A Word (or two) About Brake Design

Once again, I am not engineer by trade.  But, like most Gear Heads and Old Guys, I’v learned a thing or two over the years.   I’v done my research, read a lot of books, worked on a lot of cars, and learned about braking at the edge on the race track.

Heat

The essential function of brakes is to convert rotational energy into heat energy.  Remember the First Law of Thermodynamics:  “Energy can neither be created nor destroyed.  Energy can only be changed or transferred from one form to another.”   The more efficiently it can be converted or transferred, the better your brakes will work.

When the pads clamp on to the rotor, it converts the rotational energy of the wheel into heat energy in the rotor.   That heat energy is then transferred into the atmosphere.  This is the reason for large brake ducts.  The function of cooling is to move the heat energy from the brake rotors to the environment.   Your very first brake upgrade is to improve cooling.  Race car, street car, tow vehicle, doesn’t matter.   Make that your first upgrade.  It’s easy and inexpensive, and it will support any other brake upgrades you make later on.

Bigger rotors will make big improvement in two ways.  First, it’s a bigger diameter (I know, “Duh”).  That places the caliper further away from the center of the wheel, and that acts like a longer lever.  Give me a long enough lever and a fulcrum, and I can move the world.

The other advantage is the added steel.  More real estate can handle more heat.  There’s that conversion thing again.

Rotors

Since we’re talking about rotors, lets mention holes and slots.  In the olden days, hot brake pads on the race track would outgas.   Think of steam coming off of boiling water.   So much gas was produced that it would actually push the pads away from the rotor surface.  No contact means no transfer of energy.    To combat this outgassing, racers drilled holes in the rotors to give a place for the gas to go.

The downside of the holes is that it created stress risers, and cracks would propagate from the holes.  The rotors don’t last nearly as long.  Not a big deal if you plan to replace the rotors a couple of times a season.    It is a problem on a street driver, though.  Stress cracks in your rotors are a bad thing.

Well, welcome to the 21st century.  Modern brake pads are made out of different materials than they used to be.  Remember asbestos brake shoes?  Not pads, but shoes.  Yeah, me too.  Anyway, modern pads do not outgas.  Hence, there is no need for holes.   Holes in rotors are strictly for bling; they no longer have a function.  That is not my opinion; that is the opinion of the good folks at Wilwood brakes – who I literally trust with my life.  And unless you plan on spending thousands of dollars for carbon fiber or ceramic  rotors,  holes will significantly shorten the life of your brake system, and set you up for a catastrophic failure.   I don’t know about you, but I consider that a bad thing.

Slots, on the other hand, actually serve a purpose.  When brake pads get really hot, they can form a slight glaze on the surface.  The glaze doesn’t work as well as the fresh unaltered pad material.  The slots act kind of like a cheese slicer, and shave off a small amount of pad material every time they pass over.   That works well on the race track.  But on the street it will shorten the life of your pads and create a lot of dust.

Little Big Brake Kit
Little Big Brake Kit

 

Take a look at these racing brake rotors.   No holes, only slots.

Cobra Racing Brakes
Cobra Racing Brakes

Balance

Stock brake systems work OK on stock cars.  They’re usually specifically designed for a specific application.   Transplanting all those bits and pieces to cars that are radically different, requires some fore thought.   For example, a Fox body mustang has about 57% of it’s weight on the front tires.   A Factory Five Racing roadster has about 49% of it’s weight on the front tires.  But both cars use the exact same bits.

For the average roadster owner who cruises around town, goes to the drive in on Saturday night, or takes a week end trip with his wife, it doesn’t really matter.  He’ll probably never perform threshold braking except in a panic situation.  And then he’ll probably lock them up anyway.

But if you push your car to the limits on the race track lap after lap, you will quickly discover that the brake balance is all wrong.  If you’re going to be racing, you’ll need to figure out a way to change the balance.   Fortunately, the Exocet has the same balance as a Miata.  So it was pretty easy to build a good braking system around mostly stock Miata parts.

Stock Miata master cylinder
Stock Miata master cylinder

I did use a “proportioning valve” in the rear brake lines.   Although it isn’t really a proportioning valve.  If you look at it’s function, it’s really just a pressure reduction valve.   I found that a couple of turns on the valve created a better balance for the Exocet.  In retrospect, I should have place it on the other side of the MC.

Master Cyl

The Factory Five Racing Cobra replica, though, is a different story.  The braking system is pulled directly from the Mustang.   But the Cobra actually has a slight imbalance to the rear.   My car is about 51% rear.  So I used a master cylinder that can balance braking force easily and accurately.

Fluid

 

Picking the right fluid is important, too.   Brake calipers get hot.  If they get too hot, the fluid will boil and turn to steam.   Steam is compressible.   Sudden, you’ll have a soft and mushy brake pedal.   So you have to pick something that will not boil at track speeds.   I’v been using Wilwood 600 for years without problems.  There are plenty of good high temp fluids on the market, pick one.

DO NOT use DoT 5 synthetic fluids.   The word “synthetic” makes it sound attractive.   You have synthetic fluids everywhere else, right?  Why not the brakes, too?

Brake systems get water in them.   They just do.  If you’re in a high humidity environment, like Florida, you’ll attract a lot of water.   DoT 5 fluid will not absorb water like other fluids will.   The water tends to clump together and form pools and pockets. Water is heavier than brake fluid, and will drop to the lowest point – the caliper.  Water works fine until it turns to steam, and then you’re in trouble.   And it happens at a very low temperature.

Brake fluid1

DoT 5 will not mix with other fluids, either.  It will separate out into two layers.  And at the junction of the two fluids, it clots.

 

Racing

If you’re racing, spend the money in the beginning for good brakes from a company like Wilwood.   Good brakes will not only make your car safer, but faster.   If you can brake later in to a corner, you’ll beat your competition into the corner and gain position.   Good brakes will significantly decrease your lap times.

Also, racing brakes are cheaper.  Yes, you’ll spend more money in the beginning.  But they will require far less maintenance, and you’ll be replacing fewer parts.   Pads and rotors will last 2 1/2 – 3 seasons.  Where stock parts last less than 1 season.