Why are automotive brakes set inside the wheels, and not on the axles?

Question

In most (maybe all?) automobiles, the drum or disc brakes are positioned at the end of the axles, inside the wheel. This increases unsprung mass, which reduces handling performance.

Would it not be just as effective to move the brakes inboard, positioning them on the axles themselves, somewhere under the chassis in between the suspension arms? It seems that this would have the same effect, but would eliminate the brakes' contribution to unsprung mass.

I've seen it done this way on go-karts with a solid rear axle. Does the fact that automobiles have sprung suspension systems necessitate the brakes' placement inside the wheels? Are there other engineering tradeoffs at play?

Answer 1

This idea isn't even feasible on a typical front-wheel-drive car. There is no "front axle" inboard of the steering joints which the front brakes could be fitted to, and the only rotating parts of the rear axle are the stub shafts holding the wheels.

Presumably on a rear-wheel-drive go-kart, in principle you could put the rear brakes anywhere along the axle between the differential and the wheels.

In any case, the reduction of unsprung mass would be limited. If one wheel on an axle goes over a vertical bump, the vertical movement of the mid-point on the axle is half as much as the wheel. If both wheels go over a similar-shaped vertical bump, the whole axle moves the same amount as the wheels.

In a car, the brakes would also occupy space and require a bigger clearance between the axle and the floor pan of the car body - and would probably be harder to access for maintenance as well.

Answer 2

I did some more research, and it turns out that this is in fact a tried & true concept:

https://en.wikipedia.org/wiki/Inboard_brake

Other answerers were accurate in their assessments, particularly in the difficulty of servicing, which seems to be the main reason why these aren't more popular. Accessing the brakes is more difficult and requires a lift or jack, and replacing a rotor requires disassembly of the axle (a lot harder than removing a wheel).

Aside from that, some pros:

• reduces unsprung weight
• reduces exposure to environmental contaminants
• eliminates torsion forces on suspension components while braking

and cons:

• harder to cool
• increases complexity; non-driven wheels require axles dedicated to braking, and cooling channels/ducts are generally required

The cons seem to outweigh the pros, since the primary advantages are performance-related, but the disadvantages make this approach mostly unfeasible for performance (i.e., motorsports) use. Racing teams tend to work on brakes frequently and may need to change brake components quickly during practice or races, which isn't possible with inboard brakes.

I do wonder if these may make a comeback with electric vehicle (EV) racing due to the decreased demand on friction brakes from the use of regenerative braking. I can imagine a set of pads lasting an entire race, in which case a racing team could swap them between races and eliminate the need for quick access.

Answer 3

1. It would be more difficult to service/repair
2. The axle of a typical car resides within a outer tube and the surface area of the axel is much smaller than the surface area of a typical rotor or a drum.
3. The heat dissipation rate would likely be lower than conventional brakes.