How do PMs record ‘passive’ standing weight ‘wattage’

So, I’ve ordered the Rally XC conversion kit for my Vectors so I can start recording power on my mountain bike.

I understand how Vectors etc record power on flattish roads etc, but how do they record when the terrain gets bumpy. I’m thinking that the pedals might think I’m pedalling when standing up absorbing successive impacts and therefore record watts. Whilst I’m not actually putting effort in to turn the cranks, I’m obviously doing other work counteracting the impacts and holding my weight up. I wonder whether the watts recorded be be similar and representative?

Generally, I’d find descending a very rocky and bumpy 5 minute descent harder (certainly on muscles) than putting out a lot of power for 5 minutes on a flat section…

Vectors have accelerometers built in to measure cadence. Maybe if there is torque measured (as you described, standing during a descent) but a cadence of zero the software records it as coasting?

Caveat: I don’t have any Vectors.

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@abrooks13 yeah, that sounds like a good hypothesis…

Just asked my brother who has dual Vectors on a road bike. Free wheeling = zero power.

Makes sense as power is force per unit time and the ‘unit time’ can only come from the cadence.

You’ll have to invent power meters for your knees to get credit for descending

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In addition to force x unit time = watts (or horsepower if we wanted!), the power meters have smoothing algorithms built in to help remove spikes unrelated to what they’re trying to measure. Anyone who has collected raw data in anything knows how much smoothing needs to occur.

@abrooks13 cheers. Yeah, I need knee, wrist and eyeball meters!

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Feature request for Garmin - convert the fluctuant signal from the strain gauges (for the periods that cadence is zero) into an equivalent power output and calorie burn called ‘descent power’ and ‘descent burn’. It was your idea originally though, I expect Garmin to reward you appropriately!

Joking aside I remember a lecture on Youtube where the guy stated that the power generated getting up from a chair was, for that short period, the same as the power output of Wiggins during his hour record. It can’t be insignificiant, the energy used going downhill. I guess an elite enduro or downhill athlete might have some idea.

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Haha, let’s get a patent out…

The energy expenditure on an uplift day where you hardly pedal must be significant, but it is strength rather than cardio oriented. It’s almost like a weights session, and thus TRs metrics such as TSS don’t apply as such I don’t believe.

I think you might be overlooking one component of the power equation. Power = Force x velocity, so essentially at zero rpm, you are producing zero watts regardless of how hard you are pushing (static force) on the pedals.

When your feet are moving slightly absorbing uneven terrain, you might cause tiny movement in the pedals, so very minor power readings. But I would guess this is really, really small in magnitude.

So unless I’m overlooking something here, the power meter won’t credit you for most of the hard work you do when descending on a mtb because most of the energy you spend does not go to “turning the crancks”

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Yes, that sounds right. So I’d be generating force, but no power. That means, as @abrooks13 suggests, the PM will record zero watts whilst descending, so the TSS for the ride will be somewhat under-estimated?

Yes, zero watts when descending and underestimating of all the work you do moving your bodyweight around is exactly what I’d expect.

In essence that’s same problem that makes power measurement for running so hard: a bike is a very “simple” mechanical system when all energy (=> power) enters the system cleanly through the bottom bracket.

Once you start dealing with systems involving springs (e.g. suspension or even Achilles tendon) and kinetic & potential energy shortly stored in, and transferred through, the body and/or bike (e.g. running, pumping a track on a MTB) things become very, very difficult to measure accuratly really fast…

If only everyone just kept spinning their legs in tiny circles really fast and doing nothing else, things would be so easy!

Correct. More specifically for rotational motion power = torque (rotational force and radius) x cadence (rotational velocity).

(Takes off engineer hat)

And yes, this completely ignores all other power you exert by applying force through the bars and pedals to move yourself and the bike around. People talk about pumping terrain for “free” speed, meaning acceleration without pedaling. But anybody that has ridden a pump track knows that it makes you really tired. You are doing lots of work still.

It’s “TSS” just like lifting weights or mental stress. It adds stress to your body that matters in training, but doesn’t show up with a power meter. You just have to mentally approximate and account for that TSS.