As an automotive engineer, I can’t help feeling frustrated when I see a poorly tuned car. Every auto manufacturer has its own vehicle dynamics department and every vehicle project has a large group of engineers dedicated to it because every single change in the car affects its handling and ride capabilities. That is why it is usually a bad idea tune a vehicle by yourself.
What do we mean by tuning?
The first image that pops into most people’s head when they hear “tuning” is a lowered, modified car with huge wheels and an out-of-proportion spoiler. Sure, this is also considered tuning. However, in the engineering world “tuning” simply means optimizing and improving
The selection of parameters to tune depends on your goal. For motorsports, our main objective is to achieve faster lap times, usually achieved by increasing grip. For road cars, the process is more complex. There is always a trade-off between comfort and performance and the vehicle profile determines where a car is on this spectrum. Rolls-Royce buyers expect maximum ride comfort. A Chevrolet Corvette costumer, on the other hand, sacrifices comfort for performance.
For that reason, there is really no such thing as a good or bad car. It depends on what comes closer to the costumer’s preference. For example, if you buy a BMW you would likely expect a sportier vehicle compared to a Mercedes-Benz.
It does not mean one brand makes better cars than the other, only that each brand has its own “DNA”. If you drive different models from the same manufacturer you are likely to find similarities in their handling and ride behaviour.
The magic of vehicle dynamics comes from the fact that no two cars are equal. Changing a single bushing or a single mass will change the vehicle`s dynamic behavior. Suspension systems are very complex, so you can achieve your objectives using distinct paths.
Although every component has influence on the overall vehicle dynamics, when you want a specific change you usually start with a specific part. For example, if your problem is on the transient part of a corner (entrance or exit) it would be a good idea to look into a damper change.
There are three fundamental terms in vehicle dynamics that you should be aware of: understeer, oversteer and neutral steer.
Every component on the car contributes to understeer or oversteer, but to simplify matters let’s focus on the CG (center of gravity) longitudinal position as the main variable.
Mechanics of vehicle dynamics
Understeer happens when you are cornering and you need to apply more steering input to keep your driving line. In a vehicle with its mass distributed toward the front (front-engine car), a higher cornering force is necessary in the front-wheels to sustain the lateral acceleration in the CG. Thus, the front-axle tends to have less grip and slides to the outside of the curve.
Oversteer has exactly the opposite effect. To keep your current driving line, you need to input less steering wheel angle. This happens when the CG position is towards the rear of the vehicle. The rear wheels need to compensate the shortest moment arm by exerting more lateral force. The rear axle slides outside of the curve and as a result the vehicle front-wheels are rotated inwards, following a shorter corner radius.
Neutral steering is exactly what it sounds like. Both axles exert the same loads and the vehicle will follow the expected corner line without steering correction. This is ideal, but of course the behavior of the car varies with velocity and due to numerous components a neutral configuration is unachievable for the whole velocity range.
So, understeer or oversteer?
Normally an oversteering car is slightly quicker to retake lost grip on a corner, but is very easy to lose control of.
A understeering car, on the other hand, is always safer. That`s why every road car has a slight tendency to understeer.
The reason for that is the better stability that characterizes a understeering vehicles’ loss of grip. Cars with this configuration lose grip on the front wheels first, which leads to a stable grip recovery.
What makes a comfortable car?
Ride comfort is related to how the vehicle transmits road imperfections to the passengers. A comfortable car is one with minimum body and seat acceleration. Wheel movement is less important here.
There are usually two main peaks of acceleration. One is between 1-2 Hz and is related to the vehicle`s body resonance, and the other is between 8-20Hz and is related to wheel hub resonance frequencies. There are several ways of minimizing those acceleration peaks and each one of them may change the vehicle in a specific way. The most common tune is changing the damping ratio, but this can lead to important changes in handling and excessive damping can result in excessively “floaty” behavior. Other options to adjust are tyre size, spring stiffness, anti-roll bar stiffness, bushing stiffness, suspension geometry and mass reduction.
The human body is more sensitive to vertical vibration in the 4-8Hz frequency range because our abdominal cavity natural frequency is in this range. If you are familiar with mechanical vibration theory, you are aware that exciting a body in its natural frequency leads to resonance (acceleration amplification) and as a result we feel discomfort at this frequency. During fore and aft vibrations, discomfort is felt in the upper torso, which has a resonance frequency around 1-2Hz. Of course these frequencies range from person to person. In fact, losing some belly fat will actually change your resonant frequency!
Comfort is also reflected in steering efforts and steering responses. A comfortable car usually has a slower response to steering wheel inputs. On the other hand, performance vehicles normally have a “heavier” steering wheel with a higher friction feel and better vehicle roll control. If you have ever driven a car that allows you to select between “comfort” and “sport” modes, you probably noticed the difference.
What makes a performance car?
For performance autos, comfort is secondary. The main objective is to keep the tyres on the ground as much as possible. To do so, minimum wheel hub vibration is necessary. The most effective way to assure the vehicle keeps ground contact is to use the softest springs possible.
You may have noticed that race cars often have very stiff springs. It’s true that their springs are much stiffer than those used in road cars, but they usually need stiff springs due to high aerodynamic downforces. Aerodynamic forces oscillate according in each part of the racing circuit and a higher spring resistance is necessary to maintain a constant attitude and ride height. As a result, the grip has minimum variance and the car behavior is more predictable.
For vehicles with lower downforce, including sports cars, softer springs are used to better follow road imperfections. Softer springs give a more even ride height and, more importantly, prevent loss of grip.
These are a few of the key elements which go into building the vehicle dynamics which help provide a smooth and fun ride. In the next article we will continue to dive into the mechanics of vehicle dynamics.