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To have or not have a rear brake on BEV. That is the question.

Updated: Nov 11, 2023

The following three-section blog post aims to shed some light on the editorial article on the SAE Automotive Engineering publication, by Michael Robinet on 10/2/2023.


Section 1 - Excerpt from the original publication

Batteries and motors get most of the spotlight, but electrification may significantly impact the engineering of steering, braking and suspension, too.

Much has been written about battery-electric propulsion’s impact on the automotive ecosystem. The level of change moves much past an overhaul to a complete re-imagination. While it can be difficult to categorize different system areas into neat buckets driven by the impact of BEV propulsion, no doubt that there are at least five major systems negatively impacted. Known as “BEV-negative” systems: internal-combustion engines, “traditional” transmissions, driveline systems, fuel and exhaust. They all face significant upheaval from a refocus on alternative systems and carryover of components, when possible, with BEV volumes rising.

Beyond the BEV-negative systems, there is a handful of other system areas significantly impacted as our world changes. Depending on your perspective – and while many may characterize the control/chassis systems of braking, steering and suspension as “BEV-agnostic” systems (still required, though altering significantly) – there are risks and opportunities in these areas as the industry adapts. Let’s tackle each from a 30,000-foot perspective.

Braking: Two main technical factors underscore electrification’s impact on braking. The shift of the center of mass lower in the vehicle (because of the battery pack) and the increase in vehicle mass alters demands on the braking system. Possibly more important is the ability for speed to be scrubbed off through propulsion-system regeneration/energy capture. This ability to slow the vehicle through recapturing energy back into storage systems is a game-changer.

Many BEVs now have reduced requirements for a 4-wheel-disc braking system as a significant portion of deceleration typically can be recaptured by the new drivetrain. In absence of vacuum supplied from traditional ICE engines, electro-hydraulic actuation systems increasingly are the norm. In the future, the elimination of rear braking is a very real possibility. Braking systems will be a focal point as BEVs become more prevalent.


Section 2 – Comments by Paul Gritt - former senior brake specialist at Chrysler

I must take issue with your comment that in the future the elimination of rear braking is a real possibility. In my opinion, regardless of the level of retarding torque applied to a vehicle, one cannot completely eliminate rear braking.

Without rear braking, the best possible deceleration one can achieve is about 0.7 G's. In a BEV with a low CG and more rear weight bias, the deceleration might even be lower. With only front tires trying to stop a vehicle, it is likely that the stopping distance will be greatly increased.

If we consider a case study where a RWD BEV is assumed, and that the vehicle could use regenerative braking to provide braking for the rear wheels and additional rear friction braking would not be required; this might in fact work to some extent.

On a different point of view, achieving the required approvals from the NHTSA and ECE regulators for a vehicle with no rear friction brakes would be a challenge. This is especially true because of the requirement to meet certain stopping distances with partial system failures.

Some testing would have to be conducted to establish if stopping distances equal to those using rear friction brakes are possible. Since regenerative braking below 5 mph is almost nonexistent, there might be a problem at the lower speed to achieve full-stop. I believe at higher speeds with newer technology batteries enough regenerative braking could be applied to skid the rear tires. Being able to skid the tires is the basic criterion for having adequate brakes.


Section 3 – Comments by Mo Eskan, PhD - Brake Academy Consultant

While there is a lot to address in this short article, there are 2 major points that stands out from the viewpoint of a brakes researcher and engineer - firstly the hypothesis on the future architecture and engineering of the BEV’s, and secondly on the necessity or redundancy of the rear brake systems. These two topics seem to be intertwined and one may consider one in conjunction with the other. Nevertheless, stating that all EV’s are heavier than the ICE equivalent with lower CG seems more of a generalization and might be in conflict with the point raised later in the article regarding the rear brakes becoming redundant.

The necessity for any system in a vehicle (and their associated required performance) will be dictated by the technical specifications of that particular vehicle. In disagreement with the approach of the SAE article, unfortunately there is not much room for generalization in this topic. Considering different classes of vehicles, they may require different systems and technologies. The notion that all EV’s are excessively heavy might hold some truth in the North American Market but certainly not relevant when one takes a more global view of the automotive industry.

Small hatch-back vehicles are (and have been) extremely popular in the European markets - both in ICE and BEV powertrains. According to European Automobile Manufacturers' Association (ACEA), 37% of all passenger vehicles sold in the EU market are A/B/C segment vehicles. Looking at the other parts of the world - In the Asian / Far East markets there is an abundance of light-weight / hatch-back EV’s which are not necessarily excessively heavy vehicles. Insider reports “Data compiled by Bloomberg shows the Sakura - a $13,000 minicar developed jointly by Nissan and Mitsubishi - is the best-selling electric car in Japan this year”. With curb weight of 1,070 kgs (2359 lbs.) this vehicle has small disc brakes in the front and drum brakes in the rear which is predominantly for parking brake.

The hypothesis that rear brakes are going away because of regenerative braking certainly holds some potential for the lighter vehicles but considering some engineering “bookends” is certainly not globally applicable for the heavier BEV’s mentioned by the author of the SAE editorial. Elimination of the rear brakes might be possible on smaller automobiles mentioned above by compensating the braking power with the larger front brakes as well as a new solution for the parking brakes that does not rely on the brake lines in the rear axle.

In summary, the necessity or redundancy of rear brakes invites two distinct areas of focus. Firstly, academic and R&D focus to develop new technology that is lighter, cheaper and more efficient to replace current rear brake set-up. Secondly, thorough design and development of the specific vehicle project where the aim is to eliminate the rear brakes. This picks up from where R&D leads to, and involves simulation and vehicle level testing of the system to achieve homologation - assuming the regulatory aspect can be reached.

Mr. Paul Gritt holds a Bachelor of Mechanical Engineering from Carnegie Institute of Technology (now Carnegie Mellon University), and a Master of Mechanical Engineering from Ohio State University. He joined the Chrysler Corporation brake engineering department in 1969 after serving two years in the U.S. Army core of Engineers.

For the past 40 years he has been involved in all aspects of brake engineering with concentrations in the areas of warranty reduction, quality, brake actuation, dynamometer testing, and system simulation and modeling.

Paul has been active in several SAE technical brake committees and has been a member of the Brake Activities Committee for over 24 years. Paul has co-organized and chaired many brakes technical sessions at the SAE International Congress in Detroit. He is the liaison between the SAE Chassis Activities Committee and the Executive Committee of the SAE Brake Colloquium. Most recently he was a member of the task force that created SAE J-2784 the recommended practice for single ended dynamometer simulation of MVSS-105 and 135.Mr. Gritt retired from Chrysler in November 2008 and is the principal of Paul Gritt Consulting LLC

Dr Mo Eskan has over a decade of experience in Brake NVH research & development and has published in various academic journals. Mo also has a comprehensive hands-on automotive design and engineering experience delivering various vehicle programs. Mo is one of the instructors on Brake Academy.

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