This looks like a valid point. That would suggest that there is slippage, stretching or shrinkage going on in the tire (or maybe a combo of 2 or all 3). I hadn't considered that - I was thinking of it more like a car tire, which has a designed contact patch that is already very flat side to side, so the contact patch primarily gets longer when under load.

In that case the question becomes - would this affect the circumference at the center of the tread? I don't have that answer.

I still think it isn't large enough to get into saying you have to do a rollout weighted down or else it won't be accurate - the error in the measurement technique most of us will be using will wash it out, so neither method should improve accuracy necessarily. My own measurements support this, even though as others point out there's simply not enough repeatability for the difference in averages to be statistically significant. I'd just like to point out that my difference in averages was very small - well under 1% (2mm), and the difference in samples was somewhat larger (+-3mm). Even adding the difference in between the largest outliers gives you a change in rolliout of less than 0.5% - still not real reliable, but probably accurate and definitely not worth worrying about when setting your computer. Yes, more samples would make this a more valid data set. FWIW, I'm the only one who bothered to post any data at all.

JB

PS and for what it's worth, comparing a deflated tire with a properly inflated tire weighted down is specious - as has already been noted, there's good reason to think simply inflating the tire stretches it, increasing the circumference.

Tire..jpg

From the side the loaded wheel should look something like the left side. The increased length doesn't matter because the wheel never has to roll over it, in other words you never see a wheel looking like the right side. Because of this, although the contact patch is longer than it would be on a perfect circle, the radius if you traced the point touching the ground through a whole revolution the circumference of the point touching the road for a loaded tire would be smaller than for an unloaded tire.

Now you're getting to another argument, which is whether there is enough of a difference to matter. Assuming a 70 cm diameter wheel, and 15% sidewall compression (given as a guide to recommended tire pressure somewhere, maybe sheldon brown's site?), and assuming a 23 mm tire you end up losing 2.3*.15=.345cm off of a 35 cm radius. This gives a difference on the order of .1%, so almost certainly smaller than the error introduced by hitting bumps, changes in temperature, changes in barometric pressure, etc. but still present.

My example was to show that putting weight on a bike does change the roll out distance. Nothing more. It does exactly that.

Sigh. When measuring circumference or rollout, you are measuring from the same point on the outside of the tire to the exact same point. That point does have to cover that flat part as the tire rolls through, then it lifts off the ground and will follow the circle until it comes in contact again at the beginning of the flat section - but you don't measure rollout when it gets there there, you measure it at the center point , where the hub is closest to that point (assuming that's where you started). If you insist on using circular measurements (radius) for non-circular shapes, you will get results like this. Think back to the tank tread. Does it have a circumference? Yup. Can you make it into a circle? Pretty close, as long as the plates are small and the hinges between them are flexible. Does that change it's circumference? Nope, unless the tread shrinks or stretches in the process. In that respect the tread acts a lot like a bicycle chain.

If you take the radius to the ground when the tire is deformed, you will get a smaller measurement to the hub. That is the radius of a circle, but a circle with too small a circumference. It's not useful for calculating anything to do with the tire circumference unless the deformation is very small - and even then you are introducing a systematic error. edit - the tank tread provides a useful example again. as you flatten out most of it along the ground, you could have the rest of the tread form a circle (odd looking tank, I know). The measurement from a point in or near the center to the ground is not the radius of a circle that has the same circumference as the tank tread. Which is relevant? The actual circumference of the tank tread = because that is the rollout (do I really need to remind everyone again that the definition of circumference of a closed curve is the linear distance covered in one revolution? The special relationships between radius, diameter and circumference are a special case and only pertain to circles. The deformed tire by definition no longer is a circle!).

JB

JB

No, it's not irrelevant. In fact, it's essential. Do the hula-hoop example, either for real or in your head. The bulging is what makes the roll-out distance the same.

Holy crap. This is some tangent...(geeky pun intended).

Incorrect.

Except the flat part moves relative to the tire. Your entire premise with the increased length of the contact patch is wrong because the flat patch is always in contact with the ground. It is stationary relative to the ground, not relative to the tire.

New analogy. If you were to roll a weighted bike over red paint, then rolled the same bike, unweighted, after the red paint dried, over blue paint, would you see:
a) only red paint
b) only blue paint
c)blue paint with a stripe of red paint between it and the rim
d)red paint with a stripe of blue between it and the rim
e)none of the above, explain

Indeed. I never thought attaching a magnet to the spokes and a sensor on the fork could be so complicated.

I don't think the tire stretches much at all. The point of measuring a deflated tire is to illustrate the effect.

Anyway, What's a "properly inflated" tire? A range of pressure is "proper".

No, with the rollout, you are measuring the linear distance traveled by one rotation of the hub. For a nondeformable wheel, this would exactly be the circumference of the wheel. For a deformable tire, this distance will be less than that.

There are two methods one could use to measure the "rollout"

1) With the tank, you can measure the "rotation" of the tread, and that will produce a large number.

2) You can also measure the distance traveled by one of the rollers. That will be a much smaller number and that number will be exactly (we are ignoring errors) related to the height of the hub center to the ground (the radius). Keep in mind that the sensor is counting the rotation of the "roller".

Because the deformation in a bicycle tire is small, you think there is isn't a difference.

In a weird way, that was exactly my point. Think of your bicycle chain. you can have a circular shape with a circumference pi*2*r. Now take the chain and flatten out part of it, like a bicycle tire, but keep the rest circular. Do you really think the circumference just changed?

BTW, I've already conceded that a bicycle tire is stretching/shrinking/slipping when loaded in the real world, thanks to phantoj's example - this is therefore now just a theoretical geometry discussion in 2 dimensions - and remember that all of my examples were assuming no stretching or slipping (since that's what I'd thought was really happening). I suspect that you all are assuming that a tire must stretch out or shrink along it's circumference - because if it's not stretching or shrinking the circumference is not changing.

JB

Sorry. It wasn't my intention to run 3 pages. I actually dialled the little bugger in during page 1.
Mike

They both result in defomation, but that doesn't mean the effect is identical.

I didn't say anything that disagrees with your second point. A deflated tire by definition is not properly inflated.

JB

No, the effect won't be "identical" (no one is making that claim).

Anyway, you haven't provided any supporting references.

The tire circumference doesn't change. The rolling circumference does.

Michelin says so.

https://www.michelinman.com/glossary/

Hopefully, Michelin is a authoritative enough source!

What he's saying is correct with the assumptions he's making. They weren't very clearly stated, and aren't particularly good assumptions, but what he's saying is correct within them.

No, it's only correct if the tire does not deform.

Note that he is correct that the tire circumference doesn't change but that isn't the number we are interested in.

We are interested in something called the "rolling circumference".

In the case of a nondeformable tire, the tire circumference is equal to the rolling circumference.

In the case of a deformable tire (the real case), it will be less and it will be smaller with less-pressure/more-load.

We don't care that the points on the perimeter of the tire take the longer path (the tire circumference). We only care how far the center of the hub moves horizontally (the rolling circumference).

\0/



There are many, many references that say this. And none that say otherwise.

https://www.michelinman.com/glossary/

Hopefully, Michelin is a authoritative enough source!

https://webcache.googleusercontent.co...&ct=clnk&gl=us

If, Michelin isn't good enough, here is what Jobst Brandt says!


==========

It appears that the "rolling circumference" is a critical issue for farm tractors (and four-wheel drive)!

For automobiles, there's even a distinction between "static" and "dynamic" rolling circumference!

Which is one of his assumptions. Like I said, they aren't great.

That's being too generous!

The issue of "rolling circumference" versus tire circumference is a big deal in some places.

By the way, the difference is 1.0% (not 0.1%).

Imagine a rim standing alone on the ground. It has a given circumference or perimeter, you can measure it with a tape. Now put a big load on it, it will deform, flat on the bottom and top. Measure the perimeter now . It hasn't changed, right?

We know that. So what? It's not the number we are interested in.

Do you know who Jobst Brandt is?

Sigmas stop reading after a few minutes rest. If forgotten to wake mine up after a break and end up short mileage. Could that have happened?

I thought the number was precisely what mattered, if it's the same regardless of deformation.