The braking demands for any performance vehicle are one of the most important considerations to accommodate when conducting a full build. Every application should be carefully reviewed before hastily slapping on an off-the-shelf big brake kit in the hopes that everything will jive. Many misconceptions exist around braking system design and technology, so it is important to educate yourself before blindly groping for the largest rotor diameter and piston count your wallet will allow.
Brakes Are Application Specific
The braking needs of a street car differ dramatically from those of a race car. Striking an appropriate middle-ground for a project like Gruppe R has been a challenging task due to the specifics of its intended purpose.
Foremost in our view is the fact that our Golf R is not a race car — but it will experience demanding and adverse surface conditions and driving situations. Only a handful of top-shelf brake manufacturers specialize in the needs of the rally car, so we reached out to the Dakar Rally-proven specialists at PowerBrake — a South Africa-based company that has been in the braking game for 30 years.
According to Delon Murphy of PowerBrake, the company is comprised of 10 employees, and started out as a supplier specifically for South African race vehicles. In the last 10 years, PowerBrake has grown into filling the niche, albeit global, market of rally and off-road brake systems. Priding themselves in personal customer relationships — we have had the pleasure of participating in their one-on-one, individually-tailored, brake package business model.
What Makes Rally Brakes Different?
For some pertinent engineering lessons on braking, we turned to Clive Murphy of PowerBrake. Better understanding the differences between a circuit race car braking system and a rally braking system served to prioritize our decision making process and tailor a package that will be best for this project.
“All (PowerBrake) calipers have to have internal fluid crossovers because rock and stone damage is an issue if you run external; all bleed screws have to be recessed for the same reason. Of course it’s always a trade off between lightweight (which is a big focus point in circuit racing) and durability, but in rally and off-road lightness is important, but durability is critical,” explained Murphy.
Following through on their purpose-built design Murphy continued, “Everything from the seal polymers to the internal wiper seals, which are not present in circuit racing, all has to be taken into account when you are designing for rally and off-road; it’s a slightly different recipe.” Dust, mud, water, ice, and vibration are all far more severe under rally conditions than any track car will experience — to ensure continuing longevity, brakes must be engineered to withstand these conditions.
How Do Floating Rotors Come Into Play?
The function of full-floating rotors may be elusive to some, but the driver who lives on the edge of brake performance will appreciate their contribution. As a solid rotor builds thermal energy over prolonged use, it expands in size — ultimately diameter. With the inner circumference mechanically or structurally bound to the rotor hat, rotor deformation occurs as the the disc hooks or curves. This presents a concave or convex surface against which the pad or friction surface must bare. This uneven surface means less surface area is available for clamping, and contributes to accelerated and uneven wear.
Full-floating rotors are rarely found on street-driven vehicles because of the excessive jangling noise they generate as the rotors are allowed to self locate slightly. For the race car, full-floating rotors offer a solution to one problem, but generate other issues.
“The majority of the industry has very closely copied the Brembo system that was launched in the ’80s for the McLaren F1. We took a different approach with some aerospace technology involved in the spring system we use. We originally designed the system for the Dakar — if you can imagine at the Dakar, a lot of the teams run a full-floating system. The problem with that is that while the cars are designed to take 20 ton impacts off jumps, rock, sand, and corrugated road surfaces — with a full-floating brake system all that vibration and the impact will knock back the pads to their full extent of rotor float — which then makes the pedal longer at the next application,” Murphy enlightened.
“The SASS-float system gives the best of both worlds in that you have zero pad knock-back. If you apply more than about 5 percent pedal pressure, it becomes a full floating disc, and as you come off the pedal again it reverts to the no knock-back situation,” he explained. This proprietary system is a clever innovation in brake technology and is finding it’s way into other performance segments.
“It became popular with our crossover track day/street market because in that application you want something that runs quiet on the road and full performance on the track,” Murphy concluded.
Buying Off The Peg Is The Past
Fitting a car for a custom set of brakes is much like having a suit tailored; your dimensions, weight, and needs are considered before diving in. In the past, consumers were left at the mercy of vendors and hearsay to select a package proven to work with their vehicle. PowerBrake takes careful measures to ensure every system they engineer will perform at it’s best for the given application.
“We developed brake simulation software in-house over a period of about seven years. What the simulation software allows us to do is send a form with 12 or 13 questions for a race team to answer — we plug that information into our simulation software and virtually brake the car at various negative g’s and simulate the weight transfer during that brake application. That allows us to look at two things: how the system is balancing hydraulically, and which friction material should be used to balance properly,” explained Murphy.
Size Isn’t Everything
Lavish endorsement of big-brake kits has grown to be a a rather vulgar contest of size which then becomes a weight and packaging issue. In a racing application, lightness and compactness are premium considerations, so volumetric and mass efficiency are at the top of our concerns.
“To dispel a few myths — the piston count in a caliper has very little to do with the final stopping power of the package. It all comes down to the hydraulic ratio between master cylinder and the overall caliper/piston size, the disc diameter (which has less effect than what people believe), and finally the friction material, which has quite a significant effect,” detailed Murphy.
Piston size is a tailored balance to optimize even pad wear which prevents poor pedal feel, side loaded pistons, and other maladies. By staggering the piston sizes tactfully, PowerBrake distributed pressure otherwise offset by outgassing pads and other considerations.
“A lot of race-pad profiles tend to be quite long, where the disc initially enters the caliper and contacts the pad — the pad tends to bite into the disc, and then as you get to the exit-side of the pad, there is a lot of brake gasses that are being formed. The long of the short of it is, you start to get tapered pad-wear; more (material) on the entry-side, and less on the exit-side,” Murphy explained.
“The way we counter that is by placing the larger piston at the exit-side of the caliper where you usually get less wear, and we put less pressure on the entry side where it’s biting and wearing more. The staggered piston bores are to stagger the pressure on the backing plate of the pad,” he continued.
Our intention to downsize the wheels in order to increase sidewall thickness and rough surface handling meant that the OEM 345 mm front rotors and single-piston caliper would no longer fit. Fearing for our stopping ability, we asked Clive what we could expect from a tailored brake system.
“In your case, we are reducing from a 345 mm to a 315 mm, in terms of torque output purely on the mechanical lever, we could easily triple the torque output with the friction material if we wanted to. The rules of the Dakar limit cars to 16-inch wheels, our air-cooled Dakar caliper which you are getting are stopping cars that weigh 5,500 pounds,” he explained.
With changes in piston count and size, questions arise in terms of fluid displacement volume. Specifically the volume the OEM master cylinder needs to be to sufficiently move the pistons and retain an excess for pedal height and feel.
“We will give you the best pedal feel that we can and to be sure that the ABS and vehicle stability controls on the car remain in place. In the case of an original equipment system, there are limits to which all the algorithms that are written around your vehicle stability controls can handle,” Murphy assured us.
Friction Compounds Are A Big Part Of The Equation
Selecting a set of performance pads can be a daunting task. Without a data base of past experience to use as a datum — once again, one is left feeling reliant on the experience of others. Because the braking simulation software developed by PowerBrake and their real-world interaction with teams racing the Dakar and circuits (such as BMW Motorsport) detail proof of concept, we can be sure our selection will be well established.
“You’ll notice more and more, particularly with European cars, that the original equipment brakes are starting to feel quite over-assisted. You’ll literally touch the pedal and you’ll have extremely high initial bite — this has been done because it sells cars. The truth of the matter is though — it’s not great for modulation,” explained Murphy. Modulation is important not only for feel but for performance.
“This particular compound would have a maximum operating temperature range of from cold to 650 degrees C (1,200 degrees F). We tend to be quite conservative on our max operating temperature which means that at this temperature you can complete laps — not that you could reach this temperature and then fall off,” he concluded.
Torque And Braking Bias Theory
There are a multitude of schools of thought when it comes to brake use, design, and bias. PowerBrake not only engineers their products to function in the harshest environments but to work in concert with the kinesthetics of the human condition. “The Human Brain modulates pressure better than movement, hence why on a race brake system you want the shortest pedal travel possible,” explained Murphy.
“Everything in one of our big brake kits is designed to reduce compliance in the system — the calipers are stiffer and so are the stainless steel braided lines. We are very particular about what we want our pedal to feel like,” he declared. “We prefer to go for a slightly progressive brake-torque curve. As you stay on the brake pedal with constant pressure it will build slightly in torque. What is allowed to happen in that first couple of split seconds is the weight transfer to take place to the front tires, then once the tires have been pushed into the tarmac and you have more grip — it builds slightly to make use of that grip.”
In the end, we have to take a step back an assess the true purpose of a brake kit, and understand what it truly does in the long run, “In general the big-brake kit is about consistency and modulation rather than one-stop, stopping power,” stated Murphy.
Temperature Reporting And Cooling
Of course we expect to see more aggressive brake usage as the application of Gruppe R expands onto the track or stage. With more spirited driving will come more enthusiastic brake use — and therefore heat. While the Golf R is not the heaviest among its contemporaries (EVO and WRX) it is by no means light.
“On a fairly heavy car like the Golf R, you’ll be compounding temperature per lap — you’ll be putting more energy into a disc than you can take out in one lap. Every single rotor we produce has a temperature reporting system on it — as well as calipers,” predicted Murphy.
Equipped on all PowerBrake rotors is temperature sensitive paint marking the change color as different temperature thresholds are exceeded. “They are expensive, especially on a higher level production basis — we decided very early on in our company history that it was critical feedback,” he relayed. Murphy emphasized that one of the misconceptions about big brake kits is that size is everything. Thermal capacity and the ability to dissipate heats is really where it’s at.
“The internal cooling veins in the rotor are more efficient than the original rotors on the car. Standard rotors tend to come with straight cooling veins, well manufactured big brake kits come with fairly advanced directional cooling veins which move a lot more air and cool the brakes more efficiently,” he explained.
Stock Driving Impressions
The stock brakes on the Mk6 Golf R are ample. 345 mm-diameter front, and 310 mm-rear rotors are clamped by large single-piston calipers. These brakes are very effective at their intended purpose but there is always room for improvement. A 60-0 mph stopping distance test yielded 113 feet 11 inches — a respectable figure given most of the world’s supercars only take a car-length less. But ultimately we are not judging our brakes on a one-stop test, repeatability and consistency under track driving are our goal.
There were no shudders other than cycling ABS, the car did not pull, and only let out the tiniest chirp of the tires right before coming to a controlled halt. However the stock brakes are heavy, iron calipers and one-piece rotors leave little concern to weight savings. When you consider that brakes are a major contributor to unsprung weight, our suspension will thank us for the diet.
The installation of the PowerBrake package is very straight forward, only simple hand tools are needed and the job takes just a few hours. With the car jacked up and a wheel removed we first disconnected the flexible stock brake line from the hardline protruding into the fender well — capping it with the supplied rubber cap.
Removing the stock caliper is just a matter of a few spring clips to free the flexible line and two 21 mm mounting bolts holding the assembly to the knuckle. With the caliper out of the way, the stock rotors are located with a single torx-head fastener. After all the hardware is removed, a few taps of a mallet liberate the rotor from the drive flange.
Comparing the stock components against the PowerBrake kit is night and day. While the new rotors are smaller in diameter, they feature directional cooling veins, a machined aluminum center hat, slotted rotors and temperature reporting paint. The new billet aluminum calipers are Type-3 hard anodized and weigh considerably less than the stock single piston VW units.
Before delving any farther into the installation, it is wise to double check your fitment by measuring the run-out of the rotor. Run-out is the amount of irregularity across the surface height of a rotating object. Using a dial-indicator magnetically attached to a fixed location we arranged the needle to bear on the face of the rotor. After zeroing the dial we gave the rotor a spin and watched for any excessive run-out.
The next phase is to affix the caliper mounting bracket to the knuckle using the original 21 mm bolts. Some minor filing of the knuckle casting parting line was required to clear the precision-machined and laser-etched brackets.
Finally, the caliper and pad assembly could be held in place as the two supplied M12 bolts are threaded through the recessed caliper holes and into the mounting bracket. Final torque values of 81 ft-lbs are applied to the mounting bolts for both the bracket and caliper.
With all the mechanicals firmly in place, and the rotor free spinning having bent back the dust shield slightly — we turned to the plumbing end of the system. Supplied with the kit are 32-strand, stainless steel-braided Teflon lines.
With the supplied banjo hardware, we attached the clock-able end to the caliper and routed the line through the bracketry on the spindle using the provided grommets. Hooking up the hard line that protrudes through the fenderwell completed the task. Bleeding the PowerBrake system takes some care as the recessed bleed screws require a socket to loosen and tighten. The calipers include inboard and outboard bleed screws to liberate air from either side of the internal crossover channels.
While it’s tempting to go out and stand on the brakes after freshly installing some new rotors, calipers and pads, some discretion must be exercised to ensure long life and optimum braking performance. Like many other automotive heat and friction areas, brakes require a bed-in procedure. At first, the car should be driven at least 180 miles using light to moderate braking, without dragging the brakes too long. “The bed-in procedure is fairly straight forward, you’ll see that after you’ve driven for a couple of miles to get good contact between the pads and rotors, there’s a 6-10 stop procedure to get some heat into he brakes,” explained Murphy.
When a rotor is cast in a sand mold, molten iron is allowed to cool at a fairly fast rate — when the grain structure of a metal is forced to solidify into a form rapidly stresses are imparted in the part. PowerBrake goes to lengths to relieve some of these internal stresses with a proprietary weathering process but further heat cycling after installation sets the final temper. The bedding process not only helps to prevent premature wear of rotor cracking, but to establish the braking surface on which we will gain friction. This surface is defined by a friction compound film that, depending on the aggression of application and temperature, can be tailored to suit different driving situations.
“All modern high-performance race and road pads work by putting a friction film layer down on the surface of the rotor. That 6-10 stop procedure is to get that film built nicely and evenly. As you drive, depending on how aggressively you drive — the color of the film on the rotor surface will change somewhat,” Murphy illustrated. After we completed the pre-bedd-in procedure we continued on to the final surface treatment. We made a series of stops ranging from about 20 percent braking capacity up to about 80 percent braking capacity, without ever coming to a complete stop to prevent pad etching of the rotor — allowing the brakes to cool sufficiently afterwards.
After putting a few miles on the new pads and rotors the pedal feel really started coming into it’s own. The sensation of the Power Brake system is one of security and minimal movement, pedal feel is very similar to OEM, with slightly less initial-bite and superior modulation. You can definitely feel the progressive mechanical advantage gain under constant pedal pressure, as aforementioned by Murphy. The pads are quiet and no noticeable noise is produced from the SASS-floating system.
Thanks to our participation in the National Auto Sport Association (NASA) High Performance Driver Education Program (HPDE), we were afforded two days worth of hard track sessions around the infield road course at Auto Club Speedway of California in Fontana. This track is notorious for eating up brake pads, due to high speed straights followed by abrupt corners.
Throughout the weekend the new PowerBrake system exhibited zero fade, and repeatable stop-after-stop performance. We garnered a real sense of security when barreling into the braking zone of a late-appex corner that our middle pedal would be there. PowerBrake came through with a brake package solution where few other offered a comparable option, if you have a unique application like ours reach out to the Murphy brothers.