There are times when your car starts "talking" to you. It usually happens when your car is approaching its limits. This phenomenon is called neutral steer or over- and understeer.
Under- and oversteer are terms used to explain vehicle characteristics, and they are important signals transmitted to you by the vehicle; it is how the vehicle communicates to you. It is the vehicle's way of telling you what you should do next. In a nutshell, understeer and oversteer are the interrelationship of the front and rear ends of the car.
To get a clear understanding of under- and oversteer, a basic discussion of the laws of physics is required. When you turn the steering wheel, there is energy pushing on the Center of Gravity (CG) of your vehicle. The amount of energy (it can be measured in Gs or in pounds) is determined by how much you move the steering wheel and how fast you're traveling. The more speed and the more steering, the more energy pushing on the vehicle. Remember from high school, "for every action there is an opposite and equal reaction." So, if there is a force pushing on the CG of your vehicle, there has to be an equal and opposite force pushing back--that force pushing back is created by the friction your tires make with the road.
If you drove around a corner or made an emergency maneuver that created 3200 lbs. pushing on the CG of your vehicle, in the perfect world your tires would be pushing back 1600 lbs. front and rear. This would be called neutral steering, and it is a characteristic seldom found in vehicles.
But what happens most often is that the vehicle will either oversteer or understeer. What follows below is a basic explanation of under- and oversteer.
If you are a NASCAR fan, this is called "push."
In a turn or emergency maneuver, understeer is the condition where the front tires lose adhesion while the rear tires remain in contact with the pavement. The car tends to travel straight ahead, even though you are turning the wheel. In our example, the front tires can only push back with 1000 lbs. and the back tires push back with 1600 lbs.
You can't learn to correct under- and oversteer by reading a book or article. But I'll give it a shot, keeping in mind that this is as basic an explanation as you can get.
In an understeering condition, turning the steering wheel more won't work and will aggravate the scenario. To fix it, reduce speed and/or reduce the amount the steering wheel is turned. You can correct understeer by reducing throttle until the front tires regain adhesion.
NASCAR fans call this "loose."
In a turn or emergency maneuver, oversteer is the condition where your rear tires lose adhesion while your front tires remain in contact with the pavement. The back end of your car tends to slide out. Turning the steering wheel more will makes things worse. In our example, the rear tires can push back with only 1000 lbs. and the fronts push back with 1600 lbs.
Let me repeat--you can't learn to correct under and over steer by reading a book or article.
To fix oversteer, reduce speed and turn the steering wheel in the opposite direction of the skid.
By far, one of the most difficult situations to correct something called "power oversteer." An example of power oversteer: you're in pursuit, or for whatever reason driving fast around a corner. As you exit the corner you apply the gas, and the back of the vehicle starts to swing out. The back of the vehicle is swinging out because you have too much gas and the back tires are losing adhesion. To correct the problem you either have to steer less or give it less gas.
Also there is another form of oversteer that is called "trailing throttle oversteer," which means that as you steer, either to drive around a corner or in an emergency maneuver, and take your foot off the gas, the back of the car swings out. Trailing throttle oversteer is due to the back tires doing funky things when you transfer weight from the rear to the front. Correct trailing-throttle oversteer by smoothly increasing the throttle (to transfer weight to the rear tires) and apply steering to counter the rotation.
So what is "dangerous" understeer or oversteer? Although under- and oversteer can both cause loss of control, many cars are designed to understeer. It is generally believed by car designers that understeer is easier to control. In fact, almost all cars you drive have understeer built into them.
From a driving standpoint, what is hard to do and considered by many to be a dangerous characteristic, is a vehicle that goes quickly from understeer to oversteer, or vice versa. You hear it often when watching a NASCAR event; they will say that the car was tight (understeer) entering the corner and loose (oversteer) coming out. This condition is one of the very few that will actually cause a NASCAR driver to slow down. >/p>
Causes of Understeer and Oversteer
The causes of understeer and oversteer are usually related to the condition and type of the tires on the vehicle and the vehicle design.
- Low front tire pressure (understeer)
- Low rear tire pressure (oversteer)
- Uneven front tire pressure (understeer)
- Uneven rear tire pressure (oversteer)
- Bald tires in the back (oversteer)
- Bald tires in the front (understeer)
Keep in mind that the correct tire pressure is a function of the weight the tire supports. A special caution for those of you that carry heavy equipment in your trunk: normal rear tire pressures may not be sufficient; you will need to carry extra air in your rear tires to support that weight. Excess weight in the trunk can cause this quick change from under- to oversteer.