Deceleration Optimization  (Improve Your Brakes)

First of all, I would like to emphasize that a good brake system is only one part of the equation that leads to impressive deceleration numbers. The right vehicle dynamics are just as important and should be optimized in order for your vehicle to "slow down faster". Here are some tips:

1) Lower the unsprung weight
2) Lower the vehicle weight
3) Lower the rotating mass
4) Move center of gravity downwards
5) Move center of gravity towards rear axle
6) Optimize the suspension

Let's look at these points in more detail:

1) Lower the unsprung weight. This is one of the most important and often overlooked opportunities to improve the deceleration numbers of your vehicle. One of the drivers in a racing series with Lotus Super 7 clones ones asked me why everyone else could brake so much later than he could, while his car had the better brakes (all cars had the same tires). When I asked him why his brakes were better, he told me that his car had vented rotors and the others where driving with solid rotors... You probably already guessed it: After changing out his vented rotors for solid ones that weighted much less (and removing the spacers from in between the two halves of his M16 calipers), his frustration had turned to satisfaction since he now was able to compete fully with the other drivers. His problems with modulation, locking up the brakes and flat spotting his tires were gone (obviously brake fade was not an issue for these light cars).

Bottom line: The lower the unsprung weight, the better (more consistent) the contact between tire and road surface.
Or put differently: A heavy tire/wheel/rotor assembly (among other unsprung parts!) introduces more energy to the spring when "bumped up" by a road imperfection than a lighter tire/wheel/rotor assembly. The shock absorber needs more time to "absorb" (= convert into heat) the extra energy stored in the spring. During this time the normal force (vertical force) between tire and road fluctuates, so friction and therefore deceleration suffer.

The lower the vehicle weight, the more important it is to keep unsprung weight as low as possible. Since tires and wheels are relatively heavy objects, it is harder to keep the unsprung weight (percentage wise!) low on a light vehicle than on a heavier one. Our example of the Lotus Super 7 is a good illustration of this.

2) Lower vehicle weight (mass) means that less kinetic energy needs to be converted into heat (E_K= ½ mv²). This can save time (= higher deceleration numbers) and might allow for certain brake parts to be smaller and lighter, which reduces unsprung weight and rotating mass, which again means higher deceleration numbers, and so on...

3) Lower the rotating mass. When a brake system slows down a vehicle, it not only converts linear kinetic energy into heat from the vehicle mass moving at a certain speed, but also rotational kinetic energy of the rotating mass (tires, wheels, rotors, hubs, lug nuts, etc.) moving with a certain circumferential speed. A smaller rotating mass contains less rotational kinetic energy and requires less brake torque to decelerate in a given time, or requires less time to decelerate with a given brake torque. This will reduce demands on heat sink capacity and cooling capacity, which might allow you to choose a smaller size rotor and caliper. This of course will have a positive effect on unsprung weight and rotating mass...

4) Moving the C/G downwards will result in less weight transfer to the front tires, which give the rear tires the opportunity to take on a bigger part of the job.

5) Moving the C/G towards the rear axle will result in less weight transfer to the front tires, which give the rear tires the opportunity to take on a bigger part of the job (ever wondered why rear engine sports cars usually outbrake front engine sports cars of comparable weight and on similar tires?).

6) Optimize the suspension. The springs should have the right spring rate for your vehicle and track conditions under which it is used. Also, the shocks should be as capable as possible in controlling (read: converting into heat) energy that is stored in the springs with every ripple or bump in the road. This means that the shocks should be a perfect match with the springs and application, like road course racing, drag racing, off road racing, slalom, etc. All have different requirements for suspension "stiffness" and articulation.

Bottom Line: Deceleration numbers (or brake distance for that matter) will always benefit from shock absorbers that are more effective (not necessarily "stiffer"). The shock absorbers should be a perfect match with the springs, and the springs with the weight of the car and its specific use.
About suspension many books have been written, so I avoid diving deep into this subject to keep the focus on brakes. But keep in mind that without the right suspension for a particular application, a brake system can be quite perfect without being able to slow down the vehicle adequately and effectively...

As a side note: on most vehicles the tires can be seen as springs as well, because they absorb part of the shocks caused by small bumps and ripples in the road surface; even on sports cars and race cars. But first and foremost, they are looked at as friction material.

Probably the biggest improvement to your deceleration numbers: Tires with a higher coefficient of friction...

 

Back now to brakes:

In case of an under performing brake system, the following two areas can be looked at:

1) "stopping power" or the deceleration numbers
2) "thermal capacity" or the ability to handle heat that is generated while using the brakes

Let's look further into these two areas:

1) "Stopping Power": If you can lock up the front and rear brakes simultaneously on a clean tarmac or concrete surface, while applying a manageable force on the pedal, and with the "stickiest" tires available and the highest down force (if applicable), there is nothing wrong with your cars "stopping power". However, this does not mean that you can not improve your cars deceleration (see the six tips above).
Note that the highest deceleration during braking occurs when the tires just start to slip, but keep rolling at the same time. This situation is called "threshold braking".

2) "Thermal Capacity": To handle the thermal energy that is generated during braking, heat sink capacity and cooling capacity both need to be adequate for the application. Heat sink is the ability to easily and quickly absorb the heat generated by friction between the pad lining and brake rotor. Cooling keeps the temperature of the brake parts low enough to allow for quickly absorbing more heat and to prevent damage to rubber seals and boiling of the brake fluid.

About brake fade many articles have been written. I would like to keep it short, and simply mention the following:

   A soft pedal (or worse: pedal on the floor) is caused by overheated/boiling brake fluid (or frothed silicone fluid that foams when subjected to vibrations and should not have been used for high performance applications in the first place).

   A hard pedal but not enough deceleration is caused by brake pads that have exceeded the thermal limit of their intended application.

Since brake lining wear can increase substantially with rising brake temperatures (especially above the thermal limits of the binder that keeps the friction material together), improving the resistance to wear is simply a matter of improving the thermal capacity of your brake system and/or using brake pads with a higher maximum operating temperature.

Heat sink improvements:

• Bigger/heavier rotors (this increases unsprung weight).
• Larger brake pads (this requires bigger calipers which can increase unsprung weight).
• Clean (with wire brush/sand paper) the contact area between hub and wheel and apply temperature conducting paste. This is a last resort to keep the grease from running out of your wheel bearings when racing undersized brakes at a brake torturing track on a hot day (I've seen it work!). Wheels are great heat sinks, but keep in mind that with every 10°F increase in wheel temperature, tire pressure increases 1 psi (with every 10°C tire pressure increases 1.8 psi; 1 bar ≈ 14.7 psi, so 1.8 psi ≈ 0.12 bar).

Cooling improvements:

• Vented rotors vs. solid rotors (vented rotors increase unsprung weight).
• Curved vane rotors vs. radial (straight) vane rotors (curved vanes pump about twice as much air as straight vanes).
• Air scoops that collect air and guide it to the center of the vented rotors.
• Larger brake rotors (larger surface area radiates more heat and has more exposure to cooling air, but usually increases unsprung weight).
• Aluminum vs. cast iron calipers (aluminum cools down faster than cast iron and reduces unsprung weight).
• Cross drilled rotors expose more rotor surface area to the outside air which make them cool down faster (and also reduce unsprung weight a little, but the down side is the sensitivity to cracks, since each hole is a stress riser).
• Alloy wheels vs. steel wheels (aluminum or magnesium alloys heat up and cool down faster than steel and can reduce unsprung weight; although aluminum is three times lighter than steel, cast aluminum alloy wheels are usually not lighter because they contain so much more material than steel wheels; forged alloy wheels however, usually are lighter than steel wheels).
• Wheels with a more open design to allow for better air circulation.

As you can see, many of these improvements have down sides too, so the challenge is finding the right compromise that works best for you.

 

Another area for improvement not mentioned yet, is the ability to deal with heat energy being absorbed by the brakes that is transferred into the brake fluid. Here are some tips:

• Use Brake fluid with a higher boiling point.
• Flush brake fluid before every race or track day (the boiling point of brake fluid goes down with every heat/cooling cycle).
• Route brake lines further away from hot engine parts and exhaust parts.
• Use heat shields to protect lines and fluid reservoir(s) against heat from engine and exhaust system.
• Use heat barrier plates (SS or titanium) between brake pads and caliper pistons (may introduce some extra pedal travel)
• Use calipers with SS, ceramic or titanium pistons (which have a lower thermal conductivity than iron and aluminum pistons).
• Use titanium "buttons" inside pistons, to separate them from the pad's back plates.
• Use light alloy calipers (instead of cast iron calipers) with cooling ribs or even liquid cooled calipers.
• Install a dynamic bleed system that replaces hot brake fluid in the calipers with cool fluid from the reservoir in small increments, every time the brakes are used.

 

A few words about slotted and cross drilled rotors:

Both do a good job wiping brake dust from the pad surface, preventing this dust to hinder friction between the pad and rotor surface. Both also do a good job allowing gas developed by overheated binder materials (that keep the friction materials in the pad together) to escape instead of form a buffer between the pad and rotor surface. But cross drilled rotors have an advantage over slotted ones when it comes to cooling capacity, because the holes increase surface area exposed to air.

However, cross drilled rotors have a tendency to form cracks (originating from the holes which act as stress risers) when subjected to extreme temperature fluctuations over and over again. Those fluctuations will almost certainly take place during racing. Small hair cracks are no problem, but as soon as the first one reaches the outside edge of the rotor, replace it for safety reasons! And always replace rotors in pairs.

Slotted rotors do not seem to have a tendency to form stress cracks, more than plain rotors do. But eventually any cast iron rotor will show small hair line fractures in the surface of the wiped area, when subjected to extreme use long enough. If your rotors are worn before cracks dictate replacement, heat sink and cooling are obviously sufficient.... meaning there is room for improvement in one or more areas: for example lighter/smaller rotors or a faster engine.

 

A few words about carbon ceramic brakes:

Carbon Fiber Reinforced Ceramics, or just Carbon Ceramics (CC), when used as a material for the rotors and pads, have several advantages over cast iron rotors and conventional brake pads:

• Lower weight (a CC rotor weighs only approximately 25% of the weight of a similar cast iron rotor)
• Higher maximum allowed operating temperature
• Higher resistance to thermal shock
• Higher resistance to wear
• Lower coefficient of expansion

Disadvantages are the higher costs and lower friction at temperatures below optimum temperatures of operation.

 

A few words about brake boosters:

One disadvantage of a booster is that it usually does not assist from zero to maximum in a linear way, thus making it harder to prevent wheel lock-up when aiming at maximum deceleration and threshold braking. If you have a booster (often used with a pedal ratio of around 3:1 or 4:1) and want to get rid of it, then keep in mind that you will need to change out the brake pedal assembly for one with a higher ratio (usually 6:1 or 7:1) to keep the pedal pressure required for maximum deceleration at a comfortable level.

 

And finally a few words about ABS (Anti-lock Braking Systems):

ABS has no influence on any of the calculations on this web site. In fact, it only comes into play when you try to balance your brakes by observing whether your front brakes or rear brakes tend to lock up first, which will be close to impossible when your ABS is functioning properly. In order to do that test, you will have to temporarily pull the ABS fuse or relay.

On brake systems with modern ABS lacking a vacuum booster, the function of the booster has been taken over by the high pressure ABS pump. However, you can still determine the "boost factor" the same way as you would with a vacuum booster (see "?" button on the "Booster Assist Ratio" line from the brake calculator).