If you have ever walked down a street and observed the wheels of parked cars you may have noticed how front brakes are always larger than rear ones. And it's not just the diameter of the brake disc, it's also the brake calipers, which are larger and often have more pistons in the front.
If you had the opportunity to observe brakes of cars with their wheels removed you may also have noticed that front brake discs are usually ventilated whereas rear ones are not.
So why is this the case? Well the answer is that if rear brakes were larger than front ones, braking would often result in your car spinning out of control and likely crashing.
To understand why this would happen we must observe what happens with the car during hard braking. As you can see the front of the car or the nose of the car dives. This happens because of weight transfer.
Weight transfer occurs because of inertia. The car wants to keep going in the same direction but the brakes and the tires are trying to stop it. By doing so they become the point of rotation for the vehicle. A nice way to visualize weight transfer is to imagine a car with a single wheel in the middle. You can imagine how such a vehicle would hit the ground with its nose upon hard brake application. Motorcycles are also great for visualizing weight transfer and the brakes becoming a point of rotation of the vehicle, because this is exactly what happens when a motorcycle performs a stoppie.
Now this weight transfer is an extremely important factor to overall braking performance and it greatly impacts the performance of your tires. Why?
Well that's because weight transfer provides increased grip to front tires.
As you probably know tires keep the car on the road through friction between the rubber and the road surface. But friction is a very fickle phenomenon. Despite what intuition may say, friction is not influenced by surface area. In other words it doesn't matter how you drag a plank across the floor, whether on it's side or on it's face - friction will be the same even if surface area is not the same.
Friction only cares about the force acting on the object, which is usually the weight of the object and the friction coefficient which is determined by the surface roughness of the object material.
So in the case of a car coming to a hard sudden stop the material obviously remains the same but weight does not. Weight transfer puts more weight above the front wheels, in other words the force acting on the wheels pressing them down into the ground has increased. This means that the front tires now have increased friction and thus increased grip available to them.
Now for any brake system to be efficient and safe it must be capable of overcoming the maximum grip potential of the tires. What does this mean? It means that all brakes on all cars made in the past 40 years or so are capable of overpowering the tires. In other words when you slam on the brakes the brakes are strong enough to cause the tires to skid or slide acorss the surface. Now tire skidding is bad news because a tire that is skidding has much less friction than a tire that is still maintaining it's grip on the road. This is why ABS was invented. Using sensors on the wheels the ABS control unit can „see“ when a tire is about to skid. To prevent this from happening the ABS system rapidly releases and grabs the brakes in order to maintain grip and prevent skidding. This is the vibration you feel in the brake pedal when abs engages, it's the rapid releasing and grabbing that the ABS system is performing to achieve maximum braking potential and prevent skidding.
A step up from this can be found on some race-cars and it's a brake proportioning valve that can be controlled by the driver. This means that you can change brake bias on the fly while driving.
But ultimately all of these purely mechanical systems are limited. This is why we invented something known as EBD or Electronic brakeforce distribution. This is a system that is capable of constantly monitoring what is happening to each wheel and constantly changing the amount of brakeforce allocated to each individual wheel. A system like this the optimal way of maximizing braking potential and preventing the under-utilization of rear brakes.
EBD can increase brake force on the rear wheels before weight transfer occurs and then reduce it as weight transfer occurs to prevent the rear from skidding out of control. It can also allocate more brakeforce to the outer rear wheel during cornering because this wheel has more grip and then after weight transfer it can allocate more to the outer front wheel to maximize braking.
A special thank you to my patrons:
Daniel
Pepe
Brian Alvarez
Peter Della Flora
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Cole Philips
#d4a #brakebias
00:00 Weight transfer and friction
03:51 Proportioning valves
06:10 CX-30 vs MX-5 vs MR2
09:12 Manual vs Electronic bias