The Canadian Grand Prix is synonymous with a debris-strewn track, some of it from crashes and other incidents, but the majority of the litter being classed as "marbles." Marbles are what we call the small blobs of rubber thrown from the tyres; usually marking the middle of a race with only the racing line kept clean of the rubber.
Marbles are a self-defeating result of soft racing tyres. As it stands, teams want soft tyres for maximum grip, however the softer the tyre, the more marbles are created. Unfortunately, these bits of rubber create havoc with the driver’s ability to go off line and even cause problems with the cars themselves.
With a road car tyre, long life is a crucial factor; the tyre needs to last tens of thousands of miles of road use. To do this, harder rubber compounds are used, so the tyre wears out slowly. Despite the billions of miles driven on the roads every year and the amount of tyre tread worn down, it’s quite surprising there aren't actual piles of worn rubber piling up at the side of the road.
On the other hand, a race car tyre's grip comes not only from the large slick surface, as F1 tyres also have an adhesive-like grip. So soft is the rubber compound, that the tyre is quite literally sticky. When heated by lapping the track at racing speeds, the tyre surface is in a near molten state. Marbles are created as the car goes around corners, with the rubber squeezed towards the tyres' edge, resulting in small blobs of the rubber flying off.
To make racing more exciting and unpredictable, Formula1’s tyre supplier Pirelli is tasked by the FIA with making tyres with an artificially short life. So Pirelli has a family of four dry weather tyres, each with a different hardness. At each race, two tyre types are selected, and the teams have to run both in he race. For the Canadian race, just like in Monaco, the tyres selected are super soft; the softest and shortest-lived of the family. This makes the tyres particularly prone to creating marbles.
The first issue with marbles is a serious one: when they litter the track, they live up to their name, as driving over them is actually like driving on marbles. The small balls of rubber prevent the racing tyre from sticking to the road surface, and the driver can suddenly lose grip. Not only do they lose their grip while running over the marbles, but the marbles also stick to the sticky racing tyre, such that the driver then has to drive several corners trying to clean this old rubber off the tyres' surfaces.
In a race, the leading drivers will come to lap slower cars. The backmarker will be shown a blue flag and will often leave the racing line to allow the leaders through. So the already slow car will have to slow down even more; suffer the loss of grip over the marbles and then have to clean the tyres up. This compound effect makes life for the backmarker insufferable, as their race pace slows.
There is a time when this sticking effect is used to a driver’s advantage. When the race is finished, all the cars must come into the pits to be weighed as part of the post-race scrutineering process. With disqualification being the penalty for an underweight car, the drivers use a ploy where they will purposely try to pick up marbles on their tyres on the slow-down lap, to gain a little extra weight. It might only be a kilogram of marbles that the tyres pick up, but the margins of being overweight or underweight are very small. Saying that though, the scrutineers are wise to this trick and have the right to fit another set of worn tyres to the car, to obtain a more accurate post-race weight.
But this shift to softer tyres also means that as the marbles come flying off the tyre, they will stick to any surface they hit. Often this means they stick to the car they came off of. In F1, where perfection and spotless bodywork is paramount, these marbles end up as unsightly black splatters stuck to the pristine paint work. This in itself is not a great detriment to performance, but these marbles will also end up in more aerodynamically-sensitive areas. Cooling inlets for the radiators and brakes can easily be clogged by marbles, so teams fit mesh screens over them to prevent the ingest of rubber.
Last year at the Korean GP, we saw a perfect example of marble problems when Lewis Hamilton found he lost grip from the front of his car during the race. When the car was inspected after the race, it transpired that the front wing had become clogged with marbles. Front wings have thin slots between the aerofoils to improve the aerodynamics; these slots were blocked with rubber and prevented the wing working as well. Since then, you will now see a mechanic run a thin spatula-like tool inbetween the front wing slots at the pit stop, to ensure no marbles have built up. This is the level of detail teams go to in making sure their race performance is not compromised in any way.
One rare situation where marbles getting stuck to the car has proven to be a benefit was just this year, when a leading team found the car actually gained pace through the race. An insider explained to me that the hole in the back of the bodywork used to pass the external starter motor through became clogged with marbles. When this opening was blocked off with bits of errant rubber, the car gained downforce! This led the aerodynamics team to investigate the airflow in that area and come up with a solution.
So for this weekend's Canadian GP in Montreal, watch out for the telltale black rubber littering the track, and keep your eyes firmly peeled as you watch how the drivers try and avoid going offline. Also keep an eye out for the mechanics raking out the slots in the cars wings. If the weather remains dry, marbles could be a deciding factor in this weekend's race, and make or break a team's future.
Image Credit: Sauber F1