Open-Source Sports Electric Vehicle (OSSEV®)
In a sports car, it is critical that the braking system provides great feel and feedback. EVs are not known for this as most of them are very heavy and have all sorts of electronic systems involved, including regenerative braking. For this reason the OSSEV® will have a standard disc-based braking system with minimal servo-assistance and interference. An automatic handbrake will be applied intelligently and the driving profile will give the driver full control over the regenerative braking.
With the MX-5e we are having to work within the constraints of the existing braking system. We have replaced the discs and pads, added braided hoses and have refurbished the calipers as new. The existing cable operated hand brake is retained.
We are using the Tesla iBooster in the MX-5e, to provide programmable levels of brake assistance. The bulkhead will be stiffened and a sturdier/lighter brake pedal will be fabricated with proper bearings. There is no ABS.
The Vehicle Control Unit (VCU) can be configured on the move, to allow configurable regenerative braking based on the driver profile and driving profile. This is controlled by the Driver Control Unit (DCU).
The hand brake will remain a manual feature for now but, we are looking at 'auto hold' options.
Our road testing to date has shown that the new braking system (combined with the Yokohama tyres) on the MX-5e provides great stopping power, good feedback and instills confidence. You really can brake very late 😎 Based on our experience doing track days in Yoko, we know that the standard braking system can over heat if used hard on track but, the MX-5e is being designed and build as a 'fast road' car and the standard braking system is perfectly adequate for this purpose.
Modern cars use a mechanical vacuum system to provide brake assistance. This basically helps you when you press the brake pedal and makes the pedal feel lighter. Rob's Fisher Fury R1 was light enough (450kg) to not need this but, in heavier cars it is essential.
An EV will typically use an electric vacuum pump and a vacuum reservoir tank as an alternative way to provide the assistance but these are noisy, even when the vacuum pump is demand driven.
Bosch have developed the iBooster to provide brake assistance in EVs. It is a "vacuum-independent, electromechanical brake booster that meets the demands of a modern braking system". The pedal characteristics can be adjusted from sporty to comfortable. The control principle behind the iBooster is similar to that of vacuum brake boosters: in vacuum brake boosters, a valve controls the air supply to provide a boost to the force applied from the drivers foot. With the iBooster, the actuation of the brake pedal is detected via an integrated differential travel sensor and this information is sent to the control unit. The control unit determines the control signals for the electric motor, while a three-stage gear unit converts the torque of the motor into the necessary boost power. The power supplied by the booster is converted into hydraulic pressure in a standard master brake cylinder.
Depending on the active driver profile and driving profile, the OSSEV® can use regenerative braking to capture energy and recharge the battery. The amount of regenerative braking will be user configurable for each driver profile and driving profile.
In a rear-wheel drive car, you really don't want to be over-braking the back end, especially in corners.
The level of regenerative braking is dependent on the battery state. With a fully charged battery, there is no capability to use regenerative braking to push energy back into the battery. As the charge level increases, the level of regenerative braking reduces. Our CUPRA Born uses a nice graphic on the display to show how much regenerative braking is currently possible and we would like to replicate this functionality in some way.
The OSSEV® will feature an automatic handbrake. In our experience, these work really well and avoid the need to clutter the center tunnel. They are also an essential part of the Adaptive Cruise Control (ACC) system.
Our CUPRA Born has an automatic handbrake (called 'auto hold') and this is a nice feature to have in our view. It can be switched on and off though, which is essential in our view. One thing you can't do though is set the delay before it kicks in.
In the MX-5e we have a traditional handbrake and whilst we would like to investigate an auto hold feature, this is going to be low priority.
In the MX-5e we have replace the existing brake pipes and fittings with new ones. We are using 3/16" Cupro Nickel hardline kunifer brake pipe.