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Design Strategies and Tips


Tips: Suspension


The following tips and information focus on how to optimize a race car suspension. Because of the numerous types of suspension, we suggest you read some excellent books that cover this topic in much more detail. Depending on class rules, these suggestions may or may not be valid. Always check your regulations.

General Suspension Design Principles

Suspension Design Tips

General Suspension Design Principals

Unsprung Weight

Unsprung weight is a measurement of the weight of everything outboard of the wishbones or suspension links, plus 1/2 of the weight of the wishbones or links and spring/shock. It has a great effect on handling. The diagram below demonstrates why unsprung weight is so important:

Diagram of unsprung weight

The more weight outboard of the car, the more force bumps exert on the suspension (and ultimately the chassis). This force must be dealt with using springs, dampers and anti-roll bars (described below), and the more force, the more difficult it is to keep the tire planted on the road. This is especially true of lighter weight cars. In the example above, if the car weighs 1000 lbs, a 2G bump would result in a vertical force of 10% of the car's weight. This will at the very least reduce the grip of the car, because the weight of the car is what keeps the tire planted, and pushing a car up into the air with that much force will inevitably reduce the weight on the tire, and hence grip.

Tires

As the first point of contact with the road, the tires work in conjunction with the suspension geometry and weight transfer dynamics to provide grip. Many different types of tires exist, but provided you are building for a specific class, you can easily select a particularly good or popular tire.

The grip provided by a tire is linked to the coefficient of friction (Cf) of the rubber compound and to the tire's construction (Radial/bias). This coefficient indicates the lateral grip the tire is capable of providing for a given weight being placed on it. Racing slicks are very high Cf tires, in the range of 1.0 or more. Street radials, on the other hand, rarely even approach 1.0. So what is in a number? If you were to place 500 lbs weight onto each of four tires with a Cf of 1.0, you could expect 2000 lbs (actually a little less) of lateral grip. Without aerodynamic aids to add to vehicle weight, the car would almost achieve a 1G turn.

Wheels

Of course, the wheel is what the tire mounts on. Wheels also come in a myriad of widths, sizes and materials.

The primary types of wheels used in racing are alloy and steel.

Alloy wheels can be constructed to very minimal weights, as alloying materials such as aluminum and magnesium can be used. They are also generally much more expensive than their steel counterparts, but they also lack the dent resistance of steel wheels. An alloy wheel, when struck by a curb will sometimes shatter, and possibly worse, crack (only later to fly apart!). Nonetheless, for most motorsports series, alloys are the choice.

Steel wheels can also be constructed to amazingly low weights. Their cost is quite a bit less than the alloys, due mostly to lower cost construction. Steel wheels are deformable when struck, and will usually allow air to leak out of the tire, as opposed to shattering. NASCAR, and the general stock car scene use steel wheels due to the extreme forces encountered by 2 ton cars.

Uprights (Wishbone suspension)/Knuckles

The upright or knuckle attaches the wheel, brake rotor, hub, brake caliper and steering arm to the car (of course, the wishbones and control arm(s) do the final attachment to the chassis)

The upright or knuckle determines the king-pin inclination, and the final camber, caster, and toe settings of the wheel and tire. These various factors are demonstrated in the diagram below.

Diagram of suspension geometry factors

Kingpin Inclination determines steering feel to a great extent. In the front view above, the red line on the right represents the center line of the tire/wheel. The kingpin inclination is several degrees, the angle between the center line and the line running through the upright or knuckle. The kingpin inclination determines steering effort, and feedback.

Scrub radius is the distance from the centerline of the tire/wheel to where the kingpin line intersects with the road surface. The larger the distance, the more effort is required to turn the wheel, as the wheel has to "scrub" slightly to turn around the kingpin axis.

Camber is the angle between vertical (perpendicular to a flat road surface) and the "lean" of the tire/wheel. In the diagram above, negative camber of about 2 or 3 degrees is shown. Negative camber is often used to offset the normally positive change in camber as the wheel moves up. The concept of camber is simply to keep the tire contact patch as large as possible through the complete range of suspension motion.

Toe-In/Out is a slight steering angle that is preset into the suspension. Toe-in has the tires pointing slightly toward the center of the car's front. Toe-out has the cars pointing slightly away from the car. In the diagram above, there is zero toe-in/out. Toe-in/out is used to offset the natural change in toe position caused by braking and accelleration.

Caster is the angle from vertical of the upright/knuckle, when viewing the wheel/tire from the side. This angle is used to create a gyroscopic effect on steering. This is easily demonstrated by turning the steering wheel in the car and then letting go of the wheel (Do this in an empty parking lot!). The caster causes the steering to correct itself back to straight ahead, instead of turning, without the need for driver input.

As you can well imagine, all these factors work together to produce a varying contact patch and steering feel in the car. Software exists for designing suspensions, and a computer makes it easy to see changes and how they affect the contact patch of the tire.

Like the wheel and tire, weight here plays an important part as well.

Wishbones/Control Arms

Wishbones and control arms connect the previously mentioned upright or knuckle to the car chassis. The wishbones or control arms (depending on suspension type) affect the previously mentioned factors as well. Camber, castor, and toe are all affected to some degree.

Essentially the wishbones connect to the chassis with rod-ends or spherical bearings, allowing the wishbones to pivot up and down with the wheel's movement and triangulating the suspension to prevent the wheel from moving fore or aft of it's designated position. Outboard, at the upright or knuckle, there are two ball joints, one for each wishbone. See the diagram below for a better visual representation:

Diagram of wishbones/toe link

The toe link, shown in blue above, is attached to the steering rack at the front of the car, and to the chassis at the rear. Toe adjustments are made by varying the length of this link.

Suspension Design Tips

  • Use aerodynamic wishbones on open wheel cars. Open wheel cars, round wishbone tubes will create turbulence and drag. The prefered tubing is oval, which in profile allows the airflow to diverge and converge nicely without turbulence.
  • Use at least a little scrub radius. Some books suggest eliminating the scrub radius. In race cars this can be a problem, as much of the feel about the car's handling can be lost. The scrub radius allows the driver to feel when the tires lose traction, without being "too far gone" to recover.
  • Use strong, high quality rod ends and other fasteners. Fasteners and rod ends can be expensive, but one of the last places you want to save money is on the suspension. These parts are often lamented by amateurs because the parts they use break too often. When selecting rod ends, bear in mind the angle, mounting and location of rod ends has an impact on their longevity.
  • Protect the driver from the suspension. Broken chassis mounts can be deadly or at least crippling. Using aluminum plates along the sides of the chassis will prevent broken wishbones from perforating the driver's legs.
  • Radial vs. Bias ply tires and Camber. Radial tires are more tolerant of static negative camber, or camber that is built into the suspension. If the suspension's range of motion is substantial (more than 2 or 3 inches of bump travel, and 2 or 3 inches of dip), then using more negative camber to compensate for the positive change introduced by the suspension helps. Radial tires will work better with this situation.
  • Minimize Unsprung weight. Unsprung weight, or the weight comprised by tire, wheel and suspension affects how well the tire follows the bumps and dips in the road surface. Using lighter wheels, tires, uprights, wishbones or control arms, and other parts will reduce the weight. The weight of these suspension parts by itself is not so critical as the ratio between the car's sprung weight (chassis, driver, engine, etc) and the unsprung weight. The lower the unsprung weight in relation to the sprung weight, the easier it will be to control the tire/wheel via the springs, dampers (shocks) and anti-roll bars.

Check Back Later For More Suspension Tips.

Got a suspension design tip? Send us feedback.

Read some good suspension design books...

 

(c) 1999 Matt Gartner