Brakes

From Trackpedia

The brakes are far more powerful than the engine or steering wheel. However, it is not the brakes' job to stop you, that's what the tires are doing.

Brake Systems

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All the horsepower in the world is of no use if you have no way to stop it. In this section we'll examine all of the key elements of braking systems, and discuss how they come together to provide a complete system. For hints on using the braking system, head over to the driving tips section.

Once you've assembled the mechanicals and fitted them to your vehicle, you need to ensure that they are all working well on your vehicle. There are some common issues that you are bound to encounter. Below, read some suggestions on how to handle them.

Brake upgrades

Pedals (all) -- change them to allow heel/toe down shifting. (See the articles on Pedals and Heel-toe Downshift in the Driving Techniques tab for more information on this).
High temperature racing brake fluid -- prevents fluid boiling and the fading of brake pressure that results
Stainless steel brake lines -- prevents the fade caused by overheated and stretched stock rubber lines.
Carbon or other racing pads (used during the event only) -- provides extra braking grip, and will not fail due to the high heat generated with repeated braking (however they will wear rotors faster).
Slotted rotors -- reduces fading caused by buildup of brake dust between the pad and rotor.
Increased rotor size -- provides greater leverage which increases braking power (and less wear)
Multiple-piston calipers -- provides greater squeeze more uniformly over the pad for greater braking power (and less wear)
Possibly improve or add brake boosters, to increase response from the pedal.

Common Issues

Managing Heat

Nearly every issue brake system issue has one thing in common: Too much heat. While there are non-heat related issues, such as pad knock-back, almost everything else comes from excessive heat build up.

The two main situations that arise from heat management problems are Fade and Boiled Fluid. You can read about both of these situations on the Heat Management page.

Soft Pedal

A soft or mushy feeling pedal is typically the result of boiled brake fluid or pad knock-back.

Pad Knock-Back

The following is paraphrased from the article "A Common Racing Phenomenon" by James Walker, Jr. of scR motorsports. The full article can be found at StopTech's website.

As the wheel, hub, and wheel bearing deflect during cornering, the rotor hat sandwiched in-between is forced to go along for the ride. Because the caliper (red in the illustration) is attached to a more rigid suspension component – the upright – the parallelism between the rotor face (gray in the illustration) and the brake pads (yellow in the illustration) is altered. In so many words, the deflection of the rotor relative to the brake pads actually forces the brake pads away from one another. This spreading action pushes the caliper pistons (blue in the illustration) back into their bores a tiny amount (horizontal green arrows in the center illustration) so that when the deflection goes away (when the cornering event is over) there is not enough springback in the piston seal to push everything back together again (green arrows in the rightmost illustration). The pads are now pushed off the rotor and will stay put until the brakes are next applied.

Pad Knock-Back In Action

Unfortunately, the next time the brakes are applied the initial travel will be used exclusively to push the pistons back up against the pads and rotor. This requires forcing fluid into the system, so the pedal feels as if it drops away toward the floor. Because this action does not build any line pressure, no torque is generated and for a brief moment the car will not slow down.

So what can you do about knockback?

There are many schools of thought on addressing knockback, each with their own pros and cons. We’ll list them here not in order of preference or recommendation, but rather to assist you, the reader, in making your own best decision.

1. Minimize wheel end deflections during cornering

While it may sound obvious, making sure that your wheel bearings are fresh and tight is the first major step toward addressing knockback. Following the wheel bearing itself, upgrading hubs and other suspension components to achieve less deflection during cornering will also serve to minimize knockback. At times, heavy-duty or race-specification components may be available for bolt-on installation. A little bit of research here can go a long way.

Most professional race teams will actually take the time to disassemble, blueprint, reassemble, and shim their wheel bearings prior to use. In addition to minimizing clearances and running gaps, the grease is upgraded as well to make sure that the bearing generates as little deflection as possible while on track.

Note that if you are experiencing knockback only after either right-hand or left-hand turns, it may be indicative of a single wheel bearing on the fritz. As right turns load left bearings and vise-versa, a little on-track analysis can sometimes lead you directly to the suspect component(s).

2. Tap-up the brakes when necessary

This practice may not sound glamorous, but you might be surprised to find how many professional road racers use this technique on a regular basis. All that is usually necessary is a one- or two-tap application of the brake pedal just moments before your braking zone arrives. It takes some practice to get used to, but like heel-toe downshifting eventually it just becomes habit. Note that if you are applying the brakes hard enough to feel the car decelerate you are applying too much pressure – you only need enough pressure to seat the components, not to build torque!

3. Install active knockback springs

In some applications the best solution is to install a spring behind the caliper piston to actively push the pad against the rotor face, even when the brake pedal is not applied. While this creates a situation where the running drag of the system goes up significantly (the brake are always applied!), it can be of great help in solving otherwise terminal knockback issues. It should be noted that this is not typically an applicable solution for street-driven vehicles – it’s primarily found on track-only cars.

4. Increase master cylinder diameter and/or reduce caliper piston diameter(s)

Both of these changes will alter the hydraulic ratio of the braking system in such a way that for a given amount of deflection, the amount of fluid displaced is reduced. While this might sound like a good solution at first pass, keep in mind that the fundamental brake system characteristics will be impacted as well! Both of these changes will require the driver to apply more pedal effort for a given level of deceleration and will certainly impact the front-to-rear bias of the braking system at the same time.

Bias/Balance

we're talking about a street car in predominantly stock form here used for both street and track use. The principles carry through to racier cars, but the numbers change enough to yield a different outcome. We'll explore that at the end.

To understand why it is important to upgrade all four corners at the same time, whether it be pads, rotor diameter, or both, we can make the simple statement that the auto manufacturer balanced the braking force of the front and rear wheels to start with for the best braking performance and stability with the given parts. This balancing takes into consideration the car's weight, weight distribution, center of gravity, wheelbase, and weight transfer during hard braking conditions. Unless you make substantial changes to the car's weight distribution or center of gravity location, whatever brake upgrades you make, you want to keep the ratio of the braking forces the same as the stock setup.

To show the effect of changing front pads only, front rotors and pads only, or all four rotors & pads, we'll look at a series of calculations to show the net effect of these changes on the force applied at the road and tire interface.

First, we will assume a certain clamping force generated at the caliper based on the driver's pedal pressure, pedal leverage, master cylinder, and caliper piston size. We will assume this as a constant input to see what happens throughout the rest of the braking system by changing pads and rotor diameter. Of course, the actual braking force at the wheel is variable based on the pressure the driver applies to the brake pedal.

The net result of the calculations of our theoretical setup shows that we start with a stock system designed to have a 64% front and 36% rear distribution of force. By changing only the front pads to a high friction racing pad, this braking distribution changes to 71% front and 29% rear distribution. If the front rotors are upgraded in size and the racing pads are used, the distribution changes to 74% front and 26% rear. This is a dramatic change from the intended balance of the stock setup. One very noticable effect is that the car will be less stable under hard braking -- the back end will wiggle around requiring the driver to control it. With an out-of-balance setup like this, stopping your car with huge front brake upgrades would be like stopping a bicycle or motorcycle using only the front brake. It's very unstable (and nerve racking).

Another noticeable effect with a setup like this, where only the front has been changed, is significant front brake and tire wear, and little rear brake and tire wear. This is often misinterpreted as "the more front brake you have, the less you neeed in the rear." The rear system wears less, and therefore appears to be needed less. In fact, the reduced wear is not because the rear brakes are needed less, it is because they are used less. We will prove this a little further on with some numbers:

So, let's start looking at some calculations. First, let's understand this front & rear balance thing with some numbers. Let's review the calculations to show the braking forces at the wheel. First, we need an input clamping force at the rotor. This is created with a certain pedal pressure, pedal leverage, master cylinder design, and caliper piston size. We're not trying to show the actual engineering here, just the principles with enough math to back it up, so we'll start with a set of assumptions that result in a certain caliper clamping force at the front & rear calipers. The components involved to this point will remain constant through our theoretical rotor and pad upgrades, so we'll use that same clamping force for all instances.

The clamping force is multiplied by the brake pad coefficient of friction. This results in a net clamping force on the moving rotor. Next, we have to convert the static force of lbs into a ft/lbs of torque (our rotor is rotating, and a rotating force is defined as torque). Next, to translate the rotor torque into force at the road surface, we have to account for the diameter of the tire. This ends up back to a simple linear force in lbs at the road / tire interface.

The table below shows the results of these calculations on four scenarios: the stock setup, front pad upgrade, front rotor and pad upgrade, and all four rotor and pad upgrade. The important net result is the balance of force between the front & rear, not the actual forces (which are variable based on driver's foot pedal pressure).

OK, so now we see the relationship of how changing common bolt-on upgrades in the braking system affect the balance of front and rear braking forces. What does this do at the tire / road interface where the stopping action actually occurs?

Braking in racing is concerned primarily with performance during limit braking. With this type of braking, the brakes are applied with enough force to be just short of locking up the tires. One thing you must be very clear about, is that the frictional forces of the tire on the road is what slows a car down. Rotors, calipers, and pads do not stop the car. The brake rotor is used to apply a torque on the tire. That torque is translated into a linear force at the tire and road interface that resists the car's forward motion at the road surface. This is what stops the car. Therefore, when looking at brakes you must look at the impact of the forces applied at the road surface. All the other calculations in between are simply balancing the engineering for designing parts to get to that point.

A tire will generate a certain coefficient of friction based on the rubber compound, the size of the contact patch, the road conditions, etc. Another major factor in a tire's stopping power is the amount of downward force on the tire. Higher downward force increases a tire's traction. Because we're dealing with limit braking (the maximum end of the braking scale), we can simplify all the factors involved into two numbers. The downward force applied on the tire (which is a combination of a vehicle weight distribution and weight transfer under braking), and the tire's coefficient of friction (which reduces all the other factors into one number). If we multiply the downforce by the coefficient of friction, we get the maximum force the tire can create before it loses traction.

Let's look at some numbers and see how this actually affects braking.

Let's take a 3,000 car that has a static weight distribution of 55/45. That's 1,650 lbs on the front tires (825 each), and 1,350 lbs on the rear (675 each). Now, let's say under limit braking, we have an added weight tranfser shift towards the front of 265 lbs. That is now a total downforce of 1,915 lbs up front, and 1,085 lbs in the rear. Under braking, weight distribution is now 64/36.

Under our limit braking example, the front tires will have a total of 1,915 lbs of downforce. Multiply this times the coefficient of friction of a good sticky street tire of 1.3 and you have a total of 2,490 lbs of force (1,245 lbs for each tire). The rear tires contribute 1,085 lbs x 1.3 for a total of 1,410 lbs of force (705 lbs for each tire). For all four tires to be at or near their braking limit, this also means that the brake torque applied by each brake assembly must be the same as the tire forces above. So, from the arbitrary starting point in the table above, we actually need to increase pedal pressure to acheive 1,245 lbs of force at the front brakes, and 705 lbs at the rear brakes.

Now, be careful to remember that this amount of force could be generated by any size rotor, and any brake pad. The pedal pressure, master cylinder, and caliper could all be changed to account for various rotors & pads to generate this braking force. At this point, it really doesn't matter what the brake design is -- they can all lock up a tire. So, be careful not to get hung up on the fact that our sample table above shows different available brake torques. These are not maximum values -- they are sample values at a given brake pedal pressure (and other factors as mentioned).

If we take either the stock or the example four wheel modified brake system (both have 64/36 force ratios), and increase pedal pressure to generate 1,245 lbs of force up front, this will result in a force of 711 lbs in the rear (we used the four-wheel modifed brake system for the numbers). This is a simple ratio calculation using (1,245 / 819) * 468. That's less than 1% off -- almost perfect, and close enough for real world use. In a racing setup, this kind of small difference could be dialed out with a front / rear bias valve. Using either of these brake systems, the front and rear tires would be very close to their maximum potential at the same time under limit braking.

What happens when we use the front-only modified setup with a larger rotor and race pads? Again, the pedal pressure is increased to generate 1,245 lbs of front brake force. However, the rear brake force only generates 429 lbs of force at that pedal pressure ((1,245 / 819) * 282). For the rear tires to be at their braking limit, 705 lbs are needed. With only 429 lbs available, the rear tires are nowhere near their maximum braking potential. The rear tires are contributing far less to the overall braking performance than they could be. So with big front brakes, the rear brakes are not needed less, they are used less! Under racing limit braking, the big fancy brake upgrade may actually increase stopping distances! Brakes don't stop the car -- tires do. If you're not using 100% of all four tires, stopping performance suffers.

Available braking force; 705 lbs. needed at rear to maximize braking

Front-only upgrades may feel like they stop faster on the street. In fact, brake upgrade suppliers always tout shorter stopping distances. If brakes don't stop the car, how can this be? Until you reach limit braking (where the tire is 100% in control of stopping distance), the stopping force generated by the torque created at the rotor dominates the available stopping forces. With bigger rotors, you can generate maximum braking force with less pedal pressure, less pedal travel, and therefore less time. It's this reaction time and the faster ramp up of the torque applied at the tire that creates the shorter stopping distances. When you're traveling at 60 mph, you travel 88 feet per second. If your brakes reached peak torque .1 seconds faster, then you just reached limit braking 8.8 feet sooner. So in that respect, bigger brakes can shorten braking distances. Another way brake upgrades feel better on the street is that the bigger rotor will generate greater torque for a given brake pedal pressure. Therefore, the driver can press lighter on the pedal to get equivalent stopping power. It feels like the brakes make a big improvement in stopping power -- but they really don't. You just don't have to push as hard to get the same power.

The goal of brake upgrades is therefore thermo-durability. That is, the ability to sustain through high temperatures at great efficiency.

Stock style brake pads are not designed for the sustained high temperatures created by repeated high force braking. Their coefficient of friction drops significantly, and the material begins to fall apart with the high temperatures involved. On a track with several hard braking zones, brand new stock pads can wear down to the metal backplate in two hours of track time. Racing pads will last 4-to 6 hours of racing time, and their co-efficient of friction will remain consistent at the high temperatures.

Stock rotor sizes, likewise, may not have the ability to dissipate the heat generated by race-duty braking. The repeated, high-temperature heat cycles may destabilize the mechanical integrity of the rotor. It could warp, or generate numerous small surface cracks. Larger rotors act as larger heat sinks. Their larger mass reduces the peak temperature they reach, and they stay mechanically stable. In a nutshell, they last longer.

For the part time racer, racing brake pads are almost always needed for road racing. Rotor upgrades depend on the car, and it's original design purpose. A Vette's rotors will be fine. A Honda's rotors may not. Whether you need bigger rotors depends on the track you're racing at, and how hot the rotors get. If they warp after every couple race events, then larger rotors could save you money in the long run.

There are conditions when this "keep the balance the same as stock" conclusion does not hold true. This is when the makeup of the car has been dramatically altered from it's stock form. Primarily this involves major changes in the car's CG position, it's total weight and / or weight distribution, or in its weight transfer characteristics under braking.

If you have gutted the car, outfit it with much wider than stock tires, or use very sticky race tires, the net result of the braking balance can be quite different than the stock ratios. The ratio of front and rear braking forces changes typically reducing the amount of rear brake force required.