More Grip

There are two basic factors that govern traction. These are based on the friction formula:

the grip of the driving tire (how sticky it is)
the load on the driving tires.

To put this in perspective, road racing tires have a coefficient of 1.3 to 1.5 and drag racing tires can be 3.0 to 4.0. By comparison, high performance “street road tires” do not have nearly the grip as track tires do. Of course, we still would like those great 0-60 and quarter mile times.

There is a subclass of race tires actually approved by the Department of Transportation for the street. “D.O.T race tires” are designed for road racing, autocross and drag racing. D.O.T race tires (by BFG, Hoosier, etc.) test at 1.25 and better. However, even the manufacturers often do not recommend them for the street because of their high wear rates and lack of puncture resistance.

On a high performance street car, the horsepower may be increased more and more, but you are still limited by the grip of the street tire. You can change the tires at the track but not while you are driving on the street. Even though more horsepower is put into the car, the grip limit stays the same. The speed at which the available power falls below the grip limit, I call the grip crossover point. This is typically at a shift point where the higher gear has less mechanical advantage than the previous gear. By increasing the horsepower, the “mountain of excess power” builds over the grip limit and the grip crossover speed is pushed to a higher and higher velocity.

The second part of the grip formula is the load on the driving wheels. The percentage of the car’s weight that can be applied to the driving wheels determines traction and straight line performance. The percentage of car weight that cannot be applied to the driving wheels is excess “baggage.” So on a rear wheel drive (RWD) car, that amount of car weight that cannot be transferred to the rear just has to be pushed along by the driving wheels. In this case, the more weight that stays on the front tires, the more the acceleration is degraded. This is a key principle for straight line performance.

The weight on the drive wheels is affected by weight transfer during acceleration. As the vehicle accelerates, weight is transferred to the rear tires due to two things:

the moment created by the forward force of the drive axle on the car’s inertia at the center of gravity
the moment created by the reaction of the axle torque on the drive axle housing (straight axle housing, independent rear suspension third member, or transaxle)

Both of these twisting moments lift the front wheels and transfer weight to the back wheels. In excess, you have a wheelie.

With weight transfer in mind, there are four factors that affect the weight on the drive wheels (as depicted in the diagram):

location and quantity of drive wheels
static weight distribution
center of gravity (CG) of the car
wheelbase

With the weight transferring to the rear, RWD has an obvious advantage over front wheel drive (FWD). Since higher static weight on the rear gives better traction, a mid-engine or rear-engine layout has a grip advantage over a front-engine car. CG height has a dramatic affect on traction because it increases the drive axle/CG moment arm. But since a high CG has an adverse effect on cornering, a low CG is preferred in the less specialized “well-rounded” super car. Shorter wheelbases have a minimal advantage for a low CG car.

Drag racers capitalize on these effects to carry the minimum “baggage” weight on the front tires. A drag racer transfers weight to the rear driving wheels so the highest percentage of weight can be applied to traction. A very small percentage of vehicle weight is being “carried along for the ride” degrading acceleration.

In a low CG super car, the weight transfer is less so a higher percentage of vehicle weight must be “carried along for the ride.” An effective way to obtain better acceleration in a high horsepower, low CG super car is all wheel drive (AWD). While AWD does add some weight, there is very little “baggage weight” in the car. The higher the horsepower, the greater the effect of AWD because it utilizes the excess “mountain of power” which is pushed up to higher grip crossover speeds.

Look at this in terms of “traction efficiency.” This can be defined as the power a car actually applies to the ground (up to the grip crossover speed) divided by the power that could be applied if the car was four wheel drive (4WD has all four wheels locked with no center differential). A front engine RWD car with a weight distribution of 55/45 may have a traction efficiency as poor as 60%. That means 40% of the power is useless below the grip crossover speed. A mid or rear engine RWD car might have a traction efficiency of 75%. An AWD car with an optimized center box differential ratio (CBDR) can have a traction efficiency of over 95%. Only 5% of the power is unused below the grip crossover speed.

Apart from the location of the static weight of the car and the effects of weight transfer, there is an additional factor which affects the load on the driving tires. Aerodynamic lift can take weight off the driving tires by lifting the car and aerodynamic downforce can increase load by pressing down on the driving tires. Aerodynamic load increases with the square of velocity so with current aerodynamic techniques, there is little effect at low speeds where traction is needed to apply available power. At high speeds, however, aerodynamic techniques such as wings, spoilers, and ground effects have all had very positive effects on cornering.