Introduction | Suspension Performance Comes Down to What the Four Tires Can Do
Suspension performance is often judged by how much a car rolls in corners, how little it dives under braking, or how quickly it responds to steering inputs.
Yet the forces that actually accelerate, slow down, and turn a car are generated by its tires.
To understand suspension performance, we need to consider how much force each of the four tires is capable of generating at any given moment.
As a car accelerates, brakes, or corners, the vertical loads on its four tires change. Road irregularities also cause the wheels to move up and down, altering tire orientation and contact with the road.
The suspension manages these changes, helping the tires make effective use of their available grip.
To understand how it does this, we will examine load transfer, tire characteristics, and the mechanical functions of the suspension.
1. Load Transfer Is a Consequence of Vehicle Motion
When a car brakes, vertical load increases on the front tires and decreases on the rear tires.
During acceleration, the relationship reverses.
When cornering, vertical load increases on the outside tires and decreases on the inside tires.
These changes are known as load transfer. They result from the forces and moments associated with accelerating, braking, and cornering.
Even if a vehicle had no suspension travel, braking and cornering would still redistribute vertical loads among its tires because the forces involved create moments about the vehicle’s center of gravity.
On level ground, assuming steady acceleration or cornering and no significant changes in aerodynamic loading, the overall amount of load transfer is determined primarily by vehicle mass, center-of-gravity height, acceleration, wheelbase, and track width.
For example, if the same car brakes at the same deceleration rate, the total longitudinal load transfer is essentially independent of suspension stiffness under these simplified conditions.
However, suspension characteristics affect how lateral load transfer is shared between the front and rear axles.
During transient maneuvers, springs and dampers also influence body motion and how vertical loads develop over time at each tire.
It is therefore useful to distinguish between two aspects of load transfer:
- The overall load transfer produced by vehicle motion
- How that load transfer is distributed among the four tires and how those loads change over time
The suspension plays a major role in the second.
2. How Vertical Tire Load Affects Grip
Tires generate the forces needed for acceleration, braking, and cornering through their contact with the road.
As the vertical load on a tire increases, the maximum force it can generate generally increases as well.
However, the relationship is not directly proportional.
This characteristic is known as tire load sensitivity.
Consider two identical tires on the same axle, initially carrying equal vertical loads.
During cornering, load transfers toward the outside tire.
The outside tire can now generate more lateral force because its vertical load has increased.
The inside tire, meanwhile, loses some of its lateral-force capacity as its load decreases.
For typical tires, the gain in the outside tire’s maximum lateral force is smaller than the loss at the inside tire.
As a result, assuming other conditions remain unchanged, the combined lateral-force capacity of the two tires tends to decrease.
This leads to an important principle:
Load transfer changes the grip capacity of individual tires, and the way load is distributed affects the total grip available at each axle.
If the front axle takes a larger share of lateral load transfer, the difference in vertical load between its inside and outside tires increases.
Tire load sensitivity then tends to reduce the front axle’s combined lateral-force capacity.
If the rear axle takes a larger share, the corresponding reduction occurs at the rear.
This difference contributes to a car’s understeer and oversteer characteristics.
For a more detailed application of tire load sensitivity and weight distribution, see Why Front-Wheel-Drive Cars Tend to Understeer ★ Weight Distribution and Acceleration.
The maximum combination of longitudinal and lateral forces that an individual tire can generate can also be represented using the friction circle.
As vertical tire load increases, the friction circle generally becomes larger, indicating greater available force.
This relationship is explained in What Is the Friction Circle? ★ Understanding Braking, Cornering, and Acceleration on Track.
3. Three Ways Suspension Influences Tire Grip
The suspension affects how effectively the tires can generate force through three closely related mechanisms.
3.1. Distributing Load Transfer Among the Four Tires
Suspension springs, anti-roll bars, and geometry all influence how lateral load transfer is shared between the front and rear axles.
The distribution of roll stiffness is particularly important.
Increasing front roll stiffness generally increases the front axle’s share of lateral load transfer.
Increasing rear roll stiffness generally shifts more of that load transfer toward the rear axle.
Through these changes, suspension setup affects the balance of available grip between the front and rear tires.
3.2. Controlling How Tire Loads Change Over Time
Road irregularities and driver inputs produce motion in both the vehicle body and the wheels.
Springs generate restoring forces in response to displacement, while dampers primarily generate forces in response to the speed of relative suspension movement.
Their characteristics influence body pitch, roll, wheel movement, and fluctuations in vertical tire load.
Appropriate spring and damper characteristics help manage the response of the body and wheels to changing inputs.
This allows the available tire grip to be used more consistently.
3.3. Managing Tire Orientation
As the suspension moves, wheel camber and toe can change.
The way these angles change is determined by suspension geometry.
Tire orientation affects how the contact patch works and how much lateral force the tire can generate.
Suspension design must therefore consider not only load distribution and load fluctuations, but also the orientation of the tires as the wheels move.
The relationship between static wheel alignment and suspension geometry, including camber, toe, roll centers, and bump steer, is explained in What Is the Difference Between Alignment and Suspension Geometry? ★ A Beginner’s Guide.
Together, these three mechanisms determine how the four tires can use their available grip and contribute to the vehicle’s handling characteristics.
The structural design of the suspension also affects how the wheels are guided and how their orientation changes during travel.
For a comparison of MacPherson struts, double wishbones, multi-link suspensions, and other common layouts, see Six Suspension Types Explained ★ A Beginner’s Guide.
4. Keeping Tires in Contact as Vertical Load Decreases
During braking, vertical load decreases on the rear tires.
During cornering, it decreases on the inside tires.
As these tires become unloaded, the maximum forces they can generate become smaller.
Road undulations and body movement can further reduce their vertical loads if the wheels cannot follow the road surface adequately.
In some circumstances, a tire may lose contact with the road entirely.
This is where rebound, or droop, travel becomes important.
For example, during hard braking, the body pitches forward and the rear suspension generally extends.
Available droop travel allows the rear wheels to move downward relative to the body as needed to follow the road surface.
Even when a tire carries relatively little vertical load, maintaining contact allows it to generate force according to the load that remains.
If the suspension reaches its extension limit, further downward wheel movement is restricted. Depending on road shape and vehicle motion, the tire may then lose contact.
There is also a separate physical limit: if braking becomes sufficiently severe for the rear tires’ vertical loads to reach zero, the rear wheels can lift regardless of how much suspension travel remains.
For a tire losing vertical load, the suspension’s road-following ability helps preserve the opportunity to use whatever grip remains available.
5. Making Effective Use of Grip as Vertical Load Increases
Now consider tires receiving additional vertical load.
Under braking, the front tires carry more load.
During cornering, the outside tires carry more load.
These tires take on a greater share of the forces required to slow down or turn the vehicle.
Their maximum force capacity generally increases with vertical load, but making effective use of that additional capacity requires appropriate tire contact conditions and vehicle response.
For example, when a car crosses an uneven surface at speed, rapid vertical inputs act on its wheels.
Depending on spring and damper characteristics, the wheels may bounce excessively or experience large fluctuations in vertical load.
Even a tire carrying substantial load on average may temporarily lose part of its available grip when its instantaneous load decreases.
Rapid braking or steering inputs introduce another challenge.
They change body pitch and roll while also changing the forces demanded from the tires over a short period.
Consider the front tires under braking.
As longitudinal load transfers forward, their vertical loads increase, generally increasing the maximum forces they can generate.
At the same time, they are using part of their available capacity to produce braking force.
As the driver begins steering into a corner, those tires must also generate lateral force.
The demands for braking and cornering therefore need to be managed within the tires’ combined force limits.
Suspension characteristics influence the associated body motion and vertical-load fluctuations, helping determine how consistently the available tire capacity can be used.
For the practical relationship between brake release, steering input, and available tire grip, see Understanding Brake and Steering Timing Through the Friction Circle ★ How to Corner Faster on Track.
6. Suspension Travel Provides the Range Needed to Follow the Road
Suspension travel is the range through which a wheel can move vertically relative to the vehicle body.
This movement range allows the wheel to accommodate changes in road height and body position.
Droop travel provides room for the wheel to extend downward, which is particularly important when the suspension becomes unloaded or the road surface drops away.
Bump travel allows the wheel to move upward relative to the body, accommodating suspension compression and road irregularities.
When the suspension reaches either end of its available travel, further relative movement becomes restricted.
Near the compression limit, for example, engagement of the bump stop can substantially change the effective spring characteristics and the forces transmitted through the suspension.
At the extension limit, the wheel has exhausted its available downward movement relative to the body.
Suspension performance therefore depends on more than the total amount of travel.
It also depends on how that travel is distributed between bump and droop, where the suspension operates within its range, and how quickly it moves under different conditions.
Spring rates and damping characteristics must be considered together with suspension travel.
On adjustable coilover suspensions, preload and ride-height settings can affect the suspension’s operating position and available travel, depending on the design.
The relationship is explained in Spring Preload and Ride Height ★ Why the Relationship Changes Between Coilover Designs.
7. Road Cars and Race Cars Are Optimized for Different Conditions
Production-car suspensions are designed to work across a broad range of conditions.
They must accommodate different road surfaces, speeds, vehicle loads, comfort requirements, and safety considerations.
Their suspension travel, spring characteristics, and damping therefore need to cope with a wide variety of inputs.
Purpose-built race cars, such as Formula cars, are developed around a much narrower set of operating conditions.
Tires, ride height, aerodynamic characteristics, chassis stiffness, and suspension geometry can all be engineered together for the expected racing environment.
The same principle applies when tuning a production car for track use.
For example, increasing spring stiffness can reduce body pitch and roll during braking, acceleration, and cornering.
However, if the resulting setup reduces the wheels’ ability to follow rough surfaces or curbs, fluctuations in vertical tire load may increase, making available grip harder to use consistently.
Lowering the vehicle can reduce its center-of-gravity height, but it also changes the suspension’s operating position and may affect available travel and geometry.
Track-oriented suspension tuning is therefore a process of selecting and coordinating suspension characteristics to suit a particular range of operating conditions.
It involves choosing an appropriate compromise among body control, wheel movement, tire orientation, and load distribution.
Anti-roll bars deserve particular attention.
An anti-roll bar resists differences in movement between the left and right suspensions and contributes to the vehicle’s overall roll stiffness.
Changing front or rear anti-roll-bar stiffness alters the front-to-rear distribution of lateral load transfer.
This affects the difference in vertical load between the inside and outside tires at each axle, and therefore the balance of available grip.
Anti-roll-bar tuning must also account for its effects on wheel independence and road-following ability, particularly over uneven surfaces.
The different functions of strut bars, chassis braces, and anti-roll bars—and why they can make a car faster or slower—are discussed in Do Strut Bars and Anti-Roll Bars Really Work? ★ Why They Can Make a Car Faster—or Slower.
Conclusion | Judge Suspension by How Effectively the Four Tires Can Work
Whenever a car accelerates, brakes, or corners, the vertical loads on its four tires change.
The suspension influences how load transfer is distributed, how tire loads evolve over time, how the wheels follow the road, and how the tires are oriented.
Together, these effects determine how effectively the tires can use their available grip.
For tires losing vertical load, maintaining road contact helps preserve the ability to use the remaining grip.
For tires gaining vertical load, stable contact conditions and appropriate tire orientation help make use of their increased force capacity.
Changing the suspension also changes what the driver feels through the vehicle body.
Yet the effect on actual performance ultimately depends on how those changes influence the work performed by the four tires.
When body roll decreases, how has vertical load distribution changed?
When steering response becomes sharper, what has happened to the forces the tires can generate, and how those forces develop?
Asking these questions makes it possible to evaluate suspension stiffness and vehicle response in terms of their relationship with actual tire grip.
Understanding suspension performance ultimately means understanding how the four tires share the work of controlling the vehicle—and how effectively each tire’s available capability can be used.
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