Page 20 - PCA Metro NY Region POST | 2019-September-eMag
P. 20

Track Ramblings, Continued from Page 16
the mid-point of the front and rear tires’ contact patches, and only a smaller weight transfer to the front occurs.
Back to the realm of cars, when you are cornering there is a weight transfer to the outside tires.
The height of the center of gravity affects this weight transfer – the higher the cg, the more weight transfer. Another point to understand is that a greater weight transfer causes the car to lean towards the outside. This lean affects the suspension and causes the tires to lean also, changing their camber and their grip potential. So, once again a higher center of gravity has adverse effect on a car’s performance capability.
Let’s say you are in a corner and you feel like you are going too fast. Your immediate reaction may be to lift off the gas pedal. By doing so, you are exerting a small braking force on the car.
This causes a small weight transfer from the rear tires to the front tires. If you apply the brakes in this turn, you cause more weight transfer. This weight transfer adds grip to the front end of the car and reduces grip at the rear. The effect may be to provoke the rear of the car to swing wide, away from the apex, and for the front of to aim for the inside of the turn more than it was a moment earlier. The car may go into a TTO spin, which is not desirable, to say the least.
Acceleration in a car, which in physics is defined as a change in velocity – in any direction – speeding up, braking or cornering, causes a weight shift in the car, which adds weight to certain tires and reduces weight on others. The elements determining the amount of weight shift are the acceleration amount and the height above ground of the car’s center of mass (often called the cg, or Center of Gravity). The car’s center of gravity is that location where, for convenience in calculations, its total weight may be thought to be concentrated. A tall SUV has a higher center of gravity than a low-slung sports car; a rear-engine Porsche 911 has a more rearward center of gravity than a front engine front-wheel drive sedan.
Trailing Throttle Oversteer (aka TTO)
 Let’s consider braking. When you apply the brakes, a force is applied to the tires in the rearward direction. This braking force is located where the tires are, on the ground somewhere between the front and rear tires. (Its exact location depends upon the contribution of the front and rear tires’ braking force.) As Newton promised, an equal but opposite reaction is applied to the car in the forward direction. This forward force is located at the center of gravity of the car. The quantity of this force is determined by the weight of the vehicle and the amount of the negative acceleration. (F=ma, for those who remember their physics.)
 But the amount of torque (the twisting force) on the car is affected by the length of the ‘lever arm’ – the distance between the center of gravity and the location on the ground of the braking force. (Remember the breaker bar example I discussed a few issues ago – the longer the breaker bar [the ‘lever arm’] the greater the torque.) The meaning of all this is that you get more weight transfer
At this point I want to make two observations. The first is that since the mid-2000s all Porsches have been equipped with stability control, called PSM
in Porsche-speak, sometimes derogatively called “nannies” by purists. These nannies may intervene to save your bacon from the perils of TTO by automatically applying the brake on your outside rear wheel. The second is that above I said that “you feel like you are going too fast.” The likelihood is that your superb Porsche and its excellent high-speed tires probably have more cornering capability than you thought, and that if you don’t do anything abrupt (like hitting the brakes), and
the higher the car’s center of gravity is. But the important point to understand is that you always get weight transfer to the front tires under braking.
As you apply more weight to a tire, that tire can produce more grip. This is why serious race cars have aero wings, fins, etc. The high-speed air pressure is used to make the car weigh more.
Let’s now consider a bicycle and why it is a bad practice to slow the bike using only the front brakes. The bike has a very high center of gravity since most of the weight is the rider and the rider sits very high off the ground. If you do hard braking using only the front brakes, the center of gravity wants to pivot around the contact patch of the front tire, and you may find yourself propelled in the air onto the ground. If, by contrast, you slow down with equal force, but use both the front and rear brakes, the center of gravity wants to pivot around
if you perhaps unwind the wheel (i.e., reduce the steering) a tiny bit, all will be well. It is difficult to override your instinct to hit the brakes when you feel you are going too fast but sometimes it is important to do so.
I’ll see you at the track.
You can always contact me to discuss my articles at [email protected]
 18
METRO NY REGION PCA * PORSCHE POST SEPTEMBER 2019















































































   18   19   20   21   22