Chapter 5: Braking and Entering
Block 3: Decelerating While Turning (Trail-Braking)
As stated earlier, there has been, and still is, a philosophy that
slowing for corners should be done on the straight. We disagree. Since the Skip Barber Racing School began in 1975, we have trained drivers to use not only straight-line braking but also the brake-turning ability inherent in every vehicle. Our philosophy is that it is easier in the long run to learn the skill and sensitivity required to brake and turn right from the beginning rather than learn a technique that will then have to be modified at a future date. If you choose to continue braking past the turn-in point, be it 150 feet or two feet, we call this "trail-braking."
In reality, all the argument over whether to trail-brake or not is a bit over-blown. Using the braking and turning ability of a car is just another tool in the effort to lower lap time. You don’t use a hammer for
repair, but it’s the perfect tool for some jobs.
No one is sure who coined the phrase, but the first well-known driver to talk publicly about braking and turning was Mark Donohue. In the late 1960s, Mark
|"This whole issue of trail-braking or not trail-braking is bullshit because every quick driver trail-brakes, whether they talk about it or not. On fast racetracks, like you find in Europe, you might do it less, but in the States you have more tight turns where you have to carry the brakes in there to help point the car and to gain an advantage by going deeper. There should be no question about it. That’s that."|
|- Robbie Buhl|
and other members of the Road Racing Drivers Club held seminars on racing technique and car preparation; it was in these seminars that Mark first began the heretical public discussion of using the brakes beyond the straight-and-narrow confines of conventional racing technique. The prevailing wisdom was that one was courting disaster by mixing the steering wheel and the brake pedal. Mark, a Brown University-trained mechanical engineer, based his premise on a graphic conceptualization of a tire’s traction capabilities, which he called the "friction circle."
The Friction Circle
Let’s build a friction circle graph. For the sake of simplicity we’sll graph a 1 G tire. Under acceleration, this tire could create 1 G of accelerating force. Under braking the force would be the same, 1 G, but in the opposite direction. Cornered at its maximum, the tire would create 1 G of cornering force.
In Fig. 5-5, we graph acceleration on the vertical axis below the point where the two axes meet. The tire’s braking ability is shown on the vertical axis above the 0 point. To the right of the origin is the tire’s right cornering force; to the left, left cornering force. Since this tire’s maximum is at 1 G, you can pinpoint on the graph the maximum abilities of this tire at four points. As the driver, you can choose to use the tire at less than its maximum if you like. At point A, for example, you would be using a portion of the tire’s braking potential. By pressing harder on the brake pedal, your level of force would travel up on the graph, closer to the tire’s maximum limit. The same is true of cornering. You could choose to use a portion of the tire’s cornering ability, represented by point B; or by driving faster or cornering on a tighter arc, you could use more of the tire’s cornering ability, up to the maximum available.
You can, however, mix abilities. Tires can develop a combined accelerating force and cornering force. Still, they won’t provide you with cornering force if you are using maximum acceleration. At point A on the graph in Fig. 5-6, you would have zero potential for cornering. If you accelerated at a point short of maximum, the tire would have some traction left over to provide some cornering potential.
|Fig. 5-5. A “friction circle” graph identifies the tire’s maximum capabilities in the three forces it is capable of producing: accelerating grip, decelerating traction and cornering force.
Carl Lopez and Cathy Earl
Chapter 5, page 81
|Fig. 5-6. By plotting all the combinations of Acceleration and Cornering force a tire is capable of, the lower half of the graph becomes a semi-circle.
Carl Lopez and Cathy Earl
Chapter 5, page 81
If you graphed all of the combinations of accelerating and cornering force available, those points would form a semicircle, starting at point B, going through point A, and ending up at point C.
|Fig. 5-7. By graphing all of the combinations of braking and cornering effort the tire can produce, you form a friction circle.
Carl Lopez and Cathy Earl
Chapter 5, page 81
Using the Friction Circle
The friction circle allows you to think about what happens to one ability of the tire as you increase or decrease the demands for another ability. A lot of this is intuitive, but the graph helps you visualize what is happening with the tire. The bottom half of the circle in Fig. 5-6 represents the accelerate-turn combinations. The braking and turning forces available from this tire are represented in Fig. 5-7, as another semicircle above the x-axis.
What you now have in Fig. 5-7 is the complete friction circle, a graph of all the potential combinations of ability available to you. For a moment, take a look at the brake-turn part.
Under threshold braking, at point A, there’s no cornering force available. But if you relax the brake pedal effort so that your demands for braking force are reduced, say, to point B, you can trace a horizontal line out to the circle and see that, with this reduced demand on the tire’s braking force, you get a considerable amount of cornering potential. The key, again, is not to identify specific points but to understand how you can affect the cornering potential of the car by the pressure on the brake pedal.
More Than Just a Concept
The friction circle is a real thing, not just some conceptualization in a text book. Our data collection system has the capability of drawing graphs which plot lateral G vs. longitudinal G--in essence, drawing friction circles for a real car on a real racetrack. Fig. 5-8 is a direct copy of one of these graphs drawn for the
|Fig. 5-8. An actual 1 G friction circle graph of a Formula Dodge pass up to and through the Sebring Hairpin.
Chapter 5, page 82
braking entry of the Sebring hairpin. The traces are exactly as Mark Donohue theorized.
How Much Braking and Turning?
Corner entry technique as taught at many other racing schools would not include the topic of carrying braking past the turn-in point. We have found that in most racing situations, it is advantageous to do so. The question is not
you"re going to do it, but
There is a wide range of brake-turning efforts. At one extreme there is the aggressive brake-turn right down to the apex, which you might use either at the end of a long straight leading to a compromise corner or in a passing maneuver. At the other extreme there is the light modulation off the brake pedal that you might use just past the turn-in for a fast sweeper.
Significance of the Throttle Application Point
When trying to choose the appropriate degree of brake-turning, an important variable is the location of the throttle application point. The later you apply the throttle, the more you’sll use brake-turning.
As discussed earlier, you should begin the acceleration through the apex and onto the following straight as early as possible. This yields higher corner exit speed and consequently greater straightaway speed. The end result is improved segment times from the apex of the corner to the entry of the following turn.
If this is the case, why not begin accelerating
the turn-in point--you can’t get any earlier than that! The problem is that, depending on the acceleration potential of the car and the radius of the corner, starting the acceleration that early may create so much speed that you can’t make the second half of the turn. As a rule, the throttle application point is where you can begin acceleration
and still make the corner
. If you have to feather the throttle at the exit of the corner to stay on, then you’ve begun acceleration too early.
You will find that, for the same corner, lower powered cars will have earlier throttle-application points than cars with more horsepower and greater acceleration potential. The speed range of the corner also affects the throttle application point since the car’s ability to accelerate changes with speed.
In one of our Formula Dodges, for example, the car can gain over 5 m.p.h. per second in the 60 to 70 m.p.h. range, but this falls off to less than 2 m.p.h. per second between 90 and 100 m.p.h. The primary reason for this drop-off is an increase in aerodynamic drag as speed increases. As the car goes faster, more horsepower is used just to push the air out of the way, leaving less for acceleration. As a result, the throttle application point will move earlier as the speed of the corner rises.
|"I absolutely use ‘The Procedure’ we teach—all the time. I can’t see that there’s any other way to approach it. If you try to brake late before you’ve ID’d the brake level available, you’re wasting time. You start by braking a little early and working on the level that’s available to you. Once you’ve ID’d how much is there you can start working it down. As you get more experience and confidence you can do both—you can step the brake level up and move it down toward the corner at the same time."|
|- Jeremy Dale|
How Corner Angle Affects Brake-Turning
The total amount of direction change required in a corner affects the throttle application point and therefore the extent of the brake-turn. Take a look at the three 55 m.p.h. corners in Fig. 5-9.The radius of the arc required to negotiate each corner is in the 200 foot range and a 1 G car can reach 55 m.p.h. on this arc. As usual, the radius would expand at the exit, as would the speed, but the constant radius arcs we’ve drawn will be fine for illustration purposes.
In each of these three cases, let’s say that the
you can manage to begin acceleration away from the corner is 225 feet from the exit. Any earlier and the car falls off the road; any later and exit speed is lost.
In corner A, a 75-degree corner, you’sll find that the 225 feet takes up 65 degrees of the change of direction. The car only needs to change direction 10 de-
|Fig. 5-9. The throttle application point varies with the total amount of direction change required in a corner.
Carl Lopez and Cathy Earl
Chapter 5, page 83
grees before the throttle application point. The brake-turning portion of the corner entry is going to be brief if not nonexistent. Since the throttle application point is barely 30 feet into the corner, you may choose to go directly from straight line deceleration (Block 2) to the brake-throttle transition (Block 4). In this case, in the 55 m.p.h. range it takes barely .4 seconds to go from the turn-in to the throttle application point, so you can skip the brake-turn phase.
In example B you have a 90-degree corner. The throttle application point is still 225 feet from the track out, but the difference is that the distance from the turn-in point to the throttle application point is now 86 feet. It would be wise to use some brake-turning to cover this distance at the highest possible average speed.
In corner C, as the corner angle increases to 135 degrees, the throttle application point moves further into the corner, right down around the apex. The length of the arc from the turn-in point to the throttle application point is over 240 feet. In the 55 m.p.h. range the car will be in the zone between turn-in and throttle application for around three seconds. The way you balance the car on a combination of braking and cornering will be very different than the braking technique used in the other two corners.
Pedal Force During the Brake-Turn
There are two basic ways of using the brake pedal in brake-turning. The first is to constantly relax the pedal pressure as the car approaches the throttle application point.
Let’s say, for example, that at turn-in pedal pressure is at 140 lbs., and the brake-turn portion of the corner entry is going to take .7 seconds, as it does in this run into the carousel in Fig. 5-10. In a uniform brake modulation, you would release 20 lbs. of pressure each tenth of a second, going from 140 lbs. to 0 lbs. in a steady progression. When you take a detailed look at the brake pressure graphs of different drivers and different cars, you’sll most commonly see this kind of brake-turning. In fact, the term "trail-braking" undoubtedly comes from explaining the process of trailing away the braking loads as the car decelerates while turning.
The second general way you can use the brakes while turning is by relaxing the brake pedal effort to a certain level and holding it there. In the example shown in Fig. 5-11, the driver turns in with 140 lbs. of pedal force and, over the first .3 seconds, relaxes the pressure to 70 lbs., then holds it there for a bit before relaxing the last 70 lbs. We have found that in corner entries where the brake-turn segment is longer (as in the 135-degree corner we looked at earlier), you try to find a specific combination of braking and cornering effort that slows the car
puts it on the right path for the apex. Once you’ve found the level of pressure that results in good speed loss and a solid cornering balance, you stick with it up to the point where the brake-throttle transition needs to take place.
|Fig. 5-10. This graph of brake pedal pressure depicts a uniform relaxation of braking pressure, decreasing braking force and increasing cornering potential.
Chapter 5, page 84
|Fig. 5-11. When the brake-turn segment is longer, a constant level of braking pressure, below threshold, provides a balance of deceleration and cornering.
Chapter 5, page 84
Another corner entry where you see this kind of brake-turning is where there is little or no opportunity to brake the car in a straight line approaching the corner. Since most or all of the slowing is done while turning, you try to brake at a level that accomplishes the speed loss but still leaves enough cornering force.
These situations are often found where there is a string of two or more corners, and the last corner is slower than the previous one. In Fig. 5-12, the left-hand corner leads to the right hander without an intervening straight. The curved braking zone will require braking and turning at a constant level of pressure.
|Fig. 5-12. Where there is no straight line braking segment, a constant level of brake-turning, subtly done, mixes deceleration and cornering.
Carl Lopez and Cathy Earl
Chapter 5, page 85
This type of corner entry is tricky. At the beginning of the braking zone the car is already turning right. If you snap off the throttle on the way to the brake pedal, it is likely that the tail of the car will slide out toward the left. The throttle-brake transition needs to be done smoothly and over time to reduce this tendency.
The same goes for the initial brake application--smooth and progressive is the key, building to a constant level until it’s reduced in the brake-throttle transition. This slowing-down process is going on quite close to the edge of the racetrack, and if, for one of any number of reasons, you get in a little fast and the car wants to go straighter than you intend, you have to modulate the braking loads to create some cornering force to keep the car on the road. This is a standard part of braking and turning--being able to control the degree of cornering force available by increasing or decreasing the brake pedal pressure.
Although we"re concentrating primarily on road racing, drivers progressing up the ladder toward Indy Cars will encounter oval racetracks and their special demands. Entering long corners like those on ovals is very much like the situation in Fig. 5-12. The throttle application point, especially with more powerful cars, is relatively late in the corner, and there is significant lap time to be gained by carrying straightaway speed into the corner entry. The transition off the throttle onto the brake has to be done delicately, and many drivers smooth out this transition by learning to use their left foot on the brake pedal. For those of us accustomed to braking with our right foot, this is a skill that takes time to develop, but one worth learning if oval track racing is in your future.
|"On ovals, most of the time, you drive the car down into the corner on power, then slow it down, then pick up the throttle for the exit. The car is already light down in the middle of the corner, so it’s a delicate deal—I had to learn to use my left foot on the brake and, until you do it for a while, you find that you have a lot less feel with your left foot. It took a while to get good at it."|
|- Danny Sullivan|
Shape of Corners
In decreasing-radius corners that
at the end of straights you are most likely to see constant-level braking and turning. This is because the more gentle arc in the first part of the corner often puts the turn-in point past the point where the road curves.
In Fig. 5-13, the car is first turned to match the arc of the edge of the road (point A), well before the turn-in point where the car is committed to the apex (point B). Since it is a long way between turn-in and the throttle application point, it is likely that you’sll use constant-level braking and turning. Whether this will follow straight-line braking depends upon the total speed loss required. If the speed loss requirements are high, the process will likely start with threshold brak-
|Fig. 5-13. Constant level brake-turning is more likely used in decreasing radius turns.
Carl Lopez and Cathy Earl
Chapter 5, page 86
ing. Then, at the first turn of the steering wheel, reduce the brake pedal effort to a level that allows the car to negotiate the arc between A and B. At the real turn-in, the level may have to be reduced again to accommodate the tighter arc of the line toward the apex. In both segments, though, the pressure is likely to be constant-level rather than a bleed-off.
As speed loss requirements decrease, the braking point gets closer and closer to the corner. Especially in decreasing radius corners, the initial braking point may end up
the corner. If so, the cautions and requirements of starting the deceleration process while turning are the same as we outlined in the "connected corner" situation.
When Not To Use Brake-Turning
Starting to slow for a corner
you have turned into it is not the most common corner entry procedure, but it
one you should have in your repertoire. But there are times, even if you can do it well, when you might choose not to. The cases I"m thinking about are those which we dealt with when looking at small corner entry speed losses.
|"I don’t think I ever, ever, release the brake before the point of turn-in, but sometimes it’s not so much to save time in braking. On fast sweepers, you don’t want rotation, you don’t want big weight transfer, you just want stability. So you knock off the speed before you get to the corner and have it as stable a platform as possible at the entry. It’s a balance deal."|
|- Dorsey Schroeder|
For example, if you only need to lose two m.p.h. of speed, why do it with a breathe on the straight when you could drive into the corner, brake-turn lightly, then transition back to power? The reason not to do this has more to do with handling considerations than with chasing the latest possible braking point. The sticky point is the throttle-brake transition.
When speed loss requirements are low, there isn’t much time available to do a smooth throttle lift after the turn-in and, smooth or not, the initial lift off the throttle is going to tend to create oversteer. If the brake-turning process is going to be a long one, the application of the brakes settles down the oversteer by substituting a proportionately balanced loss of cornering traction, front to rear, for the unbalanced situation of using engine braking at the rear tires while the fronts are at full potential cornering traction.
For now, let’s agree that you are going to carry some deceleration past the turn-in point of all corners where speed loss is required. In fast corners and small speed losses, this may mean the continuation of a throttle breathe ten feet past the turn-in. In some cases you will be losing 60 or 70 m.p.h. between the turn-in and throttle application point.
Using Throttle Before the Turn-In
As you’ve seen in the chapter on car control, simultaneous inputs of steering and throttle most often create understeer. This is the fundamental disadvantage of a throttle-then-steer entry. Once the understeer is created how is it defeated? Most often, a lift off the throttle, which comes
what would have been the throttle application point, takes care of this problem. The down side is lower exit speed caused by the delayed throttle application.
Brake-Turn vs. Corner Type
Your decisions about how much brake-turning to do are also affected by whether you"re dealing with a Type I, Type II, or Type III corner. As you no doubt recall from the chapter on the racing line, we said that the line could change depending upon whether a corner led onto a straight, came at the end of a straight, or led to another corner.
In corners leading onto a straight, exit speed is king. You want to bias the throttle application toward the early side, so you would tend to do a shorter brake-turn in Type I corners.
In Type II corners where exit speed is not the prime consideration, you could make use of the braking and turning ability of the car and decelerate up to or past the apex, making up time in the braking zone.
In Type III corners, you are more likely to not only extend the brake-turning zone but to find yourself in situations where the slowing down process will have to begin while the car is still turning.
|"Late in my career I drove a Porsche Carrera RSR at Watkins Glen and it was a beautifully balanced car—you could do anything with it.... My habit on the approach to the 90 at the Glen was to pitch the car at the entry, get a lot of opposite lock and try to be really aggressive with the power trying to get the best shot up the back straight. On one lap—I think I was passing someone—I really carried the brakes down toward the apex. The car was much more stable than usual and I found since it was more stable that I could apply more throttle sooner and the speed at the trackout was much better."|
|- Skip Barber|
Car Capability Affects Brake-Turn
The handling predispositions of your car affects how aggressively you choose to brake and turn. If the car’s setup is biased toward the oversteer side, whether intentional or not, the brake-turning aspect of corner entry has to be de-emphasized. This oversteer tendency may be generated in any of number of ways. It could be that the rear sway bar is too stiff, the rear ride height too high, the front sway bar broken or missing--any of a number of chassis-related thrillers. It could be the rear tires going off or the brake bias proportioned too much to the rear. If the car can’t be changed back toward neutral handling, you have to adapt. One of the adaptations for driving an oversteering car is to reduce the level of aggressive braking and turning, if not abandon it altogether.
In these situations you"re trying to reduce the braking traction demands on the rear tires so that they can supply more cornering force, settling down the oversteer. You’sll find that, likewise, the accelerating and turning ability of the car is going to suffer and that more sensitive throttle application will be necessary to keep the rear end behind the front.
Block 4: The Brake-to-Throttle Transition
The "brake-to-throttle transition," a fancy way of describing how you take your foot off the brake and put it back on the throttle. Let’s look at the variables.
The first variable is how long the process takes. You can take your own sweet time going from your initial braking level (be it threshold or not) down to zero brake pressure. You could also pop your foot off the brake pedal in a nanosecond.
Sometimes, as you can see in Fig. 5-14, you gradu-
|Fig. 5-14. In this graph of braking and throttle application at the Sebring hairpin, the driver gradually reduces braking force from 140 lbs. to zero over 1.1 sec. then makes an instantaneous switch from the last bit of braking to the application of throttle.
Chapter 5, page 87
ally reduce braking effort down to zero and the right foot instantly picks up the throttle and begins to squeeze it on. This is exactly the same motion as a brake-turn done in the bleed-off style--that is, with a progressive reduction in brake pressure. It’s the same as a slow, deliberate brake-throttle transition. The two are interchangeable.
If the brake-process is one where brake pressure is more or less constant, the brake-throttle transition can be slow and gradual as it is in Fig. 5-15, where it takes 1.25 seconds to go from roughly 80% braking down to zero.You
do it suddenly, as in Fig. 5-16, where everything else is similar but the brake-throttle transition is more abrupt.
Effects On Handling
The length of the brake-throttle transition will affect how the car sets its cornering slip angle. A slow brake-throttle transition allows the reduction in braking effort to keep in sync with the changes in front-to-rear loading. These changing forces work together over time, maintaining (or at least not upsetting) the overall cornering balance of the car. A sudden, sharp reduction in the braking effort delivers instant cornering traction to the front of the car by suddenly giving
of the front tires’ grip over to cornering force. The car is likely to point aggressively toward the apex, increasing the car’s yaw angle. Sometimes you want this to happen, sometimes you don’t.
In a sharp, slow corner, having the car rotate an extra 5 to 10 degrees might be helpful--it keeps the car out of understeer. In a 95 m.p.h. sweeper, however, you might not enjoy the kind of excitement that 10 degrees of extra yaw angle provides.
The second element you can use toward the same end is to choose to go directly to the throttle to stabilize the yaw you have created, or you can pause before picking up the throttle.
The "pause" is an intentional trailing-throttle oversteer maneuver. If the car has some rotation going, but not enough for your liking, a pause before going to throttle will help the car increase its yaw angle. If you like the extent of the slide you’ve got going, go immediately to enough throttle to stabilize the yaw angle. Once at this point, you"re now playing the corner exit game of squeezing on as much power as you dare, trying to maximize your speed coming onto the straight.
Assumptions We’ve Made
The type of car you"re racing will affect your braking choices. Many racecars, especially the ones that a beginning racer is likely to start with, have excellent brakes. On a purpose-built racecar like a Formula-Ford, Spec Racer, Ford 2000, Formula Atlantic, or Sports 2000 car, the brakes should be able to work at their maximum level every time you go to them, from
|Fig. 5-15. The smooth brake-throttle transition in this example takes 1.25 seconds.
Chapter 5, page 87
|Fig. 5-16. Similar to Fig. 5-15, but the transition from brake to throttle happens in .4 seconds, which affects car rotation oat the corner entry.
Chapter 5, page 87
|Fig. 5-17. Here the driver apparently didn’t like the amount of yaw and paused for .35 seconds between the brake release and the application of plenty of power. Note the position of the lower steering wheel on the graph—it’s turned significantly left in this right hand corner. The “pause” worked.
Chapter 5, page 89
beginning to end of your race.
Expecting a showroom stock car to perform like a Formula car under braking is a big mistake. In a car designed primarily for around-town use, the brakes are not up to the task of stopping a heavy vehicle time after time for hours on end. Although threshold braking may be desirable and achievable for a few qualifying laps, the faster and heavier the car, the more likely it is that you will have to modify your approach to braking to compensate for deterioration in the braking system.
Remember, during a long race, the pursuit is not for the single fastest lap but for the fastest
lap time over the race distance. An intelligent racer has to make the distinction between the technique which is usable in a sprint race in a bulletproof car and the driving required in order to save a car which has deteriorating performance in one or more of its systems.
|"Sure, you’d like to try and go into every brake zone as hard as you can, but in a long race you can’t drive every one at 100%—98% is more like it. If something changes, and it always will, or if you run into problems, you have a one or two percent margin. It doesn’t have to cost you speed."|
|- Jeremy Dale|
The Key Points of Corner Entry
Corner entries are exciting--more exiting than corner exits, to be sure. Coming out of a corner, you feel more fully in control, squeezing on the accelerator. If anything unusual or threatening takes place, you just don’t push the throttle down as aggressively. Coming into the corner you feel a bit more like a passenger. You’ve made a commitment at your braking point that your technique this lap has to be very close to the last lap’s or you’sll arrive at the corner going too fast to make it. Consistency in brake point and the level of pressure used on the brake pedal is critical.
It’s important to keep in mind what you"re trying to accomplish at the corner entry. Remember that taking the speed off in the shortest amount of time is only part of the goal. Slowing down to the right speed--the maximum speed the car can support on an arc that makes the apex-- is even more important because, especially in entry level cars with limited horsepower, this speed frequently determines the cornering speed and your speed down the next straight.
Different corners present different speed loss situations and you can count on the fact that you won’t use the brakes the same way in every corner. A little thinking and planning before you go out in the racecar can help you prepare for improving your corner entry performance.
|"A lot of people jump on the brakes very hard. I was always a guy who braked, for the most part, very easy. I didn’t use up the brakes at all since I tended to roll off the throttle and onto the brake more easily.
“To put it in perspective, at Laguna Seca, which is hard on brakes, Rick Mears and I were teammates at Penske and Rick finished the race with only 70 thousandths of an inch of brake pad material left. I only used 70 thousandths of the pad in winning the race. People brake differently but can still run the same lap time, especially in a race."
|- Danny Sullivan|
End of excerpt
©2002 Robert Bentley, Inc. We encourage visitors to link to this page if you’d like to share this information with others. Please do not copy this excerpt to other web sites. It is protected by copyright and represents significant resource investment by Bentley Publishers.