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 at least as important and should be as good as possible in order for your vehicle to "slow down faster". Here are some tips:
1) Lower the unsprung weight
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.
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 (EK= 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 engined sports cars usually outbrake front engined 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.
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 many brake systems: 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
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).
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).
• Vented rotors vs. solid rotors (vented rotors increase unsprung weight).
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.
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.
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, a faster engine, stickier tires, etc.
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)
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) 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).
|Last Update: 12/01/2019||
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