Project STi - Putting a Stop to it all

SUBARU STi WRX brake improvement - USA

The old engineers joke that “speed doesn’t kill, rapid deceleration does” applies to all vehicles capable of attaining dangerous velocities.  Since we have been concentrating on improving project STi’s acceleration and handling, we have defiantly achieved the ability to reach deadly velocity on the track.  In our first foray onto the grounds of Willow Springs raceway we noted that perhaps we were starting to drive beyond the capabilities of the stock brakes due to the higher speeds that we were able to attain on the straights.

The STi is blessed with excellent, large, competition worthy Brembo brakes from the factory.  However these Brembos are somewhat hamstrung by the requirements that the STi be a civilized street car.  Street drivers usually don’t tolerate any sorts of squeaks and squeals that high performance brake pads are prone to.  Thus the brake pads that the car is equipped with from the factory are compromised to be less than optimal in hot braking performance with quietness and effectiveness when cold.  Usually a brake pad works the best within a given temperature range.  A pad that works best when hot, often works poorly when cold and vice versa and Subaru compromised in the conservative cold direction.

When driven on the track the stock brakes were not horrible as stock brakes often fade away to nothing when pushed hard.  We did notice that the brakes did fade, losing about 25% of their effectiveness then stabilizing for the duration of the track session.  Although this was not horrible for OEM brakes, it was not exactly confidence inspiring either. Although we could have simply installed a super duper mega huge brake kit, we decided that we did not really want this approach for several reasons.

The first reason is that the 2004 STi is prone to suffer what is called brake caliper knockback.  The 2004 tends to wear its wheel bearings.  This causes the wheel and brake rotor to wobble around when subjected to cornering load.  This wobble knocks the brake caliper pistons back into the caliper.  When the brakes are reapplied, the brake pedal height is now lower than usual because the pistons have been knocked back into the caliper and must be pushed back out to contact the rotor.  In extreme cases the pedal may drop to the floor and must be pumped to regain braking power.  Subaru recognized this problem and the 2005 and later models have bigger hubs and wheel bearings to eliminate this.

If a larger brake rotor is installed the effects of knockback are greatly magnified and the brakes are more sensitive to any sort of run out in the hub bearings.  Since the STi is blessed with the equivalent of a racing brake upgrade stock, we decided to work around the stock rotor size to help reduce the effects of knockback.  This evolved swapping the stock Brembo rotors with some highly effective ones from Disc Brakes Australia.  When traveling to Australia several months ago in part two of our project, we drove a highly modified STi at the track, Whiteline’s infamous P25.  No matter how hard we abused the brakes on the P25 they worked consistently with no signs of fade.  Driving the P25 around on the streets during the week demonstrated that the brakes also worked well when cold with little noise or fanfare.

When queried about the brakes excellent performance, Jim Gurieff, Whiteline’s Technical Director gave much of the credit to the rotors on the P25 supplied by a fellow Australian company DBA or Disc Brakes Australia. The DBA rotors have many unique features that are truly functional and help the brakes work better over a wide range of conditions.  Impressed as we were by the P25, we ordered up an identical set of rotors for Project STi.

Part of the secret to the effectiveness of the DBA rotors is their patented Kangaroo Paw venting system. The Kangaroo Paw design features a series of 144 diamond and tear drop shaped pillars and columns, individually spaced between the rotor faces. This system provides two key advantages. One advantage of this design is the ability to cool the rotor faster than before. This works on the same principal as a turbocharger impeller, designed to draw air through the rotor. As the disc rotor rotates, air is moved through the rotor by centrifugal force. Aerodynamic turbulence generated around the rotors individual pillars and columns helps improve the removal of heat from within the internal areas of the rotor. Whiteline offered some data showing a rotor surface temperature drop of over 100 degrees generated during track testing of the DBA rotor.

The second advantage offered by the Kangaroo Paw rotor is increased dimensional stability across the disc friction face, achieved by the equally spaced pillars and columns. This helps prevent rotor warpage and pedal pulsation during use. These advantages increase the DBA rotor’s efficiency by as much as 20 per cent when compared to conventional rotor designs. A 20 per cent increase in disc rotor efficiency, translates to reduced brake operating temperatures, brake fade, disc rotor stresses, disc thickness variation, run-out and shudder.

We replaced our front rotors with DBA’s top of the line 5000 series rotor. The 5000 series rotor is a two piece design featuring a lightweight aluminum center hat with a corrosion resistant black anodized finish. The Aluminum center hat reduces unsprung and rotating weight by several pounds per rotor.  This improves acceleration, braking and suspension performance. 

The rotor itself is available separately so the hat can be reused should the rotor wear out. The rotor is positively held to the hat with aerospace NAS bolts.  Although the 5000 series rotor comes in a cross drilled configuration we opted for a more competition worthy slotted surface.  

When brakes are used at high speeds, the brake pad material vaporizes and the pad can hydroplane on a layer of gaseous vaporized brake pad. This makes the brakes much less effective. This type of brake fade often encountered, especially with new unbedded brake pads.  Holes drilled into the surface of the rotor give the gas somewhere
to go, reducing the hydroplaning effect


and eliminating this type of brake fade.  The trouble is, although drilled rotors look sexy, they often crack between the holes.  Thus for track use, many feel that a slotted rotor is better.  The slots act like tread on a tire and give the gas a place to go.  The slots do not have any tendency to crack. For this reason we decided to go with slotted rotors.

For the rear, we opted to use DBA’s 4000 series rotor.  This is a one piece rotor made necessary by the STi’s use of a drum type parking brake that is integral into the rear rotor.  This makes the use of an aluminum hat impractical.

The 4000 slotted model has a unique 6x6 Wiper Slot design for exceptional venting of vaporized brake pad. Unlike many aftermarket rotors, the 4000 series has a coating of heat resistant paint on the center part of the rotor so if you have open wheels you don’t see a nasty rusty looking rotor.

Both of the DBA rotors have stripes of thermal paint on outer edge of the rotors.  By observing which stripe of thermal paint burns off, you can determine what temperature the rotors reach and choose an effective brake pad for the temperature reached on the track.  This is a practical and neat detail that no one else to our knowledge does.

Having good brake rotors means nothing if the operating temperature range of your brake pads is being exceeded on the track.  It was obvious that we were right at the edge of the stock pads effectiveness with sticky tires and increased horsepower at Willow Springs, a track noted for being easy on brakes.  At any other track the brakes would probably fade into oblivion.  Although the car sees track use, it is also a daily driver so having a brake pad with a wide effective temperature range was also a must.  For help and advice on all things braking, we turned to our friend and supplier of brake pads for our race team Monty Holt of Bits and Pieces Motorsports.

Monty recommended the Hawk HP+ brake pad.  For our road racing program we use Hawk products supplied by Monty exclusively and have always been impressed by their performance and durability.  The HP+ is a semi metallic formula modified with the addition of iron and carbon powder.  This gives it a consistent coefficient of friction over a wide range of temperatures.  The HP+ can still stop while cold and works well enough when hot that it can be track driven, especially with a car with large factory brakes like the STi that just needs a little help to prevent fade.

To improve pedal feel Stoptech supplied some of their braided steel brake lines.  The braided lines don’t balloon under pressure like the stock rubber lines do, thus every bit of pedal travel goes to moving the pistons in the calipers.  We were surprised how much difference these lines made.  The pedal feels rock solid like a real racecar, impressive considering the stock pedal feel was quite good as well.  The car’s ABS system seemed to like it as well, seemingly cycling faster with a higher resolution.  We don’t know if that’s true or not but our seat of the pants told us so. 

The Stoptech brake lines are DOT approved.  Many braided steel lines are not as the steel braid fatigues and breaks during the dreaded DOT whip test.  During the whip test the brake line is mounted by one end to a rotating fixture and spun at a certain rpm for a given time.  The brake line cannot fail during the test period.  Stoptech lines have a molded plastic reinforcement around the end fittings that reduces the fatigue of the line in this area, allowing the passing of the whip test with ease.  We also noted the quality of the fittings and the superior fit of the Stoptech parts in general.  This is a welcome change after many years of jury rigging most braided brake line kits with zip ties, Adel clamps and other devices to get a safe fitment.  It is refreshing that the Stoptech lines simply slide right in with no hassles or fiddling around

Last in line for our brake upgrades we replaced our stock, stale never been changed before Japanese fish oil brake fluid with Motul RBF 600.  Motul’s RBF formula is one of the best fluids, performing nearly as well or better as uber expensive exotic stuff like the legendary F-1 brake fluid standards Castrol SRF and AP650.  When brake fluid gets really hot it boils.  Boiling fluid has bubbles in it that are filled with compressible gas. The compressible gas causes the brake pedal to drop to the floor.  This is another type of brake fade that must be avoided.

Motul’s dry boiling point is an amazing 600 degrees F.  Motul works so well that our teams race cars need only bleed the brakes once a season, instead of the typical once a race.  Most brake fluids absorb moisture from the atmosphere.  When the brake fluid absorbs moisture its boiling point rapidly drops rendering it less effective.  Even though the brake fluid is contained within the brake system, it still can quickly absorb a surprising amount if moisture from the atmosphere that is inadvertently drawn into the brake system. Some brake fluids can deteriorate greatly from this phenomenon with as little as one heat cycle of the brake system. Motul resists absorbing moisture from the atmosphere better than most brake fluids so its inherent high boiling point lasts longer than most fluids.  If you do manage to boil Motul, surfactants in the formula cause the bubbles formed to rapidly dissipate and you get a recovery of your brake pedal feel.

The change to the feel of our modifed brakes is confidence inspiring.  The brake pedal is now firm and high.  Modulation of the brakes is now much easier due to the improved pedal feel.  Under track conditions we are not observing any fade at all.  The only negative is a slight amount of brake squeal for the first few minutes of operation on the street when the pads are dead cold, an acceptable trade off for greatly improved fade resistance.  From our initial track test it appears as if our brake pad life is greatly improved as well.

Our last mod for this installment is the addition of Whitelines front and rear Strut Tower Braces.  Although the STi has a strong WRC inspired chassis that exhibit good rigidity to begin with, it like most cars responds to an increase in stiffness.  The Whiteline braces are made of aircraft aluminum alloy with bracket that are designed to place the brace in an area that does not interfere with the Whiteline Group 4 struts damper adjustment. Despite the stiff chassis, we noted a difference in the car handling.  Steering response is slightly improved and the ride is also improved, a sure sign that the braces are working well.  We also noticed that the car is now more sensitive when it comes to making adjustments to the damping and swaybar stiffness.  This is another sign that the bars are doing their job.

We felt that the rear bar surprisingly seemed to make a bigger difference than the front bar did.  Although the braces did not make a huge world stopping difference, they did make a positive contribution to Project STi’s handling and are the icing on the cake rounding up what has become a very good handling car that also has decent road manors to boot.