interesting. I am going to contact stages today and see what they say, I too read something about updated cranks…

I have a question though and wonder if you could help. I have had my SB20 for a couple of years and although I do ride outside, any testing has always been inside on the SB20, I have never had an outside PM. I suspected it was overreading slightly after coming from a Kickr and having a little bump in FTP straight away. My FTP on the SB20 is 315 - Assioma Duo-Shi pedals would probably read that about 290 lets say. Because the issue is with the cranks handling the PCO - is it likely my FTP is really 315 and it is the PCO of the new pedals creating different readings? If not and my FTP is really around 290 I will be pretty gutted. Doing my first serious race this summer I have been focussing on and this would really effect where I thought I was and how I thought I would perform.

Thanks

Just some old Shimano PD-5700, not sure on the difference in q-factor but it is substantial. I don’t have any other pedals to compare the assioma power readings to. I was going to take them to a friends to put on his indoor bike but its also an SB20 so not much point in that!!

Thanks for the fast response! I would be happy with that solution tbh. But indeed it is annoying that the powermeter can’t be switched with another easily.

My SB20 vs Assiomo remains off by 10% (ish). Annoying A.F., and I rarely ride the SB20 anymore as the Kickr is frankly just a better experience in almost every regard. Maybe for people using Zwift or something the SB20 would be good. For me… it is a regret.

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As I have said before, I just use my SB20 with my Assioma Duo’s - works perfectly. Same power indoors and outdoors which is my priority.

This must mean you have to take the pedals off and on everytime you want to go inside/out?