Understeer

Understeer

[n. uhn-der-steer; v. uhn-der-steer]
Understeer is a term for a car handling condition in which during cornering the circular path of the vehicle's motion is of a greater radius than the circle indicated by the direction its wheels are pointed. The effect is opposite to that of oversteer. In simpler words understeer is the condition in which the front tires don't follow the trajectory the driver is trying to impose while taking the corner, instead following a more straight line trajectory. Understeer covers several different phenomena, in particular, there is a big difference between linear range understeer, typically between 0 and 0.4g, and limit handling understeer, which is at higher lateral accelerations, and is what racing drivers are talking about.

The latter 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 sources such as mechanical traction, aerodynamics and suspension.

Classically, understeer happens when the front tires have a reduction in traction during a cornering situation, 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.

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. However, if the owner fits new tires to the front axle only, they will tend to reduce the understeer margin, which can cause handling problems, as claimed in San Luis Obispo County Court Case CV078853, and others. The recommendation from most manufacturers when replacing only two tires is to fit the unworn ones to the rear, and the best of the old ones to the front axle, for this reason. However, this is not ideal either.

Physics

Under all high speed (greater than approximately 10mph (16 km/h) for a typical automobile) cornering conditions a wheeled vehicle with pneumatic tires develops a greater lateral (i.e. 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.

An old bit of racing humor says that an understeering car goes through the fence nose first, an oversteering car goes through the fence tail first, and with a neutral-steering car, both ends go through the fence at the same time.

Linear range understeer

In a straight line, or when cornering gently (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.

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. However, in certain cars and under certain circumstances (e.g when there is a small amount of grip still available) the understeer can be reduced by applying full power to the wheels rather than braking.

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.

Understeer values

How much a car understeers can be measured in the number of degrees more the steering wheel have to be turned per G of lateral acceleration. Note that cars have different steering ratios - one with a higher ratio will require more rotation of the steering wheel to get a given change in front wheel angle. Most road cars have fairly similar ratios, typically around 14-15:1, but some may be substantially higher or lower. Here are the measured linear range values for some cars. The higher the number the more the car understeers.

Car model Understeer value
Nissan 350Z 19
Mazda MX-5 21
Nissan 350Z Roadster 21
Ford Mustang GT 22
Renault Megane Sport 22
Maserati Gran Sport 23
Mini Cooper S 23
BMW 330i 25
Alfa Romeo 147 GTA 27
Corvette C6 27
Corvette Z06 28
Lotus Elise 111R 28
Porsche 997 Carrera S 28
Saab 9-3 Aero combi 28
Škoda Octavia RS 28
Porsche 997 Carrera 29
Porsche Cayman S 29
VW Golf GTI 29
Porsche 987 Boxster S 31
BMW M6 32
Range Rover Sport Supertech 32
Ford Focus ST 33
Mitsubishi Lancer EVO8 34
Porsche 968CS 34
Audi RS4 35
BMW M5 35
BMW Z4 Roadster M 35
BMW M3 Comp Package 36
Opel Astra OPC 36
BMW Z4 3.0i 37
Subaru Impreza WRX STi 37
Subaru Legacy 3.0 aut 38
Volvo V70 T5 39
Audi A4 2.0 T Q 41
Mercedes E55 AMG 42
Audi A4 V6 3.2 Q Avant 43
Porsche Cayenne Turbo 45
Smart ForFour Brabus 45
Mercedes SLK 350 47
TVR Tuscan 49
Alfa Romeo 159 2.2 JTS 51
Morgan Roadster V6 71

Notes

External links

An online guide to understeer with illustrations

See also

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