Tech Update: Rotors For Track Days

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Accounting for heat capacity and thermal stress management, here’s how to select brake components that correspond with different driving styles.

Choosing the correct disc brake setup for a fun day at the track can be confusing and expensive if you use a product that is not fit for the purpose. 

The two most important factors when it comes to disc brake rotors used in extreme applications such as motorsports is heat capacity and thermal stress management. 

Heat-capacity design is a compromise between having enough metal mass in the friction ring to accept and process the kinetic energy transferred into the brake system to decelerate the vehicle and trying to minimize the rotating mass of the wheel assembly (which is unsprung weight). 

Thermal stress management is all about minimizing fatigue to prolong the life of the disc rotor. Regardless of the manufacturer or disc design, gray cast iron will fatigue in motorsports applications. The thermal expansion from heating and contraction when cooling initiates a stress-and-strain cycle that is essentially metal fatigue. It is common practice for professional race teams to discard the disc rotors after a known period, whether it be a certain number of laps or hours in race conditions, to avoid any unpredicted failures due to metal fatigue. 

Rotor Selection

There are several types of rotors used in motorsports. Some drivers like to maintain the factory setup with plain discs, and this is okay. This choice does require more scrutiny in friction material selection to avoid surface glazing and declining friction performance. 

Many brake companies offer disc rotors with slots in them. The purpose of the slots is to evacuate waste material (dust and gas) from the friction surfaces. The slots will ensure consistent friction performance throughout the life of the disc and pad.  

This leaves us with drilled or drilled and slotted discs. 

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Disc Brakes Australia (DBA) manufactures disc rotors with slots, which are designed to evacuate waste material, such as dust and gas, from the friction surfaces. The slots allow for consistent friction performance throughout the life of the disc and pad, according to a company source.

Drilled disc rotors are not recommended for track use of any kind. The drilled holes are focus points where stress and fatigue will accelerate and eventually result in crack formation. Drilled disc rotors have proven to improve braking performance on the street, where normal driving conditions rarely exceed 500 degrees F and stress is not a concern.

In summary, we feel the best choice is slotted rotors wherever possible. All of Disc Brakes Australia’s disc rotors are available in various surface configurations, including slotted. 

Post-production Treatments

There are several post-production treatments or processes applied to disc rotors marketed in the industry, including heat treatment and vibrational stress relieving. Cryogenics is another method of treating the discs post manufacturing. The discs are often sourced direct from an external or unknown manufacturer and processed later by the distributor, who may not have detailed knowledge of the disc rotor’s material properties. The risk of not receiving the desired performance is real. 

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DBA’s 4000 Series and 5000 Series (seen here) disc rotors undergo a thermal stress cycling procedure during the manufacturing process to ensure the metal structure is set for optimal performance out of the box.

DBA’s 4000 Series and 5000 Series disc rotors undergo a thermal stress cycling procedure during the manufacturing process to ensure the metal structure is at its optimum for maximum performance out of the box.

Friction Materials (Brake Pads) 

There are hundreds of different friction materials available for road and track vehicles. Material choice is a personal one, as driving styles differ with confidence and experience. 

It is important to cover some basics when it comes to making the best choice. Many OE friction materials are designed for comfort and moderate performance in normal street applications. Often these are Non-Asbestos Organic (NAO) class pads that are gentle on the rotors and make little noise. These pads are not intended for prolonged high-temperature applications. 

Sintered metal, carbon metallic and semi-metallic pads, as the names suggest, have varying amounts of metal as part of the compound. These pad materials offer a high-friction coefficient and higher temperature resistance. Each of these pads has different performance characteristics, so it pays to do your research.

Final Considerations

Here are some key friction material metrics to focus on:

Initial bite: The friction coefficient upon the first application. 

Peak effectiveness: The point where the material reaches its highest performance. Usually related to temperature.

Release speed: Responsiveness when releasing the pedal.

Modulation: A technique of applying and releasing the brake pedal to effectively decelerate while maintaining maximum control and applying the least amount of stress in the brake system. 

Wear: Wear and tear are critical for product life and cost effectiveness. 

Below are friction material characteristics to consider:

Super Sprints: High initial bite with a peak effectiveness at temperatures of 200 degrees F to 300 degrees F. This may support a late-braking driving style for short sprints. 

Circuit racing 10–15 laps: Moderate initial bite increasing to a peak effectiveness at 550 degrees F to 750 degrees F. This may provide the most effective brake and tire friction at the same time, typically lap number three or four, for strategic, competitive driving. 

Endurance: Moderate initial bite with a very flat peak effectiveness curve from 550 degrees F to 900 degrees F. Typically slightly lower coefficient of friction than the above race pads to ensure longer pad and rotor life. 

Please note: Each person’s driving style is different, and it is the responsibility of the driver to determine which braking products are best suited to their driving behavior. The above information is a guide only to assist the driver in conducting their own research.

Brian Johns is currently Head of Innovation and R&D for DBA. He has spent more than 15 years in various automotive roles across the globe for many of the major OE manufacturers. He also has 20 years of experience in designing and engineering products for defense and high-tech applications, as well as in the consumer sphere.

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