Automobile handling: Difference between revisions

Car handling and vehicle handling is a description of the way wheeled vehicles perform transverse to their direction of motion, particularly during cornering and swerving. It also includes their stability when moving in a straight line. Handling and braking are the major components of a vehicle's "active" safety. The maximum lateral acceleration is sometimes discussed separately as "road holding". Handling is an esoteric performance area because rapid and violent manoeuvres are often only used in unforeseen circumstances. (This discussion is directed at road vehicles with at least three wheels, but some of it may apply to other ground vehicles.)

Handling is a property of the car, but different characteristics will work well with different drivers.

Weather affects handling by making the road slippery. Different tyres do best in different weather. Deep water is an exception to the rule that wider tyres improve road holding. (See aquaplaning under tyres, below.)

Cars with relatively soft suspension and with low unsprung weight are least affected by uneven surfaces, while on flat smooth surfaces the stiffer the better. Unexpected water, ice, oil, etc. are hazards.

Front heavy cars tend to understeer and rear heavy cars to oversteer. This can be compensated, at least mostly, by using wheels and tyres with size (width times diameter) proportional to the weight carried by each end.

The center of gravity hight, relative to the track, determines weight transfer (side to side and front/rear) and causes body lean.

Automobile suspension has many variable characteristics, which are generally different in the front and rear and all of which affect handling. Some of these are: spring rate, damping, straight ahead camber, camber change with wheel travel, roll center hight and the flexibility and vibration modes of the suspension elements. Suspension also affects unsprung weight.

Many cars have suspension that connects the wheels on the two sides, either by an anti-roll bare and/or by a solid axle. The Citroën 2CV has interaction between the front and rear suspension.

In general, larger tyres, softer rubber, higher hysteresis rubber and stiffer cord configurations increase road holding and improve handling. On most types of poor surfaces, large diameter wheels perform better than lower wider wheels. The fact that larger tyres, relative to weight, stick better is the main reason that front heavy cars tend to understeer and rear heavy to oversteer. The depth of tread remaining greatly affects aquaplaning (riding over deep water without reaching the road surface). Increasing tyre pressures reduces their slip angle, but (for given road conditions and loading) there is an optimum pressure for road holding.

The track provides the resistance to sideways weight transfer and body lean. The wheelbase provides resistance to front/back weight transfer and provides the torque lever arm to rotate the car when swerving. The wheelbase, however, is less important than angular inertia (polar moment) to the vehicle's ability to swerve quickly.

The suspension only directly affects the part of the car that it supports. The unsprung weight is kept in contact with uneven road surfaces only by the springiness of the tyres (and wire wheels if fitted). To aggravate this, for fuel economy and to avoid overheating at high speed, tyres have limited internal damping. So the "wheel bounce" of the unsprung weight moving up and down is poorly damped. For these reasons, high unsprung weight reduces road holding and increases unpredictable changes in direction on rough surfaces.

The main factors that improve unsprung weight are a sprung differential (as opposed to solid live axle) and inboard brakes. Aluminum wheels also help. Magnesium wheel are even lighter but corrode easily. Inboard brakes are found on Citroëns, some racing cars and some other production cars.

This is mainly effective at high speeds, especially very high speeds. Like darts, airplanes, etc., cars are stabilized by fins and other rear aerodynamic devices. Racing cars use downward "negative lift" to improve road holding, but the value of this at ordinary road speeds is questionable.

The coefficient of friction of rubber on the road limits the vector sum of the transverse and longitudinal force. So the driven wheels or those supplying the most braking tend to slip sideways.

Unless the vehicle is very short these are about the same. The yaw angular inertia tends to keep the rate of change in pointing direction constant. This makes it slower to swerve or go into a tight curve, and it also makes it slower to turn straight again. The pitch angular inertia detracts from the ability to keep front and back tyre loadings constant on uneven surfaces.

Angular inertia is an integral over the square of the distance from the center of gravity, so it favors small cars even though the lever arms also increase with scale.

This increases the time it takes to settle down and follow the steering. It depends on the (square of) the hight and width.

Having to take up lateral "g forces" in his/her arms interferes with a driver's precise steering.

Depending on the driver, steering force and transmission of road forces back to the steering wheel and the steering ratio of turns of the steering wheel to tuns of the road wheels affect control and awareness. Play — free rotation of the steering wheel before the wheels rotate — is a common problem, especially in older model and worn cars. Another is friction. Rack and pinion steering is generally considered the best type of mechanism for control effectiveness. The linkage also contributes play and friction. Caster — offset of the steering axis from the contact patch — provides some of the self centring tendency.

The steering effort depends on the downward force on the steering tyres and on the radius of the contact patch. So for constant tyre pressure, it goes like the 1.5 power of the vehicle's weight. The drivers ability exert torque on the wheel scales similarly with her size.

Power steering reduces the required force at the expense of feel. It is useful, mostly in parking, when the weight of a front-heavy vehicle exceeds about ten times the driver's weight, for physically impaired drivers and when there is much friction is the steering mechanism.

Rear wheel steering (supplemental to front) has begun to be used on road cars (Some WW II recognisance vehicles had it). It relieves the effect of angular inertia by starting the whole car moving before it rotates toward the desired direction. It can also be used, in the other direction, to reduce the turning radius.

The severe handling vise of the TR3 and related cars was caused by running out of suspension travel. (See below.) Other vehicles will run out of suspension travel with some combination of bumps and turns, with similarly catastrophic effect.

Since automobile safety is mainly a control issue, one should expect a largely electronic solution. Apparently there has already been some advance in this direction.

Of course things should be the same, left and right. Toe in affects steering because a tyre tends to move in the direction the top of it is leaning.

The frame may flex with load, especially twisting on bumps. Rigidity is considered to help handling. At least it simplifies the suspension engineers work. Some cars, such as the Mercedes 300SL have had high doors to allow a stiffer frame.

When any wheel leaves contact with the road there is a change in handling, so the suspension should keep all four (or three) wheels on the road in spite of hard cornering, swerving and bumps in the road. It is very important for handling, as well as other reasons, not to run out of suspension travel and "bottom" or "top".

It is usually most desirable to have the car adjusted for neutral steer, so that it responds predictably to a turn of the steering wheel and the rear wheels have the same slip angle as the front wheels. However this may not be achievable for all loading, road and weather conditions, speed ranges, or while turning under acceleration or braking. Ideally, a car should carry passengers and baggage near its centre of gravity and have similar tyre loading, camber angle and roll stiffness in front and back to minimise the variation in handling characteristics. A driver can learn to deal with oversteer or understeer, but not if it varies greatly.

The most important common handling failings are;

• Understeer - the front wheels tend to crawl slightly or even slip and drift towards the outside of the turn. The driver can compensate by turning a little more tightly, but road-holding is reduced, the car's behaviour is less predictable and the tyres are liable to wear more quickly.

• Oversteer - the rear wheels tend to crawl or slip towards the outside of the turn more than the front. The driver must correct by steering away from the corner, otherwise the car is liable to spin, if pushed to its limit. Oversteer is sometimes useful, to assist in steering, especially if it occurs only when the driver chooses it by applying power.

• Body roll - the car leans towards the outside of the curve. This interferes with the driver's control, because he must wait for the car to finish leaning before he can fully judge the effect of his steering change. It also adds to the delay before the car moves in the desired direction.

• Weight transfer - the wheels on the outside of a curve are more heavily loaded than those on the inside. It is likely to contribute to understeer or oversteer. Weight transfer (sum of front and back) in steady cornering is determined by the ratio of the height of a car's centre of gravity to its track. Differences between the weight transfer in front and back are determined by the relative roll stiffness and contribute to the over or under-steer characteristics.

• Slow response - sideways acceleration does not start immediately when the steering is turned and may not stop immediately when it is returned to centre. This is partly caused by body roll. Other causes include tyres with high slip angle, and yaw and roll angular inertia. Roll angular inertia aggravates body roll by delaying it. Soft tyres aggravate yaw angular inertia by waiting for the car to reach their slip angle before turning the car.

For ordinary production cars, manufactures err towards deliberate understeer as this is safer for inexperienced or inattentive drivers than is oversteer. Other compromises involve comfort and utility, such as preference for a softer smoother ride or more seating capacity. High levels of comfort are incompatible with a low centre of gravity, body roll resistance, low angular inertia, support for the driver, steering feel and other characteristics that make a car handle well. Inboard brakes improve both handling and comfort but take up space and are harder to cool. Large engines, tend to make cars front or rear heavy. In tyres, fuel economy, staying cool at high speeds, ride comfort and long wear all tend to conflict with road holding, while wet, dry, deep water and snow road holding are not exactly compatible. A arm or wishbone front suspension tends to give better handling, because it provides the engineers more freedom to choose the geometry, and more road holding, because the camber is better suited to radial tyres, than MacPherson strut, but it takes more space. Live solid axle rear suspension is mainly used to reduce cost, but, in general, cost is a relatively less important factor.

In addition, lowering the center of gravity will always help the handling (as well as reduce the chance of roll-over). This can be done to some extent by using plastic windows (or none) and light roof, hood (bonnet) and boot (trunk) lid materials, by reducing the ground clearance, etc. Increasing the track with "reversed" wheels will have a similar effect, but remember that the wider the car the less spare room it has on the road and the farther you may have to swerve to miss an obstacle. Stiffer springs and/or shocks, both front and rear, will generally improve handling, at the expense of comfort on small bumps. Performance suspension kits are available. Light alloy (mostly aluminum or magnesium) wheels improve handling and ride as well as appearance.

• Porsche 911 — the inside front wheel frequently leaves the road during hard cornering, although it is still considered to have acceptable handling. When the rear wheels do lose traction, however, the extreme rearward position of the center of mass causes the rear to snap around unstoppably, so that the vehicle is headed backwards. Ironically, since the car has been engineered to such a high standard of road-holding, when it finally does lose traction it is typically at extremely high speed and cornering forces, leading to a tremendous impact.
• Triumph TR2, TR3 and TR4 — began to oversteer much more suddenly when their inside rear wheel lifted.
• Mercedes-Benz A-Class — early models showed excessive body roll during sharp swerving manoeuvres, most particularly during the Swedish moose test. This was later corrected using active suspension.
• Volkswagen Beetle — Since it was designed in the 1930s as the minimum family car that would be reliable at motorway speeds, it is not surprising that it was top-heavy and rear-heavy. Since they were produced for so long, with stickier tyres and more powerful engines, people who drove them hard fitted reversed wheels and bigger rear tyres and rims. Ralph Nader blamed the swing axle suspension.
• The gaudy 1950s American "full size" "dinosaurs" responded very slowly to steering changes, because of their very large angular inertia, soft but simple suspension and comfort oriented cross bias tyres. Contact with Europe and the 1970s energy crisis have gradually relieved this problem. (Large trucks, also, cannot be made to respond quickly because of their angular inertia.)
• Dodge Omni and Plymouth Horizon - these early American responses to the Volkswagen Rabbit were found "Unacceptable" in their initial testing by Consumer Reports, due to an observed tendency to display an uncontrollable oscillating yaw from side to side under certain steering inputs. While Chrysler's denials of this behavior were countered by a persistent trickle of independent reports of this behavior, production of the cars was altered to equip them with both a lighter weight steering wheel and a steering damper, and no further reports of this problem were heard.
• The Suzuki Samurai was similarly reported by Consumer Reports to exhibit a propensity to tipping over onto two wheels, to the point where they were afraid to continue testing the vehicle without the attachment of outrigger wheels to catch it from completely rolling over; once again, they rated it as "Unacceptable", and once again the manufacturer denied that it was any sort of problem "in the real world", while reports by owners who had experienced such rollovers steadily trickled in. The vehicle was eventually taken off the market before any changes were made to the handling. As SUVs became popular, however, it became evident that their high center of mass made them more likely to tip over than passenger cars, and some even did so during Consumer Reports' testing; but none other than the Samurai showed such a readiness to roll over that they were rated unacceptable. This was in all probability due to the Samurai's being exceptionally short and narrow.
• Ford SUVs then were cited as having a dangerous tendency to blow a rear tire and flip over. Ford and Firestone, the makers of the tires, pointed fingers at each other, with the final blame being assigned to quality control practices at a Firestone plant which was undergoing a strike; it was widely surmised, however, that at least part of the problem was caused by Ford specifying lower than optimum pressures in the tires in order to induce them to lose traction and slide under sideways forces rather than to grip and force the vehicle to roll over. An internal document dated 1989 states

Engineering has recommended use of tire pressures below maximum allowable inflation levels for all UN46 tires. As described previously, the reduced tire pressures increase understeer and reduce maximum cornering capacity (both 'stabilizing' influences). This practice has been used routinely in heavy duty pick-up truck and car station wagon applications to assure adequate understeer under all loading conditions. Nissan (Pathfinder), Toyota, Chevrolet, and Dodge also reduce tire pressures for selected applications. While we cannot be sure of their reasons, similarities in vehicle loading suggest that maintaining a minimal level of understeer under rear-loaded conditions may be the compelling factor.[1]

This contributed to buildup of heat and tire deterioration under sustained high speed use, and eventual failure of the most highly stressed tire. Of course, the possibility that slightly substandard tire construction and slightly higher than average tire stress, neither of which would be problematic in themselves, would in combination result in tire failure is quite likely. The controversy continues without unequivocal conclusions, but it also brought public attention to a generally high incidence of rollover accidents involving SUVs, which the manufacturers continue to address in various ways.

• Tim Skelton racing (Retrieved 4 July 2004)

• DOT rollover ratings by vehicle type