What are the factors that determine tire wear over a race?

[captainmorgan]

is it linear with grip, so if a car has a whole load of downforce does it or does that not wear the tire more?

Is it a static vs kinetic friction issue?

[BreezyRacer]

Tire wear is end result of just about every aspect of a race car. Certainly the more downforce you have the better wear, but chassis balance is even more important, especially the balance in the faster corner exits. Everyone tries to get their car balanced but often a race car cannot be balanced in all corners, as diffence in speeds make cars handle differently.

Also the amount of scrub built into a chassis has an effect on wear. Keep in mind that scrub is often used to put some heat into the tires too, so it's not always a bad thing.

And of course there is also gearing and powerband, which can affect whelspin, and therefore wear.

As you can see a race car is a true rubicks cube.

BTW, I expect tire wear to far less of a factor this year, with a single source supplier in the mix.

[ketanpaul]

Even I wanted to ask a question on tyre wear, what makes just one specific tread disappear during a race?

[DaveKillens]

Generally, if a tire skids or slips, wear and abrasion happens. Then usually, more heat is generated, that helps make matters worse. If the tire gets too hot, wear is accelerated as well as loss of grip. It's a vicious cycle.
But usually, if you increase downforce, tire wear is less because of less slipping.
Tire construction, materials, the road surface, and driving style are also factors. And some car geometries are more forgiving than other car geometries.

[RH1300S]

Even I wanted to ask a question on tyre wear, what makes just one specific tread disappear during a race?

I think I understand your question.

Camber - with negative camber the inside edge of the tyre COULD get more wear than the outside. I am sure it is sometimes beneficial the set a car up with added negative camber despite the way the tyre may look.

Tyre pressure - high pressures will wear the centre part of the tread, low pressures the outer edges. TBH I would be amazed if this ever happened by accident with an F1 team.

[bizadfar]

[QUOTE=BreezyRacer]BTW, I expect tire wear to far less of a factor this year, with a single source supplier in the mix.[/QUOTE]
That entirely depends on how tyres are used, ie suspension design and settings.

[QUOTE=DaveKillens]But usually, if you increase downforce, tire wear is less because of less slipping. [/QUOTE]

That's logical for frontal downforce as there is less scrubbing, but for rear downforce the tyre is being pushed down more-> more wear? I can see where you're coming from with the idea of less slip/abrasion.

Another factor is the surface, every track surface(well, most) are very different from eachother, low grip, hi grip, abrasive, smooth etc.

[Jersey Tom]

Generally, if a tire skids or slips, wear and abrasion happens. Then usually, more heat is generated, that helps make matters worse. If the tire gets too hot, wear is accelerated as well as loss of grip. It's a vicious cycle.

I would venture to say there isn't much heat generated by tire scrub ("skidding"). Heat in tires is generated by elastic deformation (onset of slip angle or slip ratio). This is why if you look at instantaneous tire temps going through a corner they can jump 40F easily.

But usually, if you increase downforce, tire wear is less because of less slipping.

Disagree. How do you figure there is less slipping? Regardless of how much downward force there is on the tire you're still driving it to the limit of adhesion in high-performance racing. As you push from the peak grip level of the tire to the ultimate grip level (grip falls off slightly from peak) the contact patch is no longer purely gripping at slip angle but starts to slide as well. I would say higher downforce = higher tire wear rates due to the fact you're putting that much more load on the tire and driving it to its absolute limit of adhesion.

While camber and tire inflation are big contributors to uneven wear distribution, I'd say car imbalance has a huge effect on tire wear. If you're under- or over-steering an appreciable amount your tires on the poorly-gripping axle start to grain real bad, particularly if you're running a pretty soft compound, and even worse on a poor track surface. And once youve gotten a deep grain pattern on your tread, its damn hard to get rid of.

Exceeding the tire's operating temperature range also accelerates tire wear in a big way. Can potentially be a big problem if you're in intermediates and a dry line comes out. Need to drive a wet line on straights to cool the tires off.

[DaveKillens]

Well, next time you're running the racer, just remove the wings and see how long the tires last.

[Mikey_s]

Jersey Tom,

Heat in tires is generated by elastic deformation

This is not correct! Elastic deformation is recoverable and energy is not dissipated. Heat is generated as a result of viscous, or plastic flow and is non-recoverable. If you look at the stress/strain properties of the rubbers used in tyres you will see that at particular frequencies there is a significant viscous component in the strain response and this leads to the loss of energy as heat.

Having said that, I think you and Dave are in agreement of sorts. Under most circumstances these cars are traction limited and therefore are unable to apply their full power to the track as the wheels will spin (leading to abrasion). Adding downforce increases the traction - which should reduce wheel spin (and thus abrasion), but at low speed and in lower gears, there is still more power than the tyre is able to transmit, nevertheless if the wheel spins the stresses are higher with higher downforce... However, taking the wings off will inevitably lead to lots of wheel spin and therefore lots of abrasion.

[persovik]

Jersey Tom,

Heat in tires is generated by elastic deformation

This is not correct! Elastic deformation is recoverable and energy is not dissipated. Heat is generated as a result of viscous, or plastic flow and is non-recoverable. If you look at the stress/strain properties of the rubbers used in tyres you will see that at particular frequencies there is a significant viscous component in the strain response and this leads to the loss of energy as heat.

Are you referring to viscous flow as a permanent reorganisation of the molecules?
It has always been my belief that tyre heat was generated by friction between the molecules during elastic deformation. I have seen tyre-engineers from Goodyear explain that it was generated by permanent deformation, but find that hard to believe as it would mean that you would be constantly releasing energy that was part of the tyre's "formation" energy in the chemical or structural sense and would soon find yourself driving around on something different to what you mounted on your car.
Am I mistaken?

[Mikey_s]

Persovik,

I'm not an expert in tyre technology, but I have looked at tyre rubbers in the lab (caveat that these were commercially available rubbers, not F1). When you prod these materials using a rheometer (a lab instrument to look at how materials respond to stress/strain) you can measure a variable known as the complex modulus - a kind of stiffness measurement.

The modulus is composed of elastic and viscous components, and also plastic components, especially for composites (including tyres!). Essentially the phrases refer to recoverable and non-recoverable components of the stress/strain response. Elastic, or delayed elastic components are recoverable and therefore energy absorbed by the system is stored and returned on removal of the stress (or strain). Viscous, or plastic components are non-recoverable and the energy is dissipated - invariably as heat.

If you've ever picked up one of the "marbles" from a race track you will see that the tyre compound does have a significant viscous component as the rubber can be deformed permanently. Tyre compounds are intriguing materials insofar as the behaviour also varies depending upon the frequency of loading - at short loading times the behaviour is almost purely elastic, but at longer loading times there is s substantial viscous, or plastic component (hence the reason why the teams put the cars on jacks when they are stationary for any length of time - otherwise they would flat-spot the tyres just as a result of the car sitting on the tyre). Graining is a good demonstration of the viscous flow - and the viscous component becomes less viscous (i.e. has a greater tendency to flow) with increasing temperature - another explanation for why some tyres go through a tender period after a few laps. Remember the Michelin illegal tyre discussions of a few years back? the tyres were deforming to give a wider contact patch - very clever use of the flow properties.

and would soon find yourself driving around on something different to what you mounted on your car

When the tyre is manufactured the compound is a mixture of rubbers, fillers and other stuff which is "cured", or crosslinked, chemically. The reaction is initiated by heat and the manufacturers stop the reaction when the desired properties are reached. However, there are still reactive sites in the compound which can be re-activated by heat. Part of the knowledge the teams build up with their tyre supplier is how the tyres react to further heat cycles (either through track action, or by sticking them in an oven). Therefore the tyre does not have fixed properties and the compound can, and does change its properties during use.

[persovik]

Mikey_s;
OK, I see what you mean. I agree with what you are saying about the "half baked" racing tyres.
However, there is still the case of the carcass, made up of more stable elements, even at racing temperature. Surely the heat generated in this part of the tyre has to be purely from internal friction between stable molecules? And this should also apply to the entire thread of a road tire?

As for the actual race compound, up to a certain temperature, are you observing a viscous element, or is it just an unchanged structure temporarily "sticking" in a deformed state. (energy of the deformation stored and waiting to be released)?

[Mikey_s]

P,

think of it in thermodynamic terms; elastic, or delayed elastic responses are entirely recoverable, only the response (relaxation) times vary, therefore energy is not lost in the system - just stored.

Like most real life situations there are many factors at work complicating the situation, but if we exclude heat generated by friction (slippage/abrasion), the heat generated in the tyre by deformation of the compound arises from dissipative processes which are viscous, or plastic flow. Elastic processes do not generate heat as the energy required to deform the structure is returned on removal of the stress. Tyres are made up of composites (not including the carcass at this point), there are particles of carbon black, silica, sulphur, zinc oxide and other chemicals used to react the rubber compound. When these are forced past each other by deformation of the tyre energy is lost as heat. Furthermore, next time you go to a race meeting pick up a piece of the tyre rubber and play with it... it's really not very elastic, especially at long loading times (give it a long, slow pull and it is really quite ductile and remains stretched).

The carcass of the tyre is clearly a different matter altogether; the carcass introduces what rheologists (geeks who study flow behaviour) refer to as a non-linear element into the stress/strain picture. The carcass is there to permit the tyre to resist the huge centrifugal, lateral and longitudinal forces that they tyre must counter during acceleration, braking and cornering and high speed. I can imagine that friction between the fibres of the carcass will generate some frictional losses and thus some heat, but equally i can envisage that the construction would attempt to minimise this as it would lead to wear and potential failure of the fibres. In any case the carcass must permit deformation in certain directions whilst limiting it in others.

I hope the above makes some sense, in summary the heat is the result of dissipative processes - by definition these cannot be elastic in nature as elastic responses are not dissipative.

[persovik]

Mikey_s,
a steel spring, like in the suspension of a car, will heat up as a result of "internal friction". The nature of this internal friction, I'm not too sure about, but it exists in all elastic materials. Take a Squash ball, made up of a rubber that doesn't seem to be of a particularly plastic nature, it gets really hot during play, and you preheat it by kneading or squeesing it.
The nature of the internal friction might be viscous or plastic flow, I don't know, but the important bit is that the material behaves in an elastic manner, and heat is generated.
The race rubber is being baked while being used, and thus changins its structure as it ages. Forgetting that for a while, the rubber radically changes its properties with temperature, being quite elastic at room temperature and turns to syrup around 100 deg C. When it rolls off the tyre, it has obviously been stretched beyond the breaking point of its elastic properties, as well as being subjected to oxidation, contamination, and finally rapid cooling. When you pick it up from the track or pick it off your tires in the pits after the race, it is quite different from the fresh compound of the racing tyre.
So much for my stubborn opinions.

A softer compound definitely feels more plastic than a harder compound at room temperature.
Could this partially explain why a soft tyre heats up quicker? That the compound absorbs (and converts it to heat) more of the energy that is being put in? I used to think that it was simply a case of being able to drive it harder from cold, but you got me thinking.

[ReubenG]

A squash ball is not a good example to use to describe temperature changes during repetitive deformations - squash balls are made of a rubber-like material, just like tyres. Rubber is a bit of a pain for engineers - it has its own class of material model, generally known as "hyper-elasticity". The repsonse of rubber to a load is generally non-linear wrt strain, dependent on temperature, strain rate, and exhibits hysteresis : the stress-strain curve on loading differs radically from the unloading curve. The integral of the stress-strain curve gives the work converted to heat during one load-unload cycle.

Also, please remember that the general descriptions of a material as elastic, plastic, viscous (and many others) is not complete for every loading regime - and that these models are just mathematical descriptions that closely fit the behaviour of materials we observe in experiments. Describing a material's response completely for all values of strain (up to failure), at different temperatures, different rates of loading etc requires the combination of more than one model.

The wikipedia article on "Equation of state" gives a good idea of how a gas can be described simply (i.e. in certain pressure-temperature regimes) using the ideal gas law, and builds to the more complex equations used to describe it at higher pressures and temperatures.