The subject of preheating tyres is, ironically, a hot topic in the run up to the Centenary Le Mans 24 hours, the cornerstone of the FIA World Endurance Championship (WEC). As part of the drive to become carbon neutral, the FIA introduced a tyre road map for the 2023 WEC season. The aim was to ban the energy-sapping practice of pre-heating tyres. However, this has been thwarted by two high-profile accidents where Toyota’s Brendan Hartley and Ferrari’s Antonio Fuoco crashed at the Spa 6 hours earlier in the season.

Both accidents were attributed to leaving the pitlane on cold tyres. This prompted an in-depth investigation where the FIA and ACO have agreed to reverse the regulation and authorise tyre warming for all WEC classes for this year’s 24 Hours of Le Mans only.

Why do tyres need to be preheated?

Preheating tyres in its crudest form has been a part of motorsport since the 1970’s. Apparently, at the 1974 Formula 1 Canadian Grand Prix, teams stripped the duvets from their hotel beds to wrap around the tyres. Today, tyre warming is exploited across almost every level of car and bike racing, from Formula 1 and MotoGP, down to trackday bikes and radio control racing.

To understand why tyre preheating has become such an important practice, we first need to understand how a tyre generates grip. The viscoelastic behaviour of tyre rubber means that at low temperatures the modulus of the rubber is high which makes it brittle and rigid. Whereas at high temperatures, the modulus of the rubber is low, making the rubber flexible and elastic. The more elastic the rubber, the more contact it makes with the track as it moulds into the grooves of the asphalt.

When a driver leaves the garage, their main priority during the outlap is to bring all four tyres up to temperature consistently. This means avoiding subjecting the tyres to large longitudinal or lateral loads, so minimising heavy braking and accelerations as well as reducing speed around long corners.

> How does a tyre generate grip?

If a driver pushes too hard before the tyres are within the optimum temperature window, the surface of the tyre is too cold and brittle to generate grip, resulting in the tyre sliding which damages the surface. This can lead to graining which reduces the amount of rubber in contact with the track and ultimately the available grip.

What are tyre blankets, tents and ovens?

The different approaches to pre-heating tyres is defined in the regulations of each championship. Typically, single seaters use tyre blankets and closed wheel categories favour tyre tents or ovens. This is predominantly due to the difficulties of fitting a tyre blanket within the wheel arch of sportcars.

A tyre blanket consists of a flexible heating element contained within a heat conductive gel. The blanket is sized to encase the entire circumference of the tyre and once fitted to the full set of tyres, the blankets can then be connected to a thermostatic control box which is used to monitor the heating process.

Tyre tents or ovens are large enclosures that house several racks of tyres. Hot air is blown into the tent, usually by means of a fuel-based space heater, which gradually heats the tyres. Both tyre blankets and ovens consume large amounts of energy, and in both cases can take approximately 1 to 2 hours to heat the tyres to the desired operational temperature.

How long should tyres be preheated?

The time tyres spend in a tyre blanket or oven is defined by the regulations of each championship. In Formula 1, tyres are only allowed to be preheated prior to a session in which they are intended to be used. Slicks can be preheated for a maximum of two hours at 70degC (158degF), intermediates can also be heated for two hours but only up to 60degC (140degF) and wets are not allowed to be preheated. These temperatures limits refer to the temperature of the surface of the tyre’s tread or sidewall, measured with an IR gun, not the temperature set on the blankets themselves.

Should preheating tyres be banned?

The issue with banning preheating in championships which are used to this practice is that drivers, engineers and tyre manufacturers all need time to adjust. Furthermore, preheating a tyre and rim also increases the tyre pressure which is critical to the structural integrity of the tyre sidewall, particularly on racecars that generate significant downforce. Without preheating, tyre pressures will be much colder at the start of a run, which could be a structural safety risk.

There is also the added implication of lower pressures affecting the ride height and therefore the aero platform. To avoid this issue, cold starting pressures could be boosted. However, tyre pressures increase significantly throughout a run, so simply boosting starting pressures could mean the tyres become over-pressured later in the stint, which can then lead to a myriad of overheating and wear issues.

The solution is to develop tyre compounds and constructions that can provide the support and grip at colder temperatures and lower pressures, without compromising performance. Formula 1 tyre supplier, Pirelli has tried to achieve this with a step-by-step approach. Pirelli originally targeted 2022 to ban tyre blankets alongside the new 18inch low profile tyres, however this has now been implemented in several stages.

The 2021 season saw the maximum pre heat temperature reduce to 100degC (212degF) for the fronts and 80degC (176degF) for the rears. This has now been further reduced to 70degC (158degF) for 2 hours, and the number of blanket sets for slick tyres reduced to 7 per car. This approach is giving Pirelli time to develop tyres that can cope with starting from cold. All teams will vote on the proposed ban of tyre blankets by the 31^st July, following the two day test after the British Grand Prix.

Does preheating tyres result in better racing?

Not all high-profile championships preheat tyres, and yet still deliver competitive and engaging racing. For example, the likes of IndyCar and Formula 2 have successfully banned the use of tyre blankets. In fact, the lack of tyre blankets in IndyCar actually generates more excitement around the pitstop windows.  The  offset of cold, new tyres against hot, heavily worn tyres constantly changes the effectiveness of the undercut or overcut and therefore the pitstop strategy.

The British Touring Car Championship (BTCC) is another good example of a race series working well without any form of tyre heating. Unlike many other championships, BTCC allow Front Wheel Drive (FWD) and Rear Wheel Drive (RWD) cars to compete side by side. The absence of preheating tyres typically favours FWD cars in the early stages of a race because the front tyres are bought up to temperature much faster than a RWD car. However, RWD cars tend to have a more even spread in tyre wear hen compared to a FWD car and therefore has more grip towards the end of the race.

The future of preheating tyres

There is an argument to say that as a professional racer, driving to the limit of adhesion offered by the tyre regardless of circuit grip level, tyre life or in this case tyre temperature should be par for the course. This coupled with the fact that several high-profile professional championships already operate without any form of tyre preheating, would suggest that WEC and Formula 1 could successfully follow the same path.

What is clear however, is that if tyre blankets and ovens are banned, tyre suppliers and teams need time to adjust to this new way of racing. Getting this right is not only vital for the safety of competitors, but is also imperative to the quality of the racing. It is also publicly important that the environmental reasons for banning preheating is not cancelled out by the carbon footprint of repairing accident damage due to cars crashing on cold tyres. Perhaps the more graduated approach applied by Formula 1 and Pirelli could have been utilised by WEC to avoid this sticky situation surrounding tyre warmers during the build up to Le Mans.

What parameters can we change and think about how these parameters influence the performance of a vehicle in a balance of performance (BoP) regime? The physics discussion helps us link parameter changes to the primary modes of operation of a vehicle around a circuit, traveling in a straight line and cornering. 

Different racing series have different options available to adjust performance of vehicles, but the general BoP variables typically include mass, total power output, minimum ride heights, aerodynamic elements, and fuel capacity, and to some extent tyres. 

Mass 

As implied by Newton’s Second Law, the mass of a vehicle directly, and inversely, impacts the ability of a vehicle to take advantage of the propulsive forces to accelerate. Whether we are talking about longitudinal or lateral accelerations, any increase in mass will reduce the acceleration capacity in those directions, while any reduction in mass will improve the acceleration capacity of a vehicle. 

Because of the direct impact of mass on longitudinal and lateral accelerations, we can increase a vehicle’s mass to slow it down or reduce a vehicle’s mass to speed it up. A good rule of thumb is that a 10 kg increase or decrease in mass will result in a 0.15% increase or decrease in lap time, respectively. So, on a 100 second lap, 10 kg will have a 0.15 second impact. 

Before we leave the topic of mass, think about how mass then influences the lap time of a vehicle as fuel is consumed. A vehicle with a 100 L fuel tank will be carrying approximately 72 kg of fuel at the beginning of a stint. By the end of the stint, and assuming no tyre degradation, this vehicle should be approximately 1.08% faster (1.08 seconds on a 100 second lap). 

Total Power Output 

As we discussed already, a vehicle’s power unit is responsible for generating the longitudinal propulsive force for the vehicle. This force, when all the resistive forces are overcome, is what drives the vehicle forwards though space, and defines how quickly the vehicle can accelerate longitudinally. A higher capacity for longitudinal acceleration leads to a reduction in lap time, while less acceleration capacity yields a slower lap time. 

There are many configurations of power units encountered in racing, normally aspirated internal combustion engines, turbo/super-charged internal combustion engines, hybrid engines, and fully electric motors. I am going to focus here on normally aspirated and turbo-charged engines. 

With normally aspirated engines, the total power output is primarily controlled by inlet air restrictors with a specified minimum diameter. The minimum diameter controls how much air flows into the engine, which in turn determines how much air is available to mix with fuel for combustion. Increasing the minimum diameter of a restrictor increases the volume of air that flows into each combustion chamber, which means a higher volume of fuel can be mixed with the air, and a bigger explosion can be created.

So, a larger restrictor diameter (more air) equals more power, while a smaller restrictor diameter (less air) equals less power. Engine restrictors come in two varieties, sonic and non-sonic. Sonic restrictors have a continuously curved profile along the length of the restrictor – much like the outlet of a trumpet – where the minimum diameter is found somewhere along the curved profile. Non-sonic restrictors typically have a conical inlet and outlet with straight walls and a flat cylindrical central section where the minimum diameter is found.

A small radius is applied where the straight walls meet the flat cylinder, and the length of the cylinder is prescribed by the sanctioning body. Non-sonic restrictors will influence the output power over the entire RPM range, while sonic restrictors only reduce power once the air flowing through the restrictor starts to choke at higher engine RPMs. The power output for turbo-charged engines is typically controlled by a boost limit, or a boost limit curve where increasing boost pressure results in a power increase and reducing boost pressure reduces output power.

A boost limit applies a single maximum boost level across the entire engine RPM band, while a boost limit curve assigns a maximum allowable boost as a function of engine RPM. A boost limit acts in a similar manner to a non-sonic restrictor in that the limit has an impact across the entire RPM range. A boost limit curve allows a sanctioning body to shape the power output across the RPM range. With boost limit curves it is possible to add or subtract power where it is needed, which is highly desirable from a BoP perspective. 

In my personal experience, I have been able to successfully align the power outputs of normally aspirated and turbo-powered cars by first ensuring the power outputs of the normally aspirated cars are matched using inlet air restrictors, and then fine-tuning the output power of the turbo-charged cars by tuning the boost limit curves for those cars. 

Engine power output is influenced by several other factors that may be used to balance vehicle performance. For example, sanctioning bodies may specify ignition angles to increase or reduce spark advance and impact the engine’s power output. Likewise, an air/fuel ratio (lambda) may be specified to control how much fuel can be delivered to the engine to add or reduce power. In cases where the engine ECU is locked or cannot be reprogrammed, it is possible to increase or reduce maximum RPM limits to control power output. If this cannot be programmed into the ECU, this would involve a team setting the shift lights higher or lower and the sanctioning body scrutinizing the shift RPMs through further data analysis following a session or event. 

For a 500 HP vehicle, a good rule of thumb is that a 10 HP change in power output will result in a 0.31% change in lap time, i.e. increasing power by 10 HP will result in a 0.31 second reduction in lap time on a 100 second lap. Of course, this factor is highly dependent on the circuit layout, as there are circuits that are much more sensitive to power than others. 

Minimum Ride Heights 

We say “minimum” ride height because a sanctioning body will typically want to try and restrict a car from going any lower than the minimum prescribed ride height. These ride heights are typically static ride heights, so there is nothing stopping the vehicle from going lower dynamically while on track. Unfortunately, minimum ride height regulations can have unintended consequences on vehicle setups. Teams may start to introduce elaborate bump rubber, spring and damper settings as a way to pass the minimum ride height rules during technical inspection, but to still achieve a desired dynamic ride height while on track. 

Ride heights have several impacts on vehicle performance. For all vehicles, increasing or decreasing the minimum ride height will impact the center of gravity height of the vehicle dynamically. An increase in CG height causes increases in lateral and longitudinal load transfer when accelerating laterally and longitudinally. Increased load transfer tends to degrade vehicle performance because of the influence it has on the vertical tyre loads when accelerating. For example, a higher CG in cornering causes a significant reduction in the vertical load acting on the inside tyres that acts to reduce the total lateral force the tyres can generate across the axle. As we have already seen, a reduction in lateral force on the tyres reduces the lateral acceleration capacity, which results in a slower cornering speed. 

For aerodynamic cars, changes in ride height influence both the total downforce and the total drag. In most cases, increasing ride height causes a reduction in available downforce. This reduction in downforce then has an impact on the vertical loads on tyres acting to reduce the lateral or longitudinal force the tyres can generate. The opposite is true for reducing ride heights. So, increasing minimum ride heights can have the effect of increasing lap times due to reduced aerodynamic forces. The combined CG and aerodynamic effects of minimum ride heights make it very difficult to have any sort of rule of thumb for these changes. 

Aerodynamic Elements 

Aerodynamic devices are often used to control the downforce or drag of a vehicle. Downforce has an impact mostly on the cornering and combined acceleration components of a circuit, while drag mostly impacts the straight-line speed of a vehicle. 

While we’ve already addressed the influence of ride heights, the aerodynamic properties of a vehicle may be changed with wing angles, wickers or gurneys, dive planes, splitters and the myriad of other potential aerodynamic elements that may be attached to or removed from the vehicle. There is usually no free lunch with aerodynamic devices, so you cannot add more downforce without also increasing drag or reduce drag without also reducing downforce. So, this needs to be taken into consideration when modifying the aerodynamic characteristics of a vehicle. 

For properties such as wing angles, a sanctioning body may prescribe a range in permissible angles or define a minimum allowable wing angle. In general, increasing a wing angle acts to increase the downforce on a vehicle while also increasing the drag. Whether or not this change makes the car faster or slower depends on the sensitivity of the circuit to changes in downforce and drag. As there are circuits that favour higher engine power, there are circuits that favour higher downforce at the expense of increased drag. 

Another simple element that can be changed to influence downforce and drag is a wing wicker or gurney. In most cases an increase in gurney height increases drag while increasing downforce. I have used gurney height as a tool to manage a vehicle’s top speed on several occasions. The impact of various aerodynamic elements on lap time is highly specific to each device, so it is also exceptionally difficult to have a general idea that may be applied to most situations. 

Fuel Capacity 

Fuel capacity does not fit very well with the discussions on Newton’s Second Law, but it does have a significant impact on the outcome of races. Fuel capacity defines how far a vehicle can go between pit stops. In many cases – especially where tyre warmers are not allowed – there are significant gains to be made by going one or two laps further on fuel stint. Likewise, in series where full course yellows can interrupt green flag running there is a definite advantage to being the first car to pit last. As such, teams, and manufacturers demand equality when it comes to how far they can travel on a full tank of fuel. Of course, the driver and fuel maps still come into play to ultimately determine how far one can go, but it is important that everyone is on a level playing field to begin with. 

Tyres 

Tyre dimensions and specifications are not something that change often in BoP Tables, but these changes may still occur. For example, I have experienced times when a new tyre for a car simply does not work with the vehicle, and a reversion to an older specification was required. In addition, I have seen changes to tire specifications where the tire dimensions are increased or reduced to influence the cornering capacity of a vehicle. Again, these changes are rare, but they do occur.

At the very highest level, a vehicle’s performance around any circuit applies Newton’s Second Law, F = ma, coupled with some equations of motion. The description that follows below is basically how vehicle dynamics simulations work. 

Starting with the equations of motion, the motion of a vehicle around a circuit is dynamic, where the vehicle travels through three-dimensional space over time. If we break this motion through space into smaller and smaller time intervals, we can start to think about the vehicle’s state for each time interval as having a set of initial and final conditions. 

When the time intervals are reasonably small enough, it is possible to approximate the change in the vehicle state from the initial to final conditions as a constant acceleration problem. Using this approximation, we can apply the SUVAT equations of motion from physics to get from the initial vehicle state to the final vehicle state. SUVAT is an acronym where s = displacement, u = initial velocity, v = final velocity, a = acceleration, and t = time.

For now, we will assume we already know the vehicle’s acceleration, so if we also know the initial velocity and time step, we can apply the second SUVAT equation to calculate the vehicle’s displacement during the time step, i.e. s = ut + ½ at2. In addition, we can apply the first SUVAT equation, v = u + at, to calculate the final velocity of the vehicle at the end of the time step. For the following time step, the initial velocity is the final velocity from the previous time step, and we can proceed to evaluate each time step sequentially. However, we cannot do this until we know the vehicle’s acceleration for each of these time steps! 

We now need to consider Newton’s Second Law. From the above application of the equations of motion, we can see that the velocity of a vehicle at any point around a circuit is governed by the vehicle’s ability to accelerate; therefore, it is best to think of Newton’s Second Law expressed in terms of acceleration, i.e. a = F/m. We must consider this equation as the acceleration equaling the sum of all forces (total force) divided by the mass. These total forces include those available for propulsion and those forces resisting propulsion.

For example, a block sitting on an incline will have two forces acting on it: a gravitational force and a frictional force. The component of the gravitational force that is parallel to the incline’s surface will pull the block down the incline. Still, the friction force between the block and the incline’s surface will resist this gravitational force component. If the gravitational force component is smaller than the friction force, the block will not move. Only once the gravitational force component is greater than the friction force will the block begin to accelerate down the ramp. So, acceleration cannot happen until the total force – the sum of propulsive forces minus the sum of resistive forces – is great enough. Hang on to this concept as we start applying it to a vehicle. 

The idea of breaking forces into directional components applies to the motion of vehicles. We cannot work with the SUVAT equations or Newton’s Second Law until we break the forces acting on a vehicle into two components. We need to break the equation a = F/m into its respective longitudinal and lateral components, yielding two equations: longitudinal acceleration ax = Fx/m, and lateral acceleration ay = Fy/m. Considering the concept of total forces, the longitudinal acceleration equals the total vector sum of all longitudinal forces divided by the vehicle mass. The lateral acceleration equals the total vector sum of all lateral forces divided by the vehicle mass. 

Longitudinal Acceleration 

The longitudinal acceleration, ax, is the acceleration that we feed into the SUVAT equations above and is the only acceleration we need to consider when looking at a straight-line acceleration problem. We know that longitudinal acceleration is the sum of longitudinal forces divided by the vehicle mass. The longitudinal propulsive force for a vehicle comes from the vehicle’s power unit. A typical internal combustion engine’s output torque is fed through a drivetrain (clutch, drive shaft, gearbox, differential, axles, hubs, wheels) to the vehicle’s tyres.

This torque acting through the tyres results in a force parallel to the road that attempts to drive the vehicle forwards. This is not the only force we need to consider, though. Just as there was a frictional force resisting the motion of a block on an incline, several forces resisted the propulsive force from the engine and tyres. These resistive forces include frictional losses from the drivetrain, rolling resistance from the interaction of the tyres with the road, aerodynamic drag, and any applied braking forces.  

Lateral Acceleration 

The lateral acceleration component, ay, does not directly impact the SUVAT equations. Still, it does indirectly impact them in that the longitudinal acceleration of a vehicle is limited by the total possible combined acceleration, i.e. the vector sum of the lateral and longitudinal acceleration. Before considering combined acceleration or forces, think about a pure cornering situation around a constant radius corner. In this scenario, the vehicle corners at a constant velocity related to the lateral acceleration through the equation ay = v2 / R, where v is the constant velocity around the corner, and R is the corner’s radius.

We are still dealing with lateral acceleration resulting from the total lateral force divided by the vehicle mass, and we still have propulsive and resistive forces in the lateral direction. The propulsive force, or the force that is driving or pushing the vehicle towards the instantaneous centre of curvature, comes from the ability of the vehicle’s tyres to generate a lateral force between the tyre and the road. This frictional force increases as the vertical load on the tyres increases. The resistive force comes from the inertia of the vehicle. This inertial force wants to push the vehicle back to travelling straight, pushing it away from the instantaneous centre of curvature.

Like all bodies in motion, the vehicle does not want to turn because it wants to keep travelling along happily in a straight line. When the lateral force from the tyres equals the lateral force from inertia, the vehicle is balanced and can travel around the curved path. If the inertial force exceeds the available tyre force, the vehicle leaves the curved path, which often ends spectacularly poorly for the vehicle’s occupants. When the inertial force is less than the lateral force potential of the tyres, the vehicle can speed up and travel around the corner faster or take a smaller radius line around the corner. 

Combined Acceleration 

On the topic of combined forces, where you have both lateral and longitudinal vehicle accelerations or lateral and longitudinal tyre forces, we are talking about the ability of a tyre to generate combined force. A tyre is just a big elastic, and an elastic generates force when it is stretched. Unfortunately, an elastic can only stretch so far before it fails. Longitudinal forces stretch the tire parallel to the direction of travel, while lateral forces stretch the tire perpendicular to the direction of travel.

The total stretch, or total force the tyre can generate, is the vector sum of the lateral and longitudinal components. This concept is demonstrated through a tyre’s friction ellipse, where the outer limits of the ellipse define how much combined stretch/force the tyre can handle. When the combined force exceeds this boundary, the tyre either loses grip by snapping back to a less strenuous amount of stretch or fails where the rubber in the contact patch falls apart. The point here is that a tyre can only generate a fraction of the maximum possible longitudinal force for a given amount of lateral force. 

Returning to the SUVAT equations, we can now see how lateral force and acceleration impact the available longitudinal force a tyre can generate. This limits the longitudinal acceleration available to calculate each time step’s distance travelled and final velocity.  

Summary 

So why have we spent this much space discussing the physics of vehicle performance? How is this related to the balance of performance? Well, the balance of performance is simply a physics problem. When attempting to balance vehicles, we are manipulating a vehicle’s ability to generate longitudinal and lateral forces, which determines how the vehicle accelerates longitudinally and laterally. I want to emphasize this point because if we think of BoP as a physics problem, we can begin to have a much better understanding of how changes to vehicle parameters will influence the overall performance of a vehicle. And the better we understand physics, the better we will be at making changes! 

One of the biggest changes to the World Endurance Championship (WEC) regulations for 2023 is the loss of tyre warmers in all classes in the WEC. The governing bodies, Automobile Club de l’Ouest and the FIA, announced that tyre warmers would be scrapped from both the WEC and European Le Mans Series this year in a move “designed to reduce the teams’ environmental impact”.

Although this was common in other racing series, such as IMSA’s WeatherTech Sportscar Championship, it’s new to the world championship and there has been resistance from some teams, saying it does not help the environment if a car crashes on cold tyres. In response, the regulators point to the fact that qualifying has been increased from 10 to 15 minutes to help generate tyre temperature, increasing the time on track.

However, tyre supplier, Michelin, offers the same product for both series, despite the fact the WEC tracks are traditionally more abrasive than those in the US. It says tyre warming should not be an issue.

‘The regulation is the same for everyone,’ says Vasselon of the new rules. ‘Even if it sets a challenge to all the teams, we all know it’s a necessary challenge. There is no discussion on that. Probably the reason why the decision came so late was that we all had to make sure our tyre supplier was ready on time.

‘We also have to consider that it’s more of a challenge in winter, especially for us when we are only testing in Europe for cost reasons. Europe is on the cold side in winter, and this makes it more difficult. We will pay attention to managing the cold tyres, but it will be less of a challenge to manage the cold tyres with 35 degrees on the ground at Sebring than at Paul Ricard with two degrees at night.’

The tyres are also different compared to last year, and in the early 2023 races, Porsche and Cadillac stole a march on the LMH manufacturers, having received them in December for a test at Daytona and then raced them in the Daytona 24 hours in January.

That extra mileage in race conditions may help them on the low-grip circuit at Sebring, but the LMH manufacturers have tested on European tracks, unlike Cadillac, which so far has only tested in the US.

‘The tyres definitely require some special handling,’ concludes Vasselon. ‘From the car set-up side, as well as the driver side, but I’m not going to elaborate on it.’