Understeer

Understeer is a term for a car handling condition in which the application of cornering force (lateral force) also applies a rotational torque (or moment) to the car in the opposite direction of the turn.

The effect is opposite to that of oversteer.

In simpler words, understeer is the condition in which the vehicle does not follow the trajectory the driver is trying to impose while taking the corner, instead following a less curved trajectory.

Understeer is a dynamically stable condition; in other words, if control is lost, the vehicle continues to point and travel in the direction that it is already pointing and traveling in.

Understeer is also often referred to as pushing, plowing, or refusing to turn in. The car is referred to as being "tight" because it is stable and far from wanting to spin.

As with oversteer, understeer has a variety of causes such as mechanical traction, aerodynamics and suspension.

Classically, understeer happens when the front tires have less traction during a cornering situation than the rear tires, thus causing the front end of the vehicle to have less mechanical grip and become unable to follow the trajectory in the corner.

In modern race cars, especially open wheel cars, understeering is caused mainly due to the aerodynamic configuration. In this respect, the lack of a heavy aerodynamic load (downforce) in the front side prevents the front tires from gaining enough traction. At the same time understeer can be caused by having a heavier aerodynamic load at the rear end of the car giving the rear tires more traction than the front tires. Also, suspension balance should take into account the types of surfaces being driven—differing levels of friction in each surface influence the potential understeer behavior. Camber angles, ride height, tire pressure and centre of gravity are important factors that determine the understeer/oversteer handling condition

.

Ideally, the vehicle will travel along the line marked with green dots. If the vehicle understeers, the front wheels lose traction and the vehicle tends to follow the red dotted line

Common practice

It is common practice among automobile manufacturers to configure production cars deliberately to have a slight linear range understeer by default. If a car understeers slightly, it tends to be more stable (within the realms of a driver of average ability) if a violent change of direction occurs, improving safety. If the owner fits new tires to the rear axle only, that will tend to decrease the understeer margin so the vehicle is actually less stable at high speed, but probably still with some understeer. However, that only applies for lower lateral acceleration. More importantly when on wet surfaces hydroplaning might occur and when the tires get near their friction limit, the new tires with better grip from the deeper tread will give better limit handling of understeer, as claimed in San Luis Obispo County Court Case CV078853, and others.^{[citation needed]} When replacing only two tires, manufacturers therefore recommend fitting the unworn ones to the rear, and the best of the old ones to the front axle (assuming identical tires front and back); however, this is not ideal.

 Physics

Under all high speed cornering conditions (greater than approximately 10mph (16 km/h) for a typical automobile), a wheeled vehicle with pneumatic tires develops a greater lateral, or sideslip, velocity than is indicated by the direction in which the wheels are pointed. The difference between the circle the wheels are currently tracing and the direction in which they are pointed is the slip angle. If the slip angles of the front and rear wheels are equal, the car is in a neutral steering state. If the slip angle of the front wheels exceeds that of the rear, the vehicle is said to be understeering. If the slip angle of the rear wheels exceeds that of the front, the vehicle is said to be oversteering.

[edit] Linear range understeer

In a straight line, or when cornering gently or moderately (typically up to 0.4g) the characteristic is called linear range understeer. This is a difficult characteristic to sense directly, but is responsible for many important facets of the handling in this regime, including step steer response, frequency response, and yaw gain linearity. Usually this is developed using a Bundorf analysis.

[edit] Limit Handling Understeer

Any vehicle may understeer or oversteer at different times based on road conditions, speed, available traction, and driver input. Limit handling is the regime of vehicle performance where the tire(s) are approaching the limits of their grip. While not often used on public roads by most drivers, it is the usual state for a racing car except when traveling at high speeds in a straight line. As cornering loads increase further the vehicle will tend to go into a particular "terminal" condition. "Terminal understeer" refers to a vehicle which, as a function of its design, tends to understeer when cornering loads exceed tire traction.

Terminal handling balance is a function of front/rear relative roll resistance (suspension stiffness), front/rear weight distribution, and front/rear tire traction. A front-heavy vehicle with low rear roll stiffness (from soft springing and/or undersized or nonexistent rear anti-roll bars) will have a tendency to terminal understeer: its front tires, being more heavily loaded even in the static condition, will reach the limits of their adhesion before the rear tires, and thus will develop larger slip angles. Front-wheel drive cars are also prone to understeer because not only are they usually front-heavy, the transmitting of power through the front wheels also reduces the grip available for cornering. This often leads to a "shuddering" action in the front wheels which can be felt in the car as traction quickly shifts between being used for turning and motor torque. This is why rear wheel drive cars tend to handle better as the rear wheels' main job is to handle the motor's torque and the front wheels' job is to steer.

Although understeer and oversteer can each cause a loss of control, many automakers design their vehicles for terminal understeer due to the experience that it is easier for the average driver to control than terminal oversteer. Unlike terminal oversteer, which often requires several steering corrections, understeer can often be reduced simply by reducing speed. A slight danger in some cars which traditionally understeer is actually sudden oversteer: for example if a car is moving fast and understeering, the driver will be tempted to take his foot off the accelerator (increasing the steering effectiveness of the front wheels as there is no engine torque to deal with) which can cause the car to snap oversteer and spin, with very little warning. Not many current production cars react like this, as it is not a desirable characteristic.

Understeer is not just present during acceleration through a corner, it can also be found during heavy braking. If the brake balance (the strength of the brakes in terms of the front and rear wheels) is too heavy at the front this can cause understeer. This is caused by the front wheels locking and losing any effective steering. The opposite is true if the brake balance is too strong towards the rear wheels causing the rear end to spin out (like a child skidding on a bicycle). In ordinary road cars a safe brake balance (tending towards slight understeer) must be found.

Racing drivers, on asphalt surfaces, generally prefer a neutral condition (with a slight tendency toward understeer or oversteer, depending on the track and driver preference) because both understeer and oversteer conditions will scrub off speed while cornering. In rear wheel drive cars understeer is generally faster on a circuit because the rear wheels need to have some grip available to accelerate the vehicle out of the turn.