Seattle Forrest

A lot of optical, wrist-based HRMs will get confused sometimes and report your cadence instead of your heart rate. DCRainmaker has plenty of examples from Garmin, Fitbit, and others, apparently this is such a common problem that he tests all oHRMs for it now.

If an HRM made primary for runners can't remove running cadence, I'm skeptical that a pressure sensor can tell pedal strokes apart from cobblestones.

79pmooney

I'm still struggling. If this goes by tire pressure, how does it know when the rider has shifted his weight? Some of us pull forward, sit "on the rivit", when we are going hard and uphill, out of the saddle, I ride with just enough weight on the rear wheel to keep the tire from spinning. Wouldn't my power show as negative, like I was braking?

Ben

MinnMan

We are far off topic now, but the number of moles would be nearly the same, assuming that all of these gases have equations of state that are small departures from the ideal gas law at a few hundred kPa and 25 °C. P= nRT/V.

THe masses of the gasses would be different. And things like their gas permeabilities will affect their behavior in the tube. But the relationship between force and tire pressure should be (approximately) the same.

TimothyH

I tend to agree with this.

Their own explanation of the product on their website makes it pretty clear.


-Tim-

FlashBazbo

Spoiler alert: The pressure inside the tire remains the same when you do those things. Certainly not enough difference to measure. Just about the only way to increase air pressure inside a closed, flexible tube is to heat it.

wphamilton

We're way into the hypothetical here since as rpenmanparker said, we don't really know how it works, if it works. More than that, we don't really have a clue how it works.

But hypothetically, assuming that it does work, my own hypothetical intuition is that it doesn't use the absolute values of pressure at all. I think it would look at transient changes. In fact, I don't think you'd even try to figure it out from base principles. What if you were curious to see a plot of pressure in the tire as a function of time, used some low-pass filter to knock out road irregularities, some other logic on the wave form to eliminate shifts due to weight and balance, and you're left with a periodic series of complex blips that corresponded to the power portion of the stroke? I'm not saying you necessarily would get there, but if you did, those blips might have characteristics that corresponded to your power output. Sure it's probably a reach and I sure wouldn't spend real money with that end goal in mind, but there's a somewhat plausible path to it.

wphamilton

Or, change the volume of the tube, by deforming it for example

Seattle Forrest

That's a fair point.

I don't think any of us are being closed minded, though. We're asking if what this company claims is possible, looking at the evidence available to us, and saying "probably not." If things had been different, we'd all be lining up to buy one.

Doge

In your equation if the V goes up so does the pressure - which isn't the case.
I assume a mistype of the Ideal Gas Law as PV = nRT.
Its been a while so if you are up on this I'm open to correction.
That law, as I understand it applies to the same "Ideal" gas. How much helium and nitrogen vary from Ideal - I don't know. And if you have charts, I'd like to see them. I recall some track team was using helium - think it was USA 1976.
Someone who fills tanks should know if you can cram more moles of helium into a tank than nitrogen.


From the quick Wiki search. I don't know what high temp means here, but I think the tire pressure is a factor.

https://en.wikipedia.org/wiki/Ideal_gas_law:

There are in fact many different forms of the equation of state. Since the ideal gas law neglects both molecular size and intermolecular attractions, it is most accurate for monatomic gases at high temperatures and low pressures.

Doge

I have cheap tires that are measurably larger at 100PSI than at 50PSI.
I have better tires that I cannot measure the difference.

If you blow up one of long string balloons the pressure starting is much higher than once it starts expanding. I can tell by degree my eyeballs pop out of my head. While I'm sure we are talking about tire changes way under 1% there are reasons other than just bottom formation tires have resistance. And as they turn a bunch looking into it adnauseam may not be so useless.

smarkinson

This is why:

"developing a patented air pressure differential technology based on the pitot tube design, from the F-117 combat aircraft,"

It's not surprising that when someone hears pitot tube they think of the powerpod which uses that method (and needs a bunch of other data to estimate power).

If they wanted to avoid confusion they probably should just tell people how it works rather than go for the PR BS.

69chevy

Before arguing about tire pressures, did anyone notice that this quack gadget is based on a pitot tube?

That means it has NOTHING to do with reading tire pressure. You guys kill me. A pitot tube measures air velocity. It just mounts on the valve stem.

FlashBazbo

You've got to do more than deform one side of it. The pressure inside a tire remains the same whether it is loaded or unloaded. It's that whole equal-and-opposite forces thing. When the forces grow on one side of the tire, the tire moves upward (or gets fat at the bottom) and the pressure stays in equilibrium. This is a classic Physics Lab parlor trick. (All the Freshmen get it wrong. This and the "Which falls faster, a brick or a feather?" It's very instructive in getting them to measure rather than trusting intuition.)

Now, squeeze it from all directions at once . . . then you've increased the outside forces, thus increasing the inside forces necessary to oppose them . . . and you've got something! The pressure of the air goes up.

wphamilton

If the tires expand somewhat with higher pressure, it only means that the pressure differential from deformation is somewhat less than if the tire only expands very little.

This isn't really talking about rolling resistance.

FlashBazbo

I doubt very seriously that it has anything to do with readying ANY air pressure. (It doesn't have a pitot tube, either.)

My theory is that the only function of the device mounted on the valve stem is making the purchaser believe the readings aren't all coming from the GPS-based phone app.

Doge

Correct. I was going off-topic.
I apologize.

wphamilton

Tires typically do not expand appreciably, not like a tube. Calculate the area of a square with perimeter P, and the area of a circle with circumference of the same P, and multiply it by the length of your (linear) tube. That's the volume of your tube. Deforming the circular tube is less drastic, but you will certainly see a different volume. Now apply your gas law.

wphamilton

No problem, we all are. I just didn't want to imply that it was related.

kc0bbq

Also pressure locally isn't the same thing as pressure in an entire closed system, or else there'd be no weather (taken to an absurd degree, not claiming that my GP4000s have any sort of meaningful climate). Or sound - compression waves are a thing. You could probably do something similar with a microphone inside the rim if you could separate the noise (in this case literally) from the data, which is also noise.

Not sold on whether this is measurable in a meaningful way, but in theory it could work. Kind of comes down to measurement accuracy, how clean the data ends up being, and how much computing power it takes to make sense of it.

Homebrew01

It's gotta be a joke.

MinnMan

You are absolutely right - it was a mistyping of the ideal gas law. P = nrT/V.

kc0bbq

A pitot tube does no such thing (in fact it doesn't do much of anything). Airspeed is calculated based on pressure differences between two pitot tubes, one in clean oncoming air, another in a location on the aircraft that is protected and has ambient pressure. Another pitot tube in ambient pressure is used for altitude. There are bunches of the things on military aircraft because of so many layers of redundancy, it's why some aircraft look like a dog that has picked a fight with a porcupine.

A pitot tube is just that, a tube. A rigid tube that extends far enough from the skin of the aircraft to get to get away from turbulence. Saying that it's based on a pitot tube is just marketing.

smarkinson

If anyone is interested here is a link to various documents for the device:

https://fccid.io/2AJGD-AROFLY01

Includes the user manual (which tells you nothing), schematics (which might mean something to someone who can read them) and internal and external photos of the device (which are pretty to look at).

OnyxTiger

I'd order a refund pronto. 2 week delivery estimate, and now they push out to February? Missed details like this aren't too promising for a business purporting to provide a break-through product at an unbeatable price. Besides, you can always reorder.

CliffordK

There is a lot of speculation about inside the tube vs outside the tube pressures.

I'm convinced that one could make a fairly accurate power profile based on real-time speed, plus perhaps rider/bike weight, although that may be able to be derived from other info. Say a speed measurement 4 to 8 times per pedal stroke.

Say if one is running with 3:1 gearing. Then at minimum, 2 to 3 speed readings per wheel revolution, or probably a bit more to actually hit close to the max and minimum speeds on each pedal stroke, or a stroke profile.

With all the talk about a "pilot tube", it sounds like they're actually measuring outside air flow rather than internal pressure, which would make sense, there would be a lot more data. Or, perhaps they measure both.