The secret to winning is well known: Don’t slow down! Well OK, at least try to minimize the amount of non-speed that you generate while braking.
Learning how to use all of the braking capability of your car is hard to do. It is a scary, high g event that starts when you are going as fast as you will ever go. If your car produces a significant amount of aerodynamic downforce, it is even harder because the maximum potential braking rate at the beginning is very high and falls off rapidly as the downforce and drag disappear. Even without downforce, the range of tire slip velocity that you have to maintain is very narrow and there are some harsh penalties for exceeding that slip velocity range: flat spotted tires, sliding off the line, getting passed, or a high speed spin.
Yes, you can learn to do it. Every kart driver does it, and if you have a dog-clutch racing gearbox, you have no excuse because there is no need to use the clutch after the car is moving. Left-footing it has major advantages, such as:
• Instant, perhaps even slightly overlapping, transition from acceleration to braking
• Continuous readiness for instant emergency braking
• The ability to use both power and braking simultaneously, which stabilizes the car to enable highly aggressive steering inputs in chicanes, slaloms, and emergency maneuvering
• No more heel-and-toe nonsense, which enables fine control of braking through the whole braking zone
• No lag between braking and throttle application
If you are racing a road car, using the clutch on upshifts and downshifts is unfortunately not optional, but you can left foot brake any time that a shift is not required. What is optional is downshifting directly to the gear you need for the corner exit, and that is a good idea. Heel and toe throttle blipping is also optional, and it is usually a waste of time and effort. Since you have synchros, you may as well use them for something and save yourself the extra workload. Just hold the clutch down and downshift directly to the gear you need.
Everyone will tell you that smooth is fast, but there is one exception. The power-to-braking transition at the beginning of a straight-line braking zone can and should be as abrupt as you can manage. That is how to make a pass stick and it is worth a little bit of lap time, so you want to slam the brakes on at the beginning of every straight braking zone. Of course the brakes had better be warmed up first. If your car produces a significant amount of aerodynamic downforce, you might not be able to push the brake pedal hard enough to lock the tires at the beginning of the braking zone. If that is the case, you have found a performance area to develop. Delaying your braking to the last possible point becomes critical when you are involved in a passing attempt, either attacking or defending.
BRAKES ARE TIRE HEATERS
Particularly if your car has racing slicks, braking as hard as the car is capable of is very important for another reason. Racing tires are very sensitive to surface temperature, and that temperature can change at up to 50°F per second! At the end of any straight, the surface temperatures of the tires are a lot colder than they were at the beginning of that straight. The energy input from braking rapidly heats the tires back up into their operating range, but they will only be heated enough if you brake hard enough. In my cars, if I was insufficiently brave about selecting my braking mark, the car would be slow and skittery all the way through that corner because the tires were too cold. In this case, there is a clear reward for using all of the car’s braking potential.
BRAKE BIAS ADJUSTMENT
I developed a simple method of setting brake bias that works really well. For me, the ideal brake bias results in the same cornering balance with the brakes applied or released in a steady-state corner. If you have too much front brake bias, adding braking will add understeer. If you have too much rear brake bias, adding braking will add oversteer. The reason to do it like this is to eliminate a handling interaction for the purpose of improving the driver’s ability to use the car effectively. It’s also close to ideal for uniform lockup with the major advantage of never having to risk flat-spotting the tires just to set the brake bias.
If your car has a proportioning valve instead of a bias bar and dual master cylinders, your cornering balance will change depending on brake system pressure. A cockpit adjustable prop valve will not fix that. I would prefer to steer the car with the steering wheel, not the brake pedal. That is why prop valves are significantly inferior to dual master cylinders with an adjustable bias bar.
A critical safety feature that is missing from many bias bar assemblies is a rotation stop that will prevent the bias bar from rotating past a certain angle. A stop like this will enable one of the braking circuits to function after the other one fails.
Do not use rear-heavy brake bias to reduce turn entry understeer. That will increase your risk of spinning in every braking zone. It will also force you to brake artificially early.
BRAKE HEAT FLOW
Energy is conserved, always. During a braking event, the brakes convert the kinetic energy of the car into thermal energy in the brakes, which we then waste by slowly dumping it overboard into the airstream. The majority of the braking heat is stored in the rotor while we are decelerating. Every pad compound is designed to be a good insulator, so only a small percentage of the heat ends up in the pads. Also, the rate of air cooling of brakes is far smaller than the rate of energy input during hard braking, so the brake heat is transferred to the air gradually during the following straight.
Since carbon-carbon and ceramic brakes are still hugely expensive, we have to respect the limitations of iron rotors. Iron is very good at both storing and conducting heat, but there is a limit to the rate that iron can conduct heat through itself. That is important for us to understand because although all of the heat is generated at the rotor faces, the thermal mass of the rotor is almost uniformly distributed all the way through its thickness. Because iron has a relatively high coefficient of thermal expansion, the heat conduction rate limit causes the iron at the faces of the rotor to try to expand more than the iron at the center of the rotor thickness while the brakes are in use. That difference in thermal expansion causes stress in the rotor, which can cause the rotors to warp or crack. It’s the “almost uniform” distribution of thermal mass that makes rotors vulnerable to warping and cracking. There is a small variation of material composition and material properties within every casting, and also a variation of wall thickness of ventilated rotors because no casting is perfect.
All of this is particularly relevant for race cars because the rate of heat input during hard braking with sticky tires vastly exceeds the conduction rate limit of iron. So, the way to deal with this issue is to gradually warm up the brakes during the out lap so that the middle of the thickness of each rotor is already somewhat warm when the first hard braking zone arrives. That reduces thermal stress, which reduces the risk of warping or cracking the rotors.
Cryogenic rotor treatment does a very good job of reducing internal stress that was caused during the casting solidification process. That reduces the risk of warped rotors by creating a more uniform stress field. So, it reduces the risk of rotor warping, but it does not eliminate it.
In circumstances where pre-warming the brakes is not possible, you have to limit the rate of heat input when the brakes are cold. The first stop is a lot harder on the rotors than the second one. The worst case for this is drag racing. You can warm up the drive axle brakes during the burnout, but the rotors on the other end are very likely to warp when you are braking unless you are very gentle on the brakes.
Carbon-carbon and ceramic brake materials have a near-zero coefficient of thermal expansion, so they are practically impervious to thermal shock. Despite the very high cost, carbon brakes are allowed on Indycars because the brakes are not used at all on superspeedways until it’s time for a pit stop, so the thermal stress caused by braking from 230 mph to zero using brakes that were stone cold at the beginning, then sitting stationary during the pit stop with no air cooling, would occasionally make iron rotors crack all the way through and disintegrate after a few pit stops. Bad things happened after that, especially if the car was back up to speed when the rotor finally let go.
BIGGER IS NOT BETTER
A surprising number of people think that bigger brakes or different pads will allow them to brake later or harder. Also, some people think that upgrading from iron to carbon or ceramic brakes does the same thing. Assuming that the brakes can generate enough torque to lock the tires, and they had darned well better be able to do that, only the tires limit your rate of deceleration. The brakes only slow the wheels. The tires slow the car.
One of the things that bigger brakes do is increase the thermal mass of the braking system, so the rotors and pads will not get as hot at the end of the braking zone. Also, the increased surface area will cool them slightly faster. For a given combination of brake rotor and pad materials, there are three parameters that size a braking system:
1. The maximum rate of braking energy input, which is tire grip limited, determines the required pad swept area.
2. The maximum energy input in a single braking zone determines the required thermal mass, which is proportional to rotor weight.
3. The cooling air flow rate is sized by the total braking energy input during a full lap and the surface area of the rotors that is exposed to air cooling flow.
Other things that bigger brakes do are increase the weight of the car, the inertia of the rotating unsprung mass, the yaw inertia of the car, and the variation of contact patch load over pavement ripples. Everything depends on everything else! Carrying around heavier rotors than the car requires will noticeably penalize your lap times. If your car is only used for autocross, there is performance to be gained by adding lightness to the rotors, wheels, and tires.
If you have problems with pad fade or fluid boil, you do not have adequate hardware. So, you need to do whatever you can within the rules to improve brake cooling. The only reason to consider bigger brakes is if you have done absolutely everything that is possible to improve air cooling flow and you still have brake overheating problems. The thermal capacity of the brakes must be sufficient to allow you to use all of the braking performance that the tires can generate, but absolutely no more than that. For road cars, this is one of the most likely examples of inadequate race track tolerance.
LESS IS FASTER
Most cars can decelerate harder than they can corner or accelerate. That’s why it takes highly developed skills to avoid braking more than you absolutely have to. As your skills and brake system tuning improve, you will find that you use the brakes harder, but for less time and distance in each braking zone. Mastering the art of deceleration is tough to do, but it is highly satisfying, and it adds to your on-track safety by enhancing your contact avoidance skills.
Neil Roberts is an aerospace engineer, a 25 year SCCA member, and a professional race car design engineer. Neil competed successfully in autocross and SCCA club racing for 20 years, then worked as a professional race engineer with the Hall/VDS Racing Indycar team for 4 years. Neil has been a major contributor to every Swift race car design since 1996, specializing in suspension and structural design optimization. Additional projects that Neil has led at Swift include chief engineering the design and construction of the Eclipse Concept Jet in 200 days and designing advanced concepts for DARPA and the military services.