ReThinking The Wheel And The Brake

For the past few decades, pretty much every car on the road has utilized the same idea: a brake disc mounted to the axle with calipers that press high-friction pads onto the disc’s surface, slowing its rotation. It’s a tried-and-tested formula, one that car makers adopted from the aerospace industry as a better solution than the venerable drum brake. A team at Continental (the tire company) have been rethinking the standard way of doing things, specifically in the context of small and medium-size electric vehicles.

The focus on EVs is logical, since, in their case, deceleration is often achieved via regenerative braking using the electric motor instead of the brakes. Obviously, EVs can’t ditch the conventional brake entirely. There needs to be a redundant system for situations when regenerative braking isn’t possible, like when the battery is full and can’t accept more energy. A consequence of using regenerative braking is that the friction brakes get much less use than in a conventional car, so they tend to last a lot longer. But there is a downside to this: a buildup of rust that can impair their performance when you need to use them, according to Continental. This is an issue with cast iron disc brakes. Of course there are also the carbon ceramic discs, but not often seen outside of the hypercar crowd, due to cost.

“In EVs, it’s crucial that the driver expends as little energy as possible on the friction brake,” said Paul Linhoff, head of brake pre-development in the chassis & safety business unit at Continental. “During a deceleration, the momentum of the vehicle is converted into electricity in the generator to increase the vehicle’s range. That’s why the driver continues to operate the brake pedal — but it certainly doesn’t mean that the wheel brakes are active, too.”

With Continental’s concept, there’s a wheel rim to which the tire is mounted, and then an inner component called a carrier star — the bit with spokes that mounts to the axle. Instead of mounting a brake disc to the axle, they connected it to the carrier star, with the caliper attached to the inside. That means that the disc can be much larger than a conventional brake disc, which needs to be small enough so that there’s room to fit the caliper without impeding the wheel itself.

Their concept brake rotor is made of aluminum rather than cast iron, solving that rust problem. But Continental says there are some other advantages, too. For one thing, aluminum is lighter than iron, and any weight you can save from an EV loaded down with batteries is a good thing. It’s estimated that the weight savings is about 4.4 lbs per wheel. Second, aluminum is a good conductor of heat that’s more quickly dissipated than with a conventional set up. And finally, the larger brake disc means a better leverage effect from the caliper, which in turn means a smaller caliper is possible.

Continental has found, after the results of the initial practical tests, that the aluminum disc itself is not subject to wear, unlike cast-iron disks. With the New Wheel Concept, abrasion only takes place on the pads, and the design of the Concept makes replacing them – and the wheels – much easier.

The New Wheel Concept has yet another positive: “Because the brake disc is fixed on the outside and the brake engages from the inside, the brake caliper can be designed particularly light and stiff. The force is transmitted largely symmetrically into the center of the axle, and this has a favorable effect on the noise behavior of the brake,” says Linhoff – and that’s a side effect which is particularly attractive in a quiet EV.