Vehicle Handling
by Jim Kerr

How does your car handle? Straight roads, city streets, or flat landscapes don't often challenge a vehicle's handling, so many of us would answer "fine". However, you would probably find many differences if you compared your vehicle to the way a new vehicle handles.

Some of these differences in handling can be attributed to improvements in vehicle design, but many of the differences are caused by wear. The wear occurs over a long period, so often we are not aware of how poor our vehicle really does handle. Improving your vehicle's handling is relatively easy. Some of the improvements can be costly, but many can be accomplished even on a modest budget.

Tires play a major part in any vehicle's handling. The tire "foot print", rubber compound, and sidewall stiffness all affect how the car responds. The "foot print" or contact patch of tire on the road is responsible for transferring all the driver's inputs into actual movement of the vehicle. For maximum acceleration, cornering ability, and braking, a large footprint is desirable. Performance cars are often fitted with wide tires to increase the car's ability to stick on the road, but large tires also cause problems.

Rougher ride, less traction on snow and ice, and higher fuel consumption are some detriments of wide tires. Drivers interested in maximum traction will overlook these bad points however.

Softer rubber compounds are used to improve a vehicle's grip on the road. Some of the most expensive performance tires can wear out in only a few thousand kilometres because their rubber compounds are so soft. Normal passenger car tires use a harder rubber compound that gives satisfactory grip on the road for normal driving and a much longer life.

Tire sidewall stiffness affects both ride and handling. A soft sidewall flexes more while cornering so a vehicle handling feels sluggish and mushy compared to the same vehicle equipped with stiffer sidewall tires. Many new performance cars use 16, 17 or 18 inch tires (this refers to the rim or wheel diameter) instead of the more common 15 inch tires. The overall outside diameter of all these tires may be the same, but the 16 to 18 inch tires have shorter sidewalls to prevent flexing and improve handling. The drawback is a rougher ride.

Before spending money on new tires, maximise your current tire's handling by adjusting the tire pressure. Many manufacturers recommend tire pressures between 24 and 28 psi (check the tire pressure label and maximum rating on the tire). Increasing tire pressure slightly can improve cornering but too much can cause uneven tire wear and a rough ride. Adjust pressure in small steps.

Keep tire pressure at the front and rear of the vehicle close to equal, but follow the vehicle manufacturer's recommendations. Tire pressures that are too high or low will cause those tires to have less traction. When the front tires slip sideways on a corner we say the vehicle is "understeering". This means no matter how far you turn the steering wheel the vehicle will not turn the corner any better. If the rear wheels slip sideways, then the vehicle has "oversteer". The vehicle turns the corner sharper than the input from the steering wheel. Most of us have experienced these conditions when turning corners on icy streets. Ideally, we want neutral handling where both ends of the vehicle have the same traction.

Correct wheel alignment keeps maximum tire contact with the road at all times and ensures optimum vehicle handling. The wheels and tires should be pointed straight-ahead and vertical. During cornering, the body of the vehicle rolls and the top of the tire tips outwards from vertical. This decreases tire contact and traction. Setting the alignment so the top of the tire is tipped inwards slightly can increase cornering traction. This technique is often used on race cars but will cause uneven tire wear.

Shock absorbers and struts designed to keep the wheel and tire assembly from bouncing after rolling over a bump. The more time the tire stays on the road, the better the vehicle handles.

Shock absorber wear is often not noticed until they start to leak oil or the vehicle begins to bounce badly. Testing of shock absorbers and struts is usually done by pushing down firmly on each corner of the vehicle and letting go quickly. If the vehicle bounces more than 3 times the shock absorbers should be replaced. This test will identify bad shock absorbers, but will not show problems with shocks or struts worn in their normal operating range. If you notice yourself correcting the steering to maintain vehicle direction when driving over small bumps, then new shock absorbers will help.

Heavy-duty shock absorbers are a good investment. They use larger and stronger parts that last much longer and help control the vehicle better. Gas filled shocks use a pressurised gas to prevent the oil inside the shock from foaming. This feature will help if you drive on rough roads where the shocks have to work rapidly and at a high frequency.

Sway bars are part of the vehicle suspension designed to reduce vehicle lean or roll during cornering. They typically use rubber blocks to mount the bar to the body, and metal rods with rubber bushings on the ends to connect to the suspension. The bushings and metal rods are the most common problem area. A visual inspection can quickly locate broken or worn parts and the cost of replacement parts is low.

Finally, how you sit in the vehicle makes a big difference to vehicle handling. A seat that supports well and seatbelts that hold the driver in position will enable the driver to control the steering wheel and vehicle rather than just hanging on for dear life!