One of the most important concepts in race car dynamics is that of weight (or load) transfer, and many other avenues of race car development rely on being able to manipulate the car’s weight. But what does that even mean?
A race car is a very dynamic system, and a key point to remember is that the suspension elements are made of spring-like components. Your suspension and dampening together are what help regular cars glide effortlessly over bumps and irregularities in the road surface. However, this means that when you are accelerating, braking or turning, the whole system can pitch and roll. When you slam on the brakes in a road car, you (and all your passengers) will feel you fly forwards in the car as you slow down. Under braking, the nose of the car has pitched down as the front suspension elements compress, and overall weight of the car has shifted forwards towards the front. Likewise, accelerating will cause you to be pushed back against your seat as the weight of the car transitions towards the rear, compression the rear suspension elements and relaxing those at the front. Turning the car will cause you to lean towards the outside of the car, the overall weight moving laterally left or right. These three combinations of movements combine, making the overall weight of the car shift depending on the situation and environment.
The terms used in the motorsport world for these movements are “dive” (when we slam on the brakes and the front of the car dives forward and towards the ground), “squat” (when we accelerated and the rear of the car falls backwards and towards the ground), and “roll” (the lateral weight transfer due to turning). These can be seen in the diagrams.
These diagrams were adapted from Driver61's video regarding weight transfer, found here.
Where the weight of the car is distributed will determine how much grip is available to each of the tyres at any one time. In simple terms, the greater vertical load you have on each tyre, the more 'grip' it has (tyres are explained further in the next blog post!).
When the car is diving when the brake is applied, the shift in load over the front axle reduces the load on the rear axle. As the suspension at the front of the car compresses, it is relaxing at the rear as load is reduced. This is causing the front tyres to increase their vertical loads and hence grip, and the opposite is true for the rear of the car. This makes the rear of the car feel much lighter, and is more prone to over-steering. Excessive steering lock on the car at this point is likely to cause a spin, as the rears have little grip to keep the car pointing forwards.
The same principles apply when the car is squatting under acceleration. The vertical loads over the front tyres are reduced as the weight of the car sits back on the rear axle. This causes the car to tend to understeer, where the front tyres are having to work harder to keep the car pointed in a given direction.
When we adjust the setup of a race car, we are often trying to manipulate the rate at which dive, squat and roll forces are acting for a particular circuit, or even corner. In general, we want the platform of the car to be as stable as possible, ie minimising the affects of dive, squat and roll. This is ensuring that the car is a responsive as possible to the driver. In order to do this, the setup of these suspension elements are set stiffly - high longitudinal and lateral acceleration forces have a reduced affect on the weight transfer of the car due to the stiff suspension setup, and therefore ensures that the car is very responsive. Race cars often have an adjustable setup element known as an 'Anti-Roll Bar', which helps reduce the roll of the race car. Excessive roll is undesirable, as it can reduce the surface area of the tyres in contact with the ground, and hence reducing the available grip. There are even specific elements designed to oppose dive and squat, known as anti-dive and anti-squat systems, but they are a little more complex to explain in this post. However, there are often times where we may want to soften particular areas of the suspension or dampers, or allow the car to roll more freely.
This image shows the Anti-roll bar, highlighted in red. It was taken from the CarThrottle article on how ARB's work, found here.
For example, if a driver has mentioned that when they try to turn the car into a corner, they are experiencing understeer (a loss of grip on the front tyres, causing the car to drift wide of there intended line around the track), then one method of reducing this could be to soften the front anti-roll bar. This will allow the front of the car to roll a little more, making it easier for the tyres to load through the corner and increasing their lateral grip whilst turning.
There is always a fine balance when it comes to setups, and softening the anti-roll bar by too much could cause adverse affects on other parts of the system, such as lacking responsiveness.
Thank you for reading! If you have any more questions about the basics of weight transfer in race cars, let me know! The next post about vehicle dynamics will be about tyres!
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