How stiff are F1 tyres?

[xpensive]

...
There are people on this board that well fit the other definition of "stiff"

There might even be people on this board who are not what they appear to be.

[WilO]

I find this topic very interesting, and it is my hope that it can continue to be discussed in a fashion that benefits all...A little civility demonstrated by all would go a long way.

I found JT's comments intriguing and I hope that he will return to expand on them. For what little it might be worth, I believe that he does work for a US tire manufacturer supporting a distinctly US series...

Thanks to everyone who has contributed thus far; excellent topic.

Wil

[riff_raff]

WilO,

I am also interested to hear JT's input regarding this topic. Most of the posters here are not F1 engineering experts, me included, and don't claim to be. We come here just to discuss technical topics of interest to us, which admittedly are sometimes a bit beyond our level of collective knowledge.

While I do have an engineering background, my only intention is to do my best to explain my understanding of a particular topic, and hopefully it's accepted for what it's worth- simply my un-expert opinion. If I make an incorrect statement, I welcome those with more specific knowledge, like JT, to correct me. Hopefully with some understanding and patience.

I agree with JT that a tire is a composite structure (fibers in an elastomer matrix) with complex, non-linear structural properties. As I noted, static sidewall stiffness will increase with inflation pressure. But sidewall stiffness will also increase dynamically, as the tread mass will create uniform radial CF forces in the sidewall, which increase at the square of the wheel speed.

Regards,
riff_raff

[xpensive]

Quite so riff_raff, but when some members claim to have inside knowledge within a certain field, xpectations rise.

Anyway, on the question of air-pressure's influence on tyre stiffness, I wouldn' be surprised if you could find my "tyrepatch times pressure equalling load" anology in entry level schoolbooks on the subject, isn't it that obvious?

[DaveW]

Xample, load supported by sidewalls disregarded here:
- A vertical load of 6 kN and a pressure of 130 kPa (19 psi), would indicate a contact-patch of 45 000 mm^2.
- An area of 45 000 and a tyre-width of 300 mm, means a contact length of 150 mm.
- A contact length of 150 and a tyre-radius of 330, gives a geometrical deflection of about 9 mm.
- Conclusively; A lower inflation pressure means a larger contact patch and higher geometrical deflection.

Acting on the rim however, is only forces from the sidewalls, as air-pressure is evenly distributed all around.

I like that analysis, X. Assuming it to be correct (& I haven't checked) that would yield an estimate for "stiffness" of just over 650 N/mm. Now that is much too high for an F1 tyre (if the dimensions are appropriate), which would be expected to be rather less than 220 N/mm (same definition of "stiffness", I think). It follows that:

a) deflection of the wheel rim relative to the tyre (required to create the tyre wall stress distribution to react the applied load) &

b) compression of the material between the pressurized air & the contact patch (probably minor)

both add to the 9mm you calculated. My "guess" would suggest that the additional deflection would be around 18 mm, or double the pressure component.

BTW I used "stiffness" (in quotes) to indicate that I was referring to "secant", rather than "tangent" stiffness (see an earlier post). The latter would be expected to be around 265 N/mm.

[xpensive]

Thanks W, it's a start which makes some sense I think. (What's with this forum anyway, since two weeks or so everybody is so polite, what happened?)

Anway, if we stick with 6 kN and continue the analysis, the same load is then transfered from the contact-patch to the rim though the sidewalls, which will be in stretch somewhere from nine to three, while in compression from three to nine.

And this is of course why the sidewalls bulge out at around the contact patch, perhaps your missing millimeters W?

Furthermore, the pressure (still 130 kPa or 19 psi) inside the tyre will preload the sidewalls making them stiffer?

[Jersey Tom]

I guess you'll then have to explain to me how increasing the tire pressure by a pound or two is the equivalent of increasing the spring rate of the vehicle..if it doesn't make it stiffer.
Then explain how say a soccer ball is not harder,,therefore stiffer when you add pressure.
I think you are wrong...unless you can find a reasonable way to explain those things.

You're getting two things mixed up here. When inflation pressure is added to a tire, that tire's vertical springrate does increase. The air does not carry load to the rim though!. As the air pressure goes up, the force on the inside of the tire trying to make it expand also increases. This has to be resisted by something, since the tire's volume doesn't double when you double the inflation. It's the carcass plies. They get additional preload.

The comparison to squeezing an inflated ball isn't really a good analog. When you put load on a tire you're not trying to compress it's volume as much as you're trying to move the beads vertically with respect to the ground. It's a shape change, and in this case there is a huge difference between 'compressing' and 'flexing'. That's the definition of tire rate.. the force required to displace the beads/rim vertically.. not the force to squeeze the tire into a smaller volume.

xpensive hit the nail on the head earlier-

Acting on the rim however, is only forces from the sidewalls, as air-pressure is evenly distributed all around.

Basic stuff.

So! To summarize and get back on track here...

• The plies are what support the load in an F1 tire, and really in any tire. Don't be deceived by the fact that when uninflated you can crush one of the tires pretty easily in your hands.
• How stiff is an F1 tire? As stiff as you want it to be, its an independent variable to some degree in chassis setup and design. However, as far as "ball park" estimates go.. there were numbers provided much earlier in this thread which are pretty reasonable.

[xpensive]

Thanks JT, most rewarding when xperienced engineers on the forum make an effort to elaborate with a credible logic.

However, I'm still cvurious of any comments on my lay-man's anology with the contact patch area?

[strad]

What happens if you let the air, or nitrogen, out of the tire??? See what I mean? Gets real damn un-stiff don't it?

[xpensive]

Look strad, after being quite sceptical myself of JT's references, I'm beginning to belive he's the real McCoy.

Having worked in the US myself, I can appreciate if he's a bit cagey on technical stuff, let the man xpress himself?

Contact patch theory please!

[Jersey Tom]

What happens if you let the air, or nitrogen, out of the tire??? See what I mean? Gets real damn un-stiff don't it?

Yes. Yes it does. Without sufficient inflation you can't preload the cords in tension, and cords in compression have no stiffness. Cords in tension have stiffness.

But the air itself cannot carry load.

[DaveW]

However, I'm still cvurious of any comments on my lay-man's anology with the contact patch area?

I thought I had put your "lay-man's analogy" into context, X, but there you go....

Here is an exaggerated sketch to clarify the difference between "secant" and "tangent" stiffness. Perhaps it also illustrates how an apparently low "stiffness" tyre is able to support a large load.

[xpensive]

A bold guess is that you're not working within marketing JT, stuff like the above doesn't help your case any more than
name droppings such as "Planck's constant" or "Avogadros number", surely you can do a little better than that?

A special note for W; I did get it that you cought on, sorry about me not mentioning that.

[Jersey Tom]

If you took my reference to Avogadro's number seriously, in anything other than the most sarcastic sense, ... I don't even know what to tell you.

I just don't know how to make this any simpler. Air is not a load-bearing material. It does not transmit any net force to the rim.

[strad]

OK..you can dis me...But Steve Matchette?
Just coincidentally he spoke to this today in qualifying...
Click on photo to play video