Formula 1 has been using 13-inch wheels for over two decades while road cars and other racing categories have gone to bigger wheels (15 inches for Indycar, or around 18 inches in LMP1 for example). Why is F1 sticking to this particular wheel size, and should it change?
What is ‘Wheel Size’?
If you want to measure a tyre very roughly, there are three dimensions that matter : wheel size (or wheel diameter), tyre size (or diameter) and width. Width is not going to change in the foreseeable future (except if F1 chooses to become less aero-dependant and switches to wider tyres so that the cars rely more on mehanical grip). The tyre size is the external diameter of the tyre. This is actually not the same for all F1 tires : wet-weather tyres are slightly larger than dry-weather tyres (raises the car so that the floor of the car is less likely to touch the road and standing water).
Wheel size is – to make things simple – the external diameter of the rim. What Pirelli is going to try with it’s new 18-inch tyre concept, is to increase the wheel size while keeping the tyre size the same. This results in a tyre that looks *thinner*.
18-inch wheels would change a lot of things
Today, an F1 tyre+wheel assembly does a lot more than just “grip the road”. Brake cooling, aerodynamics, and suspension are all heavily influenced by the tyre and wheel. This also explains why the switch to larger wheels is not just a matter of changing a number in the regulations : the switch to bigger rims will be expensive.
Suspensions will have to be redesigned, almost starting from scratch, because the current 13-inch wheel assembly is very flexible and plays a major role in the suspension. With larger wheels, the tyre will play a less important role in this area. This will require a lot of work from the engineers, but it also gives them more control over how the suspension behaves : you cannot change the behavior of a tyre to make it stiffer or softer, but if it’s role gets less important, the teams will have more setup options for their cars.
Alastair Staley/Lotus F1 Team
Aerodynamics are also heavily influenced by the wheels. The airflow that goes through them can have a major effect on the back of the car. In 2009, many teams introduced static wheel shrouds that allowed them to better control the air flow going through the wheels. They were banned the next year (with some less extreme variations appearing in 2010), and the teams had to find other ways to control this airflow. This year (and in the years before), we see teams experimenting with blown axles (hollow wheel hubs) to achieve what they want. The larger 18-inch wheel will certainly change everything again, and the aerodynamicists will have even more work in this area.
Finally, brake cooling will also be impacted by a switch to larger wheels. With more space available around the brake discs (if their size stays the same and the FIA does not allow larger brakes), they will be easier to cool down. But today, brake cooling is not just about controlling the temperatures of discs and pads. Teams try to optimize their systems to transfer heat from the brakes to the wheels (and to the tyres) to keep the tires in their optimum temperature window. With more space between the brakes and the wheel, this will be much harder to achieve. Add to this that Pirelli and the FIA are considering banning tyre warmers, and getting the tyres up to the right temperature quickly and keeping them there could become a real problem for drivers and engineers.
What We Learned From The Silverstone Test
Not much. This test was – according to Pirelli – only a way to see what a real F1 car (as opposed to 3D renders they had already published) would look like with 18-inch wheels. The Lotus that was equipped with these tyres was well off the pace, but this doesn’t really mean anything as this is a car that was built to run the current 13-inch wheels.
This test doesn’t tell us if and when F1 will switch to larger wheels. But it did give us some good photos to look at, and we now have a much better idea about how F1 cars would look if F1 switches to larger wheels.
For Pirelli, a move to bigger wheels makes sense from a commercial point of view as road cars tend to go towards this as well, and the company has already announced that it could be ready to introduce the larger wheels as soon as 2016.