Part 2: Four More Steps To Better Handling
In part one of this series (SCC, June '05) you learned four basic steps to improving your car's handling. Those steps were easy. They involved the use of basic performance suspension parts available for most cars and, no doubt, many of you have already taken those steps. This month we explore more advanced suspension tuning through alignment, chassis stiffness and suspension geometry.
Step Five: Add Negative Camber
For a tire to grip well, it must use all of its contact patch. Thanks to problems like tire distortion and compromised suspension geometry, this rarely happens. When a tire is subjected to side load, its sidewalls flex, digging the outside tread into the ground and lifting the inside.
If you drive hard, you've probably noticed the outside edge of your tires gets chewed up much faster than the rest of the tread. That means the tire isn't using all of its contact patch effectively.
As a car rolls in a corner, the chassis rolls the tire onto its outside edge, making the problem worse. Keeping the tires flat on the road is the primary reason to reduce roll. In part one we listed several ways to do this; the easiest ways are to increase spring rate or use larger anti-roll bars.
The primary tool, however, used for combating tread lift is to dial in more negative camber. Camber is the inward or outward tilt of the tires when looking at them from the front. If the top of the tire leans outward, camber is positive. If the top of the tire leans inward, camber is negative.
Dialing in negative camber helps combat tread lift and wheel tilt. The trick is to add just enough negative camber so the tread stays flat and 100 percent engaged with the ground under side load and roll. But, adding too much negative camber will hurt more than it helps. Too much negative camber will:
1. Reduce braking traction
2. Reduce acceleration traction if it's applied on the drive wheels
3. Increase the tendency to follow cracks and grooves in the pavement
4. Increase wandering caused by road crown
5. Affect tire wear; the insides of the tire tread will wear faster with more negative camber if you don't corner hard. Conversely, if you constantly corner hard, your tires will wear more evenly and last longer
Your car and your driving style together determine how much negative camber you need. Aggressive drivers should use more. Those concerned about tire life should use less. Suspension design also matters. MacPherson strut cars need more negative camber to work well under cornering load; unequal-length A-arm and multilink suspensions generally need less negative camber. Softer suspended cars that lean over in turns generally need more negative camber to grip well.
The following table provides a rough guideline on how much camber to use based on your driving style and accounting for tire wear. Surprisingly, the baseline settings are roughly the same for all typical chassis layouts: front-engine front-wheel drive, front-engine rear-wheel drive, front-engine all-wheel drive and mid-engine rear-wheel drive. Keep in mind that many drifters prefer a road-racing setup to create even tire wear and allow the most possible control.
Unfortunately, camber is not adjustable on most modern cars. Even if camber is adjustable, it's rarely adjustable enough to align a lowered car correctly. The best way to adjust camber on the typical MacPerson strut is to use a camber plate. MacPherson strut camber plates use an adjustable top mount that locates the upper shock mount in a retainer plate that slides laterally on a slotted track.
A less expensive (but inconvenient) way to make your camber adjustable is to weld a U-channel bracket to the lower control arm mounts at the chassis and install a lower control arm bolt with fixed eccentric cams riding in the U-channel. Turn the bolt and the eccentrics move the lower control arm in and out to change camber.
Simply enlarging one of the two lower mounting bolt holes in the strut housing about 1/16-inch with a drill, leaning the upright into the strut and retightening the bolts can give you quite a bit of no-cost camber adjustment.
Avoid using undersized shaft or eccentric bolts sold as crash bolts. Crash bolts are sold as a cheap way to adjust camber on crash damaged cars. Because of the small shaft diameter, they usually stretch and allow the camber adjustment to slip under the load of hard driving with sticky tires.
Cars with a multilink or unequal-length A-arm suspension can sometimes use adjusting shims in the upper control arm mount to adjust camber. Many multilink or unequal-length A-arm cars have adjustable camber from the factory. And for some popular cars like the 240SX and 300ZX, there are plenty of adjustable links on the market to adjust camber.
Adjusting camber is well worth the effort. Optimizing the camber for your car and driving style can often make a bigger difference in the amount of grip the car can generate than any other mod except tires.
Step Six: Tune Your Toe
Toe refers to the direction a car's tires are pointed relative to each other when viewed from above (see graphic, pg. 150). Toe-in means the front of the tires are closer to each other than the rears. The opposite is toe-out. Toe is measured in inches relative to straight ahead, or zero toe. With zero toe, a car's tires are exactly parallel to each other.
Fine-tuning toe settings will allow a measure of control that's often overlooked. It also has a significant effect on how a car behaves in a corner. Front toe settings make a big difference in how a car handles in the first third of the turn, the critical turn-in phase where cornering force is initiated. Rear toe settings can be critical for allowing the driver of a rear-wheel-drive car to accelerate harder and sooner out of a corner.
Like all chassis tuning, too much of a good thing will cause problems. Too much toe-in or toe-out will create tire wear on the inside and outside edges of the tire. Any toe setting past 1/8-inch will cause excessive tire wear. Aggressive toe has probably ruined more tires on lowered cars than any other chassis adjustment.
Below are guidelines for setting toe and how it can affect feel and handling.
Front Toe-Out
Just Right
Reduced understeer at turn-in Improved steering response Counteracts natural tendency for front- and all-wheel-drive cars to toe-in under throttle load
Too Much
Instability during braking Straight-line instability, especially over single-wheel bumps or split-traction surfaces Unrecoverable understeer
Front Toe-In
Just Right
Generally helps make the car feel more stable
Too Much
Wandering under braking Refusal to turn in or rapid turn-in followed by understeer
Rear Toe-Out
Just Right
Easy midturn rotation. Less front tire load
Too Much
Violent on-throttle oversteer on RWD cars. Can help drift cars
Violent lift-throttle or trail-braking rotation
Rear Toe-In
Just Right
Easily controlled power oversteer in rear-wheel-drive cars
Too Much
Sluggish response. Midcorner understeer Instability at turn-in
Every car is adjustable for front toe through the steering tie rod ends. All multilink and some strut cars are adjustable for toe front and rear. Cars that use a live or beam rear axle must have the axle or axle housing bent to adjust toe by an experienced chassis shop.
Below are some typical toe adjustments for different cars, tire wear expectations and styles of driving.
Aggressive Street Driver
Front
FWD/AWD: 0 RWD: 1/16" In
Rear
FWD/AWD: 0 RWD: 1/8" In
Weekend Hot Lapper
Front
FWD/AWD: 0-1/8" OutRWD: 0
Rear
FWD/AWD: 0-1/8" OutRWD: 1/8" In
Racer Only or serious drifter
Front
FWD/AWD: 1/8-1/4" out RWD: 0
Rear
FWD/AWD: 0-1/4" out RWD: 0-1/8" in
Step Seven: Make It Stiffer
Chassis stiffness is a critical element in suspension tuning. A flexible chassis doesn't allow the suspension to keep the tires in contact with the road and is less responsive to critical suspension changes like increased spring and anti-roll bar rates.
The best way to combat chassis flex is by seam welding every spot-welded panel in the unibody and installing a welded-in roll cage. Unfortunately, these are also the least practical ways to solve the problem. Chassis braces are better.
The most common brace is the strut tower brace, which connects the strut towers in the engine compartment. Triangulated strut tower braces are the most effective and tie both shock towers to the firewall. There are also lower crossmember braces and subframe braces available for most cars. Harness bars, which are stout bars that connect to the upper shoulder harness bolts and the floor, also significantly stiffen the chassis. Hinge braces tie the shock towers to the sturdy base of the A-pillar via the door hinges. These make a huge difference.
Any chassis bracing has the potential to bump your car up several classes in virtually any competition series (especially autocross), so be sure you read the rules if you plan to add braces to a car you race.
Another way to stiffen a chassis is to inject Foamseal-brand two-part catalyzed polyurethane structural foam into the hollow structural members of the unibody. Although it's time consuming and messy, it can produce significant gains in chassis stiffness without resorting to a roll cage. Some manufacturers use this treatment to increase chassis stiffness from the factory.
Beware of the all-too-common inferior chassis brace. These are usually spindly-looking devices with small tubes and no gussets.
Contrary to popular Internet wisdom, it's impossible to make a chassis too stiff. Fortunately, chassis braces are rarely expensive and have few negative side effects. Certain chassis braces in combination with aggressive suspension tuning will cause handling problems, which should be tuned out through suspension adjustment. You paid good money for your adjustable suspension, so be sure you adjust it correctly.
Step Eight: Adjust Caster
Every car's front wheels turn on pivots attached to the suspension. Caster is the angle of the imaginary line drawn through the pivots. It's measured in degrees relative to vertical.
If the top pivot point is behind the lower pivot point so the caster angle slopes backward like on a bicycle (as viewed from the side), the caster is positive. If the angle slopes forward (which it never does), the caster is negative.
Kingpin Inclination Angle (KIA) is the angle of the line drawn through the same pivots as caster but viewed from the front of the car. KIA always slopes toward the center of the car and is expressed as degrees from the vertical plane. KIA is a design constraint and is not adjustable. Caster and KIA together affect straight-line stability and camber while the wheel is turned.
Increasing positive caster projects the Dave Point (the point where the steering axis meets the ground) further in front of the tire's contact patch. This distance is called caster trail. When the tire's contact patch is behind the Dave Point, the tires want to stay centered behind the Dave Point the same way a shopping cart's casters naturally align its wheels in the direction of travel.
Like the shopping cart caster, the distance between the Dave Point and the tire's contact patch creates a torque reaction, which causes the steering to self-align. The driver perceives this reaction as greater stability and on-center steering feel. More positive caster means a bigger torque reaction as well as increased stability and feel.
Unfortunately, this virtual lever arm also increases torque steer on front-wheel-drive and all-wheel-drive cars because the force is reversed when the wheel is driven. This is why most front- and all-wheel-drive cars don't have as much caster as rear-drive cars.
Lots of positive caster causes the outside wheel to gain camber in a turn when you need it most. Think of a parked chopper with the wheel flopped to the side. That's an extreme example of negative camber gain with positive caster. Too much positive caster can increase tire loading and understeer.
KIA increases stability by making the axle path travel in an upside down, U-shaped arc (when viewed from the side) as the steering wheel is turned. The axle is at the apex of the arc when the steering wheel is centered (see graphic, pg. 154). As the wheels are turned, they actually lift the front of the car. This lifting effect increases effort the more the wheel is turned, which contributes to steering feel and straight-line stability. KIA also tilts the wheels outward in a turn, which reduces camber.
Positive caster and KIA are both huge considerations for design engineers. Balancing positive caster's ability to increase camber in a turn with KIA's ability to decrease it is critical to achieving the right combination of stability and steering feel.
Caster can be adjusted with tension rods, adjustable arms and universal (racing) camber plates like those from Ground Control. These parts are available for almost any car that's been raced.
Here are some basic guidelines for adjusting caster:
Positive Caster
Just Right
Improves straight-line
stabilitySharpens turn-in Improves traction everywhere in the turn
Too Much
Very high steering effortProvides sharp turn-in but increases understeer from midturn onward Increases torque steer in front- and all-wheel-drive cars Caster adjustment guidelines
Caster Adjustment Guidelines
Degrees Positive
FWD/AWD: 3-4RWD: 4-10
In the next installment, we'll continue our discussion on suspension geometry, and begin an in-depth look at dampers.
In the next installment, we'll continue our discussion on suspension geometry, and begin an in-depth look at dampers.
Photo Gallery: Suspension Tuning Negative Camber - Sport Compact Car Magazine
