Cars. What do they do? Three things fundamentally. They go. They stop. And they turn corners.

How? Well, you might say a car goes because of the engine, stops because of the brakes and turns because of the steering wheel. Of course, this is way too simple. If it were this simple we'd all be setting up our own Formula 1 teams and vying for the services of a Mr J Button for 2005. But we're not. And as well as being too simple, this explanation misses out something fundamental - that a car can only go, stop and turn because of the interaction caused by friction between the road and the tyres. In other words: grip.

Many people think of grip as being important only when cornering. As a recent Michelin TV advert said: "All that matters is that your tyres grip the road corner after corner after corner." True, it's important but it's not 'all that matters'. You need grip to accelerate under power and decelerate under braking. If the tyres were not gripping the road, speeding up or slowing down, the wheels would have no effect on the car's velocity.

Grip is what you're after when you're designing, setting up and driving a Formula 1 car. You want as much of it as possible all of the time. The more grip you have the faster you will accelerate, the harder you will be able to brake and the quicker you'll be able to go around corners.

Furthermore, every performance deficit in these areas results from a loss of grip. Wheelspin under traction? That'll be loss of grip. Wheels locking up under braking? Loss of grip. Under- / oversteer? Loss of front- / rear-end grip. Notice a pattern emerging?

The problem is that there are an awful lot of variables that can affect grip. Tyres are far and away the biggest factor, hence Michelin's ad campaign. Assuming, however, that tyres are kept constant there are still hundreds of things to take into account, especially in Formula 1 - aerodynamics, suspension set-up, ride height, the power being fed to the wheels... Add to these the fact that the requirements for a car to have maximum grip change with each phase of driving (braking, turn-in, power-out) - not to mention with track surface, track temperature and weather conditions - and maximising grip is suddenly a very complicated proposal.

Turning a corner

Cornering grip is the key to performance in Formula 1. It is also the most difficult thing to get right, since every corner requires a different combination of braking, turn-in and traction out, which entails a different pattern of weight transfer and therefore grip.

Getting a Formula 1 car to go around a corner is far more complex than you might think. First, as the driver turns in, the car will naturally want to carry straight on, or 'understeer'. Then, as the power comes on and the driver winds off the steering lock to exit the corner, the car will naturally want to keep turning, or 'oversteer', rather than straighten up. What solves this? Answer: the right amount of grip at the right time in the right places.

Much of an F1 race team's time on a grand prix weekend is occupied with setting up a car to 'dial out' these perennial handling problems and create a car that, as far as possible, is neutral in its handling. Jenson Button, Taku Sato and Anthony Davidson will come back into the Lucky Strike BAR Honda garage from each run and explain to their engineers where, when and why they are under and oversteering. There are myriad things the engineers can change on the car - ride height, wing settings, fuel level, engine mapping and gear ratios to name but a few.

But if they feel they've got the basic set-up right, there's only one place to look to further tune their cornering performance: the differential.

Differential Equations

Known more affectionately as the diff, the differential is a much underestimated bit of F1 kit even among aficionados of the sport. That's because it is exceedingly subtle in the way it works.

Put very simply, the diff splits the power produced by the engine between the two rear wheels. It controls how the torque is distributed between the two driveshafts at the rear of the car, so the outer wheel can be fed more power than the inner wheel while cornering. Why? Because the outer wheel naturally needs to turn faster to get around the corner, so it needs to be independent of the inner wheel. Also, the weight transfer through a corner means that the outer wheel becomes far more heavily loaded than the inner wheel, which means more power can be fed through the outer wheel without losing grip. Stick too much power through the unloaded inner wheel and it will slip, losing power that could be better directed through the outer wheel.

The job of the diff is crucial in finessing the precise handling of a Formula 1 car but it is often misunderstood. This isn't helped by the fact that the deepest insight in the specialist motorsport press often comes from the occasional 'so and so retired because the diff broke'. A new diff would never engender the same feverish excitement as, say, a new front wing or a radical nose design. Yet it is just as ingenious in its own inconspicuous way, nestled at the rear of the car, hidden beneath body work, getting none of the technical limelight of the world's most glamorous sport.

A bit like Tony Osgood.

Who? Exactly.

Diff lemon

Tony is BAR's head of transmission/hydraulic design. It is his job to head up the design of the gearbox, including the differential. Talk to Tony about the diff on the BAR 006 and you'll soon share his excitement about the technology involved and how crucial it is to the overall performance of the car.

"If you've got something rotating at up to 3000rpm that's over 200mm in diameter, high-up and far back in the car, then you know that that's going to have a significant influence on the car's performance and therefore design," he says. "It is intrinsically part of the gear box and it is a high inertia component, so we're always trying to make it as light and compact as possible."

Torque about a revolution

Modern diffs weigh about 8Kg and are hydraulically controlled. Until the mid-1990s, however, hydraulically controlled diffs were banned in Formula 1, so teams had to use cumbersome, heavy and infinitely less sophisticated mechanical units.

The main problem with a mechanical diff is that any change to its settings can only be achieved by changing mechanical components - which is both slow and limiting in the scope of what a diff can do.

"Mechanical diffs generally had serious compromises in how they operated," explains Tony. "You'd have a diff that was only suited to a particular type of circuit and often the ability to tune those was restricted by the mechanical system. It was a diff a week at one point and quite common for teams to have two or maybe even three diffs designed into the car per season, all at massive expense."

Now, to change a diff setting, the driver simply turns a dial on his steering wheel. This allows the diff to be tuned while the car is still out on track, saving vital track time during both testing and grand prix weekends.

The specifics of the hydraulic control actuated in the diff are now down to the imagination of the people writing the electronic control code. Each setting has a different diff map that actuates automatic control in a certain way in response to data gleaned from all over the car. Wheel speed, brake pressure, throttle position, engine torque and steering angle are just some of the parameters that will determine how the differential feeds torque to the rear wheels at any one moment.

This is why mechanical systems could never emulate a modern hydraulic unit. As Tony says: "There is no mechanical diff in the world that can take into account how much steering lock you've got on."

The requirements of the diff can change very suddenly, owing to circuit conditions, a new set of tyres - all sorts of things. So the driver has several options of diff maps already uploaded onto the car so he can actually tune the handling himself.

Diff-erent strokes

So how much difference does the diff actually make to a driver's lap time?

"That's difficult to answer really," says Tony. "I and experienced race engineers regard the differential as more of a fine-tuning tool. If you get the diff map wrong you can lose appreciable time on a lap, but changing a diff setting isn't going to suddenly find you a second."

"A lot of the time it's a driver confidence thing - maybe they'll talk about the car oversteering on corner exit so we'll introduce more diff-locking to dial that out so the driver will feel the car to be more neutral. He'll then have more confidence that the car isn't going to swap ends on him and he'll be able to accelerate more aggressively out of the corner."

Lock, stock and two smoking tyres

The diff is 'locked' when the two drive shafts are forced to turn at exactly the same speed. This gives the car maximum rear-end stability. Tony explains:

"On corner entry if the diff were completely free and open the driver would be totally dependent on his brakes to give him a balance of lock on the two tyres. If you lock the diff you have a better ability to match that energy across the axle. This stops the car from moving side to side under braking, so the driver feels on corner entry that the car is massively more stable.

"You then have to reduce the amount of diff locking to allow the car to turn-in - if you're forcing the two rear wheels to turn at the same speed essentially the car doesn't want to turn. So you have to balance off the stability that the locked diff gives you with the amount of corner entry understeer you induce. This is the first phase of the corner.

"The second phase is the exit, where you are back on the power. Your load path instead of from road through tyre to transmission is now going in the opposite direction, from engine to transmission to tyre to road. This is the point where you're looking to control power oversteer. So you begin locking the diff back up. But the more you lock it up the more the car will tend to want to drive off and understeer out of the corner."

So, like in all areas of F1, use of the diff is all about finding a good balance. That's why selecting the right diff map can be so important for the driver to have optimum confidence in the car.

Future torque

Although diffs have halved in size and weight over the past ten years, the basic concept has stayed the same. Next year, however, BAR hope to take a major step forward in diff design.

"We're working on something better for next year," says Tony. "I'm quite excited about it anyway!" he adds. He can't divulge any further, of course, and rightly so, such is the necessarily secretive nature of cutting-edge F1 engineering.

However, even if what BAR are planning works, don't expect to see it on the front cover of any magazines. It'll just be quietly getting on with its job, the way diffs do: out of sight, out of mind but very much in control.

Feature courtesy of the BAR Honda Lucky Tribe media site.