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Untiring for Tyres

FEATURE BY MAX NOBLE
03/11/2016

Who can ever have too much discussion about tyres? Well Formula One and Paul Hembery it would appear. They all seem far happier with those black rings quietly going about their business this season, as opposed to the Operatic carry-on of previous years. How can they be so shallow and dismissive?

Personally I loved the "Tyre War" seasons. Goodyear and Bridgestone reduced each other to a bloodied pulp of spent engineers and exhausted budget in an attempt to capture a few desperate tenths around each circuit. True the precise detail of the advanced materials engineering going on was not revealed to the great sporting public, but we all understood enough to respect a good knock 'em down, drag 'em out fight when we saw it. And this was a fight about real physics on planet Earth, not some parallel universe invented by the FIA.

So what happened?

Well, to simplify the issue for basic discussion many people in motor sport use what is called the friction circle to frame tyre performance issues. Now this view of tyre performance falls in and out of favour, but it gives a neat view of how tyres provide benefit to a vehicle and driver. Imagine a neat circle on the floor in front of you. When your vehicle is at rest the potential performance envelope of your tyre is the circle and your tyre sits in the middle of the circle. From here you ease gently into forward motion. Once in motion you have three, and only three, options for what to ask your tyres to do next. One, accelerate forward. Two, brake, and depending on your Physics vocabulary, either decelerate or experience negative acceleration. Or, three, turn either left or right.

That's it.

Now the reason for our circle is that as you perform one of these actions you move the current performance point of your tyre from the centre of the circle towards the circumference of the circle, with the direction and amount of movement being dependant on the action performed, and the magnitude of that action. As you accelerate you move toward the front of the circle, brake, and the point moves backward, turn and the point moves in the direction you turn. Clearly, you can do two of the three things at once. Unless you're Pastor Maldonado you cannot turn left and right at the same time, so please select just one. One can then either accelerate or brake at the same time as turning, but not too much. This will move the performance point of the tyre along a radius between the pure turn and pure acceleration or deceleration directions.

Now the total diameter of our circle defines the performance limit of the tyre. As soon as you move outside the friction circle you are deemed to have broken traction and you are either spinning your wheels, due too much acceleration, under rotating, due too much braking, or sliding, due to turning too much too fast. Refer to 0previous Pastor Maldonado reference for what generally happens next.

What influences this simple picture? Well, and this is why I love tyres, the answer is... lots!

The structure of the tyre, the chemical composition, the tyre pressure, the tyre tread design, the weather, the track temperature and composition, the sprung and unsprung mass of the car, the car's centre of gravity, and finally, the inputs of the driver and their driving style. And especially so for a Formula One car, deformation caused by the aero-dynamics generating significant downforce. Each has a significant and inter-relating impact on tyre performance.

Add to this picture that tyres degrade with use, far faster than any item on the car other than fuel or brakes, both of which join tyres in limiting performance over race distance, and one has a complex picture of a complex problem, and a delightfully wide-open solution space.

Current racing slicks, as with slicks of the past, do not have a tread pattern to generate heat, and inconsistent distortion under cornering stresses, but they do have a physical construction, and a tyre surface depth that influence performance.

Indeed much of the complexity of "switching on" the current generation of tyres comes from generating sufficient heat in the tyre to get maximum performance for as long as possible without wearing the tyre out while warming it, or over-heating it and getting a sudden unrecoverable drop off.

Again as the tyre ages and the surface wears away, there is less movement in the tyre structure and warming it becomes increasingly difficult. Hence the desire of drivers to have a great out-lap and ensure the tyres are briskly heated into their optimum range. Get caught in traffic, come out directly into a safety car (virtual or real) or simply have deteriorating weather abruptly cool the track surface and that beautiful tyre strategy can suddenly become a no-heat, no-performance nightmare.

Then we have driving style. Each driver has a unique manner in which they extract maximum performance from the package. Some like a pointy car; that is one that turns in sharply, and possibly results in a touch of oversteer from the car on corner entry. Others prefer a car that tends to understeer and they then ease the car into the corner with a rock solid rear and a responsive front.

The we have the club of the joyful late brakers. Racing folk tend to call this trail braking. That is the driver sneaks a last few moments of retardation after commencing turn-in to the corner. This is a very risky approach, as the tyres are really focused on taking the turn, and do not take kindly to giving some of that turning performance to braking.

Thankfully Miss Physics is on hand to apply the precise same laws of motion, heat and friction to all tyres and drivers on the race track. Balance this compromise of trailing the brakes into the corner, with a fraction less early turn-in, and if (as this humble author assumes) you are compromising this corner for a faster entry to the next, or setting up an over-take, then all will go well. Witness some of the amazing late brake over-takes of Lewis, Max, or Daniel for how this can work as a moment of sheer genius.

Witness some of the ham-fisted bumper-cars performed by the nameless others, to protect the guilty, to see how misjudging where one's tyres are within their friction circle, and their useful life, can often lead to either a cheerless trip across the Astro-turf, or an alarming coming together between cars leading directly to the site of the crash...

Similarly with early acceleration out of a corner. As steering lock is unwound, returning performance to the friction circle, one can redeploy the available tyre performance to acceleration. As one eases the throttle open, until the moment is reached where the car is pointing straight, and, with a nod to breaking traction, one car briskly floor the accelerator and arrow directly to the next corner.

Again great feel for the car, and a great feel for not generating wheel spin on corner exit, are vital if the driver is to obtain maximum acceleration for minimal tyre wear. Too gung-ho and the tyres are shot in a few short laps. Too Grandma off to Sunday choir, and the tyres will be just fine, except all the others cars will be passing you and the entire point of racing has just sailed out the window. The art of the compromise calls every corner, entry and exit!

Under the current regulations inflation levels and rolling resistance are of increased importance because, in the first instance, Pirelli and the FIA are grumpy about minimum inflation levels. More importantly higher inflation levels decrease tyre deformation and as a result produce less rolling resistance which, as you may be guessing, then influences fuel use.

Under the current regulations fuel use is on the limit for the cars over race distance, so anything that can edge fuel use downward is a benefit. Yet again this is a compromise. Too high an inflation pressure and the tyre will not warm-up correctly, the contact patch, the area of the tyre in direct contact with the track surface, will reduce, and in extreme cases the wear-rate and pattern, and aero dynamics of the car due to increased ride height, will all be impacted. All from simply adjusting tyre pressure!

Cover the track in water and the picture complicates further. Tread patterns are now required to channel water away from the contact patch to allow the tyre to make direct contact with the track surface and provide grip. Tyre heat is now a problem as the water cools the track surface and the tyre significantly.

Which then gives the reverse problem of far too much heat when the track moves to a drying cycle. It is the delicacy of the fine balance around the friction circle that makes wet driving such a fine skill, and frequently marks out the legendary drivers from the merely exceptional.

Just as dirt driving in rallying is a fine art requiring a stunning mix of skill and feel for the car, so wet races, especially drying tracks, present possibly the toughest environment for a driver to know the limits of the friction circle and push hard enough to extract maximum performance, while not crossing into the realm of instant spin, or aquaplaning into the far distance.

All of these issues are directly impacted by the construction and chemistry of the tyres. And tyres are one of the few items Formula One and road cars still directly share. Lessons on wear rates, compounds and construction techniques can have input to road tyre design. So the FIA mandate a single tyre supplier and rob us of the delights of a bare fisted tyre manufacturer brawl every race weekend. Shame on them!

Add to this Pirelli becoming increasingly prescriptive in their bounding of how to use the tyres and it is no wonder that some in the paddock are increasingly tired of tyre talk.

Shame say I! Can we have our tyre war back please?

Max Noble.

Learn more about Max and check out his previous features, here

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READERS COMMENTS

 

1. Posted by Max Noble, 07/12/2016 12:03

"@Spindoctor - glad you enjoyed the article. I also used to read JLK and loved his mix of technical and cheeky writing. One phrase of his that remains with me is his description, having just visited, of Sydney as "East Cheam with parrots". Brilliant! Tyres... love them!

"

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2. Posted by Spindoctor, 04/12/2016 14:07

"Thanks for an interesting insight into what had, for me at least, been a completely closed book. The late great LJK Setrightfrequently used to discuss tyre related matters in "Car" magazine back in the 1960s and 70s. Since then it's a topic that seems pretty much to have dropped out of the mainstream.

I too knew the days of a bit of competition between tyre makers, if only because in those "halcyon days" tyre manufacturers tried to to make the best tyres they could manage. Unfortunately, today they are mandated to to produce tyres with unpredictable grip and durability which wear out after a few laps.
O tempora, o mores!"

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3. Posted by Max Noble, 30/11/2016 11:39

"@Hardliner - Different people draw them differently. However a basic introduction can be found here...
http://www.auto-ware.com/setup/fc1.htm

And one with a bit more racing detail around can be found here:

https://books.google.com.au/books?id=1X3P8L72Nb8C&pg=PA130&lpg=PA130&dq=graphical+plots+of+tyre+friction+circles&source=bl&ots=42xon1KFNh&sig=NNxGkqHqzfztGskMDQnOCjpxSz0&hl=en&sa=X&ved=0ahUKEwiGrcb0sNDQAhVBCpoKHQFyBZgQ6AEIPDAF#v=onepage&q=graphical%20plots%20of%20tyre%20friction%20circles&f=false

I've no doubt the highly complex simulator software used by all the top teams is based around the same simple theory, but with highly developed maths models replacing these simple concepts."

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4. Posted by Hardliner, 23/11/2016 4:07

"Are friction circles plotted graphically, and if so might we see one? In yacht racing we plot something called a polar, which is an indication of the VMG of a boat for a given wind speed and given sail combination. It is very useful indeed. Interesting stuff to those of us for whom F1 is more than just entertainment, thanks "

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5. Posted by Willie, 04/11/2016 1:01

"It's really a matter of cost vs. benefit, and F1 no longer allows enough technical latitude to make it worthwhile for the big tire companies to invest the necessary R&D effort. And the publicity results of failure can be devastating (Michelin at Indy.)
Personally, I miss the days of Englebert, Continental, and Dunlop."

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