How stiff are F1 tyres?

[DaveW]

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

You might like to think about the "Tweel" posted by 747. The pressurised air of a conventional tyre is, arguably, replaced by the "spokes" connecting the belt to the wheel rim. The load reacted by the tweel is still contact patch area * average contact patch pressure. Here, however, the "spokes" immediately above the contact patch are buckled, & so cannot contribute significantly to contact patch pressure. So why does the tyre not collapse?

I would argue that the belt reacts the contact patch pressure directly, the unbuckled spokes act to maintain the basic circular shape of the belt, and the spokes transferring load between the belt the wheel rim are those furthest away from the contact patch.

I am not suggesting that the tweel is a complete analogue for a conventional tyre, but it does, I think, illustrate that more than one mechanism can exist to transfer load across a tyre. It also suggests why, for example, two tyres of the same dimensions on the same rim & same pressure can differ in vertical stiffness by a factor of 2, why some tyres increase stiffness as pressure is reduced, & how a "run flat" tyre might work.

[xpensive]

But it is in its own way Dave, only the spokes are not necessarily arranged in a pre-tensioned fashion, but you will see the same stress-differential in the spokes as in the pre-tensioned tyre walls.

[segedunum]

Thanks everyone. I've managed to wrap my head around that, which is a bit worrying. I always wondered what that bulge was that you see on many tyres, especially non-low profile tyres, and it's a clever triangled balancing shape that takes the load in concert with the air (gas) pressure forces. You'd actually think it was a weakness in the tyre, but it's anything but. Interesting. That one interesting function of the tyre could be a topic of discussion in itself.

Tyre dynamics is insane. There are so many variables and so many areas of development.

[DaveW]

But it is in its own way Dave...

Agreed, X. However, it has been stated several times here & elsewhere that an applied load is reacted by contact patch area * contact patch pressure, & contact patch pressure is that of the pressurized air. The first is clearly true. The tweel demonstrates that the second is not necessarily so.

I imagine that the belt of a tweel would have a higher bending stiffness compared with that of a conventional tyre. If true, that would imply, I think, that the tweel would be subjectively "harsh". Interestingly, perhaps, the "harshness" of conventional tyres produced by different manufacturers does appear to vary significantly, & does not always correlate with overall vertical stiffness.

p.s. Just had a random thought - Lunar Rover tyres?

[Belatti]

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

I know nothing about top fuels, but the difference between pics may have something to do with the fact that they heat tyres before they run. Also, when the wheel is under high acceleration I guess there may be a big component of centripetal acceleration both in the tyre and the air.

Did you take those pics, riff? In wich step of the run was the car in each pic?

[Jersey Tom]

Those Top Fuel photos... it's not so much that the tire becomes stiffer as the rotational speed pulls the tire out to a larger diameter. Centripetal force.

[strad]

Centripetal force.

Do you mean centrifugal forces? If so you are correct.
They only run about 8 psi.

[strad]

Can I ask...Do you imply that the cords somehow act like cables on a suspension bridge?

[Jersey Tom]

Can I ask...Do you imply that the cords somehow act like cables on a suspension bridge?

Something like that.. in the sense that cords can only sustain a load under tension.. and as you add inflation pressure it increases the tension in the cords.

[747heavy]

not 100% related to the topic, but maybe some of you find the informations
about pressure & shear/stress distribution in the contact patch interesting.

http://diss-literatur.lima-city.de/Lite ... _Patch.pdf

about some of the influences of centrifugal forces on tire shape/performance in roadcar tires, maybe have a read here:

http://www.jaxquickfit.com.au/new-michelin-suv-tyres



stiffness of different tire components in a motorcycle tire

[riff_raff]

Those Top Fuel photos... it's not so much that the tire becomes stiffer as the rotational speed pulls the tire out to a larger diameter. Centripetal force.

JT,

I agree. The preload tension in the sidewall fibers is increased by rotational CF, and has a similar effect as internal inflation pressure. Whether due to inflation pressure or dynamic forces such as CF, the loaded structural shape of the tire is mostly due to the ratio of wheel load to sidewall preloads, and not a change in stiffness.

An analogy would be applying a compressive load to a preloaded coil spring. The coil spring will not deflect until the applied compressive load exceeds the preload. The preload does not change the spring stiffness, only the deflection from an applied load. A tire sidewall is the same. Increasing inflation pressure only increases the sidewall preload, and not the stiffness.

Am I right?

riff_raff

[KSP]

I read here that vertical stiffness is about 300 N/mm. But I'm also interesting in the lateral and longitudinal (torsional) stiffness of the tyres. Any opinions about this?

[marekk]

Can I ask...Do you imply that the cords somehow act like cables on a suspension bridge?

Something like that.. in the sense that cords can only sustain a load under tension.. and as you add inflation pressure it increases the tension in the cords.

After reading all of the thread i wondered about the numbers, so i did the quick basic math. With a very roughly approximation (13" rim, 13" thread witdh and 26" diameter, square profile to make it easy to calculate ) i'v got some unexpected to say the least numbers:

At 18 psi total force acting on inner sides of tire/rim system is 10000 pound
1 additional psi is worth about 2400 pound. That's a lot of tension to handle all the loads. And you need a lot of load to change the shape (volume) of this tire.

[Ciro Pabón]

Well, everybody is wrong. xpensive ALMOST got it right but then he blew it (ha, ha! How funny I am!) by saying that is the pressure what keeps the vehicle above ground. Wrong, X.

I know this will come as hard to understand, but the pressure in the tyre is not what supports your car. Please, read this twice, in bold and large, as I have a tendency to write like this the things that I think are important to understand:

The pressure of the air inside the tyre is NOT what supports the vehicle.

The vehicle HANGS from the tyre through the cords that form the tyre. The air pressure simply keeps the TOP of the tyre from collapsing.

Now, the picture I always provide (besides the one thousand words it is worth).

Tension is inversely proportional to radius of curvature!


So, it is exactly like a suspension bridge. The top surface of the wheel is the equivalent of the suspenders, the walls are like the suspension cables. The top of the wheel is stiffened by the air pressure inside. So, the walls in the upper circumference of the wheel is what keeps the vehicle from falling down.

How could it be otherwise?


No, it isn't


As xpensive pointed out, pneumatic tyre works like a spoked wheel in a bicycle. The wheel is not supported by the spokes in the lower part of the tyre, it is hanging from the spokes in the upper part of the circumference.

Now, don't give me theories about bulging sides keeping the wall from collapsing, X. It's the top, not the bottom.

How is the load transmited to the rim? Simple: the walls at the bottom flex and the TOP of the tyre becomes more flat. This raises tension on the cords at the TOP and it diminishes tension at the BOTTOM cords.

Any civil engineering student can explain to you that in any structure that works in tension under pressure (that is, inflated), the tension of the wall is inversely proportional to radius of curvature.

And having said that, I go out of this thread, as I have no idea of the stiffness of an F1 tyre. Let's pray to the god of atheist for an actual tyre engineer to come into this thread.

Hrumppph... tension in the wall increased by bulges... I cannot believe the things one has to hear these days! Were you asleep in class? Was your teacher asleep in class?

[xpensive]

Maybe this has been covered before, perhaps on another thread or in another lifetime, what do I know with Alzheimer?

Maybe the truth lies with God himself, you know, the big guy that makes me fall flat on my hard earned washboard-belly whenever one of my delightful but mischevious mistresses thinks it's funny to pull the plug on my inflatable mattress?

Life itself is such a mystery, why has Dolph Lundgren not yet been awarded with an Oscar, the hour is late for that?

The plot thickens.