Test The Limits

Considering light-weighting your brakes? Read this first.

By Brad Duncan

August 29 2016

Brake rotors are a prime target for light-weighting. Why? For starters they are heavy, around 21 lbs per wheel for a large sedan. This mass has a significant rotating inertia which needs to be overcome through torque, 18 lbf-ft, each time the vehicle accelerates or stops. This might not seem like a lot but a 2015 Prius can only supply 105 lbf-ft. If the Prius had these heavy brakes, there would not be much torque left over for accelerating you and your stuff. Despite this, light weight brakes have seen little adoption due to cost, temperature and friction requirements. New developments in Exa’s PowerFLOW solution help address these needs just as corporate average fuel economy (CAFE) bring renewed interest.

 

“Recent trends in brake disc design have favoured weight reduction, to aid compliance with CO2 emissions targets, along with improving NVH and dust issues.  These goals have been achieved through the use of discs which both require a greater cooling air flow and are more sensitive to surface water contamination. “

 

Weight reduction is achieved through reducing the size of the brakes and changing the rotor materials. The reduction of material increases the cooling requirements for the brakes. This is because the heat capacity of the brakes reduces proportionally to the mass reduction.  If we don’t increase the cooling flow the brake performance, for long downhills and performance driving, will be compromised by brake fade. Lighter weight materials such as carbon/silicon carbide ceramic, which can be substituted for heavy iron, are more sensitive to wetting. It is not unheard of for the coefficient of friction to drop by more than 80% when wet, which results in a proportional increase in braking distance. This means that if we use ceramic brakes we want to ensure that they stay dry.

 

Increasing brake cooling while keeping the brake rotor dry is challenging. This is because brake cooling is usually increased by reducing the size of the brake dust shield and ducting air towards the brake rotor, both of these changes are likely to increase brake wetting. In order to ensure that changes made to increase brake cooling are not going to increase brake wetting, potential designs need to be tested for both criteria. Experimentally this is a major hurdle as the setup for thermal testing and brake wetting testing are very different. Also climatic tunnel test facilities are expensive and often need to be booked months in advance. PowerFLOW simulation overcomes all of these obstacles and allows for concurrent design. Thermal and brake wetting simulations can be done at the same time with the results driving towards an optimum design.

 

“The fact the good correlation has been achieved indicates that the tyre spray model used in this work represents the key features of tyre spray required for this application.”

 

Application of Exa’s brake soiling methodology allows for the relative soling performance of different brake cooling solutions and dust shield designs to be assessed. This capability was the topic of a paper presented jointly with JLR at the  SAE World Congress 2016. Results of this study show how PowerFLOW can reproduce experimental trends for brake shield changes. Combining this brake soiling solution with existing highly accurate thermal and aerodynamic simulations means that Exa now offers a multi-disciplinary platform for designing lightweight brakes.  This allows cooling targets to be met without a substantial increase to drag, while ensuring that the brake rotors stay relatively dry.

 

Adopting a multi-disciplinary approach is the only way to ensure a successful light-weighting solution for a brake program.  Failure to consider thermal, aero and soiling aspects of the brake performance problem can result in catastrophic failure. Attempting to do this without simulation is too costly and does not allow for fast enough turn around or design flexibility. Coming up with a cost effective design for light weight brakes is still a challenging problem but Exa’s PowerFLOW solution now provides the tools needed to make it possible.

 

References:
Schembri Puglisevich, L., Gaylard, A., Osborne, M., Jilesen, J. et al., "Application of CFD to Predict Brake Disc Contamination in Wet Conditions,"SAE Int. J. Passeng. Cars - Mech. Syst. 9(2):800-807, 2016, doi:10.4271/2016-01-1619.