Race Car Suspension Basics and Design Tips
The suspension on a race vehicle serves multiple purposes:
- It provides a stable platform from which to control the vehicle
- It provides a way to isolate the chassis and driver from the shocking jolts that the tires experience going over anything but a glass-smooth surface.
- It provides a way to keep all the vehicle’s tires in contact with an uneven surface.
- It provides damping of oscillations that rubber tires, springs and uneven surfaces naturally create.
Many versions of suspension have been created over time to resolve deficiencies, but in general they all seek to control the movement of the tires in three ways:
- Laterally – Controlling side-to-side movement
- Longitudinally – Controlling forward/backward movement
- Vertically – Controlling up and down movement
Suspensions accomplish this using links and structures that locate the wheels/tires in a specific “geometry” relative to the race vehicle. The geometry dictates the behavior the tires/wheel and chassis exhibit when accelerating, braking and turning.
- 1 Suspension Components
- 2 Suspension Characteristics and Geometry
- 3 Suspension Types
- 4 Suspension Design Tips
Let’s have a look at the components that make up a suspension.
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 every tire relies upon its contact patch with the road (Shown in diagram T1 below) to create the friction needed. Generally, the larger the contact patch, the larger the amount of friction created.
The grip provided by a tire is also based on the coefficient of friction (Cf) of the rubber compound and 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. A Cf of 1.0 means it is capable of providing 1 lb of lateral grip for 1 lb of vertical load on it.
Racing slicks (tires with no tread) are very high Cf tires, in the range of 1.0 or more. Street (treaded) radials, on the other hand, rarely even approach a 1.0 Cf. If you were to place 500 lbs weight onto a tire with a Cf of 1.0, you could expect 500 lbs (actually a little less) of lateral grip. Without aerodynamic aids to add to apply further weight to the tire, the vehicle could almost achieve a 1G turn.
The wheel is what the tire mounts on and each type of wheel has its own particular characteristics depending on its width, diameter and construction 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 crack. Nonetheless, for most motorsports series, alloys are the choice.
Steel wheels can also be constructed to very low weights and 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.
Wheels, aside from their width and diameter, have an important design characteristic called “Offset”.
In the wheel/tire cutaway diagram WH1 below, the sample wheel shows a red line that represents the mounting face for the wheel—the face with the lug holes that we bolt onto the hub of the vehicle.
The yellow dotted line represents centerline of the wheel and “Zero offset” from the centerline. If we move the mounting face toward the vehicle, as show on the left in the diagram, we create “Negative Offset”. If we move the mounting face away from the vehicle, as shown on the right in the diagram, we would create “Positive Offset”
Offset is important in relation to the design of the upright/knuckle, as it determines scrub radius (see more info below).
It goes without saying that while the gas pedal on your race car is the preferred pedal to push, the brakes are of vital importance as well.
Two types of brakes are available—Disc and drum. Both types use friction to turn the kinetic energy of the moving vehicle into heat. What makes one type of brake better than the other is the effectiveness of each type in dissipating or shedding the heat generated. Too much heat and the brake pad/shoe material will generate less friction, leading to what is termed “Brake fade”.
The disc brake, shown in diagram B1 below, produces more reliable stopping power under racing conditions because its rotor (the surface against which the brake pad generates friction and heat) is exposed to the air flow. This dissipates heat to the open air quickly.
A disc brake system works as shown in figure B2 below. The driver presses the brake pedal, which forces a piston in the master cylinder to compress the brake fluid (Yellow). The fluid runs inside a brake line to the caliper (Green) where two pistons (Blue) with attached brake pads (Red) are forced against the spinning brake rotor (Grey), generating friction and slowing the brake rotor and its attached wheel.
The drum brake, shown in figure B3 below, utilizes semi-circular shoes that are forced against the inside of the brake drum by a slave cylinder.
With the brakes released, there is a small air-gap space between the shoes and the drum as shown in figure B4 below.
With the shoes engaged as shown in figure B5 above, the brake creates high levels of stopping power through large amounts of friction. However, because the drum is “closed” compared to the exposed rotor on the disc brake, more heat is retained, which leads to brake fade sooner.
Diagram B6 below shows a drum brake in a hydraulic system. The driver presses the brake pedal, which forces a piston in the master cylinder to compress the brake fluid (Yellow). The fluid runs inside a brake line to slave cylinder (Blue) which contains two pistons (Pink). These pistons are attached to the brake shoes (Red/light blue). The pistons force the brake shoes against the drum (Green), generating friction and slowing the brake drum and its attached wheel.
Drum brakes are cheaper to manufacture and are generally used in conjunction with a live axle. However, disc brakes are the preferred for any type of race car where they can be fitted as they have less mass and better cooling.
The upright or knuckle attaches the wheel, brake rotor, hub, brake caliper and steering arm to the vehicle as shown in diagram KU1 below. The upright also locates these components in space.
The design of the upright or knuckle determines the geometry on the “outboard” side of the suspension. (The mount points on the chassis and wishbones/links form the “inboard” side of the suspension, and provide their own contribution to the overall geometry of the suspension.)
Diagram KU1 shows an example of an independent wishbone suspension that is not driven. The upright (Yellow) is attached to the vehicle using the upper and lower wishbones which have ball joints or rod-ends. This allows the upright to move vertically and to rotate about the king pin axis (See below).
Integrated into or attached to the upright is the spindle. Bearings (Orange) are inserted into the hub (Red) and it is slid over the spindle and held in place by a retaining nut. The brake disc (Blue) slides over the lugs (Threaded bolts) extending from the hub. The brake caliper (Light blue) is attached to the upright using a bracket.
Controlling the steering or toe angle of the upright is the steering/toe link which has a rod end that fastens to an arm (Purple) on the upright.
Diagram KU2 shows an example of an independent wishbone suspension for a driven wheel. As with the non-driven version, the upright (Yellow) is attached to the vehicle using the upper and lower wishbones which have ball joints or rod-ends. This allows the upright to move vertically and to rotate about the king pin axis (See below).
In order to drive the wheel, a half-shaft or driveshaft (Gold) extends from the chassis and uses a CV joint to enable suspension movement while driving the wheel. A splined shaft (Green) extends from the CV joint and passes through the upright. Two bearings (Orange) are used to support the shaft inside the upright. The hub (Red) slides over the spline on the shaft and is secured using a nut.
The brake disk (Blue) then slides over the lugs extending from the hub. The brake caliper (Light blue) is attached to the upright using a bracket.
Controlling the steering or toe angle of the upright is the steering/toe link which has a rod end that fastens to an arm (Purple) on the upright.
Wishbones, links and axles connect the previously mentioned upright or knuckle to the car chassis. Depending on suspension type, they behave in different ways, but always with the goals of controlling lateral, longitudinal and vertical motion of the wheels.
Wishbones look just like the name suggests. Diagram WL1 below shows a wishbone highlighted in yellow.
Links (sometimes called radius rods) are rods that are used to enable the wheel to move in a particular axis. Diagram WL2 below shows how a live axle uses links to control its movement. The parallel rods allow the wheels to move up and down. The lateral rod controls lateral movement of the axle.
Axles are used to connect the left and right wheels at the front or rear of the vehicle. One of the oldest ways of suspending a vehicle, the axles as shown below have been used extensively on road cars and trucks and on race cars, especially those based on production vehicles.
Diagram AX1 below shows a driven live axle. This axle is used primarily at the rear of a rear-drive vehicle. The differential accepts power through an input shaft, and transfers it through the axle shafts to the wheels. The axle shafts are housed inside of axle tubes which provide protection from the environment and the strength needed to support a vehicle chassis. The axle shafts connect to hubs at either end and these in turn mount the wheels and tires. As shown above in diagram SC2, the live axle uses links and springs/dampers to connect to the chassis.
Diagram AX2 below shows a beam axle, which can be used as a rear axle in front-wheel drive vehicles or as a front axle in rear wheel drive vehicles. It uses a simple beam that attaches to the chassis through links and springs/dampers. At the ends of the beam can be steerable knuckles/hubs (for a front axle) or non-steerable hubs (for a rear axle.). In the example AX2 below, the beam has stub axles at it ends to which hubs are mounted on bearings.
Diagram AX3 below shows a go-kart axle, which uses a one piece shaft without a differential. This is the “Simplest” of axle types, and consists of a single hollow tube or solid rod supported by bearings which are housed in supports that attach to the chassis. To the ends of the axle are affixed hubs for mounting wheels/tires. Mid-mounted on the axle is a sprocket hub to which a chain sprocket is bolted and driven by the go-kart motor.
Due to the single axle, the inside wheel travels at the same speed as the outside even when going around corners. This would normally have a detrimental effect on turning because the rear wheels would push at the same speed in a straight line while the front wheels are trying to turn. Go-karts get around this problem by lifting the inside rear wheel in corners using a special front suspension geometry. The inside rear wheel is spinning above the road while the kart is turning.
In addition to be vital to controlling the race car/vehicle, the steering components are an integral part of the suspension system and suspension geometry. As shown previously in Diagrams KU1 & KU2 in the Knuckles/Uprights section, the upright or knuckle has attached to it a “Toe Link” or “Steering Link” that controls the angle of the wheel/tire. This steering link needs to connect to the steering system to enable the driver to change the steering angle and turn the car.
There are two primary types of steering systems—Rack and pinion and Recirculating Ball. The Rack and Pinion system is pictured below in diagram ST1. In a rack and pinion system, there is a flat gear (the “Rack”) shown in yellow. Attached to the ends of the rack are rod-ends which connect to the steering links.
Meshed to the rack is a circular gear (the “Pinion”) which is attached to the steering wheel via a shaft. When the driver turns the steering wheel, the pinion gear turns and moves the rack either left or right. This in turn moves the steering links left or right and because they are connected to the upright/knuckle, the wheel and tire change their angle.
The advantage of rack and pinion is its simplicity and light weight, both of which are desirable in a race car’s design. The Recirculating Ball system is more compact than the rack-and-pinion, but it is usually heavier. Some race vehicles use Recirculating Ball, especially production-based trucks and cars.
Steering systems can also come with power-assist which provides most of the effort to steer the vehicle. Power assist is provided by a hydraulic pump driven by the engine. When the driver turns the wheel, valves within the steering system use the hydraulic pressure to assist in moving the steering gears.
Despite making the driver’s job easier, the use of power assist has been noted to reduce the “feel” feedback of the vehicle’s handling through the steering system. Power-assist also adds weight, but despite this, it should be considered for vehicles where steering effort is large or the duration of racing is long (i.e. Endurance).
Suspension Characteristics and Geometry
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 therefore usually includes the tire, wheel, brake, knuckle/upright and 1/2 of the suspension links/wishbones. It has a significant effect on handling. Diagram UW1 below demonstrates why unsprung weight is so important:
As the diagram shows, the more weight outboard of the car, the more force a bump will exert on the suspension (and ultimately the chassis). The unsprung weight on the left is 30 lbs (13.6 kg) and when the tire encounters a 2G (2 x gravity) bump, it applies 60 lbs (27.2 kg) of vertical force into the suspension. Even worse, the unsprung weight of the suspension on the right is 50 lbs (22.7 kg) and when the tire encounters a 2G bump, it applies 100 lbs (45.4 kg) of vertical force.
This force is handled using springs, dampers and anti-roll bars, but inevitably the force is transmitted into the chassis. Therefore, the more “upward” 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 (454 kg), 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 upward inertia created by the bump will reduce the weight on the tire briefly after encountering the bump. As the vertical load on the tire dictates how much traction it produces, some traction will be lost.
Kingpin Inclination provides steering feel and affects the effort required to steer the car. In the front view diagram KP1 below, the red line on the right represents the center line of the tire/wheel. The red line on the left represents the kingpin inclination line, which runs through the upright/knuckle attachment points. The angular difference between the two red lines is the angle of the kingpin inclination.
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 tire has to “scrub” slightly to turn around the kingpin axis. Diagram SR1 below shows a red dimension to indicate the scrub radius.
Camber is the angle between vertical (90° perpendicular to a flat road surface) and the “lean” of the tire/wheel. In diagram CAM1 below, negative camber of about 2 or 3 degrees is shown. Depending on the suspension type, camber can change as the wheel/tire rises and falls with suspension movement. If the geometry of the suspension is well designed, the camber change will be optimized to keep the contact patch of the tire as large as possible, providing the most grip possible. Generally speaking, the contact patch and grip begin to decrease as soon as the camber exceeds a small amount, either positive or negative.
Toe-In or Toe-out is a slight angle of a wheel/tire measured from a line running longitudinally (along the length of the vehicle). Diagram T1 below shows an exaggerated version of toe from a perspective of looking down onto the wheel/tire from above. Toe-in has the wheel/tire steering slightly toward the center of the vehicle. Toe-out has the wheel/tire steering slightly away from the vehicle center.
Toe-in/out can be used to offset the natural change in toe position caused braking and acceleration and to assist a vehicle with corner turn-in.
Caster is the angle, measured from vertical (90° perpendicular to a flat road surface), of a line that runs through the mounting points of the upright/knuckle, when viewing the wheel/tire from the side. Diagram CAS1 below shows the Caster line running through the upright mount points.
This angle is used to create a gyroscopic effect on steering. This can be demonstrated by turning the steering wheel in a vehicle and then letting go of the wheel. The caster causes the steering to correct itself back to straight ahead, instead of turning, without the need for driver input.
There are dozens of variations of suspension types used on race and passenger vehicles. Depending on the type of racing, the racing surface and evolution of the vehicles, the suspension is designed to accommodate multiple requirements. Each suspension type has its advantages and disadvantages—let’s take a look at a few of the primary types and their pros and cons.
Front Suspension Types
Beam Axle Suspension
The beam axle is just as its name would suggest—It is a beam or tube connecting the two front wheels. As shown in diagram BA1, connections to the chassis are provided by link rods (in red) which allow for vertical movement of the suspension and a lateral locating link rod (in yellow) that prevents lateral movement of the axle while still enabling vertical movement. The links are usually “Leading” links as they attach to the chassis behind the front beam axle tube.
This type of suspension provides simplicity and generally low construction costs. It lends itself well to adjusting the offset of the wheels relative to the chassis, which is useful in circle track racing. It is used today primarily in Sprint and Midget cars and in pickup trucks.
Beam axles excel in two areas: their ability to maintain zero camber during body roll and their ability to provide high strength. Unfortunately, the strength comes at a price of extra weight. The way the beam axle connects to the chassis has a big impact on how this suspension behaves. For instance, steering angle can change with things like road bumps if the linkage is not designed to be neutral during suspension movement. If appropriately designed, these challenges can be largely overcome.
Many production cars (especially if they have Front-drive) use the MacPherson strut suspension. The design uses a lower A-arm and the spring/shock absorber to connect the upright/knuckle to the vehicle. Both parts provide lateral and longitudinal locating while still permitting vertical movement. They are simple and relatively low cost to construct which is why Macpherson strut suspensions are used on many economy cars. However this suspension type does suffer from some disadvantages.
Diagram SS1 below shows the basic suspension layout:
If you put wide tires on a MacPherson strut suspension, the scrub radius increases, which in turn increases not only the overall steering effort required, but also the loading on suspension members that can lead to damage.
This suspension design also requires more vertical height to the suspension packaging due to the strut/shock, which won’t work for some vehicle designs.
Unequal Length Double Wishbone
As its name aptly describes, the unequal length double wishbone suspension, shown in diagram UEL1 below, consists of two wishbones of different lengths (The top is shorter) connected to the chassis inboard and the upright/knuckle outboard. With a good top/bottom wishbone geometry and good upright/knuckle design, this suspension type maximizes the tire contact patch by gaining camber as the vehicle body rolls. It also eliminates most of the behavior problems of other suspension designs, making it desirable when precise handling is required.
The price paid for such a design is increased cost to construct, but in racing it is very much worth the additional cost.
Rear Suspension Types
Used extensively in the past on rear wheel drive passenger vehicles, the Live Axle continues to be used because of its strength and simplicity. The basic design shown in diagram LA1 below uses a tube which joins the two rear wheels. In the middle is a differential that apportions power to each wheel. As with the beam axle, links or leaf springs are used to join the axle to the chassis, allowing vertical movement and controlling longitudinal and lateral movement. However, in rear axle situations, the links are usually considered “Trailing” (They mount to the chassis in front of the axle)
Several sub-types of Live Axle suspension exist, with each differentiated by the way they connect to the chassis and the approach they use to managing suspension behavior.
Hotchkiss Drive suspensions (as shown above in diagram LA2 above) use leaf springs to locate the axle and support the vehicle. Separate shocks are used to dampen oscillations.
Link suspensions (Diagram LA1) use links that allow suspension movement vertically, but limit that movement longitudinally (front-back) and laterally (left-right).
Torque arms suspensions use a long arm compared with link type suspensions, which enables the torque applied to the axle to be controlled in a different manner.
All these sub-types (and more) essentially try to keep the wheels planted on the ground, as free of oscillations and wheel hop as possible, and as straight and true with the direction of the vehicle as possible.
Unequal Length Double Wishbone
Just as with front suspension, the unequal length double wishbone suspension can be used for rear wheels. The flexibility in the design of the suspension geometry allows the rear suspension to be matched to the front suspension, and each wheel tuned independently.
When used as a driving wheels suspension, the upright/knuckle is connected to a half drive shaft (As shown below in diagram UEL2) with an outboard CV (Constant velocity) joint that rises and falls with the wheel.
Suspension Design Tips
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, and suspension components will reduce the weight. The weight of these suspension components by itself is not so critical as the ratio between the vehicle’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.
On vehicles where the suspension is exposed and speeds are high, moving components under bodywork and streamlining those still left exposed can provide an advantage.
Use Scrub Radius For Feel
The feel of what the tires are doing in relation to the road is transmitted through the suspension, into the chassis and steering, and finally to the driver. Scrub radius helps this feel by providing feedback from the contact patch.
With a small amount of scrub radius, the tire is turned about the kingpin axis, slightly offset from the tire’s centerline. This creates a small tire/road friction resistance that in turn translates into feedback from the road. The scrub radius can enable the driver to feel when the tires lose traction, without being “too far gone” to recover.
Use strong high-quality hardware
Suspension components, especially fasteners and rod ends can be expensive. However, they are absolutely critical not only to safety, but success. You should use best practices for joint design–Using a good component in a poorly mounted way is no better than using a poor component in the first place. Strength and durability requirements should always be considered relative to the vehicle and anticipated loads.
Protect the driver from the suspension
In vehicles where wheels and suspension can enter the vehicle when broken (i.e. Driver situated beside suspension mounts), safety can be increased through anti-intrusion panels that will deflect broken wishbones or links.
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 a positive change introduced by the suspension helps. Radial tires will work better with this situation.