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.
Smarter weight saving
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.
In terms of weight saving choices, the major inputs here will be pedal arm and bracket materials – with recent trends introducing reinforced plastics into an already busy catalogue of solutions. From a design perspective, package constraints usually play a decisive role – whether RH and LH variants can share components (and therefore share investment) – whether the loading points of the pedal are in line with the pivot, and whether actuation systems can be lined up with pedal arms. Defining a strategy for pedal optics and options at an early stage can also keep weight down.
Once the pedal inputs have been sent through the firewall, things start to get a bit more interesting. There are effectively three major functions to achieve under the bonnet; boosting, modulation and regeneration. While we looked before at the topics of regeneration and actuation for autonomous vehicles and how these requirements are changing the architecture of brake actuation, it still makes sense to consider what better looks like in this area.
When we consider the boost function, we (currently) have three proven choices to provide this – a hydraulic booster, a vacuum booster, and an electromechanical booster. In each case, when these solutions were first popularised, the energy source in question was already available on the vehicle, and so the addition of the brake boost function was seen as negligible. However, as time has progressed both hydraulic and vacuum consumers on the vehicle have been reduced, meaning these booster options were less compelling. Indeed, while vacuum boosters are still popular, it is only a matter of time (and the associated creep in emissions standards) before supplying vacuum becomes uneconomic. Vacuum boosters remain the lightest option, but the rapid advances in mechatronic integration mean it’s no longer the best choice to go with separate components providing discrete functionality. So while an electromechanical booster will weigh more than a vacuum booster, it’s ability to enable significant regenerative braking (in combination with a suitable ESP unit) will make it the smarter choice.
And we can go a step further – integrating the boost function into a unit which also can provide modulation and regeneration – a so-called 1-Box brake system. While this 1 box is heavier than the equivalent ESP hydraulic unit, it is lighter than the Booster, ESP and Regen boxes it can replace. Now we can provide all the bells-and-whistles benefits of Brake-by-Wire, high levels of regen and up to L3 ADAS, all in one neat box. In terms of the box itself, the major elements here are the pump (or actuator), the valve block and associated valves, and the power electronics to drive these. From a practical point of view, the most telling choice available to save weight here is to keep the hydraulic sizing down as low as possible, to allow for a smaller pump and valve-set to control them.
Which brings us to the foundation brakes. This is the usual starting point for weight saving in brakes systems, and with good reason – the majority of mass is in the rotating parts, and next in line are the callipers. And as we saw above, choices here can lead to opportunities elsewhere.
For callipers a promising new approach to consider, which can lead to weight saving, albeit at a price, is 3D printing, or Additive Manufacturing. We have recently seen first use of additive manufacturing on road vehicles in both titanium and aluminium, which allows for forms which could not be produced otherwise, and therefore can save up to 40% of the component weight. This is a relatively niche area for today, but it shows the promise of additive manufacture – and its ability to withstand the abuse conditions of some of the most powerful brakes on the market.
In discs, there is a relatively long history of material choices and associated weight savings, but there are also a number of design features available which can bring down weight. Discs may be made from a single piece (as in our image above), or may use two different materials for the friction ring and the top-hat (which bridges the offset between the friction surfaces and the wheel hub – sometimes referred to as the bell). Such discs are known as two-piece or hybrid discs, and will typically marry a lightweight material such as aluminium in the top-hat with a dedicated friction material. Discs can have a solid friction ring (as above) or incorporate air vents through the rotor – which allows for greater cooling, and ultimately less thermal mass.
Careful consideration must be given to the friction ring material choice as it is central to the functioning of the brakes system. While both aluminium and carbon-based materials have excellent heat transfer properties, iron discs do not perform as well here – meaning a balance between thermal gradients and air flow is more critical for iron brakes. However, aluminium brakes have the lowest temperature limit (carbon has the highest – refer to embedded video above), so thermal loading has to be managed.
Thinking about the lightest brake discs, then, it would be a two-piece, vented disc using the lightest friction ring material available, an efficient mechanism for connecting the ring to the top-hat, and a simple light material for the top-hat, such as aluminium. This would incorporate all of the features described above, but would also come at a significant cost. To achieve some of these elements without “braking” the bank, then optimising the flow into and out of our water bucket is the most cost-effective element available – meaning smaller or cheaper brakes can be installed. And while this can mean aerodynamic drag if static surfaces are used, active brake ducts can have a compound effect – maximising air flow when braking while also increasing aerodynamic drag which itself can slow the vehicle.
One final thought on weight saving and braking relates to EVs and regenerative braking. By offering a way of charging the battery on the move, regen braking effectively increases EV battery energy density – or means less battery mass is necessary for a given range. And by planning for regenerative braking, it is possible to reduce brake disc mass, further increasing the vehicle efficiency.
In summary, there is ample scope for weight saving in brakes, with a lot of new materials and techniques coming to the market in the past decade. And while some of these may appear expensive or niche, it is clear that an increasing focus on vehicle efficiency will lead to wider adoption of many of these ideas.