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The Dynamics of Braking
9 years ago
General
Let's talk braking. Time was, racing games had brakes like a switch: on or off. This evolved into more analog systems where the amount of braking could be dictated, from a slight correction through a kink to full-on hauling down for a hairpin. The thing is, it's still not that simple, even if there are a lot of games and simulators that would lead you to believe that.
Let's review what I've learned over the years from GTR2. And let's go to the Circuit de Spa-Francorchamps for a great example of coming off a long, high-speed straight into a slow corner.
Cresting the top of Eau Rouge, GT cars can be going over 120 miles per hour. There's only a slight right-hand kink before dumping out onto Kemmel Straight. Almost all cars will top out on this straight, either running out of power or dancing on the rev limiter. GT cars can come into Les Combes at over 180 miles per hour. Time to get on the brakes for that right-hander. And by that I mean squeeze the brakes. One does not "jump on", "slam on" or "stab at" the brakes in a race car. One "squeezes" the brakes, very firmly but smoothly.
The reason for that is the first thing that happens under braking is the weight of the car shifts forward, putting more weight on the front wheels. This helps press them down to the racing surface and is why the front brakes usually do more of the work stopping a car than the rear brakes. The brake pressure is not necessarily biased toward the front (although it often is, even just a little) but the front brakes might be bigger than the rear brakes. If this weight transfer happens too quickly, it can upset the car, especially if driving on the edge while racing.
The second thing that happens is that the brakes begin to heat up. We're all familiar with how overheated brakes are not as efficient at stopping a car, but brakes that are too cool are similarly inefficient. Racing brakes on GT cars are most effective in a range between 700 and 1000 degrees Fahrenheit. After flying down Kemmel, a car's brakes will be relatively cool (by that I mean 350-400 degrees).
Here's the conundrum: the brake cooling ducts could be restricted to keep the brakes from cooling off on Kemmel, but when they get used through Bruxelles, Pouhon, Campus and Stavelot, they're guaranteed to overheat. If enough cooling is used to keep temperatures in check during the technical parts of the course, they will cool off to the point they're not working as well as they could after the long straights. The latter tradeoff is usually the better choice.
So when braking is first applied coming in to Les Combes, the car won't slow as well as it should for a given pedal pressure. However, as the car slows down the brakes start heating up, and as they heat up into their working range, they stop the car more easily. Braking for the corner, the brakes literally get stronger as they are used.
The third thing to consider is aerodynamic force: flying down Kemmel at 180, aerodynamics are in full effect, pushing the car down against the track. And the harder the car is pushed to the track, the more it enhances the traction of the tires. When the brakes are first applied, the car is still being pushed down by the aerodynamics. As the car slows down, that aero force starts going away, until at the point of turning, the aerodynamic force may be completely gone.
When the second and third items are combined, they create quite a dynamic: braking for a corner from a straight, the brakes are getting stronger while the traction due to aerodynamic force is going away. And because of this, a driver will need to ease up on the brakes while braking for a corner or the combination of stronger brakes and weaker traction will lead to tire lock-up.
It seems totally counter-intuitive to ease up on the brake pedal when staring down a slow corner ahead, but it's absolutely necessary to keep control of the car.
Locking a tire during racing is bad, but how bad is it really?
First, a sliding tire has almost no grip, which means all braking force is lost on that tire. When hanging it out on the ragged edge racing, braking with only three tires is not good enough. If it's a front tire that locks up, steering direction is lost and trying to wrestle a car through a corner with one tire doing the turning is a big handicap.
Second, depending on the severity of the lock-up, it creates a flat spot on the tire. This is bad for two reasons: first and most importantly, a tire that's on the verge of locking up (and they should be during braking, see below) will tend to stop and lock-up more easily at the flat spot because of the extra force necessary to get over the "edge" of the flat back to the rounded surface. Locking up again on the same flat spot makes it flatter, further aggravating the problem. Also at racing speeds and the rotational speeds tires turn, a flat spot can cause intense vibrations in the car.
Tire compounds suffer from something called hysteresis. Suppose we have a tire with a certain amount of traction force. Asking for 105% might cause the tire to turn more slowly, scrubbing over the racing surface, but not fully lock up. This is in fact called "threshold braking" and slows the car better than a tire at 100%. If a tire is pushed to 108% and it locks up, releasing braking back to 100% traction force will not start the tire spinning again. The force may need to be released back to 93% for it to stop sliding. But releasing braking force for one tire means releasing braking force for all tires, so sometimes the best policy may be to lump it and leave the tire locked through the braking zone.
Brakes fade when they get hot for a variety of reasons, but here are a few that come to mind.
Very often, the brake calipers can get hot enough to boil the brake fluid inside of them. This creates air pockets and physics takes over: air compresses the way hydraulic fluid does not. This will reduce the effectiveness of the brakes, the pedal getting spongy. There are brake calipers made with water channels cut in them, designed to be water cooled while racing and keep temperatures under the boiling point of the brake fluid.
Brake rotors can get so hot that when the pads are applied, the pad material vaporizes and creates a layer of gas between the pad and the rotor. The brake pad wants to skate over the rotor surface on this layer of gas, reducing the friction between the two and therefore reducing braking force. This is what slotted brake rotors try to reduce: the slots give that gas a channel to escape from under the pad. It's not unlike the treads of a tire letting water channel away. In fact, that's an excellent analogy: the brake pad is "hydroplaning" (aeroplaning?) over the surface of the rotor.
Finally, there are the limits of physics. Slowing a car is all about energy transfer and the kinetic energy of the car is turned into heat energy and dissipated. There does come a theoretical point where the brake rotor absolutely can not absorb any more heat energy, so the kinetic energy has nowhere to go. The pads can be applied to the rotor as hard as possible, but no energy transfer will take place, and the car will not slow down.
Let's review what I've learned over the years from GTR2. And let's go to the Circuit de Spa-Francorchamps for a great example of coming off a long, high-speed straight into a slow corner.
Cresting the top of Eau Rouge, GT cars can be going over 120 miles per hour. There's only a slight right-hand kink before dumping out onto Kemmel Straight. Almost all cars will top out on this straight, either running out of power or dancing on the rev limiter. GT cars can come into Les Combes at over 180 miles per hour. Time to get on the brakes for that right-hander. And by that I mean squeeze the brakes. One does not "jump on", "slam on" or "stab at" the brakes in a race car. One "squeezes" the brakes, very firmly but smoothly.
The reason for that is the first thing that happens under braking is the weight of the car shifts forward, putting more weight on the front wheels. This helps press them down to the racing surface and is why the front brakes usually do more of the work stopping a car than the rear brakes. The brake pressure is not necessarily biased toward the front (although it often is, even just a little) but the front brakes might be bigger than the rear brakes. If this weight transfer happens too quickly, it can upset the car, especially if driving on the edge while racing.
The second thing that happens is that the brakes begin to heat up. We're all familiar with how overheated brakes are not as efficient at stopping a car, but brakes that are too cool are similarly inefficient. Racing brakes on GT cars are most effective in a range between 700 and 1000 degrees Fahrenheit. After flying down Kemmel, a car's brakes will be relatively cool (by that I mean 350-400 degrees).
Here's the conundrum: the brake cooling ducts could be restricted to keep the brakes from cooling off on Kemmel, but when they get used through Bruxelles, Pouhon, Campus and Stavelot, they're guaranteed to overheat. If enough cooling is used to keep temperatures in check during the technical parts of the course, they will cool off to the point they're not working as well as they could after the long straights. The latter tradeoff is usually the better choice.
So when braking is first applied coming in to Les Combes, the car won't slow as well as it should for a given pedal pressure. However, as the car slows down the brakes start heating up, and as they heat up into their working range, they stop the car more easily. Braking for the corner, the brakes literally get stronger as they are used.
The third thing to consider is aerodynamic force: flying down Kemmel at 180, aerodynamics are in full effect, pushing the car down against the track. And the harder the car is pushed to the track, the more it enhances the traction of the tires. When the brakes are first applied, the car is still being pushed down by the aerodynamics. As the car slows down, that aero force starts going away, until at the point of turning, the aerodynamic force may be completely gone.
When the second and third items are combined, they create quite a dynamic: braking for a corner from a straight, the brakes are getting stronger while the traction due to aerodynamic force is going away. And because of this, a driver will need to ease up on the brakes while braking for a corner or the combination of stronger brakes and weaker traction will lead to tire lock-up.
It seems totally counter-intuitive to ease up on the brake pedal when staring down a slow corner ahead, but it's absolutely necessary to keep control of the car.
Locking a tire during racing is bad, but how bad is it really?
First, a sliding tire has almost no grip, which means all braking force is lost on that tire. When hanging it out on the ragged edge racing, braking with only three tires is not good enough. If it's a front tire that locks up, steering direction is lost and trying to wrestle a car through a corner with one tire doing the turning is a big handicap.
Second, depending on the severity of the lock-up, it creates a flat spot on the tire. This is bad for two reasons: first and most importantly, a tire that's on the verge of locking up (and they should be during braking, see below) will tend to stop and lock-up more easily at the flat spot because of the extra force necessary to get over the "edge" of the flat back to the rounded surface. Locking up again on the same flat spot makes it flatter, further aggravating the problem. Also at racing speeds and the rotational speeds tires turn, a flat spot can cause intense vibrations in the car.
Tire compounds suffer from something called hysteresis. Suppose we have a tire with a certain amount of traction force. Asking for 105% might cause the tire to turn more slowly, scrubbing over the racing surface, but not fully lock up. This is in fact called "threshold braking" and slows the car better than a tire at 100%. If a tire is pushed to 108% and it locks up, releasing braking back to 100% traction force will not start the tire spinning again. The force may need to be released back to 93% for it to stop sliding. But releasing braking force for one tire means releasing braking force for all tires, so sometimes the best policy may be to lump it and leave the tire locked through the braking zone.
Brakes fade when they get hot for a variety of reasons, but here are a few that come to mind.
Very often, the brake calipers can get hot enough to boil the brake fluid inside of them. This creates air pockets and physics takes over: air compresses the way hydraulic fluid does not. This will reduce the effectiveness of the brakes, the pedal getting spongy. There are brake calipers made with water channels cut in them, designed to be water cooled while racing and keep temperatures under the boiling point of the brake fluid.
Brake rotors can get so hot that when the pads are applied, the pad material vaporizes and creates a layer of gas between the pad and the rotor. The brake pad wants to skate over the rotor surface on this layer of gas, reducing the friction between the two and therefore reducing braking force. This is what slotted brake rotors try to reduce: the slots give that gas a channel to escape from under the pad. It's not unlike the treads of a tire letting water channel away. In fact, that's an excellent analogy: the brake pad is "hydroplaning" (aeroplaning?) over the surface of the rotor.
Finally, there are the limits of physics. Slowing a car is all about energy transfer and the kinetic energy of the car is turned into heat energy and dissipated. There does come a theoretical point where the brake rotor absolutely can not absorb any more heat energy, so the kinetic energy has nowhere to go. The pads can be applied to the rotor as hard as possible, but no energy transfer will take place, and the car will not slow down.
It occurs to me that an active system would have to be anticipatory... in that example above, braking for Les Combes off of Kemmel, it's not uncommon to raise the brake temperature 600F in 5 seconds of braking, much faster than probably even a full-flow of air could cool them off. Adding to that mix is once again, the car is slowing down. You start with 180MPH airflow but at the end (when the brakes are hottest) there might only be 70MPH airflow.
But that said... I do wonder if active systems are ruled out in the sporting regulations or are just not worth the extra hassle of development and implementation. ^_^
Sometimes I think about this stuff as I'm riding my motorcycle, though I never come close to the threshold of my brakes hehe. It is kind of funny though to think about the energy transfer, and that on a motorcycle a few finger's worth of pressure is all it takes to apply enough stopping force for your pads/rotors to stop you from normal street speeds (45-50?). And you *definitely* do not want to be >jabbing< or >grabbing< the front brakes on a bike, or you may find your rear tire over your head or your helmet face kissing the asphalt.
Wish we could go for a ride.
A friend here sold his old bike and got a new one and he's been dying to ride it. Last weekend we did a good circuit through the farmland roads and stopped at a burger joint along the way for lunch. This weekend he wants to ride again, but I'm working weekends these days, so it's not so easy to coordinate something.
Hope you're getting good rides in!