weight transfer

This is easily the most complicated subject in racing. It is thus very difficult to comprehend from the driver's seat. Weight transfers are something you learn for a particular car. They are the thing that you dial in on the chassis during practice. Point is, you'd better know what's going on and what's going to happen before race day.

Weight transfers are complicated because they are determined by a hundred changing factors, and they are driven by pure physics, something not easily contemplated when accelerating at 1g.

Thus, I'm not really going to try to tell you all the different weight transfers that can possibly happen, as there are far too many, and far too many outcomes depending on the car and your driving. What I will tell you is this, your car will rely more and less on different wheels in different situations, and you'd better have a good idea of what to expect.

Weight transfers are really misnomers, but lots of racing is misnomers, so it's ok. Weight in this case refers to force, as in the force that a particular wheel is exerting down on the ground, which changes its grip. The other two dimensions are expressed by the traction circle. Too much on a wheel, it will lose grip and spin. Too little and it will exceed its grip and slide. Both are bad, so you try to keep them at a minimum.

You minimize sliding by trying to balance the weight, which means getting it to the proper part of the car, where it can be most used. Again, the proper weighting for each car in each part of a turn is very complicated. A front-wheel-drive car is a compulsive understeerer, a light midengined race car will trade understeer and oversteer very quickly depending on the driver input.

There are Four Rules of Weight Transfer, Three lesser, one greater:

Lesser the First: Turning the car will weight the outside wheels heavily, the inside wheels lightly.

Lesser the Second: Accelerating the car will weight the rear wheels heavily, the front wheels lightly.

Lesser the Third: Braking the car will weight the front wheels heavily, the rear wheels lightly.

Using these three lesser rules in combination, one can balance the car, or at least have a clue as to the intentions the car has for you. Your driving techniques (such as heel and toe and trail braking) are developed to take advantage of weight transfers so following them will properly propel you through the turns. You must follow the traction circle, of course, it is your guide. It will tell you when you have exceeded your traction in a very impolite way. But all the lesser rules are common sense. The one Greater rule is the key to driving success, it will never fail you.

Greater Rule the Only: The car is changing, you must change with it; be smooth in all motions, in all driving.

This sound stupid? It is the key, damn you! A road course is like a giant physics equation you will work out in real time, one too complicated to comprehend by anyone but the almighty programmers of Gran Turismo! But you needn't comprehend it to solve it. You must follow the car, feeling it shift around, and sense its limits. The only way to do that is to be smooth. Sudden motions will transfer weight from one corner to the other so quickly you will not know what the car wants to do, and you will not be able to react quickly. Smooth motions allow the car to tap you on the shoulder politely and say "I'm about to crash into that wall unless you slow down," or "I think you have confused the accelerator with a pellet dispenser and I will now spin."

The most important thing about driving is understanding that you are in control (or should be) of a machine that was made to accept certain limits and tolerances, and you are purposely trying to go deep into those limits without exceeding them. Balancing the car with smoothness is your best bet to remain in control of the car, remain ahead of the pack, and remain safe.

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.