The world of car racing has plenty of intuitive concepts alongside its fair share of more complex ideas. Brake bias falls somewhere in the middle, and while it is a very important aspect of the car, it can be difficult for beginners to grasp and indeed to take full advantage of.
Brake bias (or brake balance) is essentially the ratio of the braking force that is received by the front and rear wheels. It is often quoted as a ratio of percentages, such as 60/40. In this case, 60% of the effective braking power is sent to the front wheels, while 40% is sent to the rear.
Knowing what brake bias is can make it much easier to understand why your car behaves in the way that it does. Knowing how you can adjust the brake bias and what the ideal setup is for a car is also important, and so we will go into that in more detail below.
What Is Brake Bias?
The braking system of a car is extremely important, and how it is set up can have a huge impact on the car’s performance. Brakes are often thought to simply be what slows the car down, but anyone with racing experience and indeed driving experience knows that they play a vital role in cornering too. Even a small difference in the corners makes a massive difference over the course of a full race.
A Practical Explanation
A more practical explanation involves thinking of the bias as a measure of which set of wheels will lock up first. In the real world when the brakes are applied one of the sets of tires will lock up first. In a rear biased car, it is the rear wheels that lock up first and in a front biased car it is the front tires. This is key as it will dictate how the car handles when you hit the brakes.
Understeer vs Oversteer
Which set of wheels locks up first is vital for race car drivers, as it will determine whether or not the car is more likely to go into oversteer or understeer. Oversteer is caused when the rear wheels lock up first; understeer is the opposite and occurs when the car is front biased. We will discuss each case in more detail below.
Which Is Preferable? – Driving Style
Most drivers favor a slight front bias in most cases, but it means the car tends to understeer rather than oversteer. The reason this is generally preferential is simply because it is easier to manage understeer, and oversteer is much more likely to end up in a complete loss of control, or spinning off the track.
Fernando Alonso is a good example to use, as he famously used to force his F1 car into understeer at the corners. By making the front tires lose traction early, he got past the low grip phase of the corner, allowing him to make the most of the car’s rear stability to power out of the corner quickly.
This example was highly dependent on the tires that his team, Renault, used at the time. The way that they were designed meant that they inherently offered less grip at the start of the corner, but a lot of grip when he pressed the accelerator at the exit. So, drivers will learn to use the bias of their car to their advantage, and it can be altered to suit their preferences.
The Simple Scenario
In a simplified scenario, we might go for an equal brake balance, i.e. 50/50 braking power split between the front and back wheels. You might think that this would mean that both sets of tires would lock up at the same time. However, this is never the case in the real world, mainly because we need to factor in weight transfer.
How Weight Transfer Affects Brake Bias
Weight is transferred from one area of the car to another, depending on the way that the car is being driven. It is important here to distinguish between mass and weight.
Mass vs Weight
Weight is often confused with mass, but in simple terms the mass of the car is how heavy it is without any other forces acting on it other than gravity. This is usually measured in kg or lbs, and it is what people tend to call the weight of the car. However, when the car is accelerating or braking, the weight shifts backwards or forwards, while the overall mass stays the same.
Weight can be thought of as the feeling of the mass of the car, and although the physical mass isn’t moving in either case, the weight transfer makes it feel and act like it is. The best way to consider this is by choosing one action, such as accelerating. When the car accelerates, the weight transfers to the rear of the car, making the rear end act like it is heavier, when it really isn’t.
This causes the car to almost press down on the ground at the rear tires, making them stick to the ground through an increase in grip via the downforce they experience. The opposite is true when you slam on the brakes, as you will feel the weight of the car shifts forward, along with you as well. This is why you wear a seatbelt, as if you slam on the brakes without one you will lurch forward.
This affects how much force is required to decelerate each wheel, and so you can use brake bias to account for this.
Everything In Proportion
You want the amount of braking force supplied to each wheel to be proportional to the amount of weight on each wheel when the brakes are applied. This works to correctly distribute the braking force across the four wheels, resulting in – ideally – all four wheels locking up at the same time.
This means that the braking force should be skewed towards the front of the car, in order to have all the wheels lock at the same time. However, this is where you now have to think once about the cars center of mass. The mass of the car will always be a single value, but depending on where the engine is located, the heaviest side of the car will vary, which affects weight transfer.
Usually, cars have either a front or rear engine. There are mid-engine options, which should intuitively mean the car is fairly balanced in terms of its mass. In a front-engine car, the heaviest component is situated near the front axle, while in a rear-engine car, most of the weight is at the back.
This means that, all other things equal, the front-engine car will need more braking power distributed to the front wheels than the rear-engine car for a balanced setup.
Brake Bias vs Car Bias
Remember that the simple way that biasing is determined is by which wheels lock up first. Even if a car has most of the braking power supplied to the front tires, it can still be described as “rear biased” if it is the rear wheels that lock up first. Thus, it is important to consider all of the factors involved in order to get a solid grasp of brake bias and how to use it.
Brake bias is simply the ratio of the amount of braking power split between the front and rear wheels. Car bias is the result that we really care about, and is influenced by brake bias.
Other Factors That Influence Brake Bias
Getting the right balance is essential, as it will allow the driver to have more control over their car. However, there are several other key factors that play an important role in the handling of the car, and that directly affect brake bias as well. We have already discussed the effects of engine placement, but one of the most important is the tires.
Tires Are Key
Tires play a key role in the braking of a car, as they are what is in contact with the road surface. If you use sticky tires, such as racing slicks, you will have as much surface area in contact with the road surface as possible. The stickier the tires used, the more decelerative force the car has, and thus the car shifts towards a rear biased setup.
This is because the increased decelerative force will shift more weight forward. This would put more weight over the front wheels, and less over the rears.
Maintaining the same brake bias results in the front brakes not having enough braking power (relative to the now increased weight at the front), and the rear brakes now applying the same power to wheels which have less weight over them. If the car was perfectly biased before, then using stickier tires now would cause the rears to lock up first.
Other Physical Components
Then, there are some more physical components that can be adjusted to have an effect on the brake balance. These come in the form of specific components within the braking system of the car, such as the rotor diameters and the brake caliper diameters. Increasing both of these at the front will lead to front biasing, and at the back will lead to rear biasing.
The brake pad coefficient of friction – essentially how much braking force it can apply to the wheels, or how much stopping power it has – can also be altered to increase biasing in one direction. Stronger brake pads at the front with higher coefficients will lead to front biasing, while higher coefficients of friction at the back will do the opposite.
How To Adjust Car Brake Bias
Adjusting brake bias at the level of Formula 1 racing usually involves a lot of computer programming within the car itself. Drivers can often shift the brake bias as they are driving, so that they can tune it to suit individual corners on the track. Most racers in other motorsports, and in normal cars, won’t have this ability and will need to alter the bias before they get out on the track.
The brake bias can be altered by changing the physical components that we discussed in the previous section. If you want the car to be front biased, you can keep all other factors constant and increase the brake pad coefficient at the front wheels. Or you could change the calipers and the rotors to be of a wider diameter or reduce that of the ones at the rear.
Shifting where the engine is located in the car can also be a method of shifting the weight and thus changing the bias of the car. This is often difficult to do, however some race cars do allow for small changes to the engine position.
Other Ways To Change The Weight
Other weight transfer alterations can simply involve changing the position of other components within the car. This could involve moving things like the battery and the wiring, as well as the seats or other items inside the car. Adding weight to the inside, often called ballasting, is often used in NASCAR to change the weight distribution, and thus the balance of the car.
Shifting the weight of the car is a good way to change its bias, but you still want to keep the weight low for performance reasons. The center of mass of the car is one of the most important factors that will affect the bias, and thus having a good grasp of how to move it around the car will make it much easier to control how the car handles through brake biasing.
At this point it is important to notice how both weight distribution and brake bias affect the overall ‘balance’ of the car. When testing for the optimal setup, it is important to change only one of these at a time, in small increments. This will help you to understand the effects of each of the changes being made.
Brake biasing involves finding the ideal balance of braking power between the front and the rear wheels. Front biased cars will cause the front wheels to lock up before the back wheels, tending to understeer. This is usually more favorable than an oversteering tendency, so it is generally a good idea to have a race car be slightly front biased (if you prefer slight understeer to slight oversteer).
Car bias is affected by several factors, and the ideal distribution will be influenced by the driving style of the racer. Knowing how to adjust the bias of your car can allow you to gain more control of how the car handles, can also allow you to become a better racer in the process.
It would probably be a good idea to re-read this article, paying attention to the subtle difference between brake bias and car bias. You will end up with a far better understanding than most!