I don't know whether to keep watching this thread or just walk away.

Well, I can always unsubscribe later.

Thanks njkayaker.

This what I was trying to conclude earlier. The 'deformed' radius is what must be used for the radius calculation. Agreed that in a properly inflated tire, it may not be different from the unloaded tire radius. It will most likely also be slightly different from tire manufacturer to tire manufacturer for the same size tire.

Ok, I made a mistake in my assumption, and then compounded it by trying to extend a 2 dimensional model to a complex multidimensional reality. For auto tires, it turns out that there is a derivation and experimental data to back it up (quoted from an engineering forum: http://eng-tips.com/viewthread.cfm?qid=94153):


"Distance traveled is somewhere in between what the free radius and the loaded radius predict. There's a derivation given in "Mechanics of Pneumatic Tires", that identifies the tread as being compressed in the contact patch and also over the zones immediately before and after the contact patch. It goes on to talk in terms of effective radius and effective deflection that differ from the loaded radius and actual deflection respectively. And it mentions that very little longitudinal slip occurs within the contact patch (assuming no acceleration/braking), hence there's little wear under conditions of rolling in a straight line.

Eventually the discussion gets around to providing some experimental data, with the actual distance traveled measured for a bias tire given at 96% of what the free radius predicts but the loaded radius being only 94% of the free radius (yes, this dates my reference material somewhat). For radials, it gives 98% distance traveled with the loaded radius being only 92% of loaded radius. Data for more recent tires may differ in the specific percentages, but I'd certainly expect the general relation to hold."

Assuming the data for bicycle tires would show a similar result, then my simplistic model doesn't work well in the real world, and it looks like measuring rollout is the only solution - with a loaded measurement giving the best result. mea culpa.

JB

edit: while I'm eating crow, I should also point out that I did make an error with the geometry in my 2d model. I'd assumed that the deflection acts like a secant across the circle, but that the circumference would stay the same. The only way this could work is if the radius of the circular part of the deflected tire has a radius LARGER than the original, unloaded radius. Obviously, that completely invalidates my assumption of no stretching or compression going on - the tire would have to stretch along the circumference to account for the larger radius, but then compress along the flat part to achieve the same overall circumference. mea culpa, mea culpa. I'm going to go enjoy my crow dinner now (and thanks to my son's geometry teacher for pointing out that error!).

Took you long enough. :thumb::lol:

So where did we end up?

Do we all agree now that weight on a bike changes the roll out distance?

I just measured 2 rollouts, with and w/o load, there's about 2cm (1%) difference.

No, the whole point if this thread was to establish that this isn't what matter. You didn't add anything with this "drive by posting" because people already argued that point. You just reiterated what people were arguing was wrong.

No, it will be different. The difference might be tiny (maybe, even hard to measure) but it will be there! The other thing is that "properly inflated" doesn't mean much. There are a range of pressures that are "proper".

Doing the actual "rolling circumference" measurement accounts for all sorts of things that are otherwise hard to deal with. Note that manufacturers could supply the actual circumference of their tires (but doing the actual measurement still is going to be better).

Yes, plus these important facts:

The "rolling circumference" (the standard term for this) is equal to pi*2*r, where r is the height of the center of the hub to the ground.

The "rolling circumference" is exactly the horizontal distance the center of the hub moves in a complete rotation of the wheel.

This is what the "theory" predicts.

I'm gonna dump on "poor" jonathanb715 a bit more to make some points which I think are important to make these kinds of discussions successful.

That is a reference I provided. If you didn't realize that, it means you ignored people's counter argument.

Note that for this argument, it would have been very strange if this issue had not come up before. The fact that you did not provide any references to support your position is telling. If you are going to have these kinds of arguments, providing references is important to do.

This is basically "waving your hands and saying it doesn't matter". I said earlier that the difference was going to be on the order of 1%. If you are going to have an argument about a theoretical issue, it's important to keep issues of measurement errors out of the discussion.

The "theory" predicts a difference on the order of 1%. If your measurements can't reliably show that difference, then your measurement technique is not sensitive enough!

No, it seems more reasonable to expect that a deflated tire defines the lower bound for the effect of reduced pressure on the "rolling circumference". The effect doesn't have to be "identical": it just has to be "close enough".

Your mistake here was not indicating what you thought the effect would be. You rejected it out-of-hand with "that doesn't mean the effect is identical", which might be true but might not be! It's possible that the effect is "identical" enough and you provided nothing to support your rejection of it's relevance.

You rejected it not because you had a reason (you didn't provide any) but because you could not conceive of being wrong!

The arrogance here (and in other posts) is noticeable. If you want to be arrogant, don't ever be wrong!

The only place where what the "point" does is smack dab in the middle of the contact patch directly under the hub. What the point does in any other place has no effect on the "rolling circumference" measurement. (The point could take a trip to France and it would not matter).

It's important to understand that the point in the center of the contact patch directly under the hub is a point on a circle of a rigid (nondeformable) wheel, the circumference is less than the tire circumference. The circumference of that virtual wheel is the circle whose radius is the height of the center of the hub from the ground.


You have to be very careful with the tank tread analogy. You did not read my post, where I brought it up, very carefully.

The problem with the tank tread analogy is that you don't have a hub to use that matches the "rotation" of the tread. All the available hubs are too small (this is why I used the analogy). The reason you would not use the tread is because you need a hub to put the magnet on! If you pick a roller on the ground (with the tread in between), you could put the magnet there and input pi*2*r into the computer (r is the height of the hub axis). The length of the tread doesn't matter (and is an unusable measurement).

===============

Note that I was wrong in saying that "circumference" only applies to circles (but that's a minor thing). It still seems weird to me to apply it to non-circles. Note that I never disagreed that the circumference of the tire is constant (that seems obvious to me).

:Facepalm:

This is why it's important to show your work and carry your units kids.

No big deal. I had computed the approximate difference earlier (and said it was 1% on that basis).

With the right number, the difference is about 1 inch, which is a difference people should be able to measure accurately and reliably.

Event within the same tire manufacturer.
For the 23 size Continental, front mounted, folding (Kevlar bead), 110psi, same load:

Gatorskin: 2096 (consistent for 4 different tires)
GP4000S: 2103 (only one tire)

These values result in matched distance measurements to better than .01mi for a 8.55mi route. 14 data points for the GP, hundreds for the Gatorskin.

Uh, the data from the engineering forum regarding car tires doesn't support that assertion either.

"Distance traveled is somewhere in between what the free radius and the loaded radius predict."

JB

Aren't you guys finished yet?

You (meaning "everyone here") could've gone out and done your own rollout tests by now.

Now, either get out and ride or get back to work. :p

For what it's worth (???) I believe some auto tire pressure monitors work by measuring wheel rotational speed -- an under-inflated tire can be found because it will be spinning a little faster than the others.

I'm still a little cloudy on the mechanics of why this all works.

That's true. They use the ABS system and its wheelspeed sensors to figure out which one is spinning more quickly than the others. A tire with low pressure gets squashed to the ground further and is effectively "smaller", so it has to spin faster to keep up the same speed as the other three tires.

Other TPS systems use electronic sensors inside the wheels, which transmit their pressure readings by radio to the car computer.

Think of the extreme example I alluded to -- a flat car tire. It's still a whole tire, and it looks the right size if you hold it up or lay it on its side on the ground, but when you load it, it deforms a lot.

What people are forgetting is that the radius we're looking at is ALWAYS straight from the hub to the ground. Nothing else matters.

FWIW, on the flat car tire tangent, this is one reason why car manufacturers recommend putting the "donut" compact spare wheel on an undriven axle -- if you flatted a front tire on a front-wheel-drive car, you need to replace that wheel with one of the good ones from the rear and put the donut in back. Having two different circumferences on the driven wheels makes the differential work harder than necessary (among other things). If it's an all-wheel-drive car, the best thing is to put it on a flatbed (or carry a can of fix-a-flat, or use a full-size spare).

The data from that engineering forum does not support that assertion.

"Distance traveled is somewhere in between what the free radius and the loaded radius predict."


Just sayin'.

JB

I think that's an oversimplification. As is stated above, "Distance traveled is somewhere in between what the free radius and the loaded radius predict."

No other part of the tire matters. Some of you are getting all wrapped up in what happens to the once-loaded segment as it gets twirled around in the air, wondering if it magically changes the size of the tire or stays deformed or some other weird crap that has absolutely no bearing on the distance traveled along the ground.

Now shut up and do your stupid rollout test if you care so much. I've got lunch to eat.

For a bit of perspective:
http://xkcd.com/386/

They used to before they were regulated by the (US) government. They're not accurate or quick enough to pass the standards.

Basically, the actual circumference is the over all wheel and tire combined. The effective circumference, what will be measured, is that of the deformed tire under your weight.

Really? five pages?

http://imgs.xkcd.com/comics/duty_calls.png

Yes, I think they are talking about the difference between "static" and "dynamic" rolling circumference (which I mentioned earlier)! This difference is speed dependent.

Anyway, what you are talking about is a refinement of understanding what is going on. You have to clear what I am saying as a starting point.

Pointing out that reference as showing an "error" in what I was saying proves that you still don't get the main point!

I think you're being imprecise with your terms. You're either not saying anything or not saying quite the correct thing; I can't figure out which.

The same misunderstanding that the OP had has occurred before!

The "effective circumference" has a standard designation: "rolling circumference".

(He's only saying what I've been arguing for pages anyway. That is, his comment is a "drive by posting" that doesn't add anything to the discussion (he clearly hasn't read the posts in the thread!).

Anyway, the issue his post does not address is how that value relates to the circumference of the tire!

And it may be even less of a difference for a lower mass lower speed bicycle tire relative to an automotive tire.