Brake Upgrade - What Lies Beneath

We install a new brakes system on our '72 coupe

Jeremy D. Clough Jul 30, 2012 0 Comment(s)
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In my memory, I see the green eyes first: brilliant, piercing, and Bambi-height. Not just one pair, but a forest of them, signifying a flock, a herd, a bevy--whatever you call a group of a dozen or so deer spread out across a narrow roadway. It was about two in the morning, I had just finished putting the sway bars on the 454 '71 convertible I had pretensions of road racing, and I just couldn't stand to pack it in for the night without trying out the newly stiffened suspension.

Thus it was that I found myself zinging around a curve, high up in the mountains, into...that mess. I hit the brakes, and they let me down, as one caliper locked and I glanced off a buck with the passenger side of the chrome bumper. The good news is that the deer survived the experience, and so did the car. And I became tremendously interested in the subject of brakes.

To be fair, even C3 brakes are pretty good, especially considering the era. Even the early models came with cast-iron four-piston calipers, which means that each disc gets pinched between two pairs of pistons pressing toward one another from either side. There is, however, a better way, and we find that way with Wilwood's D8 calipers. Available in a variety of different colors, the D8 caliper comes in either the usual four-piston arrangement for front and rear, or with a six-piston front and four piston rear. In this case, we'll follow the installation of the latter setup on my '72 coupe, aka "Scarlett."

Better Binders...

Before we get to the install, let's talk about the differences between the Wilwoods and the stock calipers. The first is the added clamping force. While the size of the pad itself is basically the same, the greater surface area of the pistons behind it increases the force that squeezes the rotor. While my factory calipers were too dirty to get a precise reading, on the high side, the piston diameter measured around 1.41 inches. Going back to geometry class, we can figure the area by using the formula pie X r², or, as Wilwood expresses it in the instructions, (bore x bore) x 0.785 inch. Either equation works, and works out to a total area of 6.24 inches of clamping area for a stock front caliper. The D8-6, however, has a pair of 1.88-inch pistons, a pair measuring 1.38 inches, and a pair at 1.25 inches, which comes out to a total clamping area of 10.96 inches, for a gain of roughly 75 percent.

The rears are similar. The stock calipers have four pistons of around 0.925-inch in diameter, for a total clamping area of 2.68 inches. The Wilwood D8 rear calipers' four 1.125-inch pistons give a clamping area of 3.96 inches, for a gain of roughly 48 percent.

The other advantage is that, unlike the factory calipers, which are made of cast iron, the Wilwoods are made of aluminum, which drastically reduces weight where it matters most. Since calipers are part of the suspension that rides directly against the road, without being suspended by a spring, they're called "unsprung weight," which is a critical part of how the car handles. It's a simple matter of inertia: An object at rest wants to stay at rest, and the more mass it has, the more slowly it responds to any force placed upon it.

The goal of suspension is to keep the tires on the roadway, using the springs to account for differences in load and road surface. The lighter the assembly that the spring is pushing downwards, the more quickly it responds, since it doesn't have a lot of inertia holding it in place. It is, as we say, kind of a big deal--so much so that I've heard that in terms of handling, a reduction in unsprung weight is the equivalent to removing six times that much weight from the car itself.

With this in mind, consider that the front factory calipers weigh in at a cool 11.5 pounds each, and the rears weigh 12.2. The Wilwoods, on the other hand, come in at a svelte 6 pounds for the front and 6.3 at the rear, knocking nearly 23 pounds off the total unsprung weight, an average of 5.7 pounds per wheel.

Installing the calipers is as easy as any other brake job, with the addition of changing the master cylinder. For that we used an aluminum tandem unit from Wilwood, which itself is more than six pounds lighter than the stock cast-iron one. While Wilwood offers three bore sizes for its C3-compatible master cylinders (7?8-inch, 1 inch, and 11?8 inches) I requested the 11?8-inch version. Check out the accompanying captions for a full rundown of the installation process.

...Shorter Stops

With the new master cylinder, calipers, and rotors installed, I was curious to see what kind of real-world difference the new system made, so I contacted Summit Racing and acquired an Autometer D-PIC. A standard 21?16-inch gauge, the D-PIC is an accelerometer capable of measuring parameters such as acceleration, deceleration, and lateral g's. Since the C3 isn't known for having a lot of extra dash space, and I'd already hidden a nitrous gauge in the ashtray, I looked for a way to mount the D-PIC on the A-pillar. Unfortunately, while Autometer offers a blue million gauge pods, they've inexplicably missed the C3. I did, however, acquire one for an '82-'92 Camaro/Firebird, which I was able to modify with a heat gun to fit my A-pillar and mount using trim adhesive.

Wiring it up was simple: Autometer suggests including a 1-amp fuse, so I picked one up at the local auto-parts store and wired that into the positive power, which I got by tapping into the switched-power line going to the electric fuel pump. I similarly tapped into a ground wire that went to the engine block, then wired the third white wire to the headlight circuit. While you technically only need positive power and ground to run the D-PIC, the white wire causes the gauge to dim when the headlights are on, which is a good thing, as it would otherwise be bright enough at night to be distracting.

Tuck all the wires out of the way, scrape off the excess adhesive, and off you go. Thanks to the kindness of our local sheriff and chief deputy, I was able to get a section of road closed off to allow for safe testing both before and after the installation of the new brakes. Setting up the D-PIC was easy: First I used the setup menu to make sure it was calibrated properly, then lined up the car to run it.

While the D-PIC measures 0-60 times, rear-wheel horsepower, lateral and forward/backwards g's and quarter-miles, the 60-0 feature is what mattered to me. Press the "mode" button on the left until "60-0" appears, and click "enter." The unit will show "cal," then the word "GO." Once you start accelerating, it will indicate speed until you're above 60, when the whole dial will flash and command you to "STOP," and then display the braking distance once you have.

I got in six runs with the stock calipers before picking up a nail in a rear tire. Stopping distances averaged 148 feet using threshold braking, with a best of 143 feet and a worst of 153. With the new brakes installed, my best six runs out of 11 averaged 138 feet, with a best of 135, putting the car about halfway between a stock C3 and a C4 or C5, which both stopped in something like 125 feet.

I should note, however, that the rear tires locked up before the fronts did, which tells me the bias needs to be adjusted, since it should be the other way around. With that changed, I'd expect to see the stopping-distance numbers drop even more. As it is, the brakes have a much firmer, more responsive feel. While they are by no means grabby, it requires a lighter touch to threshold brake, as the greater clamping force means they tend to lock up quicker than stock brakes, requiring more skill to hold the tires at the limit of traction. This is as it should be.

Of course, one of the greatest limiting factors on how quickly you can stop a car is how much road traction the tires give you--and I think it's about time we did something about that next.

Special thanks to Bob Ingram, April Owenby, Paul at Arrowood's Automotive, and William Stoner.

Sources

Wilwood Brakes
Camarillo, CA 93012
805-388-1188
www.wilwood.com
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