16 replies to this topic

[Aubwi]

I found an interesting article on tires here

It attemps to dispel a number of myths about tires, and explains that the only thing that affects a tire's grip is the coefficient of friction of the rubber compound. I know that tires don't obey the laws of physics for an ordinary solid object, but I think that's a perfectly reasonable simplification for most purposes.

Anyway, this all got me thinking about the grooved tires in F1 and how much mechanical grip they can produce. An F1 car has 800+ BHP, and corners at 4 g's which must produce lots of heat and wear in the tires compared to lower formulae. Plus, the grooves only make this worse. So would that mean that an F1 tire has a harder compound than other formulae? Of course, F1 tire technology is the best in the world, but I wonder if the best technology doesn't neccessarily mean the most mechanical grip, when other considerations like heat and wear become more important. So with the rubber compound being the only thing that affect grip, would an F3 car, for example, grip better in a slow corner?

Along those lines, I was also thinking about how to produce better racing in F1. Many people in forums like this say that more mechanical grip and less aerodynamic grip is the answer. I wonder if a reduction in engine power would produce both of those in an indirect way. Less power would have the teams reducing wing in order to get better straight line speed which would in turn reduce downforce. And that in turn, along with the reduced engine power, would mean less wear and tear on the tires. Which, would mean softer compounds and more mechanical grip!

Just some food for thought and discussion,... hopefully.

[Marco94]

I'll print it, and read it soon. Just started working, so time is getting in short supply. What I can say is that I think there is some potential for imporvement of the article!

[Christiaan]

I used to think about it myself till I asked why I got better handling when I used wider tyures made of the same stuff by the same company. My conclusion is that the smaller the "Normal raction force per unit area" the better the handling or the greater he "traction". If I am right it throws that article out the window

[Ben]

Well if you ran wider tyres made of the same stuff you would have a shorter contact patch (all else being equal) and this would reduce the heat build up in the tyre just as the article suggests. So your observations do tie in with what the article's saying.

Chuck Hallum has presented a number of interesting papers on tyre mechanics (one on heat generation in Nascar tyres) at various SAE conferences and I will now go off and re-read them

Ben

[MRC]

I was disappointed that the guy never talked about load sensitivity, saying that mu never changes with the tire's normal load.

[DOHC]

Does anyone know why the friction law is called "Armonton's law" in the article? It's of course ages old, and in physics it's called "Coulomb's law". Who was or is Armonton?

[Aubwi]

I guess the idea was to compare different tires, rather than the same tire under different loads, which is what load sensitivity is about. But I suppose you make a valid point, since there's always a certain amount of weight transfer happening, no matter what kind of car you're talking about. So a tire with less load sensitivity should always provide more grip.

[testarosa]

I don't entirely agree with this article; a tire's grip is not determined by F=uN because (according to Chassis Engineering) the mu of the tire decreases with increasing load. For this reason, I disagree with the statement that more rubber on the road doesn't mean more grip. With a larger, wider tire, a lower pressure can be used, decreasing the load per square inch on the tread. Because tires are more efficient at low normal force, more grip will be available.

[MrAerodynamicist]

You could of course do an experiment to test if friction force is affected by area:

Find some stiff material say some cardboard. Cut out three sqaures. Two 10x10cm, the other 14.1x14.1cm. Put the two small smaller ones face together with just the tiniest bit of glue (you could also try to put an equal amount of glue on the larger one and just let it dry). Now you should have two object witt the same friction coefficient and mass but with one having twice the surface area. [and one with a slightly higher cg but if its thin like carboard this will be negligable]
If you find some larger surface you can tilt, if contact area is not a factor, they should start to slide at the same angle of tilt.

[Aubwi]

Mr Aerodynamicist, the laws of physics for a sliding piece of cardboard are well known. The surface area has no effect on the frictional force at all. You can look that up in any basic physics textbook.

Rubber on asphalt is much more complicated though, because the tire is under pressure, and the rubber compound is not a true solid. It's more of a colloidal suspension like Jello or toothpaste. Various solids suspended in a liquid. The laws of physics for a solid object don't apply.

[Ben]

Indeed.

As I understand it, tyre friction is a combination of conventional coulomb friction but there is also an element of the hysteresis within the contact patch (certainly outside the basic f=mu*N equation).

Mr Hallum (still in the midst of the re-reading) suggests that the fall off of mu with load and increasing slip angle is to do with tyre heating. So this implies that the coulomb model is reasonably accurate but is altered due to the loads appied to the tyre which generate heat which alters the fundamental properties of the rubber.

Ben

[lateralforce]

IMO regarding the balance between more mechanical grip or aero grip or the other way round, it has to be based on the force and moment characteristics of a tire.

The force and moment generation behavior of a tire is very much dependent on a lot of factors namely the tire type (radial, bias), tire compound (the tread compound, sidewall, and numerous other tire components), tire construction and components dimension, the curing process, etc. Compouding is just one of the many areas that tiremakers tune to maximize tire performance. Factors such as the contact patch pressure distribution and the tread stiffness are also equally important to ensure that the tire always deliver its fullest potential in all areas of performance; accelerating, braking, and cornering.

A tire designer will 'play' with these variables so that the tire will perform in the desired performance envelope. For example, a high performance tire for F1 application will have a high load sensitivity (increase in lateral force with increase in normal load), while a passenger type tire will have a low load sensitivity in order to induce understeer in handling (because understeer is generally a safer handling trait in passenger car than oversteer).

I would imagine in F1 application a tire designer would gather every pertinent information he could gather such as the track temp, cornering speeds for each corner, average speed, braking distance etc and also car information (if possible) such as the roll rate distribution, suspension setup, aero setup etc and together with the team, decide on the best balance on the tire to get the most out of the car and tire as a whole package.

Regarding the grooved tires, Lauda mentioned in F1 Racing the grooved tires are a bit trickier to handle in the limits as the traction transition isn't as smooth as slicks. But I think there are some little advantages to the grooved tires (doesn't mean I like them!). Possibly, they are 1. decreased rolling resistance thus resulting in higher top speeds and better mileage 2. The grooved allows better cooling to maintain the right compound temp at higher rotational speeds....

My RM0.02 = 20 sen.

[schuy]

Do slicks have better braking characteristics?

[Jezztor]

Yes, in principle for the same reasons as concluded above, especially soft compound slicks.

Jezz

[schuy]

Originally posted by Jezztor
Yes, in principle for the same reasons as concluded above, especially soft compound slicks.

Jezz

Cheers mate.

[imaginesix]

Originally posted by Christiaan
I used to think about it myself till I asked why I got better handling when I used wider tyures made of the same stuff by the same company. My conclusion is that the smaller the "Normal raction force per unit area" the better the handling or the greater he "traction". If I am right it throws that article out the window

It's not that simple.

If you bought some wider tires to replace your old ones, the old ones will have hardened and lost grip over time. Also, It seems not only plausible but very likely that a given manufacturer will change the compound/construction of different tire sizes for the same model. Or they could simply find performance improvents year-to year.

Also, your tire up-sizing may have produced improved responsiveness rather than outright grip. Unless you're comparison-timing yourself on a skidpad with known identical tires (except for width), then your personal experience is unfortunately of no value.

[Yelnats]

Originally posted by Aubwi
Mr Aerodynamicist, the laws of physics for a sliding piece of cardboard are well known. The surface area has no effect on the frictional force at all. You can look that up in any basic physics textbook.

Rubber on asphalt is much more complicated though, because the tire is under pressure, and the rubber compound is not a true solid. It's more of a colloidal suspension like Jello or toothpaste. Various solids suspended in a liquid. The laws of physics for a solid object don't apply.

Quit true, Aubwi! A sliding tire acts nothing like a sliding solid operating well below its shear strength limits. A sliding solid will ride over the surface with very little loss of material until enough force is applied to make it begin to shear, then the co-effectient of friction will cease to obey the simple rules of friction and become sensitive to the contact area. A sliding tire always operates in the area of shear sensitivity (look at the trail of lost rubber in a skid mark!) so it never obeys the simple laws of friction and is always sensitive to contact area or pressure (force per unit of area).

The article is wrong about the contribution of contact patch length to internal heat as the example assumes a straight line operation and a properly designed, inflated and loaded tire will almost never be stressed in this situation. It is during cornering that the length of the contact patch has a greatest influence on heat generation.

For a tire to generate cornering forces it must rotate at an angle to it's direction of travel (misleadingly called 'slip' angle) introducing an element of 'squirm' in the tread at the contact patch that is proportional to the contact patch length, 'slip' angle and lateral force. It is this squirming and the resulting hysterisis friction that heats a tire much more than it's deflection to load, assuming we are talking about a high speed road or racing operation. In a contact patch under heavy cornering, the trailing edge begin to lose traction before the middle portion reaches its limit, the longer the contact patch the greater the effect of this phenomenom and the slower reacting is the tire to the turning angle. So a wider tire will feel more responsive to the driver, an important factor on both road and track with the added benifit of requiring less contact pressure (inflation pressure) per unit of contact patch length. The reduced pressure has the add benifit reducing the shear forces per unit of contact patch to improve the co-efficent of friction and allowing the use of softer compounds to further improve the situation! A win - win situation all around!