Deaglán Ó Meachair established www.Brakebetter.com to improve the efficiency of the planet’s electric vehicle fleet, by offering the automotive industry the insight and support required to move from legacy systems and grasp the vast potential available through electrification. The following is a section of his latest commentary on this matter, concentrating on autonomous vehicles. The entire piece can be read by clicking on this sentence.
The job of a brake system is to slow a vehicle by converting kinetic energy into other forms – typically heat and electricity. To do this job repeatedly and reliably, the brake system relies on its thermal capacity to soak up and subsequently dissipate the vehicles’ maximum kinetic energy at the limit of available grip – making it the most powerful vehicle system.
Brake components are dimensioned for this job based on two key vehicle characteristics; the vehicle weight and speed. As can be seen from above, speed is the dominant factor, but vehicle weight (and therefore brake system weight) plays a role.
There’s a hole in my bucket, dear Liza
A useful way to understand brake sizing is the bucket analogy. This describes the brake thermal capacity as a bucket, the brake cooling capacity as a hole, releasing stored thermal energy (water). A brake event can be thought of as a tap above the bucket turning on, and the intensity of the deceleration can be thought of as the water flow from the tap.
The size (and capacity) of the bucket could vary depending on the material it’s made from – an aluminium “bucket” may not have the same capacity as an iron bucket or a carbon ceramic bucket of equivalent dimensions. But the rate the heat can be ejected by the brakes system (the size of the hole) is also important here – so while an aluminium bucket may not hold as much as an iron bucket, if it can move the water through the bucket quicker, then the capacity of the bucket may be good enough. If we consider a single bucket material, we can definitely plan for a smaller size of bucket if we can maximise the water flow out of the bucket, or also think about how to optimise the flow of water into the bucket.
The takeaway here is that the thermal capacity of a brake system is decided by the amount of kinetic energy going in, as well as the cooling capacity of the installed hardware. When we consider weight saving, therefore, we must take a holistic view, and consider the influencing factors on the brake system design.
Often, weight saving is considered at a component level, or indeed as intrinsic to a component design. but working in isolation misses some compelling opportunities for improving vehicle performance. As eluded to above, making a lighter brake disc is possible with more exotic materials, but using a lighter brake disc can be as simple as better use of cooling air flow, or smarter control of powertrain systems. And while alloy pedal caps suggest a motorsports optic, if they are overlays on existing, fully formed pedal arms – not only do they add weight to the pedal, but they make the crash system work harder to deal with the extra mass. Finally, smarter planning of NVH activities can prevent the addition of palliative measures late in the day (which more often than not add weight in undesirable locations).
System by system lets take a closer look at what’s possible in brakes today, and what opportunities are on the horizon. Brake rotors (either discs or drums) make up the majority of the system weight, but a lot of the trends here are widely discussed, so we will save that conversation for a little later. Instead, let’s start from the opposite end of the chain – where the driver interacts – the pedal box.