Proper air ducting has become critical for effective performance of short track stock car disc brake systems. Faster cars, improved tires and better aerodynamics has changed the design requirements for brake ducting. Short tracks, and most road courses, require specific air duct considerations to maximize the brake system’s effectiveness. Channeling sufficient air from the front of the car through the front brakes is required to remove the large amounts of heat generated by severe and prolonged brake use. Just as proper brake components (calipers, rotors, pads) are required to convert the kinetic energy of the spinning rotor into heat, so must the necessary air be moved in, through, and out of the brake system to remove the heat (energy). An efficient air ducting system can prevent brake system overheating, greatly improve pad life and can make the difference between winning and losing a race. This technical data sheet discusses the particular problems and solutions to stock car air ducting for short track and road racing. Check your rule book for any limitations on brake cooling, and use the maximum allowable for optimum cooling performance.Short Track Stock Cars
New, more aerodynamically shaped stock cars have changed some of the air flow dynamics which affect brake air duct efficiency. Typically, cars have gone from a squared off shape to a more aerodynamic rounded shape. Refer to Figure 1 for a typical old style stock car/air duct configuration. Figure 2 illustrates air flow patterns associated with this style vehicle. As car design has evolved with less frontal area and more slippery front sections, the aerodynamics of the car’s front end has changed, thereby affecting the air flow entering the front air scoops (plenum). Because of this change in air flow, the standard positioning of intake plenums should be re-evaluated. Air flow can be diverted past air intakes by low pressure zones formed at the outward end of the intake opening.
Figure 3 shows typical air flow with the newly designed front ends of today’s aerodynamic efficient race cars. This results in a diminished volume of air flow into the air ducts and can greatly reduce the air flow to the brakes. The solution to this problem appears to lie in the repositioning of the plenum intakes as shown in Figure 4. By repositioning the plenum intakes vertically as close as possible to the center of the vehicle, air flow is increased. Vertically placed intakes reduce the unwanted effect of air skimming past the duct openings. Locating the intake ducts closer to the center of the vehicle positions them in a high pressure area. Note that the position of the oil cooler has been changed to allow a vertical placement of the air ducts. Protective screen mesh should be made as open as is practical. Fine mesh can severely restrict air flow. Air duct hosing should be the type with smooth interior facing. Intake plenums should be smooth and contoured for minimum air restriction, usually fabricated of aluminum, plastic or composite material. Air duct hoses should run as straight and short as possible; every turn and bend restricts air flow.
Ideally, air ducting should run straight from the front section into the front wheel plenum, with flexibility and allowance for the turning of the wheels (see Figure 5, top view). Placement of front fender support struts should be made with consideration for running brake duct hoses straight from the scoops to the wheel plenum. Avoid “snaking” the hoses as any bending reduces air flow. All three hoses should run parallel (one on top of the other, Figure 5, side view), and exit into the front wheel plenum at the front of the spindle. From a cooling stand point, it is preferred that the caliper be placed in front of the spindle so that a plenum built around the caliper allows the air to flow across the inboard half of the caliper and then into the center of the rotor. This will substantially decrease the operating temperature of the caliper (and brake fluid).
The plenum should surround the spindle and caliper assembly with the open edges as close to the inner diameter of the rotor as possible (see Figure 6). Air forced into the center of the spindle/rotor area creates a high pressure area of air which is forced/pumped through the vanes of the rotor, and to a lesser extent, the small gaps between the hat and rotor. The plenum should avoid restrictions to air flow and take advantage of all possible means of getting air to the center of the rotor.
The shock should be mounted behind the spindle to avoid interfering with brake ducts and plenum openings. Inline blowers, or blowers fitted to a “Y” pipe (see Figure 5) should be used to enhance air flow. Bumper-to-bumper racing, caution periods, pit stops - all these conditions may require supplemental air flow from inline impeller-type blowers which can be operated from inside the cockpit. Make sure to read the rules which dictate if and how many blowers may be used. Place blowers near inlet plenum. Rear air ducts are usually not required on stock car brakes. However, they are recommended for use at short flat tracks with heavy vehicles (NASCAR Winston Cup, Busch GN and Craftsman Truck Series). When used, a single three (3) inch duct run from inside the cockpit to each rear rotor is usually adequate. Install an inline blower which can be manually switched from inside the cockpit.
Wilwood pedal assemblies and integrated balance bars have been designed specifically for racing applications. Properly set-up, this assembly will allow for the precise adjustment of front-to-rear brake bias. The advantages of an adjustable balance bar and dual master cylinders are:
Brake pedals should be mounted securely. When possible, keep the master cylinder reservoir level higher than the horizontal plane of the calipers to prevent excessive fluid drain back which can result in double pumping of the pedal. If this is not possible, a two pound residual pressure valve should be plumbed into the brake line at the exit of the master cylinder to prevent fluid drain back (do not confuse the two pound valve with the ten pound version; the ten pound valve is for use with drum brakes only).
Brake pedals should be free to return when no pressure is being applied, allowing the master cylinder pushrod to return to its undepressed position. In some cases, the master cylinder spring (internal) may not be strong enough to fully return the pushrod; in this case an additional pedal return spring can be used. There are two important items for consideration:
1. The brake pedal should have an adjustable return stop on it when a strong pedal return spring is used. This prevents the master cylinder from excessively banging the snap ring stop inside the master cylinder bore (visible under the rubber boot). Adjust the stop so the pedal stops returning at the point when the master cylinder piston retracts against the snap ring.
2. The master cylinder piston must fully retract. If the master cylinder piston is not allowed to fully retract when the brake pedal is not applied, the primary inside seal will not return past the small pressure relief hole (visible within the master cylinder reservoir on some master cylinders). This can cause excessive residual line pressure and contribute to brake drag and an overheating condition.Balance Bar Adjusting
The balance bar is an adjustable lever (usually a threaded rod), that pivots on a spherical bearing and uses two separate master cylinders for the front and rear brakes. Most balance bars are part of a pedal assembly that also provides a mounting for the master cylinders. When the balance bar is centered, it pushes equally on both master cylinders creating equal pressure, given that the master cylinders are the same size bore. When adjusted as far as possible toward one master cylinder it will push approximately twice as hard on that cylinder as the other.
To set up the balance bar, thread the master cylinder pushrods through their respective clevises to obtain the desired position. Threading one pushrod into its respective clevis means threading the other one out the same amount. Sometimes this will lead to a “cocked” balance bar when the pedal is in the relaxed position, “no pedal effort”. This is acceptable as long as each master cylinder pushrod is completely free of pressure when the pedal is relaxed.
It is important that the operation of the balance bar functions without interference by over adjustment. This can occur when a clevis jams against the side of the pedal or the lever (bolt) hits the pedal bore during any point of pedal travel.
Lever movement should be unimpeded throughout pedal travel. In the neutral position, clevises should have between .20” - .25” total clearance between the side of the pedal. The large washers between the pedal and clevis should remain loose. Make sure that the master cylinder pushrods remain true in relationship to the cylinder during entire pedal travel; pushrods should not be pushing master cylinder pistons at an angle.
NOTE: In its non-depressed position, the pedal and balance bar should allow the pushrod of the master cylinders to fully return. This can be checked by feeling pushrods for very slight movement, not loose movement. Master cylinder pistons should be against the retaining snap ring (under boot).Pedal Ratios / Mechanical Leverage
Pedal ratio, or mechanical leverage is the ratio calculated from the length from the pivot point of the pedal to the center of the foot pedal, divided by the length from the pivot point to the master cylinder pushrod. Mechanical leverage is simply a means of increasing the brake force without increasing your leg effort. The mechanical leverage increases brake force without pushing harder on the pedal. The disadvantage is that the pedal stroke also increases, requiring you to push the pedal further. If uncertain about which pedal ratio is right for your application, a 6:1 ratio is an excellent starting point.
Due to the extreme operating temperatures of a high-performance brake system, standard off-the-shelf brake fluids are not recommended. Of critical importance in determining a fluids ability to handle high temperature applications is the Dry Boiling Point and compressibility.
The Dry Boiling Point is the temperature at which a brake fluid will boil in its virgin non-contaminated state. The highest temperature Dry Boiling Point available in a DOT 3 fluid is 572 degrees F.
The Wet Boiling Point is the temperature a brake fluid will boil after it has been fully saturated with moisture. The DOT 3 requirement for wet boiling point is a minimum temperature of 284 degrees F.
There are many ways for moisture to enter your brake system. Condensation from regular use, washing the vehicle and humidity are the most common, with little hope of prevention. Glycol based DOT 3 & 4 fluids are hygroscopic; they absorb brake system moisture, and over time the boiling point is gradually reduced.
Wilwood does not recommend using DOT 5 fluid in any racing applications. DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers. It is not uncommon to have caliper temperatures exceed 200 degrees F, and at 212 degrees F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel.
Whenever you add fresh fluid to your existing system (never mix fluids of different DOT classifications), it immediately becomes contaminated, lowering the boiling point of the new fluid. For maximum performance, start with the highest Dry Boiling Point available (try Wilwood Hi-Temp 570 Racing Brake Fluid), flush the system completely, and flush it regularly, especially after severe temperatures have been experienced.
As long as your pads are wearing evenly across the pad surface, the pads can be used almost down to the backing plate. Spacer plates may be added behind the pad backing plate as it wears so the caliper pistons will not have to be exposed to the abusive track dirt and grit. A regular check of the brake pads for excessive wear and taper is necessary to ensure proper disc brake performance.Caliper Bleed Screws Pointing Up
When bleeding the brakes, make sure the bleed screws on the calipers are pointing straight up so there is no possibility of air bubbles getting trapped. If the calipers are mounted on an angle, you will need to unbolt one ear from the bracket. Pivot the caliper so it points straight up and place a spacer between the pads to prevent the pistons from coming out of the housing.Weight Reduction
Finding areas to cut weight is becoming increasingly more difficult, so if you are evaluating whether or not to take weight out of your brake system, here is something to keep in mind: A caliper’s size (and weight) is largely affected by the size of the brake pad it needs to hold. If you can go to a smaller brake pad (without running out of pad before you run out of event) then a smaller lighter caliper may be for you. For example, if your pads last about 10 races, consider going to a smaller pad that will require changing a bit more frequently. Remember, if you go to a smaller caliper, you won’t affect stopping performance provided piston sizes stay the same. Also keep in mind that pad wear is heavily dependent on operating temperature, so if you are going to experiment with downsizing, make sure you have adequate cooling.Caliper Mounting
Brake calipers should be mounted square with rotor to prevent excessive piston knock-back and uneven pad wear. While looking at brake area, have someone apply brakes. Caliper should not move (square itself to rotor): only the pistons and pad should move. If caliper is not parallel with rotor, shims should be used between mounting bracket and caliper ears for proper alignment. Caliper brackets should be strong enough not to deflect under heavy braking. All caliper mounting bolts should be of the highest quality and lockwired for safety.Caliper Selection and Mounting
Most Wilwood extreme performance calipers are one directional because of the differential piston bore design (one end of the caliper having larger pistons than the other); the caliper must be mounted in a specific position relative to the rotor rotation. All Wilwood Ordering Information calipers are marked with a rotor rotation arrow on them. The caliper should be mounted so that the smaller piston end is closest to the rotor entrance, and the larger piston end toward the rotor exit. The larger piston end provides slightly greater clamping force to compensate for pad taper that can occur under extended severe use applications. An improperly mounted caliper (reverse rotation) will cause increased pad taper and reduce overall braking efficiency. Note that differential piston bore calipers cannot be used interchangeably from side to side - there is a left hand caliper and a right hand caliper. Also, calipers differ depending on whether the mounting is behind or in front of the spindle - this affects bleed screw position. Make sure to properly analyze these criteria when ordering and mounting or replacing Ordering Information calipers.Pad Selection
Proper selection of friction pads is a critical part of a high-performance disc brake system. It is important to analyze your vehicle’s braking requirements based upon track demands and driver braking tendencies, then select a pad compound which satisfies these needs. The proper compound for you can be found through a trial and error process, or contact Wilwood’s technical department for recommendations.Pad Bedding
Begin with a series of light decelerations to gradually build some heat in the brakes. Use an on-and-off the pedal technique by applying the brakes for 3-5 seconds, and then allow them to fully release for a period roughly twice as long as the deceleration cycle. If you use a 5 count during the deceleration interval, use a 10 count during the release to allow the heat to sink into the pads & rotors... After several cycles of light stops to begin warming the brakes, proceed with a series of medium to firm deceleration stops to continue raising the temperature level in the brakes... Finish the bedding cycle with a series of 8-10 hard decelerations from 55-65 MPH down to 25 MPH while allowing a proportionate release and heat-sinking interval between each stop. The pads should now be providing positive and consistent response... If any amount of brake fade is observed during the bed-in cycle, immediately begin the cool down cycle... Drive at a moderate cruising speed, with the least amount of brake contact possible, until most of the heat has dissipated from the brakes. Avoid sitting stopped with the brake pedal depressed to hold the car in place during this time. Park the vehicle and allow the brakes to cool to ambient air temperature.Caliper Piston Area
A calipers piston area is calculated by finding the total piston area from one side of the caliper (this is true for a single piston caliper also). The graph provides the piston area for individual piston diameters. Note that differential piston bore calipers will be the total piston area of the different size pistons.Caliper Rebuilding
If you race on dirt or drag race on a weekly basis throughout the year, you should disassemble your calipers mid-season and inspect the caliper seals for excessive wear or hardness caused by heat. Asphalt racers generally experience more heat and should do inspections more frequently, especially after racing on a track where high temperatures are reached. NASCAR’s Winston Cup, Busch GN, Craftsman Truck and Road Race teams usually replace caliper seals after each race to ensure proper disc brake performance. Disassembly and replacement of the seals is a simple process and can prevent catastrophic brake failure.Air Ducting for Short Track Stock Car Racing
Proper air ducting has become critical for effective performance of short track stock car disc brake systems. Short tracks, and most road courses, require specific air duct considerations to maximize the brake systems effectiveness. For complete information on stock car air ducting for short track and road racing.
Harley Davidson® Motorcycles specifies use of DOT 5 Silicone brake fluid because of its minimized impact on painted surfaces. Wilwood disc brake calipers will work with either fluid, but never mix DOT 5 Silicone brake fluid with DOT 3 or 4 fluids.
Changing brake fluid on a regular basis is highly recommended and easily accomplished when changing brake calipers. DOT 3 and DOT 4 brake fluids are superior for general brake system use as well as high-performance applications. For best results, Wilwood Engineering recommends the use of Hi-Temp 570 DOT 3 brake fluid (P/N 290-0632) or EXP 600 Plus DOT 4 brake fluid (P/N 290-6209).
WARNING: DO NOT DRIVE ON UNTESTED BRAKES. BRAKES MUST BE TESTED AFTER INSTALLATION OR MAINTENANCE
The bedding process is the final "heat cure" for the pads. This final bedding cure differs from an oven heat cure in such that the oven heat cure does not include the pressure, torque, and elevated surface temperatures that are necessary to properly condition the pad for service. As it is with the rotors, new pads must be gradually brought up to temperature and then slowly cooled. If the pads are put into hard service right from the start, damage from fractures or accelerated deterioration due to extreme temperature variations between the surface and the body of the pad can occur. Overall poor performance with the potential for rotor damage are often the results.Bedding Steps
Once the brake system has been tested and determined safe to operate the vehicle, follow these steps for the bedding of all new pad materials. These procedures should only be performed on a race track, or other safe location where you can safely and legally obtains speeds up to 65 MPH, while also being able to rapidly decelerate.
NOTE: NEVER allow the contact surfaces of the pads or rotors to be contaminated with brake fluid. Always use a catch bottle with a hose to prevent fluid spill during all brake bleeding procedures.
Once the brake system has been tested and determined safe to operate the vehicle, follow these steps for the bedding of all new pad materials and rotors. These procedures should only be performed on a race track, or other safe location where you can safely and legally obtains speeds up to 65 MPH, while also being able to rapidly decelerate.
NOTE: NEVER allow the contact surfaces of the pads or rotors to be contaminated with brake fluid. Always use a catch bottle with a hose to prevent fluid spill during all brake bleeding procedures.
All new iron rotors should be bedded-in before being used under racing conditions. Proper bedding-in will prepare the rotor surface, prolong the rotor's life and make it more resistant to thermal checking or cracking under severe braking conditions. Once the brake system has been tested and determined safe to operate the vehicle, follow these steps for the bedding of all new rotors. These procedures should only be performed on a race track, or other safe location where you can safely and legally obtains speeds up to 65 MPH, while also being able to rapidly decelerate.
Rotor runout should be adjusted as soon as you receive your Wilwood components. Wilwood rotors are precision machined to ensure the rotor surfaces are flat and parallel. Sometimes hubs, bearings or other components have runout that cause the rotor to runout. As a rule of thumb, allowable runout should be .005” - .008”. Adjust the runout by re-indexing the rotor or by placing shims between the rotor and hub, or hat. Runout should be rechecked regularly. You can assume the runout to be acceptable as long as you are not experiencing brake drag, pedal oscillation or excessive piston knock back.Rotor Wear
Rotors will eventually start to show signs of wear; how fast depends on the type of racing, the frequency, and the brake usage. Grooving and/or cracking due to severe heat and thermal cycling indicate the rotor should be replaced. Note that surface checking itself is not a sign a rotor needs replacing as this can occur on iron rotors. Always replace rotor mounting bolts and lockwire whenever replacing rotors, taking care to inspect mounting hats or hubs for signs of abuse.Wilwood Pre-Bedding Service
Getting track time for a proper pad and rotor bedding session can be difficult. Wilwood offers factory dyno-bedded pads and rotors on many of Wilwood's popular competition pads and Spec 37 GT series rotors. Dyno-bedded parts are ready to race on their first warm up cycle. This can save valuable time and effort when on-track time is either too valuable or not available at all, Dyno-bedding assures that your pads and rotors have been properly run-in and are ready to go.
LOSE YOUR PEDAL DURING RACE
CAR WILL NOT STOP
HAVE TO PUSH TOO HARD ON PEDAL
SPONGY PEDAL OR BOTTOMS OUT
OSCILLATION FEED BACK IN PEDAL