05. With the disc and hat separated, you can see the slotted mounting holes that allow the
While the cost, weight savings and sex appeal are all obvious benefits, we wanted to test the supposedly improved cooling ability of these two-piece rotors, so we took both cars to the most brutal mountain testing ground we know. The name of the road is a secret, but we can just call it Mt. Akina, since that's what the hoards of drifter wannabees think it is when they flock here on an almost nightly basis to crash their cars and attract unwelcome attention from the authorities. All the would-be Takumis like to race up a menacingly steep 4.9-mile section that climbs 1,426 vertical feet to their favorite meeting point. Only a fool would race down.
We raced down.
Sticking religiously to one lane and driving at night to avoid traffic and create extra fear-induced brake heat, we recorded rotor temperatures at the inside and outside diameter of the rotor's friction surface at the top and bottom of the hill. Science demands that we do several runs with each configuration to establish how much rotor temps vary run to run. The sheer terror of doing this downhill run more than four times was unthinkable, though, so we have no science to show you, just a story of downhill debauchery.
06. The Prodrive rotors come out of the box prettier than the STaSIS pieces, but the pads
First up was the EVO, with stock rotors and DS2500 pads. At the top of the hill, the rotors were cooler on the outer edge than on the inner, since the inside edge retains heat from the heavy, cast-iron mounting hat. On the downhill run, the car was amazing. The brakes were consistent, firm and confidence-inspiring all the way down the hill, and even worn nearly to the cords, the R-compound Pirelli P-zero Corsas offered a reassuring safety net of grip as we dove into cliffhanging hairpins at the top of third gear.
At 3.7 miles, we thought we noticed a slight change in pedal feel, as if the brakes were starting to fade, but it seemed to only happen for a moment. At the bottom of the hill, the car still stopped as if nothing were out of the ordinary--except the smell. The inner edge of the rotors was now 745 degrees, and the outer edge, a staggering 945 degres. Oh, what a pretty glow.Returning to the hill with the two-piece floating STaSIS rotors, the feel was basically the same. Under really hard braking, there was now a deep groan from the rotors, but otherwise everything felt the same. About halfway down, we momentarily felt brake fade. Looking at the odometer, it was 3.7 miles, exactly like before. Again, normal feel returned by the next corner.
At the bottom, the inside diameter of the rotors was cooler than before, at 670 degrees, but the outside diameter was hotter, some-where over the 1,000-degree limit of our infrared pyrometer.
07. The Prodrive rotors are bolted solidly to their mounting hats.
The most telling result of this test was how different the STi and EVO felt. The EVO's brake pedal was always firm and consistent, the steering precise and the tires forgiving. The STi, in comparison, was terrifying. The thousands of dollars of suspension work we've done to the STi have delivered test numbers fairly close to the EVO's (.97g vs. the EVO's .99 and 71 mph in the slalom vs. the EVO's 73.4), but the true handling envelope is nowhere near the same.
The 43mm offset of the Prodrive wheels makes the steering dart and weave unpredictably under both braking and acceleration, the very sticky Bridgestone RE070 tires still don't have the soft-edged envelope of a true R-compound and the active center diff caused such erratic handling that we simply switched it to manual to set the center diff wide open. We had the Praxis suspension set in its medium-height Sport mode where it's stiffer but still has travel, but the car was still far too sensitive to bumps. It's possible that's in part because of the extra spring rate from the very-stiff Hotchkis anti-roll bars, but we suspect it's inherent in the Praxis system. Besides, the lack of body roll from the bars is one of the car's few reassuring characteristics.
08. Only a fool would race down the maniacally twisty 1,426-foot drop of Mt. Akina. We did
Worse, though, was the brake feel. While the pedal was firm and linear once you reached the meat of the pedal, the initial engagement point seemed to move around. The STi always has an inch or so of slop before the brakes start working, but on that mountain road attack, when an extra half-inch of slop means the difference between entering a corner with confidence and entering in panic, the engagement point changes from one corner to the next. We suspect the pads are getting pushed back into the caliper when the wheel bearings and/or hub flex under hard cornering. Loose wheel bearings caused this problem on our old Project WRX, but we thought the STi's bigger bearings would prevent the problem.
We're not sure if it was the more aggressive pads, the extra power or the sharper sense of fear, but for whatever reason, the STi, which weighs the same as the EVO and has marginally larger brakes, absolutely cooked its stock rotors. At the bottom of the same road, every part of the rotor was over 1,000 degrees, and, as an extra little insult, the brake pads caught fire.