In cars, the four contact patches that tires create with the road are your only connection point with the earth. (If there's anything besides the tires contacting the road, you're doing something wrong.) You'd be surprised to see how small these patches are. Take a look at a tire on the ground, and remember that each one's contact with the road is approximately as large as your hand print.

These four hand prints of rubber need to perform a pretty vital task, allowing your several-ton vehicle to maneuver, brake and accelerate. Tire tread, condition, inflation pressure, weather, and temperature are all variables that can influence how your car feels and performs. There is one unifying concept that can help you begin to get the most out of driving enjoyment - The Traction Budget.

The Traction Budget is a simple model that visualizes the dynamics of vehicles in motion, and how tires work, to explain a lot of the phenomena you'll encounter while driving enthusiastically: Understeer, Oversteer, and a number of other concepts. In essence, the Traction Budget states that a given tire has only so much friction available to it at any given time. That amount of friction must be shared among braking, accelerating and side loads (turning).

As one force puts a higher load on the tire, the availability of friction for the other loads goes down in a fixed ratio.


This means that a tire that is busy braking, can't necessarily handle a large side load at the same moment - you get only so much friction out of a tire at one time. Drivers who grok this concept can use it to their advantage to get the most out of cornering, braking and accelerating. Those who don't will suffer from understeer, oversteer and slower lap times. The trick is to use the budget to your advantage. Use it all.

For the sake of this read, and to keep it simple, I am going to assume our example car as four good tires, all inflated properly, and that the road is dry and in good condition. There are so many variables that affect a tire's traction, that we can't go into them all here. Let's also assume this is a RWD car for the time being. (You'll see later why a lot of - but not all, performance and race cars stick with this layout. No flames please-)


It may sound obvious, but at the heart of all this is the understanding of friction. The amount of friction that your rubber tire can create with the road will ultimately determine how 'sticky' it is, and how well it performs during maneuvers. Tire rubber compound, temperature, tread design are only a few of the other factors that combine to create this force. The greatest force for our example is weight. The more weight pushing down on a tire, the greater the 'envelope' of friction can be created. Think of this conceptually, the heaver that wheel is, the more friction the tire has in contact with the road.

You see it here, as weight goes up, so does friction. A properly set up car (suspension tuning) will try to have an even weight distribution to all four tires.


For this model, Think of your tires' capabilities as an ellipse around the contact patch that changes shape and size as you drive. If there is a lot of weight on the tire, imagine the contact patch gets bigger, and lighter tires get smaller. When turning, braking or accelerating the patch can also change shape as well as size. The bigger the 'patch', the greater the grip. The secret is to manage the patch through the weight transfer in the car.

Here are a few extremes that can help explain. Most drivers won't encounter this every day. If you do, you are a true Jalopnik.

Sitting still, this car has nearly zero friction demands on the tires, and even weight distribution. The contact patches are even.


If the car is accelerating hard in a straight line, the more of the weight of the car will transfer to the rear tires, and less on the front.

The car's weight shifts to the rear, helping the rear tires gain more traction to support acceleration. (ever wonder why dragsters are generally RWD?)


When braking, some of this reverses. The weight transfers forward, and the front wheels take on more of the task of braking. (Notice that front brakes are bigger than rear ones generally for this reason.)

Now let's add in some curves in the road. In the same acceleration scenario, the front wheels aren't particularly busy and they can apply turning force easily, but the rears are putting the majority of the traction budget into acceleration.The rear tires at this point can't handle as much side load now, as with enough HP, all of the traction budget goes into acceleration. If the traction budget is spent, the tire will begin to lose traction and spin, and the car can slide from the rear easily (Think drift cars). In a turn this can also manifest in Oversteer, when the rear of the car goes wide in a turn.


The upside to the RWD layout is that tires at each end of the car get an important task. The fronts handle the turning tasks with increased side load capability, and the rears handle acceleration. Also the weight shift tends to go to the outside of the turn, helping the outside tires in the turn.

When braking hard, the traction budget in front is nearly spent in braking, so the tires can't handle much side load (turning) force at the same time. Sometimes this can be felt as Understeer, if you brake too hard into a tight turn and the front end loses traction and pushes outward. This means the front tires' traction budget is nearly spent in one direction, so they can't manage side loads well.


In a FWD car, there are challenges, as the front wheels are now burdened with all the major tasks, acceleration, braking and turning. The rears are mainly just along for the ride, keeping the trunk off the ground. During heavy acceleration, the load shifts to the rear, in effect, un-loading the front tires, reducing the traction budget when it is most needed. This is one reason FWD cars can be more prone to understeer, as the traction budget on the front tires can be used up more readily, leaving less to change direction.

Of course this is a gross over-simplification, as a vehicle in motion is a veritable live physics course, but you get the idea. There are plenty of FWD and AWD cars that can handle these forces just fine. No flames please.

While driving, keep the Traction Budget in mind. Smooth weight transfer from front to back, and from side to side actually helps the correct tire do the job. When setting up for a right hand corner, brake into the turn smoothly, letting the weight move to the front tires. Then progressively lighten up on the brake (trail braking) as you add steering input in one smooth motion. Imagine the tire patches on the ground, and that with braking, you're pressing the front of the car down into the pavement. You're increasing the size of the patches- the budget that the front tires can use by making them 'heavier'. The car will turn-in more readily and feel more responsive. As the turn begins and the tires are working to push the front to the right, gradually eliminate all braking, and add acceleration as you 'unwind' the steering, letting it go straight, in an intentional, smooth motion. This smoothly helps transition to the rear of the car, helping support good acceleration with more weight on the rear and especially the outside rear tire. That way you're keeping all tires closer to the edges of the traction 'envelope' while utilizing all the capability of the tires. You only get so much, but use it all.


The more you can visualize these patches in a dynamic relationship with the road, the faster you'll be.