Going fast comes with a cost. Everyone who's played with cars knows this. Traditionally, that cost goes toward horsepower parts. But anyone who's gone fast long enough knows the benefits of being able to stop.
The SRT-4 has damn good stock brakes. They stop the car in a respectable distance and do so consistently, even under demanding testing. But, since we've added 15-wheel hp and significantly increased grip, load on Project SRT-4's stock brakes is substantial. Plus, more power is still on the horizon. Project SRT-4 hasn't been around long enough to receive a serious mountain road or racetrack flogging, but its day is coming and when it does, it needs to be ready. Thankfully, mountain roads, racetracks and any environment known to turn original equipment brakes to smoldering piles of dust are the realm of an unusual and rarely seen creature: The brake dork.
Emerging from the smoking depths of our California Speedway test facility last winter, we ran into this most elusive of creatures. Brake dork Steve Ruiz, StopTech's engineering manager, is known for his comprehensive test procedures, brake design and most notoriously for spewing data from every orifice. He insisted he could improve Project SRT-4's braking. This month, we find out how.
The cost rationale
The SRT-4 is an inexpensive car, so spending nearly 10 percent of its purchase price on a brake upgrade is a tough nut to swallow. If you've got to go all the way, StopTech offers a four-piston brake kit with 12.9-inch, two-piece rotors (iron rotor, aluminum mounting hat), braided lines, pads and billet-aluminum brackets. The combination of two-piece rotors and four-piston calipers contribute to a weight savings of about half a pound vs. the SRT-4's stock brake parts. If you've got the green, this is the way to go. It goes for $1,895.
For $1,495, you can have the two-piece rotors, but give up the stiff, lightweight four-piston calipers, replacing them with StopTech's two-piston ST-20 caliper. The ST-20 uses a sliding design, which puts both pistons inside the rotor, allowing lots of room for wheel spokes outside the rotor. Stainless lines and pads are included.
A quick visit to StopTech's Web site (www.stoptech.com) let us figure out which brakes would fit under our wheels. We printed out the caliper templates (available as PDF files) for the two SRT-4 kits and scaled them to match the actual caliper dimensions. Then we glued them to cardboard, cut them out and confirmed our fears. The four-piston caliper hit the back of the spokes, so our only caliper option was StopTech's smaller, two-piston ST-20 caliper.
Since we had to use the two-piston caliper, we figured we'd start with StopTech's least expensive option, which replaces the two-piece rotors with 12.9-inch one-piece Aero rotors. That kit costs $1,295.
The brake dork rationale
Spouting off phrases like "elastic modulus," "standard deviation" and "relative data slope," Ruiz exploded with excitement when he found out the wheels we have on Project SRT-4 won't allow us to use the StopTech's four-piston caliper. Since brake dorks live and die by torque reaction management and caliper stiffness, Ruiz began rattling off caliper design facts fast enough to make our pencil smoke. Near as we can tell, here are the issues.
Sliding calipers make several compromises vs. an opposed-piston design like StopTech's four-piston caliper. First is the fact that they have to slide. The pistons on the inboard side of the rotor push directly on the inboard brake pad, but the outboard pad gets pushed on by a big claw that has to reach around the rotor. This claw is part of the same piece that holds the pistons. So when you mash the brakes, the fluid in the two cylinders gets pressurized and pushes the pistons forward and the cylinders, housing, claw and all backward.
The biggest problem here is the claw itself. As you mash the brakes, all that clamping force tries to pry the claw open, and since it has to have a big groove in it to make room for the rotor, it's an inherently flexy shape. That means a squishy pedal. Battling the squish necessitates the use of either more or stiffer material in the claw's construction, both of which add weight.
There's more to the two-piston design, though. If the claw has to slide, what does it slide on? It has to slide on two greased pins. But if the sliding claw squeezes a moving rotor, the brake pads will want to stick to the rotor and go with it. That's where brake torque comes from, and that's good, but those two little pins can't handle that brake torque. They'll bend and bind and get stuck. Instead there's a frame around the claw. The back of the frame mounts to the car and holds the pins, and the frame wraps around the rotor and holds the leading and trailing edges of the pads to resist that brake torque.
This separation of clamping and torque handling duties, according to our favorite dork, is the reason that in spite of utilizing all-aluminum construction, the ST-20 calipers are still heavier than the ST-40s. If the material in the frame could be used to strengthen the claw, as it does in a one-piece ST-40, the claw wouldn't have to be so thick.
|Stock vs. StopTech Brake Data|
|Brakes||Kit||Rotor||Caliper and Hardware||Pads|
|Stock||N/A||13.8 lb one-piece, 11.0-in.||11.0 lb single-piston sliding||3.5 lb|
|StopTech||$1,895 kit||14.0 lb two-piece, 12.9-in.||9.4-lb four-piston fixed||4.4 lb|
|StopTech||$1,495 kit||14.0-lb two-piece, 12.9-in.||10.6-lb two-piston sliding||5.0 lb|
|StopTech||$1,295 kit||15.7-lb one-piece, 12.9-in||10.6-lb two-piston sliding||5.0 lb|
The data rationale
StopTech is known for its comprehensive test procedures. We joined the brake dorks for a day in their natural environment to learn about stopping Project SRT-4.
Before performing any brake testing, Ruiz made a few hard stops in Project SRT-4 to see how it would cope with the abuse of testing. At full ABS activation, Project SRT-4 wandered around enough to send Ruiz into fits of brake-dork frustration. He decided the instability was caused by several problems. First, Project SRT-4's front tires were toed out too much for this kind of testing. Second, the SRT-4's soft stock bushings deflect severely under this kind of load, further enhancing the problem. The solution? A quick toe adjustment at both front tie rods and Project SRT-4 was stopping straight.
Before putting Project SRT-4 through a testing cycle, Ruiz made sure the pads were bedded in with about 10 60-to-0 stops. Apparently, StopTech has tested cars with 20,000 or more miles that didn't have their brake pads bedded properly, which produced erratic test results. When our SRT-4 returned from this exercise with its pads smoking, Ruiz declared them bedded. Then the real abuse began. StopTech's test cycle consists of 10 measured 60-to-0 stops including another 60-to-0 stop on the return run to keep rotor temperatures high. Then six more 80-to-0 stops are performed (with no stop on the return run). Front and rear rotor temperatures are recorded after every measured stop.
During these last six stops, our SRT-4 began a routine of rear-wheel hop, which made it difficult to keep the car straight. Again, the soft bushings were acting up, this time in the rear. This couldn't have helped braking distances since tires that are bouncing in the air are doing little to stop the car. Even so, 80-to-0 distances were consistent, varying about 2.5 feet over the six stops. Pedal feel was soft, but remained relatively consistent throughout the test.
On the 16th and final stop, front brake rotor temperatures soared to an amazing 1,010 degrees. Impressively, the car actually stopped shorter on this pass than it had on its first 80-to-0 stop. This kind of brake performance is a testament to O.E. testing and development. It also shows just how hard StopTech's job is when it comes to improving brake systems.
StopTech's kits all include step-by-step installation instructions with photos making it hard for even chumps like us to screw up the install. Details like the inclusion of torque settings for the bracket/upright mounting bolts and thread-locking compound for the caliper body bolts speak for the thoroughness of the kit. The ST-20 kit we used replaces the stock brake line bracket in addition to the lines themselves. The ABS wire is tie-wrapped to the bracket.
StopTech's rotors are treated with a rust inhibitor, which must be washed off before installation. The instructions with the brakes say to wash the rotors with brake cleaner or solvent, but Ruiz used water and a Scotch-Brite pad to remove to the coating. Once cleaned and rinsed, it's normal for there to be a little surface rust when the rotors go on the car.
Once installed, Ruiz began to bed the pads in but called things off when he was unimpressed with the braking response. Time to swap pads. This was the first test of this kit on an SRT-4 and StopTech hadn't finalized which pads will ship with the kit. The original pads were replaced with Hawk HP plus pads, which Ruiz felt were better suited for the heat they'd experience on the SRT-4.
With the new pads installed, we noticed the brakes sticking slightly. Ruiz figured the effect was minimal enough that we continued with the test. He also figured it was a problem that would rectify itself by driving the car. He was right. Turns out, an anti-rattle clip on the back of the pads was too tight, causing them to bind slightly and not fully release. By the time the car was driven back to the shop and disassembled, the brake function was almost normal.
With the new pads bedded, StopTech repeated the same test cycle performed with the stock brakes. The wheel hop continued and the Hawk pads produced an impressive spark show after a few stops. This is normal for Hawk pads despite how it might look--we actually like it. After 16 stops, the Stoptechs showed a 1.94-foot average improvement in 60-to-0 braking and a 5.1-foot average improvement from 80-to-0 braking.
The rear rotors averaged 36 degrees cooler over the last six stops, which means the new setup is slightly more front biased. Even though they were doing a proportionally greater amount of the stopping, the front rotor temperatures averaged 73 degrees cooler than stock. Without the sticking problem we experienced, rotor temperatures would be even lower. This added heat capacity makes the StopTech brakes less likely to overheat on the track or under hard street driving.
Brake dorks are never happy, though. Ruiz replaced the Hawk HP plus pads on Project SRT-4 with Mintex Red Box pads shortly after this test. He said the Mintex pads have better friction characteristics and a higher heat range than Hawks. Mintex Red Box pads will ship with the kit.
Pedal feel was never stellar on the SRT-4 and it's still not amazing. We can't argue with the braking performance of this kit, but we would like a harder pedal with less travel. The StopTech kit has reduced the initial slop, but under hard braking, the pedal travel is still long. Modulation control remains adequate, but it happens over too much pedal travel and the Neon's pedal position, which is much higher than the throttle, is still annoying. The solution is a larger master cylinder, which would reduce pedal movement and bring back some effort.
The SRT-4 uses a 15/16-inch master cylinder, which is the largest for DaimlerChrysler's small-car platform. There's a chance we'll find a larger master cylinder from another platform, which might work. We'll let you know. Until then, we'll deal with the soft pedal.
Project SRT-4 is now at the level of performance where every compliant bushing or mount in the car is at its limit. Last time, we discovered the motor mounts and torque arms were stressed to the point of needing replacement. This time the suspension bushings are giving up. Clearly, we've got plenty of work to do. n