How stiff are F1 tyres?

[xpensive]

I'll be darned seg, try this for a bit of an intellectual challenge?

How a Tyre Supports its Load

It is commonly thought that the air pressure in a tyre supports the rim. If you think about it, this can not be so, because the air pressure against the rim is the same all the way around.

How, then, does a tyre support its load?

First of all, the role of air pressure in the tyre is to hold the fabric under tension -- in all but one place, the contact patch with the road surface.

At the contact patch, the tread and fabric of the tyre are flattened against the road. Air pressure can only push directly outward, and so here, it pushes directly downward to balance the weight load -- they cancel each other out, and the air pressure against the contact patch makes no contribution to the tension of the fabric. The area of the contact patch equals the weight load, divided by the air pressure. For example, if the air pressure is 50 PSI and the weight load is 100 pounds,, the contact patch will be two square inches.

The threads of the tyre fabric can only transmit loads lengthwise and in tension. The air pressure forms them into a circular cross-section except where the road contract patch disturbs this. Here, the cross-section is flat at the bottom, with an arc of a smaller circle on either side.

The load is transferred from the contact patch to the tyre sidewalls by the shallower angle of the threads next to the contact patch -- they are pulling downward less. The load is similarly transferred from the sidewalls to the rim by the shallower angle of the threads of the fabric where they meet the rim. As the threads pull downward less, they also pull outward more, but the outward forces at the two sides of the tyre are equal and opposite, and cancel out.

These effects together produce the bulge seen at the bottom of a tyre under load. Because the contact patch is flat against the road, the curvature of the sidewalls is increased -- the tyre becomes effectively thinner, not counting the inactive width of the contact patch. The tension of the threads is actually slightly reduced at the bulge, in spite of their supporting the load!

With a bias-ply tyre, the load is carried lengthwise in both directions along the tyre by the diagonal threads, so the bulge is longer and less deep than on a radial-ply tyre In the early days of radial-ply car tyres, people often thought they were underinflated, because the bulge at the bottom was more pronounced.

A tyre, then, supports its load by reduction of downward pull, very much the same way that spoking of the wheel supports its load. The tension-spoked wheel and the pneumatic tyre are two examples of what are called preloaded tensile structures, brilliant, counterintuitive designs working together remarkably to support as much as 100 times their own weight.

Bias plies also help to transmit lateral and torque loads, by triangulating the connection between the contact patch and the rim -- much like the way the spokes of a semi-tangent spoked wheel transmit lateral and torque loads. With tubulars, the diagonal plies also work like a Chinese finger puzzle: the air pressure makes the tyre fatter, and so makes it shorter and helps hold it to the rim.

If you would like to get into mathematical details, there is an excellent technical description in an old Britannica encyclopedia article online.

[strad]

AS one who has had the misfortune to drive equipment with solid tires, I can tell you, you wouldn't like it.

[747heavy]

just to add some illustrations to x´s good explaination...
also note, that the tire side walls get pre-stressed when the tire is mounted
to the rim, as the diameter of the rim is larger then the internal diameter of the tire.

[xpensive]

Simply a brilliant illustration jumbo, I owe you one.

[WilO]

excellent stuff, and a special thank you to 747 for making a contribution.

[Arunas]

just to add some illustrations to x´s good explaination...
also note, that the tire side walls get pre-stressed when the tire is mounted
to the rim, as the diameter of the rim is larger then the internal diameter of the tire.

Sorry.
Sorry, just do not see here anything about air pressure carrying load.

[xpensive]

Regardless if you agree with the piece or not, there is no neet to ruin this thread's layout by quoting the whole thing, please edit asap!

[DaveW]

A couple of interesting posts by X & 747 (good to hear from you, BTW). I think they describe one of several mechanisms in play when a tyre reacts a load.

A couple of things to think about (just to keep boredom at bay). How can some tyres increase stiffness as internal pressure is decreased (it does happen)? Also, how do "run-flats" work?

[747heavy]

I don´t really have a dog in this fight/argument, so I don´t try to prove or disprove any of the statements made. I just wanted to provide some additional informations on the subject, and leave it to anybody to make up his own mind, as what is happen in a tire.
I guess, it is a rather complex system, and that there is more then "one ultimate truth".
As for Dave´s question about un flat tires. I think that there are different systems out there, so it´s a bit difficult to give an universal valid answer.
There is(was) the "mousse/foam" system used in rally tires and then there are the different systems used in road car tires.

some info about the Michelin "Mousse" tire used in motor bike off-road sports and the WRC:
http://www.motorsport.com/news/article. ... 994&FS=wrc

So just to start somewhere here some info about some of the "run flat" or "zero pressure" road car systems.

Supporting Role
A standard tire depends on internal air pressure along with sidewall construction and multiple steel belts to maintain shape while rolling down the road. The pneumatic pressure of the air inside works with the structure of the tire itself to support the weight of the vehicle. The air inside the tire also acts as a cushion against bumps and other road surface irregularities. While the sidewall is relatively strong in a normal tire, once the air leaves the containment vessel that is the tire itself, the ability of the tire to hold up the weight of the vehicle is diminished. In other words, if the tire loses air, it goes flat. A run-flat tire improves on standard tires by adding a special supporting sidewall structure that allows the tire to make it to the nearest service station completely devoid of any air pressure.
While the benefits of such tires are obvious, it is only within the last few years that they have become commercially available at a realistic price. Until recently, run-flat tires were reserved for a precious few. From Aston Martins driven by bullet-dodging secret agents, to bulletproof limousines bearing national flags, to the ultimate SUV, also known as the Popemobile, the use of run-flats was previously limited at best. Today, dropping prices have enabled wider distribution of run-flat tires, both as original equipment on many new vehicles, and as affordable replacements for older models.

Two of the areas RFT focuses on are rigidity and heat resistance, so that the tyre can support the weight of the vehicle with total air loss. Bridgestone's RFT meets this challenge with a combination of design, materials and construction changes that optimise the location, shape and physical property of each tyre component:

The sidewall is constructed with reinforced rubber, carrying the vehicle's weight at zero-pressure;

While the bead shape configuration itself is unchanged, so that the Bridgestone RFT is compatible with conventional rims, the bead wire is wider and reinforced, ensuring a secure fit on the rim even at zero pressure, and special bead filler with low heat generation is used.

some info about the Michelin PAX system (which is close to the "Mousse" system, Michelin pioneered in the WRC some years ago)

http://www.familycar.com/carcare/runflattires.htm





some of the commonly used trade names for "run flat" tires:

EMT Goodyear
RFT Bridgestone
DSST Dunlop
EUFORIA Pirelli
ZP (ZERO PRESSURE)Michelin
SSR Continental

this shows (in my opinion) quite nicely the analogy to a "wire spoked wheel" which Xpensive used in his explaination.
Michelins "airless" tire


some more info about the Tweel airless tire:
http://auto.howstuffworks.com/tweel-airless-tire.htm

[strad]

Ok...I understand guys,,and I even agree to agree, but if the air plays no part..then WHY does, and believe me it does, adding air make it stiffer in the sense of mimicking adding spring pressure?

[747heavy]

o.k. for better or worse, I will stick out my neck again.

I don´t think that anybody on here seriously said, that the "air plays no part",
as far as I see it, the discussion revoled around the definition, which part does the air(pressure) play in how the tire supports his vertical load.
I think (maybe wrongly) part of the controversy lies in the definition of the word "carry".
Does we use "carry the load" in the sense of "supporting the load" or do we mean "carry the load" in the sense of taking it from one point to another as in (transfering the load).
I guess it is the definition of the term "carry" at which point the argument started, with different people may defining the term in different ways.

To try and answer Strads question.
The answer (at least part of it) lies in the cosinus term of the function -IMO.
An increase in airpressure and therefore "tension" in the sidewalls will reduce the angle of the sidewalls under (vertical)load, thereby allowing more of the vertical load being tranfered "in a straight(er)" line towards the rim/wheel centre, with a lesser portion being transfered sidewards/sideways.

If you would fill the tire with water ("almost" incompressible fluid), you would make the tire very stiff.
By increasing the pressure, you reduce the compressability of the air,
bringing it closer to the properties of water.

In laymans terms I think it helps to picture the sidewall as a "buckle stay" like trying to compress a short plastic ruler between your fingers.
At first (no angle - cos of 0°=1), the stay is "very stiff", then it starts to buckle.
As more as it buckles, as easier it gets to compress it more (the cosinus term of the (buckle)angle times the amplied force).
The inflation pressure (by pretensioning the sidewalls more) reduces the "buckle" angle of the sidewall, so to speak.

Just my 2 cent, feel free to point out the thinking errors in my attempt.
There are surely other factors at play in the whole loadtransfer system of the tire.

[riff_raff]

I went to the NHRA drag races at Pomona today, so here's somewhat relevant post about sidewall stiffness in a Top Fuel rear tire. Compare the following two photos of the same car. Is the massive increase in sidewall stiffness and associated increase in rear ride height shown in the second photo due to inflation pressure?





riff_raff

[Mystery Steve]

why does [...] adding air make it stiffer in the sense of mimicking adding spring pressure?

When you add air to the tire you are increasing the preload in the cords. This is why the cords are able to take a compressive load because they are under high tension during static conditions. As far as the cords are concerned they are always under tension even if they are seeing a net compressive force. As you continue to add even more air, you further increase the preload and reduce the flexibility of the cord and rubber matrix. Think of it as a rubber band. If you wrap one end of the rubber band around your thumb and the other around your index finger then spread them apart and try to "strum" it like a guitar string it will flex less the further apart your finger and thumb are (higher preload), and vice versa.

So, yes the stiffness is a function of the air pressure, but the air pressure does not actually provide any load bearing ability. The tire itself supports the load, but as a result of the air pressure being present.

[xpensive]

...
- What faces the road surface load-wise is air pressure times contact patch area.
- What faces the rim load-wise is force on the bead in contact.
- In between the two contacts above, load is conveyed through the sidewalls only.

Conclusively, the way I see this very simple static analysis; The air-pressure indeed carries the load from the road surface, through the sidewalls, to the bead and finally to the rim.

Thanks to jumbo's and Misery Steve's contributions I can humbly elaborate a bit on the above:

A) Between the road surface and contact patch, load is simply tyre-pressure times area.

B) Between the rim and bead, load is obviously the same as A)

C) The load of A) as weell as C), is conveyed through the sidewalls of the tyre simply by off-loading the air-pressure dependent radial pre-tension of the sidewalls around the contact-patch.

Conclusively, I will stick to my guns that tyre stiffness is entirely air-pressure dependent, no matter how you cut or slice it.

[DaveW]

Is the massive increase in sidewall stiffness and associated increase in rear ride height shown in the second photo due to inflation pressure?

I suspect the tyres do increase in stiffness when they are spun up. However, "preload" probably also contributes (similar to increasing ride height by cranking preload into a spring).