They're black, they're round, they roll along the ground. Even the most committed Formula One buff would have no shame in admitting the above as the extent of their knowledge of the sport's tyres. The ultimate black art, tyres don't just look similar but, to all intents and purposes are identical.

That suits the tyre companies just fine, of course - and goes to explain why we are allowed to know so little about them. Like most high-technology industries, the tyre manufacturers want to give nothing away to their opposition. And in this case they are helped - more than in any other aspect of Formula One car design - just by the very fact that they do all look the same.

But it is because tyres are such a crucial part of F1 design that they have a unique ability to raise the temperature of any debate about the sport. That is particularly evident during a tyre war situation, as last month's controversy has amply shown.

Tyre improvements are the quickest way to improve the performance of any car. Generally, the time gain is equal to or greater than all the other factors put together. Put simply, all the many man hours that hundreds of workers put in at a team like BAR, coupled with all the many more hours of work by the engineers at Honda, amount to the same improvement per season as whatever mysterious changes have been applied to the four black, round things the car is standing on. For, no matter how much power a car produces, it can only go as fast as its wheels can go round. That depends entirely on how much grip a car can get out of its tyres on the tarmac.

That is why motor racing's governing body, the FIA, was so keen to introduce limits on tyre performance when they last made a radical attempt to slow down Formula One cars and stop them reaching dangerous speeds. In 1998, the classic old huge wide rear tyres were outlawed and it was stipulated that all dry tyres should have grooves - in a bid to limit the contact area with the ground, and with it the traction.

As BAR race engineer Jock Clear explains: "It wouldn't matter if we had a set of road car tyres on, we'd all go off and do it like we effectively did in 1999, when we weren't in a tyre war situation. But when, like today, there are two manufacturers battling it out, the pace of development is very quick. Generally speaking, you might improve times by a tenth of a second a month by working on the car. It's about the same with tyres, or even slightly more."

The two main variables for F1 tyres are compound and construction. If you look up rubber in the Concise Oxford Dictionary, you will be told that it is defined as "a tough elastic polymeric substance made from the latex of plants or synthetically." It comes in all manner of varieties, then, and the compound refers to its chemical make-up. After over a century of development of its use for road vehicles, that science is highly advanced, and F1 is at the very forefront.

As for the construction, the rules state that modern Formula One tyres have to have four identical grooves - when they're brand new, at least - but that's about it. And if that is as far as the F1 rulebook goes, then, by deduction, you can bet that is also where the similarities between them end. The tyre companies have a certain amount of freedom to vary how they actually build the tyre from the inside out and so top-level design is all about what goes on underneath the skin.

But the only way to check how well or otherwise any particular tyre will work on a Formula One car is to go out and check it on a track. That is why F1 teams dedicate so much time during the season to tyre testing. But with a wide range of tyres available to try, how does a team go about doing it? Cue BAR's Craig Wilson, race engineer to Jenson Button.

"When you go through a programme looking at the fundamental aspects of the tyres, you'll try to concentrate on varying either construction or compound, but not both," says Wilson. "If you're doing construction work you'll keep the same compound so you can compare them, and vice versa.

"The way the tyre works is all about the way the construction affects the contact pressure distribution and what local stresses and temperatures the compound is seeing in the tread. You're looking for what works best in what combination. So you may have a compound that works best with one kind of construction against another compound that works with a different construction.

"It's very much a moving target. You have an idea which way you want to go with the tyres, but tyre companies are always developing new compounds and new constructions, so it's always a shifting area you're chasing."

It all makes for a giant workload both for the tyre companies and the teams who have to test out all the different combinations of compound and construction. And that load increased still further this year when the rules changed. Previously all the teams with a company had to use the same choice of two tyres at race weekends. That inevitably led to situations where the tyre company's leading team would have a greater say in which tyres they wanted to bring. Now every team is free to pick and choose their own rubber to race with every weekend. In the Monza test before the 2003 Italian Grand Prix, for example, BAR's tyre partner Bridgestone brought along a total of 2000 tyres in 20 different specifications.

"In previous years when the tyre companies had to bring same to basic tyres for all their teams, it was a lot easier for tyre companies to structure their approach," adds Wilson. "Now they're able to tune their tyres to specific cars it means a lot more work for the tyre companies. It also means more work for each individual team because you've got a wider range of tyres to try and sort through to find which will make your car the best.

"We still tested a whole range of tyres last year, but now we're doing it circuit by circuit. You know that certain tyres are going to behave in a certain manner within one circuit characteristic, and you know that those tyres will not perform that well in another kind of circuit characteristic. So, at each test we go to, half our programme can be just tyre testing. But the tyre is so fundamental to the car performance that you're more than willing to spend that time on them."

With the exception of those who opted for Friday morning testing this year, teams are still only allowed to take two different dry tyres to the races. That means that all the work for any circuit has to be done in advance. And normally teams end up basing their hopes for any weekend on one construction of tyre.

They then bring two different compounds - one of which will usually be harder than the other. The main difference is that the softer the tyre, the more grip it provides - at least in the short term. But harder tyres last longer. So teams make decisions on which to bring to cover themselves for the circuit and then pick which to race based on the likely temperature and weather conditions.

Wilson says: "The most critical thing about the way the car handles is how the tyre is interacting with the road surface. The way the tyre adheres to the track surface dominates the entire grip performance. So the temperature and the conditions affect how the tyre compound works. If it's very cold then the softer tyre may give a better race performance, for example, whereas if it's very hot, hard tyres are better."

What open comparisons there are between tyres from different companies are largely based on observation. X's tyres may have a reputation of working better in hot weather, Y's may be better in damp conditions. Until we see cars of roughly equal performance trying the same tyres in the same conditions - usually only in qualifying or the race, no one outside the sport's inner circle can really tell what the comparative strengths are.

And it's all made that bit harder because tyres do not keep the same performance levels throughout even the shortest stints. Indeed, tyre performance can vary in the course of a single lap. That is why cars are often seen struggling at different times during a race, and why teams have in the past spent most of the weekend's practice sessions 'scrubbing' tyres - to work them through their poorer phases before they race them. That has been harder this year with limited practice time, and the requirement to start the race on the same tyres used in qualifying. But what is it that causes a tyre to 'drop off'?

"It's mainly down to compound degradation whereby the compound gets hotter the longer you run it, and that changes the chemical nature of the tyre," says Wilson. "But it's also due to the stresses of the tyre being worked, because it runs at slip angle (lateral motion over the ground) and slip ratio (longitudinal motion) around the track. That causes all the polymer cross-links in the compound on the tread to change their relationships, so the compound itself is changing in its fundamental characteristic."

The other main area that teams like BAR can affect in the course of a race weekend is tyre pressure. Sometimes it's hard to remember that F1 tyres are actually full of air - until you see a tyre puncture in spectacular style. But the amount of air inside a F1 is another crucial factor when you're striving to find the perfect balance to bring a car to its very finest performance.

"You vary tyre pressures depending on the kind of circuit you've got," adds Wilson. "The general rule is that you'll run lower tyre pressures at a low speed circuit like Monaco compared to places like Silverstone and Barcelona, which have a lot of high speed corners. And then you have some circuits also which are very high speed in a straight line. Then tyre pressure and temperature become critical for a tyre. Finally, if you've got a set-up issue to solve, then sometimes you'll change the tyre pressures to affect that balance as well."



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