In the final year of the ground-effect regulations, McLaren has emerged as the team to beat over long runs. The MCL39 uses a perfectly legal technology, but one that has left many puzzled due to the performance shown on track by the Woking-based team. There’s a common question in the paddock: how can McLaren have such consistent tyre performance? The secret likely lies in PCM materials, which allow McLaren to excel in tyre management.
While some teams struggle with tyre overheating and degradation, McLaren continues to lap consistently within the same tenth of a second. Rival teams have raised many doubts — Red Bull, for instance, suspected that the MCL39’s brake ducts contained water. The Milton Keynes-based team even claims to have images showing “cold spots” on the rear axle’s covers.
McLaren brushed off this provocation with a cheeky on-camera response, as Zak Brown held up a bottle labelled “Tyre water.” Yet behind this subtle humour lies a clever and fully legal innovation that allows the MCL39 to consistently stay within the optimal tyre operating window.
What is PCM and why does it matter?
The key term behind the equation that makes the MCL39 so effective is Phase Change Materials (PCM). Supporting this highly plausible theory is Martin Butchen, a former F1 engineer, who wrote a thesis on this technology while at McLaren. These materials absorb or release heat during a physical state change — for example, from solid to liquid — while maintaining a stable temperature during the process. A practical example is ice: it absorbs heat but remains at 0°C until it has completely melted.
Applied to F1, this means absorbing brake heat without large thermal fluctuations affecting the wheel and tyre area. This is crucial given that braking systems can reach temperatures of up to 1000°C, with heat radiating across the entire wheel assembly.
The theory is that McLaren has integrated these materials into the rear axle, where heat generated under braking is absorbed by the PCM. These materials, exposed to high temperatures, begin to melt and in the process absorb significant amounts of thermal energy — without increasing in temperature themselves. The result? Stable and controlled temperatures at the key point of the tyre. As Mario Isola has highlighted, optimal performance comes when the tyre’s “core” hits its ideal working window — a thermal range that’s hard to reach, but one that McLaren’s MCL39 consistently achieves.
PCM materials: no illegal active system on the McLaren
Many theories have been proposed to explain McLaren’s remarkable advantage. One suggested the presence of an active system within the brake ducts, which feature a particularly complex design. These included heat-sensitive carbon partitions, pressure-operated membranes, and other devices. Any such system, if discovered by the FIA, would have created major problems for McLaren. However, the ingenuity of Rob Marshall and the engineers at Woking led to a perfectly legal solution.
PCMs are a completely passive system, working solely due to the choice of material. Each PCM has its own specific properties, and the key lies in selecting the right one for the desired task.
It’s a technology that offers significant advantages, at the cost of a slight increase in unsprung mass (wheel assembly). But this downside is more than offset by the perfect thermal control of the tyres. This would also explain why the two MCL39s are not as dominant in qualifying as they are in race conditions — where PCM performance reaches its peak.
Other teams have begun investigating the origins of this advantage, with Ferrari reportedly working on an initial solution for the SF-25.
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