Tuesday, March 5, 2024

How do tyre blankets work?

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.

Close up shot of a white and red Toyota LMDH crashed into red barriers at the Spa 6hrs
Toyota’s Brendon Hartley crashed at the 6hrs of Spa earlier this season, with cold tyres the primary cause. CREDIT: Xinhua News Agency

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.

Graphics showing how tyre rubber moves over the road on a molecular level
There are two mechanisms of grip: molecular adhesion (left) and indentation (right). CREDIT: Michelin

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.

Close up shot of tyres in tyre blankets stacked on top of eachother connected to a thermostatic control box
A tyre blanket is fitted around each tyre and each set is then stacked and connected to a thermostatic control box. CREDIT: XPB Images

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.

A tyre oven in a garage with a stack of tyres in it
An example of a typical tyre oven or tent used in motorsport. CREDIT: Greaves 3D

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.

Screenshot of the Pirelli prescriptions which shows a bar chart and text explaining the heating time and temperature limits
The tyre blanket time and temperature limits are defined in the Pirelli prescriptions that are supplied to teams before each Formula 1 race

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.

Graphic showing the cross section of an under and over inflated tyre
Higher tyre pressures provide more structural integrity for the sidewall of the tyre, but reduce the contact patch area and therefore grip. CREDIT: Virtual Racing School

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 31st July, following the two day test after the British Grand Prix.

A Formula 1 intermediate tyre in a blanket that is half open
Tyres can be heated for a maximum of 2hrs prior to a session. CREDIT: Mercedes AMG Petronas F1 Team

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 IndyCar pitlane with two cars in the pits
The lack of blankets in IndyCar means there is more variation in pitstop strategy due to the difference in grip between old and new tyres. CREDIT: XPB Images

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.

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