Ride Quality & Ride Comfort Explained: A Quarter-Car Suspension Simulation

What Is Ride Quality (Ride Comfort)?

Ride quality, also called ride comfort, describes how well a vehicle isolates its occupants from road-induced motion and vibration. A vehicle with good ride quality protects passengers from bumps, potholes, and rough pavement, reduces driver fatigue on long trips, and helps prevent damage to cargo. Ride quality is shaped mainly by the suspension system (springs, dampers, and bushings), the tires, the vehicle’s mass, and the seats.

This article uses a quarter-car suspension simulation to show how road input, tire stiffness, and vehicle mass change the vertical acceleration passengers feel. It also explains how ride quality is measured, what influences it, and why it is always balanced against handling, cost, and efficiency.

How Is Ride Quality Measured?

Ride quality is evaluated with two complementary approaches:

• Objective metrics: vertical acceleration is measured on a real vehicle (on test roads or a test rig) or predicted in simulation, then processed with the frequency weightings in ISO 2631-1, the international standard for human exposure to whole-body vibration. Human sensitivity to vertical vibration is highest at roughly 4–8 Hz, so vibration in that band counts more heavily than the same amplitude at other frequencies.
• Subjective ratings: trained drivers and passengers rate the ride on a scale, and objective metrics are correlated against those ratings so the numbers reflect what people actually feel.

The simulation in this article takes the objective approach: it calculates the vertical acceleration of the sprung mass in response to a sinusoidal road input and plots it against excitation frequency.

What Affects Ride Quality?

Many design choices influence how a vehicle rides. The main ones are:

• Spring rate: stiffer springs transmit more road input to the body, while softer springs isolate better but allow more body motion. See our guide to coil springs in automotive suspensions.
• Damping (shock absorbers): dampers control how quickly body and wheel motion decays. Too little damping feels floaty, and too much feels harsh.
• Anti-roll (sway) bars: these mainly control body roll, but stiffer bars can carry motion from one wheel to the other over uneven roads. See sway bar design.
• Tires: tire stiffness and pressure act as an additional spring and damper between the road and the suspension, as modeled in the simulation below.
• Sprung and unsprung mass: the mass of the body versus the wheels, tires, and axle changes natural frequencies and how the wheel responds to bumps. Weight reduction strategies are covered in vehicle lightweighting.
• Seats: seat foam and structure filter vibration before it reaches the occupant. See car seat comfort.
• Noise and NVH: ride quality overlaps with noise, vibration, and harshness. Road noise, squeaks, and rattles are usually evaluated separately but shape the overall impression of comfort. See buzz, squeak, and rattle (BSR) and car wind noise.

Ride Quality vs. Handling: Why Suspension Design Is a Compromise

The purpose of this simulation is to model an automotive suspension and the effect it has on overall vehicle ride quality and comfort. Suspension systems must compromise between ride quality and vehicle handling, depending on the goals of the vehicle being designed. Sports cars have suspension systems designed for better handling by improving control of the vehicle and reaction times; however, this affects the ride quality. On a sports car, the lower center of gravity is better for handling. However, the low ground clearance also limits suspension travel, requiring stiffer springs and reducing ride comfort. A luxury vehicle will handle worse than a sports car but will provide a much more comfortable ride. If the suspension system is too soft, it can cause passengers to develop motion sickness. If it is too harsh, passengers are subject to significant acceleration levels, which can lead to chronic medical conditions. See Figure 1 below.

Ride and handling targets are set early in vehicle development through customer needs and a requirements cascade. See the automotive suspension design process and vehicle driveability for how ride and handling fit into overall vehicle performance.

As ride quality improves inside a vehicle, comfort improves, providing a better experience for passengers. A good suspension system should avoid damaging any cargo inside the vehicle and should also help reduce driver fatigue on long trips while improving stability and maneuverability. The International Organization for Standardization (ISO) whole-body vibration standard (ISO 2631-1) shows that humans are most sensitive to accelerations in the vertical direction. As a result, a good suspension system should absorb road disturbances such as potholes or debris, which can cause a driver to experience vertical velocities and reduce ride quality. In the worst case, a large disturbance can affect the driver’s ability to control the vehicle, putting anyone inside the vehicle in danger. See Figure 2 below.

Quarter-Car Suspension Model and Road Input

To simplify the analysis, the model focuses only on heave, the vertical acceleration felt by the driver. Pitch and roll movement are out of scope, so the model covers just one corner of the vehicle. If the front of the vehicle moves up, the rear is assumed to move up by a proportional distance. The model has two main energy storage elements (masses): the sprung mass, which is the mass of the vehicle supported by the suspension system, and the unsprung mass, which includes the wheel, tire, and axle. Both masses are subject to the gravitational force, which opposes any vertical acceleration. The suspension components are modeled as a spring (capacitor element) and a shock (resistor element) in parallel, connected to the unsprung mass. Similar to the suspension system, the tire is modeled as a spring and a damper in parallel. The tire transfers the road force to the unsprung mass (wheel hub). See Figure 3 for the suspension model and bond graph.

The final bond graph with code is shown in Figure 4. The same bond graph model is used for the first three simulations because the input velocity is the same.

In the first three simulations, the only input to the system is the road profile, modeled as a sinusoidal curve with bump wavelength (L) and an amplitude equal to the road roughness (R). The road profile, which equals the displacement of the tires (m), is described by the following equation:

Differentiating the road profile with respect to time gives the velocity source (m/s):

where the frequency ω (rad/s) is equal to:

The simulation assumes the tires stay in constant contact with the road surface. As a result, road roughness can be treated as a displacement or velocity source to the bond graph model. See Figure 5 below.

Ride Quality Simulation Results

Initial Response: How Does the Suspension Settle From Rest?

In the first simulation, the vehicle is at rest on a perfectly flat road with zero initial tire deflection. At the start, the only force acting on the system is gravity on both the sprung and unsprung mass, which is why the initial acceleration is -9.8 m/s². The acceleration peaks at the maximum tire deflection (~-0.025 m) and quickly decays as the system reaches a steady state. The maximum velocity, -1 m/s, occurs at the beginning, before the tire reaches its maximum deflection, as the gravitational energy on the mass is converted into kinetic energy. Once the system reaches equilibrium about 4 seconds into the simulation, the static deflection of the tire is -0.015 m. See Figure 6 below.

Frequency Response: Which Bump Frequencies Hurt Ride Quality Most?

A Bode plot shows how a system responds across a range of frequencies. For background, see this explainer on frequency response, filters, and resonance.

The second simulation found the frequency response two ways in 20-sim: numerical integration and a direct frequency-response approach. For the numerical integration, the wavelength L was varied from 0.5 to 50 m over 12 runs to determine the amplitude of the steady-state sprung mass acceleration. Figure 7 shows the first three trials (L = 0.5, 1, and 2 m).

To calculate normalized steady-state acceleration, the steady-state acceleration from each run is divided by the road roughness (0.05 m) and ω (rad/s). The normalized value is then converted to decibels (dB) by taking 20·log₁₀ of the result. The full data set is shown in Table 1.

Finally, frequency (ω) was plotted against normalized steady-state acceleration. The same plot was built using the frequency response approach in 20-sim. The two graphs are identical, though the 20-sim model is more complete since the spreadsheet plot used only 12 data points. Both graphs show two worst bump frequencies: ω = 7.9 rad/s and ω = 78 rad/s, with magnitudes of 30 dB and 18 dB respectively. See Figure 8 for the Bode frequency-dependence plot.

The greater the magnitude (dB), the worse the ride quality of the vehicle. Since the forward speed of the vehicle is assumed to be a constant 25 m/s, the only variable really affecting the frequency is the bump wavelength L. As a result, it can be assumed that if the road profile consists of many small bumps (ω < 1), the ride quality of the vehicle will be good. The reason is that the variation in the road is so small that the tire just rolls over any disturbance. However, as the size of the bumps begins to increase (1 < ω < 100), the tire is no longer able to absorb all the variation in the road. This causes the tires to move up and down very rapidly, leading to very poor ride quality for passengers. Finally, when the road profile consists of very long bumps (ω > 100), even though the tires are going up and down, the amplitude of each bump is so spread out that the variation is barely noticeable, leading to very good ride quality. See Figure 9.

Does Tire Pressure Affect Ride Comfort?

The third set of simulations examined the effect of tire stiffness and sprung mass on ride quality. In the first case, the tire was assumed to be deflated, reducing tire stiffness by 35%, from 775,660 N/m to 504,179 N/m. Deflating the tire improved overall ride quality above a frequency of 30 rad/s. The shift was small, but the magnitude of the second worst bump decreased from 18 dB to 16 dB. Because the tire was less stiff, it could deform more, which improved ride quality. During the transient response (0–4 seconds), the difference between the two cases was minor, although the vertical acceleration of the non-deflated tire was always greater. As the response approaches a steady state, the difference in vertical acceleration is much more noticeable: about 2.5 m/s² for the non-deflated tire versus about 1 m/s² for the deflated tire. See Figures 10 and 11 below.

This is not a recommendation to run low tire pressure. A softer tire raises rolling resistance (see the conclusion) and can compromise handling and tire life, so tires should be kept at the vehicle’s recommended pressure.

How Does Added Vehicle Mass Affect Ride Quality?

In the second case, five passengers averaging 70 kg each were added to the vehicle, increasing the total vehicle mass by 350 kg. The added mass shifted the first worst bump frequency slightly to the left. The magnitude of the first peak was the same, but the heavier vehicle reached its worst bump frequency at 6.88 rad/s versus 8.04 rad/s for the original vehicle. After the first peak, the heavier vehicle also decayed more quickly. The magnitude of the second worst bump was lower at 15.58 dB versus 18.03 dB, which indicates improved ride quality. In the acceleration graphs, both cases start the same because of gravity, but the heavier vehicle quickly goes out of phase with the original during the transient response and takes longer to reach the same vertical acceleration. In steady state, the vertical acceleration of the heavier vehicle is lower, at 1 m/s² versus 2 m/s² for the original vehicle. See Figures 12 and 13 below.

What Do the Results Mean for Ride Quality?

Because the force applied to the system is a harmonic force (V_r(t)), all of the graphs have a similar shape. At the beginning, each graph is largely influenced by the transient response, which is why there is so much fluctuation in the first few seconds. The response then converges to steady state. Overall, the sooner a graph reaches steady state and the lower the magnitude of the vertical acceleration, the better the ride quality of the vehicle. Likewise, on the Bode plots, the lower the magnitude and the faster the decay, the better the ride quality of the vehicle.

Conclusion: Balancing Ride Quality Against Cost, Efficiency, and Performance

When designing a vehicle, an automaker must make sure the vehicle is balanced and that the right decisions are made for the target consumer. Even though each automaker will try to optimize its suspension by calibrating the springs and shocks, there is a point where the suspension can only be improved so much, and improving it further requires additional capital. Depending on the vehicle and the program’s budget, this might or might not be the right move.

Some factors that improve ride quality are directionally incorrect for the vehicle. Decreasing the recommended tire pressure might improve ride quality, but it also increases the tire’s rolling resistance, causing increased fuel consumption or decreased vehicle range. Increasing the overall vehicle mass is another factor that might improve ride quality, but it reduces maximum speed and acceleration performance. Adding mass also decreases efficiency, reducing fuel economy and range. Automakers sell vehicles, not suspension systems, so the vehicle must be optimized for its intended hardware and well balanced across all of its systems rather than biased toward any one system.