Brakes
Stealth / 3000GT Brake Upgrade Considerations
November 2004
References
An excellent reference for 3000GT/Stealth brake modifications is Jeff Lucius's upgrade guide. I highly recommend James Walker's Pulp Friction article and the technical papers compiled by StopTech as a primer or refresher of brake system operation and behavior. Also interesting and insightful are Michael Romano's notes. If you want your brain to hurt, check out Brian Beckman's The Physics of Racing. Below is a short and hopefully digestible overview of the issues key to my experience.
Objective
Stock brakes in good working order are powerful enough to lock up the wheels and engage the ABS. Stronger brakes can squeeze the rotors harder, but if the tires are already stopped, then so what? Threshold braking represents optimal stopping power and is already possible with the stock brakes (exceptions may include wider, stickier racing tires).
The primary function of a brake upgrade is not to decrease stopping distances but to improve fade resistance (temperature management); the goal is to stop just as hard for longer. The stock brakes with aftermarket pads are excellent but can exhibit fade due to overheating under repeated heavy use; the goal is to delay the onset of fade by keeping temperatures lower and/or reducing temperatures more quickly.
When upgrading brakes, there are three primary variables to consider: rotor diameter, caliper piston area, and the hydraulic control system (master cylinder, ABS, combination valve). The stock hydraulic system is calibrated for the stock rotor diameters and caliper piston areas; if either, or both, are increased, the stock hydraulic system may be forced to operate outside of its design parameters, resulting in different and likely unfavorable behavior. It is undesirable, and in some cases legally impermissible, to make modifications to the stock hydraulic system, so careful consideration of the other two variables is critical.
Many front-only big brake kits sold for our platform offer superior temperature management but at the expense of overall braking distances; because larger rotors and pistons are more effective (i.e.- produce more torque at lower pressures), the brake bias shifts forward, reducing the relative contribution of the rear brakes.
Here are a few things to consider (not specific to any particular kit on the market):
ABS
With larger rotors, larger caliper pistons, or both, the amount of torque generated at a given pressure increases. The stock ABS system is calibrated to modulate torque by pulsing hydraulic pressure to the calipers. With larger rotors and/or caliper pistons, each pulse releases and reapplies more torque than stock, resulting in more violent brake actuation and potentially extending braking distances, setting an ABS fault, etc.
Smaller caliper pistons generate less torque at a given pressure. It seems logical that smaller pistons would allow the stock hydraulic system to accommodate larger rotor diameters (though huge rotors with tiny caliper pistons seems odd), and as long as the amplitude of the ABS pressure pulses is sufficient to effectively modulate braking, the brakes should release enough to prevent a skid.
For more information, see James Walker's technical paper on ABS and Big Brake Kits.
Bias and Proportioning
Under aggressive braking, weight shifts forward, reducing rear traction to the point where dangerous rear wheel lockup becomes a concern. To allow the rear brakes to contribute normally under light to moderate braking but not too much when weight shifts forward, a combination valve in the stock hydraulic system includes a proportioning function that reduces the rate at which pressure to the rear brakes rises above a threshold. Called a "knee point" or "split point," this pressure threshold is calibrated to a specific forward weight transfer determined by the factory for the stock weight distribution and suspension rates (the Spyder combination valve's pressure threshold is slightly higher than the coupe valve's threshold because the additional rear weight distribution permits the rear brakes to contribute more under slightly heavier braking before enough weight shifts forward to warrant attenuation).
The split point is critical even when stock bias has been maintained, yet it is seemingly overlooked by many who design and purchase big brake upgrades. When larger rotors are installed on a vehicle, the amount of torque generated at a given pressure increases. This increased stopping power causes weight to shift forward at lower pressures and can lead to rear-wheel lockup before the proportioning valve engages. Calipers with larger pistons increase torque at pressures lower still, making the vehicle dangerously unstable under even lighter braking.
For more information, see James Walker's technical paper on Brake Proportioning Valves and Bill Williams' detailed dissection of proportioning valve operation.
Recommendation
For those pursuing an upgrade, I suggest use of better (thickness, venting, etc.) rotors that are close to the stock diameter and better (more rigid, larger pads, etc.) calipers that have close to the stock piston area. I went with a StopTech four-wheel upgrade designed by Supercar-Engineering to use 332mm front rotors and 322mm rear rotors (described on my Modifications page). Unfortunately, the rear calipers selected for my kit necessitated excessive piston areas at all four corners to maintain reasonable front-rear bias, and the resulting hydraulic imbalance causes some of the problems described above. StopTech and SCE reviewed the design and provided me with adjustable proportioning valves to install between the ABS HU and the stock combination valve as a correction. This has been a valuable learning experience, and all SCE StopTech kits have been redesigned to preserve hydraulic system compatibility. I have no qualms about recommending SCE as the best source of balanced brake upgrades for our platform.
Update: April 2006
Serious Maintenance Issues
After just two winters, the aluminum hats have disintegrated where they interface with the cast iron disks, and the aluminum brackets are deteriorating where they bolt to the knuckle, apparently due to galvanic corrosion. This was an extremely disappointing discovery, as my car has been rendered unsafe to drive due to failure of a critical safety part in which I have invested heavily. Even if I rinsed the hats after every drive (impractical), the conditions out on the street with corrosive water and hot parts spinning and creating who knows what kind of electrical field would likely still lead to substantial damage. Regardless, anodization was clearly insufficient, and I am now seeking ways to avoid having to spend many hundreds of dollars every few years to maintain my expensive kit.
My brother-in-law contacted an independent materials testing, consulting and engineering firm on my behalf and promptly received the following reply:
> What you experienced in the high performance sports car is a common problem
> in all types of brakes in chloride containing corrosive environments.
> Certainly anodized aluminum by itself is not adequate due to galvanic
> action. Any alternative coating should be free from pin holes and be inert
> or cathodic to cast iron. It all depends on specific of brake design and
> type exposure. May be some type of noble (tin, copper, nickel....) plating
> would solve the problem. Currently we have a major project dealing with
> galvanic corrosion problems in friction pads. This is a major problem for
> this industry and should be researched further.
>
> Please call me to discuss possible options and the research involved.
>
> [name withheld], PhD
> NACE Certified Materials Selection/
> Design/Corrosion/Coating/Cathodic
> Protection Specialist
He called Dr. [name withheld] who sounded like he knew what he was talking about. He stressed that other than speculating, the only way to get a workable solution to this is to come up with a product and then test it. He expressed interest in working with StopTech and in doing so would be able to test some different materials and measure the results. We discussed several different approaches:
- Galvanizing the assembly by hot-dipping it. Both the hat and the disk must be galvanized, the coating would be ground off by the brakepads, and the coefficient of friction could be affected.
- Electrically insulating the hat from the disk. The problem here is that the electrical connection must be completely severed, and this would be nearly impossible while maintaining the mechanical bond between the two parts (though I wonder why something like a carbon fiber ring wouldn't work).
- Using a different material for the hat such as copper. This sounds plausible (if structurally acceptable), but again he stressed the importance of actually measuring the galvanic voltages on the actual parts to guarantee that it is a good solution.
- Plating the surface with another metal. Discussed briefly - not much time spent on this.
- Painting the surface. He said this is impractical because it's nearly impossible to prevent pinholes. However, "cold galvanizing" products such as Zinc Rich Primer Spray may help.
- Attaching sacrificial anodes - galvanic protection. This was not discussed in depth, as he quickly steered the conversation back to coating or plating. While sacrificial anodes are perhaps the most effective approach, it may not be feasible to adhere such components to a rotating assembly.
I decided to coat the mating surfaces with a zinc-rich cold galvanizing primer (cast iron) and an etching primer (aluminum) in an attempt to insulate the parts and act as a sacrificial layer. Time will tell how effective this will be.
Jim Matthews - Yorkshire, England, UK
*** Team3S, 3SI #0030, GTOUK #155 ***
http://www.the-matthews.com/stealth.html
Jet Black '94 Dodge Stealth R/T Twin-Turbo AWD AWS 6-spd