In the last couple of Sidepodcast Debrief shows, Mr C has made a point of asking why can’t engineers/regulators make cars that work in the rain when he can drive his car perfectly fine when storms are a-brewing? Whether or not Mr C was just being cheeky, this is a probably a good time to explore the mechanics of the difficulties in racing in the rain. There will be some maths and a little bit of physics, but I’ll keep it light and we’ll all get through this together, I promise! We need to learn a couple of simple concepts:

Coefficients of friction

A coefficient of friction, μ, is a very simple concept to understand. It’s the relative grippiness of two surfaces rubbing together. For example, if you try and push a brick along a pavement, it’ll be quite resistive; it’ll come to a stop very quickly if you give it a shove: the brick/pavement ha s a high μ. But if you push a brick made of ice over a smooth stainless steel surface, it will move very easily and glide for a long time if you shove it: this scenario has a low μ.

A coefficient of friction comes into play in this way: put a book on a table. Now, very gently start to try and push it along the table. At first, the book won't move at all, but as you start to push harder, suddenly the book gives in and starts sliding. This is 'overcoming the coefficient of friction' - up to that point, the friction is able to balance your pushing and keep the book still. The maximum amount of friction the book can cling on with is μ multiplied by the force of the weight of the book pushing into the table. The heavier the book, the more friction is can produce and the harder you'll need to push to get it moving.

As you are probably aware, the combination of soft racing rubber on tarmac is very grippy indeed – F1 tyres have a very high μ with the track. This is excellent for cornering, in particular*, as we'll come to in a second. But it's important to bear in mind that the tyres can only grip within the limits of the coefficient of friction - put too much force on them, and they will begin to slide. As you can imagine, a wet track is more slippery than a dry surface and this makes it easier for tyres to slide.

Max. Frictional Force = μ x weight of car on the road

Put simply: The amount a tyre is able to grip the track is dependant on the grippiness between rubber and the tarmac, and the weight of the car.

Centripetal Force

Centripetal Force is another very simple concept that probably sounds more complicated than it is. It's all to do with circular motion.

When things move in a circle, they need to be pulled in towards the centre of the (imaginary) circle to stop them flying out of the circular path. For example: when you swing a conker round and round on a string, the string is pulling the conker towards the centre of the circular whirling path. If you let go of the string, the conker flies off into the distance.

You'll see this kind of action anywhere you see a circular movement: the Moon going around the Earth (gravity pulls the Moon inwards); whizzy fairground rides (the seat/frame holds you inwards); and with driving. Whenever you (or a race car) driver around a corner, you need to pull yourself inwards, and you do this with your tyres. The tyres cling on to the road and stop you sliding off into Mrs Flannigan's hedgerow. If may not feel like they are gripping on when you drive at a moderate pace around the streets, but if you've ever driven on an icy road and found that turning that steering wheel seems to do absolutely nothing, you'll know what I mean.

Put simply: To turn a corner, a car needs enough grip to pull itself into the corner and stop it from flying/slipping away from the corner. Not only that, but the Centripetal Force squares with velocity:

Centripedal Force = (mass of car) x (velocity squared) / (radius of corner)

This means that if a car goes twice as fast, it needs four times as much grip to go around the corner, etc.

We all good so far? Okay, so:

Racing cars in the rain

Right, so for an F1 car to do its job, it needs to go fast around corners. On this much, I'm sure we all agree.

However:

1. the faster the car goes, the more grip it needs to make the corner
2. the car's grip is limited by the μ of the rubber and the tarmac
3. a wet track severely reduces the μ of the tyre against the track

This means that no matter how aerodynamically or mechanically 'grippy' you engineer the chassis, the car will always be limited by the rubber's ability to grip the road [Correction - see comments #28 and #37]. A road car will not push the limits of the tyre's rubber as much as a racing car will because it is less mechanically grip and is unable to force the tyres to work so hard.

Aquaplaning

Also known as hydroplaning, this is when a car rides along the surface water, instead of gripping the road surface. This can happen in both road cars and race cars, but is much more likely in a racing car due to their speed, low weight and their low ride height.

Don't forget that the most factor part of a car's grip is the ability of the tyre to hang on to the road. If water is forced between the rubber and the tarmac, then the tyre is no longer trying to grip the road, but the water surface (not very grippy at all!). Racing wet tyres help a lot, because they work to channel the standing water into the tread so some tarmac is able to touch the road. However, the very low floor of modern race cars means that the cars can act like boats in heavy standing water, and skim along the water, raising the entire car from the road and forcing the tyres to lose their contact with the road surface.

This is why you suddenly hear drivers screaming, 'undriveable!' into their radios as the standing water reaches a critical point at which at F1 can no longer operate at speed without skimming over the water and losing that precious tyre grip.

It is also important to note that F1 tyres are much fatter** than road car tyres and therefore aquaplane with much greater ease, especially due to their extremely lightweight bodies.

Interestingly, this is a problem that could potentially be solved: perhaps some kind of mandatory manual dynamic suspension that forces the car higher above the track if the race is declared 'wet'? Potentially, this is not too expensive a solution, though the problem of extreme weather is quite rare.

* Note: accelerating and braking are a slightly different and they are based on rolling frictions (within the tyres and bearings) and frictions in the brake components, so aren’t directly based on the relationship between the road and the rubber. Unless the wheels lock under braking, of course.

** Note: Interestingly, the fatness of the tyres doesn't have any direct effect on the coefficient of friction. Skinny tyres, or massive fat ones: you'd still need the same amount of force to stop the car sliding. But fat tyres work better because they won't mechanically break down as they can spread the effort over a larger surface area.