4. Vehicle Dynamics
Load Transfer
Understanding how load transfer works is critical to driving a vehicle properly. Whether you’re in an evasive situation or racing your sports car on a track, understanding load transfer and its effects is critical to achieving the maximum braking and handling abilities of a vehicle.
Contact patches are the only thing between you and the road. The size of the contact patches controls the handling characteristics of the vehicle.
When a vehicle is stopped, it is in a neutral position. However, with the slightest movement of the vehicle, load transfer occurs, which affects the size of the contact patches and the handling of the car. If you accelerate, where does the load go? What happens to the contact patches? Just think if you had a can of soda on the dashboard; if you accelerate, that soda is coming into your lap. Why? Because you are shifting the load of the car to the rear. By doing so, you increase the size of the rear contact patches and decrease the front.
The larger the contact patch, the more rubber you have on the road. Common sense tells us that the more rubber you have on the road, the better traction the vehicle will have. So from this we know that while accelerating, the rear end has more traction than the front. That’s why dragsters have such large tires in the rear and small tires up front. The more traction they have in the rear, the faster they can accelerate.
What happens when you get on the brakes? Loose objects in the car roll forward. That’s because the entire load is now being shifted to the front of the car. The front contact patches grow and the rear ones decrease, giving the front of the car more traction.
Understeer and Oversteer
Most drivers have experienced understeer and oversteer, whether they realize it or not. To put it simply, understeer is when the front tires lose traction. The vehicle begins to lose its turning ability and tends to go straight instead of in the direction the driver intended. This is usually caused by trying to turn a corner too fast for the conditions or by locking up the brakes. In a front wheel drive car, understeer can also be caused by accelerating too hard and causing the front tires to spin.
Oversteer is caused when the rear wheels lose traction, and the vehicle wants to spin out. This is usually caused by dramatically reducing power in a corner. This takes the load off the rear of the vehicle and causes it to lose traction. In rear-wheel drive cars, oversteer can also occur by getting on the power too hard in a corner and spinning the rear tires, which will cause the vehicle to lose traction.
It’s better to ANTICIPATE than to REACT!
A large percentage of motor vehicle collisions are caused by drivers simply failing to anticipate a potentially dangerous situation. This failure to anticipate usually results in an over-reaction, or panic behind the wheel, resulting in the creation of an understeer or oversteer situation.
What is the best way to correct an understeer situation? Although every situation is potentially different, generally, as you enter a turn, if your front tires begin to lose traction you simply need to enlarge the contact patches on the front tires. To do so just gradually ease off the gas pedal and place more load on the front of the car. Sounds easy, however, most people fail to recognize understeer soon enough and when they do, their first instinct isn’t to ease off the gas but to simply turn the wheel more. When that doesn’t correct the situation the next step is to over-react and jump off the gas pedal. By doing that, you unbalance the vehicle because you place a lot of load on the front tires, and you take it off the rear. Watch out!! Now you’re in an oversteer situation!
Driving a car properly is like ballet.
The smoother you can be with your actions, the more responsive the vehicle will be. However, the more abrupt you are behind the wheel, the more abrupt the vehible will be.
An oversteer situation is a little more complicated to control. Most drivers have been taught to “turn into the skid” if the rear end loses traction. What does that even mean? What it does mean is simple…LOOK…AND STEER…WHERE YOU WANT TO GO. It’s that simple. Your eyes act as a guide.
Skid Control
How do you control a skid? CPR is the easiest way to remember the necessary steps. Correct, Pause and Recovery. Those are the three steps involved in correcting an oversteer situation.
Video: Driver’s Edge Skid Pad Exercise |Students learn how to control a vehicle when it goes into a skid.
1. Correct
The first step is to correct. That’s the easy part. Correct simply means to turn the steering wheel in the direction of the skid (simply continue to look and steer where you want to go). Step two is the most difficult. As the vehicle slides, the vehicle’s suspension becomes loaded or bound up.
“For every action, there is an equal and opposite reaction”.
– Sir Isaac Newton
2. Pause
The pause is the split second when the rear wheels regain traction before the suspension unloads. It is during the pause that you achieve the final phase and recover.
3. Recover
Recover simply means to straighten out the wheel. If you do not catch the pause, you have a fishtail effect in which the car will begin to slide side to side. It is this last step that is the most difficult. The vehicle is already sliding in one direction. As the skid dissipates, the rear end will regain traction.
It is at that moment that the steering wheel needs to be brought back straight because the compressed energy in the suspension will unload and spring back to the other side, which can cause a fishtail effect. This secondary spin can occur twice as quickly and be twice as violent as the initial slide. This is due to the unloading of the compressed energy and the fact that you are now steering with the skid instead of into the skid.
Sounds complicated? Remember, most times an oversteer situation can be avoided by recognizing an understeer situation before it’s too late. A simple, easy to control skid can quickly become deadly if you delay your action.
Your eyes act as a guide.
If you are skidding towards a tree, DO NOT look at the tree!
Look and steer where you want to go!
During a skid, if you feel that you have any chance of regaining control of the vehicle, your feet should do nothing. No gas, no brakes, nothing. “When in doubt both feet out.” Just correct everything with the steering wheel and once the situation is recovered, THEN use the appropriate pedal. However, if you find yourself in a spin and out of control, remember “in a spin, both feet in.” That means that when you realize you can’t regain control of the vehicle, get as hard as you can on the brake and put the clutch in if you have a manual transmission.
You put the brake on to stop the car as quickly as possible. If you have a clutch, it needs to be pressed that so you keep the engine running. When the car comes to rest you may be in a dangerous position so it would be in your best interest not to stall the vehicle.
“When in doubt, both feet out.”
“In a spin, both feet in.”
Cornering
Steering your vehicle through a corner is a basic driving maneuver. Yet, a majority of all single vehicle collisions are caused by drivers failing to negotiate a corner properly. Improper cornering techniques combined with excessive speed are usually to blame for these incidents.
Every corner has a proper driving “line”. Whether a city street, mountain road or race course, there is a specific area of pavement on which your vehicle should be positioned. The proper line allows a vehicle to follow the path of least resistance through a corner. Finding the line in a racecar is crucial because it allows the driver to get as much speed out of the vehicle as possible. Finding the proper line on the street is just as critical because it allows the driver the greatest safety margin through a corner.
If you have ever watched an automobile race, you may have noticed that all vehicles tend to follow the exact same “line” or path, around the racecourse.
How To Find the Proper Line
Every corner has three key elements:
- Turn-in point; the point where you actually turn and commit to the corner.
- Apex; the point where you come closest to the inside of a corner.
- Exit; the point where you should have the steering wheel unwound and are proceeding straight again.
Establishing the proper line is basically straightening out a corner as much as possible. The same principle applies to rollerblading, skateboarding, skiing, etc. The straighter you make a corner, the easier the corner will be to negotiate. The tighter the corner the more difficult it is to negotiate, and the slower you have to go.
Using your eyes is critical to finding the proper line through a corner. Always look where you want to go. It is at the apex of a corner where you are closest to the inside. This is where you should be able to see your exit, begin accelerating and unwinding the wheel. Do not commit to a corner unless you can see where you are going.
There is no such thing as accelerating “through” a corner.
You accelerate “OUT” of a corner.
If your car is not pointed in the direction you want to go,
Why would you get on the gas to get there quicker?
Turning into a corner too early is the number one problem that most drivers face when attempting to negotiate a corner. Combined this with excessive speed and you have a combination which can be deadly.
Once you have reached the inside point (apex) can you begin to unwind the wheel? Can you see the exit? If you cannot, you have turned into the corner too early.
An uneducated driver can make the best car handle poorly. An educated driver can make a bad car handle well.
Finding the proper line is an essential part of safe driving, especially on winding and mountain roads that tend to have blind corners. In corners that are blind or unfamiliar to you, remember it is always best to slow down more than you think is necessary and turn later than you may think you have too. This will provide you with a greater margin of safety.