Rim heating is common practice in F1, but how does it actually work? We spoke to the Technical Directors up and down the pitlane to find out.
The Problem
Tyres are without doubt the most important element of any racecar. You can have all the power you want, all the downforce you want, but if none of that can be effectively transformed into grip through the tyres, then you won’t go anywhere fast. This is why tyres are a never-ending discussion point up and down the pitlane.
A tyre generates grip through excitation of the rubber due to friction with the tracks surface. There are two friction mechanisms involved:
- Molecular adhesion – The molecular chain of the rubber is stretched and due to its viscosity, this resists deformation which results in a friction force.
- Indentation – Rubber is excited by the tracks roughness and due to hysteresis does not immediately return to its original shape. This asymmetrical deformation generates a friction force.
These friction forces combine to generate grip and this grip can be increased with temperature. Think of a tyre as a piece of blue-tack. When blue-tack is cold, this visco-elastic material is stiff, brittle and won’t stick to anything. Yet, when you play around with it, the stresses induced by pulling blue-tack apart and squishing it together results in a temperature increase. It therefore becomes warmer, more malleable and starts sticking to things.
Tyres are just the same; the higher the temperature of the rubber, the stickier the rubber becomes and therefore the more grip the tyre generates. However, if the tyre gets too hot then it can degrade. The heat can either change the chemical compound of the rubber, leading to thermal degradation or the heat can physically destroy the rubber, leading to graining and blistering. Both result in less grip and therefore slower lap times.
To achieve maximum grip, the tyres need to operate within their working range, and in F1 the Pirelli tyre has a very narrow working range. This means that the tyres require a lot of conditioning and management and can also behave unpredictably.
‘The tyres have a very narrow working window and it is a real challenge to keep them in that window from the start of the stint to the end of a stint,’ highlights Andrew Green, Technical Director at Racing Point.
‘It’s easy to go over the working window at the beginning of the stint and it’s easy to fall below it by the end. Trying to keep the tyres in that middle region and all at the same time is what we aim for. It’s a big challenge and we don’t get it right all the time, infact we hardly get it right at all to be honest.’
This is why drivers often complete slow outlaps and/or a preparation lap before their qualifying attempt to try and bring all four tyres up to temperature consistently without damaging the tyre’s surface and initiating graining. That is the next problem faced by engineers; trying to get all the tyres within the working range at the same time.
‘As you’re not driving through the fronts and they see a lot of airflow, the front tyres only really generate temperature during cornering. So often the problem at the front is that they are not up to temperature as quickly as the rears,’ explains James Key, new Technical Director at McLaren. ‘Given that all your traction is going through the rears, it’s the opposite problem and you try to protect the rears from additional temperature.’
This difference in tyre temperature, and therefore grip, between the axles leads to an imbalance which the driver then has to manage out on track. As a rough rule of thumb, if the front tyres are too cold, the car will likely understeer. Whereas, if the rears are too hot, the car will generally oversteer. So every effort is put into managing not just the tyre temperatures themselves, but the temperature balance between the axles.
The solution: F1 rim heating
There are a whole host of tyre temperature tuning tricks that teams use to manipulate the behaviour of the tyre. These include utilising brake bias, tyre pressures and tyre blankets. Another major tool is F1 rim heating. This is where teams exploit the thermal energy generated from the brakes to transfer heat to the rim which then radiates through to the tyres, increasing the temperature of the rubber. This F1 rim heating effect is not revolutionary and has been used by teams for many years, across many categories. However, with the 2019 regulations reducing the window of opportunity for aerodynamicists and the tyres continuing to be so temperature sensitive, this has been a key area of development.
At each corner of the car lies an intricately packaged wheel assembly, which includes the braking system, cooling ducts and the wheel rim. The brake discs and pads can reach temperatures of up to 1,200degC while the calipers have a maximum operating temperature of roughly 230degC. Like tyres, brakes have a working window and if either the calipers, brake fluid, pads or discs overheat, brake performance can fade, effecting driver confidence and therefore laptime.
In addition to clever design of each individual brake component, the most effective way to reduce the heat of the brakes is to feed the entire system with continuous, cool air. This is achieved by brake ducts which sit on the inboard side of the wheel on both the front and rear axles. ‘The entry scoop splits the air up into several different channels and each of those channels has a different function. The first function is to cool the brake disc, so there’s a portion of air that gets sent to the inside diameter of the brake disc which travels through the cooling vent holes and that’s purely to keep the brake disc temp below its maximum,’ explains Green. ‘The second channel of air goes to the caliper which has a much lower working temperature then the disc and that actually requires quite a bit of air to keep it cool. There is very little aero performance to be gained from these two methods of cooling.’
However, as is usually the case in F1, wherever there is an airflow, there is an opportunity for the engineers to extract aerodynamic performance. ‘The third portion of air is what we’re really interested in as far as performance is concerned,’ says Green. ‘We jet this air pass the caliper and the disc and then out through the spokes of the wheel as fast as we can. This air that is blown outboard tends to drag all the dirty air that is being created behind the wheel out with it and that’s the aerodynamic part of the brake duct.’
As well as the aero gains of channelling air through the rims, teams also utilise this air to heat the rims, and then the tyres. ‘If we are at a circuit where we need to heat the front tyres up then we’ll pass the air through the inside of the rim where we’ll put small turbulators to not only increase the surface area, but also mix the air to try and heat the rim,’ explains Green. ‘It is quite low velocity air that hasn’t got a lot of energy aerodynamically, but it has got a lot of heat because it has just passed through the brake disc, so it is several hundred degrees. The hotter rim then radiates that heat through to the carcass of the tyre which is another six inches away from it. So that’s what we’re trying to achieve with rim heating.’
This is why the wheel assemblies have now become so complex with the carbon shrouds featuring inlets and outlets, with some blocked or unblocked depending on the circuit. While the rims now incorporate turbulators, fins, holes and special surface finishes to try and increase surface area to either radiate or dissipate heat.
‘Obviously there is a heat transfer from the brakes and depending on where we are with rim, air and carcass temperature, we’ll decide how we want to distribute that heat transfer. That’s been the trick for a number of years now and everyone has been trying to make the best use of that,’ explains Jody Egginton, Technical Director at Toro Rosso. ‘Its no secret that people have been developing wheel rims to increase surface area by knurling and other features and its all part of the same challenge really; to improve the control of rim temp, air temp and carcass temp. We’re always looking to explore the best way to optimise brake temp and optimise carcass temp so any development we can do, whether it’s trim a brake drum or a trade off of bypass flow or a feature on a wheel we would take advantage of that within whats permitted. It’s a lot of work, but whilst there’s a performance benefit to be had, it’s our duty to exploit it.’
Andrew Green from Racing Point estimates that every 10degC increase in rim temperature sees approximately 1degC increase in tyre carcass temperature. ‘It also increases the pressure as well and increasing the pressure works the tyre harder, which generates heat in its own right so it works through two mechanisms,’ explains Green.
Front vs rear rim heating
However, rim heating is predominantly to help the front tyres as the rear tyres often struggle with overheating. ‘So we don’t even want to blow the hot air at the spokes because that will still heat the rim which will radiate onto the tyres,’ says Green. ‘We actually insulate the disc and caliper and after blowing the cool air through these components, we’ll exit that air inboard of the tyre and keep it away from the rim all together.’
‘Tyre management is very much a science in itself. On the fronts you typically try and transfer heat from the brakes into the wheel rim, it’s not the most efficient way of heating the tyres but it’s the only way you’ve got to try and maintain front temperature and get them closer to the rears,’ says Key. This is why on the inside diameter of the rims there are a variety of strange profiles and bulges. ‘These can work both ways. On the rears whilst you’re not trying to actively channel that cooling exit airflow from the brake disc onto the rims [as you would the fronts to heat the rims], that extra surface area you get from these various profiles act as a heat dissipator rather than a conductive device. But if you had very cold conditions, such as in winter testing you would reverse that.’
‘If we’re struggling to get a front tyre into the working range we would typically try to make use of brake temperature to transfer heat to the rim and then the tyre. Now that could involve running the rim at a higher temperature. There are certain instances where you maybe can’t get the rear tyres cool enough so then you close off the drum and run the brakes as cool as you can,’ says Egginton. ‘Typically, everything is tyre driven but you wouldn’t take a huge risk on brakes in order to achieve a tyre target. If we have ten conversations, eight of them are about tyres, while two are about brakes, because we understand the brakes quite well, whereas the tyres are variable week on week, circuit to circuit.’
One scenario where you do utilise rim heating at the rear is at the end of the stint. ‘So when you get towards the end of a tyres life, the rubber is getting thinner as it wears away so its cooling down and the pressure is dropping at the same time. To increase that pressure and that temperature you can ask the driver to move the brake bias forwards to get the brakes even hotter to generate tyre temperature at the front. We have a small amount of freedom to do that, but it is small relative to what we can do at the rear. It uses more fuel, but at the rear we can change the brake bias and engine braking to put more energy into the brakes and heat the whole thing up, which we can then direct onto the rim which heats the tyre up. We’ll tend to just do that towards the end of the race and if the temperatures have fallen.’
Of course, tyre temperatures not only vary between axles, but also between circuits and the nature of the track defines the amount of brake cooling required and how much of that is used to warm the tyres. ‘All of the teams have a level of adjustability, where we can go from one extreme to the other,’ says Ben Agathangelou, Head of Aerodynamics at Haas. ‘If we have a circuit with a high cooling demand, then we have to do less work on the flow because we have to allow the flow to work wholly on its prime task of cooling the brakes. When we are at a circuit that demands less braking energy then we can interfere with these flow streams and the heat exchange can be delayed, diverted and repurposed to serve the purpose of heating the rim. Every team has got their own interpretations of how and why they do that. It has as much to do with vehicle dynamic demands as aero cooling demands.’
However, F1 in 2021 will switch to a standard wheel rim supplier, with the draft regulations not only specifying the wheel rim dimensions but also the specific shape of the wheel rim barrel, hub and spokes. Therefore, teams will have less opportunity to exploit the profiles of the wheel rim for heating the tyres. But, unless a tyre is developed with a much wider working range in 2021, the importance of tyre temperature will remain, and so will the teams desire to control it.
