Shocks can improve response and help balance the car right after the initiation of a turn; soft shocks get the car to a steady point of weight transfer faster. When stiff, they can delay weight transfer. Thus, shocks affect how the car feels at turn-in and also how it feels past mid-turn. A car with the shocks set fairly hard will turn in sharply. If the shocks are set too hard, the balance might change later in the turn in an unpredictable way as the heavy damping slows the body roll and weight transfer.
Tire sizing can also affect chassis balance. Installing a wider tire on the end that needs traction most is obvious. Rear-engine Porsches have wider rear tires to help prevent oversteer. Powerful rear-wheel-drive cars tend to have wider tires in the rear than in the front. Many front-wheel-drive autocrossers and road racers install a wider front tire to get more front grip.
At the limit of adhesion, a car that slides all four wheels without brake or throttle input is considered ideal; it also doesn't exist. Being able to provoke slight oversteer by lifting the throttle and more aggressive oversteer with slight braking while cornering at the limit is useful as well. Being able to slow rotation with slight throttle application makes front-wheel-drive cars easier to control.
Rear-wheel-drive cars should also be able to invoke oversteer with large applications of throttle. This kind of balance gives the skilled driver the most options.
Step four: Reduce weight transfer
Weight transfer is the movement of weight from the inside to the outside wheels during cornering. Excessive lateral weight transfer hurts handling. It's caused by centrifugal force working on the chassis' center of gravity, which loads the outside wheels and unloads the inside wheels.
Contrary to popular belief, very little weight transfer can be attributed to lean in a corner. Even at large roll angles, weight transfer due to roll is quite small. So lowering a car's center of gravity and widening its track width will reduce weight transfer more effectively than reducing roll angle.
Lowering is best accomplished with shorter springs. The smartest approach is to use shorter springs and shorter-bodied shock absorbers or struts that maintain stock compression travel at a lower ride height. Excessive lowering can change suspension geometry, causing positive camber during roll and contributing to increased bump steer.
The easiest way to increase track width is to use wider wheels and tires that fill out the wheel wells. This also increases the amount of rubber on the road. Using wheel spacers and wheels with a more positive offset can also increase track width. Any positive change in track width, and therefore offset, increases the scrub radius. Scrub radius (see diagram on page 128) is the distance from the centerline of the tire's contact patch to the point where the steering axis intersects the ground, also known to regular readers as "The Dave Point." Increasing the scrub radius allows forces generated by the tire more leverage to act on the steering. To the driver, this translates as torque steer under acceleration and braking.
To minimize the change in scrub radius, it's important to try to increase wheel width to the inside as well as the outside by paying close attention to the wheel offset. This puts more rubber on the road and increases the track width while maintaining the same scrub radius.
Increasing track width also changes the motion ratio of the suspension, which effectively reduces spring and anti-roll bar rates. Lastly, a very positive offset wheel puts a large strain on wheel bearings, ball joints and steering linkage, making them wear much faster. All of these are good reasons not to go overboard with this method of increasing track width. A good rule of thumb is it's safe to use the largest wheels and tires you can stuff in your stock wheel wells by rolling the inner fender flange.
A good guideline is to increase the track width and lower the car more on the end that slides first in a corner. An understeering, nose-heavy, front-wheel-drive car can use more track width and a lower ride height in the front. A powerful rear-engine car can be lower and have more track width in the rear. This play on physics will help reduce weight transfer in both cases.
In the next installment, we'll discuss more basic mods you can do to improve handling, some of the common deadly sins of modifying your suspension and basic tips on suspension geometry.
Other Installments:
Making It Stick Part 1: Four basic steps to better handling
Making It Stick Part 2: Four more steps to better handling
Making It Stick Part 3: It's all in the geometry
Making It Stick Part 4: More lessons in suspension geometry
Making It Stick Part 5: Damper fundamentals
Making It Stick Part 6: More advanced dampers
By Ti Tong
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