Tires stop a car, not brakes. Even if that's a broad statement, full of caveats and qualifiers, it contains a crucial truth. All things being equal, stopping distance is stipulated by how much grip the tires have before they're finally overcome by braking torque, resulting in wheel lock. For those with ABS, the surest and simplest way to reduce dead-stop braking distance is a simple matter of stickier tires.
So how do big brake companies go about claiming improved braking performance without changing tires? The answer: braking balance. And that's the reason why we devoted two whole days to comprehensive brake testing, one of which was completely unanticipated.
As you might have seen from various Project RSX pictures, a set of StopTech brakes were already installed when we inherited the car. It's a complete set-up, engineered to optimize braking performance through better front-to-rear balance. The kit consists of stainless steel braided brake lines, 328x28mm (12.9x1.1-inch) slotted two-piece, fully floating rotors with aluminum hats for the front, four-piston fixed squeeze-forged aluminum calipers with StopTech's patented X-brace bridge design, Axxis Ultimate pads for the larger front and stock rear calipers, and all the necessary hardware and brackets to mount the larger front brakes. We never mentioned them before because we hadn't got around to doing a full brake test involving A/B comparisons with the stock hardware. Now that the completed Project RSX is ready for its final performance figures, it was the perfect time to break out the testing gear.
Our comprehensive brake test program involves 10 repeated 80-to-0mph runs, measuring rotor temperatures on each run as well as the total braking distance. The first measurement is performed with rotor temps at near-ambient prior to the run, so we have a more complete picture of stopping performance over a broad temperature range. And since modern brakes are so damn good at stopping in spite of high temperatures, we also do an 80-to-0mph run on the unmeasured outbound trip, in order to keep brake temps up and try to get stock systems to fade.
Day OneWe took Project RSX out to the track with the StopTechs already installed, pads properly bedded and the system pre-bled with fresh Motul RBF600 brake fluid. Conditions would be more or less ideal: using the same track on the same day on the same relatively fresh Hankook Ventus Z212 maximum-grip tires. Having conducted this test countless times, we have a decent idea of what constitutes a good braking distance for a car of this weight and how much variation to look for as the brakes heat up. Project RSX averaged an 80-to-0mph braking distance of 219.8 feet (disregarding the first cold stop) with a best of 213.3 feet. We expected nothing less.
For you statistics nerds, that amounts to a standard deviation of (or 68 percent of all stops falling within) 5.1 feet from the average braking distance. This is almost nothing compared to the variations we see in most ABS-equipped cars with modified brake components. Front and rear rotor temperatures were within 160 degrees F of each other, even with the dust shields installed.
After a quick lunch to let the rotors cool, we took off the StopTech kit and installed stock RSX Type-S front rotors and calipers, using Acura's A-spec pads. We left the braided lines on to keep things fair and re-bled the whole system. With the new pads properly bedded and heat-cycled with an evenly distributed transfer layer of the new pad material on the rotors, we repeated the test.
We expected to see much higher front rotor temperatures, a squishier pedal feel and maybe even some brakes on fire at the end of the runs. With any luck, the stopping distances would be just a little longer than the numbers collected earlier with the StopTech system, but not so different as to throw out our grandiose claim of tires making the most difference.
On its stock brakes, the car did everything we expected, but also posted a shorter braking distance on almost every corresponding brake run. On average, the stock brakes stopped the car in 216.1 feet, 3.7 feet shorter than the StopTech average. It also posted its best distance of 210.6 feet on the fifth run, even with the pedal going soft and the pads billowing smoke.
Obviously, these numbers were going to piss on StopTech's parade and ours. Even though the big brakes were far superior in terms of rotor temperatures, pedal feel and modulation, the longer-than-stock stopping distances would undoubtedly raise some questions as to the validity of spending several thousand dollars on an upgrade. And even though we'd prefer some semblance of pedal feel on the track with the big brakes over a negligible difference of less than four feet (remember this is still within one standard deviation of the data collected), it's still a little embarrassing.
We consulted with StopTech's engineers and sent them our data, so they could defend their product before we blasted them with our published automotive blasphemy. They agreed our testing methodology was decent enough for a bunch of magazine weenies and that the longer braking distances were probably on account of the modifications made to Project RSX, and possibly slight changes to the ABS calibration and braking hardware on the newer 2005 RSX Type-S. StopTech had performed extensive testing of its Type-S kit on a then-new 2002 model with springs and street tires, and was able to obtain favorable braking performance in every parameter. Our car far exceeds the capabilities of StopTech's test vehicle.
Break Out The Pocket CalculatorsSo the problem really lies in brake balance. Acura did such a good job on the balance of the Type-S system that little to no room was left for improved braking distances, given the same set of tires. It already has the ideal brake balance to virtually maximize each tire's grip, while at the same time keeping the car stable during on-the-limit braking. To understand this, we're going to have to dust off the calculators and slide rules and run through some numbers.
In order to maximize stopping performance, each tire should be just near its limits of grip in deceleration. Most cars off the showroom floor have a front-biased brake system, which is much safer for the average driver in a panicked braking situation-more rear bias at the limits of grip will cause the car's tail to get loose, even in a straight line. Not a good thing when braking and turning at the same time, unless you're doing it deliberately on a track.
The cost of the greater front bias is more braking grip from the rear tires. In the case of the RSX, Acura's engineers found a way to make a safe street car that still had excellent brake balance and optimized tire grip on all four corners, making it that much harder for big brake kit manufacturers.
To maximize the work that both the front and rear tires are doing, it's logical to say that you want to generate the same amount of braking torque (created by the calipers on the rotor).
Braking torque is a function of several factors, like the frictional coefficient of the pads against the rotors, the effective radius of the rotors (since larger rotors mean longer force moments), hydraulic fluid pressure pushing against the brake pistons, and the pistons' total surface area. Multiply everything together for a simple equation that shows how much braking torque can be exerted on each corner.
It's pretty intuitive if you separate the equation into sections and have a simple understanding of physics. Friction between any two objects is a factor of frictional constant- (mu)-between the two surfaces (determined by its physical characteristics) and the force that's pushing down on it. So in the case of brakes, friction is just the total amount of force pushing the pads against the rotors times the friction coefficient of the pad material on the rotor.
Contrary to what many might think, total pad area doesn't play into the equation because (which is determined experimentally) is dimensionless, or unit-less, and already accounts for the area of the pad. The advantage of pad area is mostly in wear and increased thermal mass.
The force pushing down on the pad is the area of the pistons multiplied by hydraulic pressure of the brake fluid pushing at the back of each piston. Much like a wing, pressure pushing on one side (let's say it's 100psi) times the surface area (let's just say there are two square inches of piston area on one side of the caliper) will yield a total of 200 pounds of force pushing the pad against the rotor. If the caliper is a fixed version with identical pistons on both sides, then add a factor of two to the equation.
The amount of friction force has now been determined. That force is stopping the wheel from spinning by exerting torque on the brake rotor. Torque is a force exerted on an object at a given radius causing it to rotate.
Since a rotor is already turning in one direction and torque generated by friction is, by definition, in the opposite direction, torque created by a brake pad acts to slow the rotor down. In geek: braking torque is expressed by friction force multiplied by the effective radius of the rotor.
The effective radius of the rotor is basically the radius at which the average amount of braking work is done. Different brake manufacturers use different methods to approximate the effective radius, but for the most part, it is larger than the centerline radius of the brake pads, since the outside of the pad travels a longer distance and does more work than the part of the pad closer to the center. Multiply all this together, and you have braking torque created at the rotor.
In terms of balance, the same equation can be applied to the front and rear brakes. Even without plugging in any numbers, we know rear brakes will yield less braking torque than fronts because they have a smaller radius, less piston area and probably less hydraulic pressure from the factory proportioning valve.
There are two reasons why rear-wheel braking torque is less than the front, even though we want to get the most work out of the rear tires. In addition to safety and stability under braking, the other factor we've ignored so far is weight transfer. Under braking, weight shifts forward, so the front tires do more work. With the same braking torque at both ends, rear tires lock up as weight is shifted off and available grip is diminished. Weight transfer can also be approximated by a simple equation, though the variables are hard to determine other than through experimentation.
Knowing this, StopTech's engineering team suggested a retest using their off-the-shelf RSX non-Type-S brake system, something few manufacturers offer for cars of different trim levels but the same chassis. The only difference between the two systems is a 2mm reduction of the trailing piston diameter and a conventional cast caliper instead of the squeezed-forged process used on the Type-S caliper. Everything else, including pads, remained the same.
Going back and crunching numbers in the brake torque equation, this means a six percent reduction in torque output at the front, which further shifts brake bias to the rear.