DRIVING AT THE LIMIT
Vehicle/Driver Performance the Limit of Adhesion
Tony Scotti
When the driver moves the steering wheel while the car is in motion, a lateral or sideways force is created pushing on the center of gravity of the vehicle. This force pushing on the vehicle is called G’s. It is this force (G’s) that determines if the driver stays in control of the vehicle. All vehicles are designed to accept a maximum amount of G’s. The car magazines (Road & Track – Car & Driver – Motor Trend) list the maximum G’s for the vehicles they test.
If the driver applies more G’s than the vehicle was designed to take, life will quickly get very exciting, and in fact life gets exciting way before you get to the maximum G’s. A while back one of the car magazines came up with what happens at the various levels of vehicle usage, and the levels of skill needed and excitement created at these levels. As a side note I got to say that I agree with them, for years of teaching have taught me that it’s not the cars capability that we need to be concerned about. It’s how much of the car can the driver use.
This is what the magazine testers came up with, if a driver is using;
60% of the vehicles capability:
Requires constant attention, can probably carry on a conversation, but really have to be paying attention to the road. Handling characteristics of vehicle become evident. This is actually fun. But studies have shown that the average driver cannot use 60% of the vehicle.
70% of the vehicle capability:
Attention is very high. Driving is more fun. If people are not belted in they will start move around in the vehicle. Objects in the car that are not tied down will also start to move.
80% of the vehicle capability:
This will be at a speed at which the driver cannot afford any superfluous conversation. This is difficult, and requires skill that comes from training. The driver needs to be ready for bad things to happen. Passengers not belted in will start to be tossed around the vehicle loose objects will become missiles.
90% of the vehicle capability:
This will be too fast for anyone but a trained driver with more than average experience. Even if the passengers are belted in they will have a hard time moving there hands and arms. Small amounts of debris or imperfections in the road surface become real hazards.
100 % of the vehicle capability:
This would be driving at a speed that allows for no driver error and no unknown irregularity in the course or conditions. If sanity is intact, fear is present. Requires a great deal of luck, this is reserved for closed-course race car driving, such as qualifying for competition with the pole position in mind.
Keep in mind that all the above is scientifically measurable.
How we do it is we measure the path the vehicle takes, and measure the speed (we use a radar gun) from that we compute the G’s. To back up our calculations we install instrumentation in the vehicles that gives us a reading on the amount of G’s the driver has applied to the vehicle. Over the years we have use the phrase “he/she is a 70% driver or an 80% driver” That means that the driver could use 70 or 80 percent of the vehicles capability. To pass our courses you must demonstrate that both in steady state cornering and in an emergency scenario (These two scenarios require different driving skills) the student can use at least 80% of the cars capability.
