Weight transfer is all about the control of
traction. The first thing to remember is that everything that an
automobile does, it does through its tire
contact patches. These little spots
where the rubber meets the road make everything happen. By maximizing the size of the contact patches, you maximize available traction. The more traction you have, the faster you can go. It’s that simple, but to paraphrase
Clausewitz in
racing the simplest thing is difficult.
Every control input the driver makes, be it steering a stab on the brakes, flooring or feathering the accelerator will transfer weight in a predictable direction, around the vehicle’s roll center. Consider the approach of a race car to turn one at Nelson’s Ledges Road Course near Warren, Ohio. Turn One is relatively flat eighty degree right hander that can be taken at around 90 MPH, depending on your car. Braking before entry will transfer weight forward, compressing the front shock absorbers and slightly expanding the tire patch, maximizing grip at the front wheels, which are the wheels that turn. In turn weight will be transferred off of the back wheels, shrinking their contact patch. The reason for this transfer is inertia. The car decelerates entirely through the tires. They start stopping before the rest of the car, which is sprung. In essence the cars mass shifts in the direction it was previously travelling.
As the driver approaches the desired cornering speed he will reduce braking and feed in more steering. The steering input will transfer weight to the outside of the car, at Nelson the left side of the car, Doing so gently or-- smoothly in racing parlance --- maintains weight on the rear wheels, and on the inside wheels, which can help the car turn in provided they are touching the ground. The driver may choose to maintain light braking throughout the turn, something quite possible in a rear wheel drive car. This light braking is intended to maintain chassis loads on both the front and side of the car, effectively loading all corners of the chassis. This ‘flattens’ the car, and increases traction on the inside and rear wheels. This permits the turn to be taken at a higher speed. The technique is called trail braking. The late Jim Clark and Al Unser jr. are considered masters of the art.
As the turn continues beyond the apex the driver may use heel and toe braking to use both brakes to set the chassis forward and throttle to set the chassis rearward to balance the car. As the car exits the corner, braking will be dropped and additional throttle fed in until the car is moving straight enough to take full power.
Proper control of weight transfer requires smooth driving, which may seem a buzzword in racing, but it entirely critical. Too abrupt inputs transfer weight to radically, with potentially disastrous results. Let me provide a practical example. Until about 1995, the Porsche 911 was known as a wonderful car with a major achilles heel-- lift throttle oversteer. . It would turn, and corner at amazing speeds. But if a driver had a Maalox Moment in a fast bend and lifted bad things would happen. Let me explain why.
As a rear engined vehicle, the 911's center of gravity is very near the rear of the car. This makes it sensitive to polar motion, particularly initiating in the car's rear wheels. Porsche drivers traditionally brake before a corner, entering the turn at a low speed relative to a corvette or BMW then powering out. The acceleration transferred weight to the rear of the car, and with fat tires enough grip is generated to hold the line. Porsche has always known how to build a motor, and the 911 has enough power to point the car. But the key to driving one fast is to stay on the gas. Properly driven, they’re really fast.
But lets say that our intrepid racer was fighting it out with a Corvette, who brakes later and can carry more entry speed into a turn. And this being a race, our intrepid Porsche driver gets a bit racy and tries to out-vette the ‘vette. No matter how terrified he becomes, the 911 jockey dare not lift. If he does weight will transfer forward, off those big rear tires. At the limit they will break loose, and when that starts all that weight of the rear of the car is now moving sideways. There is no getting traction back, and our Porsche is off, probably moving backwards.
More bad things happen if you transfer weight suddenly. A driver may pitch the car, making a quick steering movement. On fresh, hot tires that can work and is a useful ‘one time’ trick. However, it transfers too much weight onto the outside wheels. Without much help from the inside wheels the car cannot travel as fast as it might. Pitching the car also wears the tires prematurely. Adding too much throttle too soon lightens the front -- the steering-- wheels and can induce understeer or push.
Good things can happen when you transfer weight gently. In my ITB Corolla I did not have to brake at all to take turn one at Nelson. But a quick confidence lift just before turn in transferred just enough weight forward to ‘upset’ the rear suspension and make it turn in.
The key in driving is to control your weight transfer.