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Not a mechanical engineer. This design is interesting. Do you think regulation is the only reason this did not happen yet? Why is nobody building this otherwise

Not a mechanical engineer. This design is interesting. Do you think regulation is the only reason this did not happen yet? Why is nobody building this otherwise?

londons_explore 4y ago
The main reasons against this are: * Regulation. Plenty of countries are very specific about the exact mechanicals of brakes, in laws that are 100+ years old from when runaway cars killed people. * Unsprung mass. Cars handle better if the wheels are very light and all heavy bits are in the body of the car. * Large diameter motors are harder to design - there is no opportunity for a gearbox, so you lose one design parameter, and need the motor magnetics to be right around the rim of the wheel to make up for it. * Difficulty of making the air gap within the motor very thin at such large diameters - you end up needing super large diameter roller bearings. However, I think despite those downsides, the benefits outweigh them.
- meragrin_ 4y ago
  What about potholes? Wouldn't they lead to the premature death of the motors if not the wheels themselves?
  - wrycoder 4y ago
    Also, more exposed to salt spray than inboard motors.
    - sdk16420 4y ago
      Drum brakes are in the same spot and can last for the life of the vehicle.
      - wrycoder 4y ago
        You've not maintained drum brakes in New England, I guess. Not as finicky as calipers, but the cylinders do rust and fail.
  - orangepurple 4y ago
    That's how they will make money after their current golden goose, automatic transmissions with "lifetime oil changes," is exhausted.
bradfa 4y ago
2 main reasons I can think of (tl;dr: cost): 1. Braking generates a LOT of energy which needs to be sent somewhere very quickly. Each wheel might need to dissipate hundreds of kW of power, simultaneously, on-demand, at any time in normal situations where mechanical brakes function fine today. This is not inexpensive. 2. Today, most EVs do not have 1 motor per wheel, which would be almost surely required to implement this type of system effectively. That's also not inexpensive and unless you can actually put the motor in the wheel-area you still need an axle to connect the motor to the wheel due to the suspension and steering movement, and then that axle would need to be able to sustain these braking forces. It surely could be done from an engineering point of view today, it's just going to add tremendous cost over using conventional mechanical brakes regardless of if those brakes are a traditional hydraulic type or if they're a brake by wire system.
analog31 4y ago
I suspect vibration, and the need for 4 motors instead of 1. I'll bet it'll be a long time before motors are cheaper than brakes. Also, braking involves a lot more torque than accelerating or cruising.
- btbuildem 4y ago
  I think having one motor per wheel is very useful in terms of all-wheel drive (and braking) -- being able to selectively apply varied amounts of power to each wheel depending on conditions. Not sure I'm convinced by the torque claim -- we've seen cars skidding / losing traction in both scenarios (accelerating and braking). ABS brakes were developed specifically to prevent that (and so was Traction Control). My experience with electric vehicles is limited to a Boosted longboard, so take this with a grain of sand -- but it seems to me that electric motors are as effective at braking as they are at accelerating.
  - analog31 4y ago
    I'm thinking in terms of f=ma, or the equivalent in terms of torque. I can live with my car getting up to highway speed within a much longer time duration than how quickly it needs to slow down if I hit the brakes. So the brakes need to exert more torque on the wheels than the motor does. Putting a motor on each wheel could have some performance benefits, but I'm not sure I'd pay for it if I could get a single-motor car for cheaper. I'm talking about "econo box" performance, which is all I want anyway.

Not a mechanical engineer. This design is interesting. Do you think regulation is the only reason this did not happen yet? Why is nobody building this otherwise?

londons_explore 4y ago

The main reasons against this are: * Regulation. Plenty of countries are very specific about the exact mechanicals of brakes, in laws that are 100+ years old from when runaway cars killed people. * Unsprung mass. Cars handle better if the wheels are very light and all heavy bits are in the body of the car. * Large diameter motors are harder to design - there is no opportunity for a gearbox, so you lose one design parameter, and need the motor magnetics to be right around the rim of the wheel to make up for it. * Difficulty of making the air gap within the motor very thin at such large diameters - you end up needing super large diameter roller bearings. However, I think despite those downsides, the benefits outweigh them.

meragrin_ 4y ago
What about potholes? Wouldn't they lead to the premature death of the motors if not the wheels themselves?
- wrycoder 4y ago
  Also, more exposed to salt spray than inboard motors.
  - sdk16420 4y ago
    Drum brakes are in the same spot and can last for the life of the vehicle.
    - wrycoder 4y ago
      You've not maintained drum brakes in New England, I guess. Not as finicky as calipers, but the cylinders do rust and fail.
- orangepurple 4y ago
  That's how they will make money after their current golden goose, automatic transmissions with "lifetime oil changes," is exhausted.

bradfa 4y ago

2 main reasons I can think of (tl;dr: cost): 1. Braking generates a LOT of energy which needs to be sent somewhere very quickly. Each wheel might need to dissipate hundreds of kW of power, simultaneously, on-demand, at any time in normal situations where mechanical brakes function fine today. This is not inexpensive. 2. Today, most EVs do not have 1 motor per wheel, which would be almost surely required to implement this type of system effectively. That's also not inexpensive and unless you can actually put the motor in the wheel-area you still need an axle to connect the motor to the wheel due to the suspension and steering movement, and then that axle would need to be able to sustain these braking forces. It surely could be done from an engineering point of view today, it's just going to add tremendous cost over using conventional mechanical brakes regardless of if those brakes are a traditional hydraulic type or if they're a brake by wire system.

analog31 4y ago

I suspect vibration, and the need for 4 motors instead of 1. I'll bet it'll be a long time before motors are cheaper than brakes. Also, braking involves a lot more torque than accelerating or cruising.

btbuildem 4y ago
I think having one motor per wheel is very useful in terms of all-wheel drive (and braking) -- being able to selectively apply varied amounts of power to each wheel depending on conditions. Not sure I'm convinced by the torque claim -- we've seen cars skidding / losing traction in both scenarios (accelerating and braking). ABS brakes were developed specifically to prevent that (and so was Traction Control). My experience with electric vehicles is limited to a Boosted longboard, so take this with a grain of sand -- but it seems to me that electric motors are as effective at braking as they are at accelerating.
- analog31 4y ago
  I'm thinking in terms of f=ma, or the equivalent in terms of torque. I can live with my car getting up to highway speed within a much longer time duration than how quickly it needs to slow down if I hit the brakes. So the brakes need to exert more torque on the wheels than the motor does. Putting a motor on each wheel could have some performance benefits, but I'm not sure I'd pay for it if I could get a single-motor car for cheaper. I'm talking about "econo box" performance, which is all I want anyway.