By Shreeharsh Parab

Xitadel CAE Technologies India Pvt. Ltd.

Read Time: 6 minutes

Target Audience: Engineers, Students, Beginners in Vehicle Dynamics

What makes a car comfortable? What makes a car a pleasure to drive along a long stretch of a highway or some curvy roads? All great automobiles worth their salt have one common quality amongst them – they make the driving and passenger experience enjoyable. All the engineering efforts that go behind crafting a car focus on these experiences along with safety, and suspension design is the key factor which will help an engineer achieve this experience for the driver. But how do we break down the definition of these experiences into engineering requirements? In this blog, we will explore how suspension impacts both ride comfort and handling, breaking down these experiences into key engineering requirements.

Suspensions: What, Why and How?

The primary function of an automobile suspension system is to isolate the sprung mass from the undulations and disturbances on the road. Isolating the occupants from the disturbances on the road is a good intuitive introduction to the concept of ‘ride comfort’. How does a suspension achieve this? A typical suspension system has springs, dampers, and linkages which act and move together to keep the wheels in contact with the ground, while ensuring that the road disturbances are not transmitted to the chassis.

A good point to start any suspension discussion is to consider the quarter car model (shown in the figure 1).

In the quarter-car model:

1. Sprung mass (M): This represents the mass of the vehicle body (including passengers) that is supported by the suspension.
2. Unsprung mass (m): This includes components like the wheels, tires, and any part of the suspension that is not supported by the springs.

1. Spring stiffness (Ks): The suspension spring’s stiffness determines how much the suspension compresses under a load. The suspension springs absorb the energy from the road irregularities, compressing and extending depending on the load acting on them. A softer spring absorbs more road irregularities, providing comfort.
2. Damping coefficient (Cs): This represents the shock absorber’s ability to dampen the oscillations of the spring. But why is a damper (or shock absorber) needed? If it were just for the springs, the sprung mass will continue to oscillate continuously after every bump on the road. Dampers dissipate the energy from the springs and prevent them from oscillating.
3. Tire stiffness (Kt): The tire is modelled as an additional spring, as it deforms under the load. Its stiffness affects how road disturbances are transmitted through the suspension to the car body.
4. Tire Damping (Ct): The damping of the tire

While softer springs may seem ideal for ride comfort, achieving a well-balanced suspension system is more complex. Comfort is just one side of the equation—handling performance is equally critical, especially when considering how a vehicle responds to driver inputs. Now, let’s dive into how suspensions affect handling and the challenge of balancing these two important aspects.

Handling

Handling is the ability of a vehicle to respond to the driver inputs satisfactorily. A driver gives multiple control inputs to the vehicle at any given time, mainly – throttle, braking, steering and gear shifting. Based on these inputs, the vehicle responds (accelerating, braking or cornering). The driver again modifies the input based on the feedback received from the vehicle. Hence vehicle handling is a closed loop as represented in the figure below.

So, what makes a vehicle respond ‘correctly’ to any given inputs? While there are a lot of possible answers to this question, we will take a detailed look at suspension effects on handling, which is the major contributor here.

Let us start by considering an example of a vehicle taking a left turn. The driver will give a steering wheel input in the desired direction. This will steer the front tires to the left, which will generate the necessary lateral force to steer the vehicle to the left. These lateral forces and their subsequent reactions are borne by the suspension components we mentioned above, along with the chassis. Additionally, during this maneuver, due to the forces acting on the chassis, the sprung mass rolls (see figure below) and weight is transferred laterally from left to right. This will compress the right-side springs while extending the left ones.

Due to the inherent properties of tires, the lateral force generated by them generally increases with vertical load. More lateral force for any given steering input generally relates to improved cornering behavior of the car, which means improved handling. How do we ensure that the tires stay connected to the ground, always maintaining a good vertical load on them for good handling? By putting stiffer springs. Stiffer springs push the tires onto the ground due to greater force generated by them.

Conclusion: Bridging Comfort and Performance

This brings us to the problem of optimizing ride and handling. As discussed, better ride is achieved with softer springs while better handling requires stiffer setup. A luxury vehicle is designed to prioritize comfort above anything else. The suspension setups on these vehicles tend to be on the softer side. On the other hand, a sports car, designed for maximum performance will prioritize handling, sacrificing ride comfort to some extent. Engineers must optimize the ride and handling performance of a vehicle considering the requirements and use cases for that particular vehicle.

Modern suspension technologies, such as active suspensions systems, are effectively bridging the gap between ride comfort and handling performance. These systems adjust in real-time to changing road conditions, allowing vehicles to deliver both a smooth ride and responsive handling. By understanding and managing these trade-offs, engineers can design vehicles that accommodate various driving styles, from comfortable highway cruising to precise control on racetracks.