Bernie Ecclestone has to decide whether he wants Pirelli or Michelin (or perhaps both) to produce tyres for Formula 1 from 2017 onwards, but he has a few factors to consider. Michelin will only supply tyres if they are able to manufacture 18 inch wheels, rather than the current standard of 13 inches. Whilst Pirelli have tested the former wheel concept a few times over the past two years, they do not seem overly fussed as to which direction the sport takes and, to top it all off, the teams would rather stick with the current design.

So why are 18 inch wheels becoming an increasingly popular size in motorsport? Formula E tyres (which are supplied by Michelin) are wrapped around the larger sized alloy rim, but what difference does this make to the car’s performance? This blog post aims to answer these exact questions.

Current F1 regulations

For dry tyres, the overall diameter of both the wheel and tyre must not exceed 660mm, which is just about 26 inches – the same as you’d find on a normal bicycle. This diameter increases to 670mm for wet tyres due to the tread block height.

The width of the front wheel can lie anywhere between 305mm and 355mm, and 365-380mm on the rear.

What’s different about the two wheel designs?

Currently the tyre surrounds a thirteen inch diameter rim, so there is lots of space to fill the tyre with air in comparison to an eighteen inch diameter rim, as you can see in this comparison view.

Highlighted is the available tyre area the two wheel designs offer. This is important when considering the volume of air inside the respective tyres, which has a significant impact on how the suspension works. In fact, most of an F1 car’s suspension travel is within the tyres flexing, so reducing the available volume increases the need for larger suspension components which take larger loads.

Let’s work out these volumes to compare the difference, using dry tyres. We’ll work in cubic metres. We will also use the maximum permitted wheel width, as there is a range of 50mm to work with on the front wheels, and a 15mm range on the rear wheels.

Whilst the tyres have a nicely rounded shoulder shape in real life, for simplicity purposes we will imagine that they are ‘caveman-like’ wheels with two simple faces – the sidewall and the round contact patch.

2015 wheel

Max. diameter – 660mm

Rim diameter – 330.2mm

Max. width – 355mm (front wheel)

Area of sidewall = area of complete wheel – area of rim face = 0.2564 m2



Inner tyre volume = area of sidewall x max. width = 0.091m3

2017(?) wheel

Max. diameter – 660mm

Rim diameter – 457.2mm

Max. width – 355mm (front wheel)

Area of sidewall = area of complete wheel – area of rim face = 0.1779 m2



Inner tyre volume = area of sidewall x max. width = 0.063m3

Performance implications

So an 18″ rim provides roughly two thirds of the tyre volume that a 13″ rim would. What does that mean in terms of performance?

Although the volume reduction is not directly linked to the suspension kinematics, a large amount of the car’s overall suspension travel would be lost. If the current suspension layout was kept, the ride would be a lot harsher and the car’s ability to ride kerbs would be affected substantially.

In slow-motion camera shots we often see the tyres oscillating on the rim as the car rides the kerbs. Due to the less available volume, tyre pressures would have to be increased to cope with the loads and so we would see less of the dramatic wobbling effect around the rim and instead a more stable tyre. Running lower pressures (to a certain extent) improves traction and grip, so the higher tyre pressures would cause performance losses on street circuits primarily.

Having considered the flexing of the tyre, larger diameter wheels therefore have a huge impact on the aerodynamics of the car – limiting the amount of flex in the tyre affects the high speed cornering ability of the car. There is less control over the ride height of the car due to the higher pressures, creating a harsher platform. Riding the bumps would cause the car to pitch more severely, stalling critical aero surfaces and losing downforce.

Teams have spent years developing tyre simulation models that show how the loading of the tyre has an effect on the airflow surrounding it, which is why we see such complicated slots and flickups along the floor edges and around the brake ducts. The thinner sidewall characteristics would be a complete game changer when optimising airflow through the diffuser and as it passes over the sidepods, which has an impact on front wing design and the nose.

Totally redesigning these components and simulation software would cost the teams a fortune. Even the top teams could be caught out by the changes.

On the flip side of the coin, there is much more room within the rim to increase the size of the hubs and therefore the size of the brakes. Bigger brakes means greater stopping power, so F1 cars would be able to brake even later, decelerating faster than ever before.

Enlarging the brakes removes the issue that some engineers have mentioned about tyre warm-up. The larger brake/hub size would bring the component closer to the inside of the wheel face once more, making for good tyre temperature management like we see today.

The larger drums would also be finetuned to suit the new wheels, regaining some of the aero efficiency losses that were evident during the Lotus test last year at Silverstone.

Of course, any performance loss created by the smaller sidewall could be clawed back by altering the compound of the tyre itself (a softer tyre). This is all well and good for qualifying, but how would they fair during the race when tyre degradation kicks in? Would the tyre life be significantly reduced, resulting in numerous pitstops? It promises to mix up the racing but not necessarily in a good way.

Conclusions

Introducing the 18 inch wheel into F1 could well be a huge success. From my point of view, the tech changes would be revolutionary and the design solutions the teams come up with – certainly for the first few years – would be intriguing to track.

However the cost of such a change could well force out some of the smaller teams. Unfortunately F1 is stuck in a bit of a rut as stability is needed to keep the sport affordable for the likes of Marussia and Sauber, but the fans (on majority) want changes to the racing. It is certainly a tough time to be a rule maker…