5 ms·

I'm also dubious. My first impression of the render is, where do the brakes go? A car can go without brakes in the rear, but front brakes are crucial in the fr

I'm also dubious. My first impression of the render is, where do the brakes go? A car can go without brakes in the rear, but front brakes are crucial in the front. This is specially true in emergency situations. I doubt an electric motor can provide enough braking power to stop any car at highway speeds in the distance required by most countries

calciphus 9y ago
Put in-wheel motors in the rear wheels and strong breaks in the front. You don't have to drive all four wheels.
- kbenson 9y ago
  Although the article does mention that they plan to put it in all wheels of some types of vehicle. A typical passenger car would have two such motors, while a heavy-duty or high-performance vehicle could have a motor in each of its wheels. So the question of how reliable braking is achieved is still open.
dsfyu404ed 9y ago
> I doubt an electric motor can provide enough braking power to stop any car at highway speeds in the distance required by most countries This really isn't a problem. A hub motor should have no problem providing more stopping force than the tire/road interface can manage. Heat is no problem either. Either dump the excess energy into a battery or just design a motor housing that can cool itself well enough. You have way more area and mass in a hub motor than in a brake rotor. You'd almost have to try to screw it up. Maximum stopping distance laws are a complete joke. Anything can stop fast the first time. It's the 5th, 6th, 7th time that's a problem. Electric motors actually have the advantage in that situation because they fade more linearly than a hydraulic system.
- hinkley 9y ago
  My research the last time I bought a new set of tires has convinced me to get new tires when I've only gone through half the rated mileage. The fall-off on braking performance on worn tires is crazy bad.
- tlb 9y ago
  Hitting the brakes in a 2500 kg car at 100 mph generates a megawatt of braking energy (100 * 1607/3600 * 2500 * 9.8 * 0.9 = 984 kW). Plausibly sized hub motors can't handle this even momentarily. Brake rotors have less thermal mass, but can get red-hot without damage.
  - deleted 9y ago
    [deleted]
  - gandreani 9y ago
    Another comment in this thread mentioned that the limiting factor is the MOSFET/Switch in the controller, instead of the motor itself
    - abakker 9y ago
      How many actuations is the motor good for? The mosfets? Brakes are great, work for a very long time, and have lots of redundancy, they are also easy to fix without diagnosing issues with mosfets, windings, switches, and wires.
extrapickles 9y ago
Some military vehicles have the brakes installed on the output shafts of the differential (see the hmmwv), but this would defeat the advantages of using hub motors in the first place. I suspect you can only install these motors on the rear wheels so the front wheels can have normal brakes.
- gandreani 9y ago
  Yes, putting brakes on shafts of differentials is probably a pain to design for. Finding space would probably be impossible for most applications of this since the vehicles would probably use Forward Wheel Drive
londons_explore 9y ago
With a PMDC electric motor, instant braking power is usually limited by the maximum current rating of ones mosfets/switches, as long as the vehicle is going fast enough. Megawatts of braking for 1 or 2 seconds is very much doable, if designed for that.
- gandreani 9y ago
  Really? I'm no expert by any means, but I'm very much interested in this field. Could you point to any readings, examples on how to design a motor for such a load?
mschuster91 9y ago
> I doubt an electric motor can provide enough braking power to stop any car at highway speeds in the distance required by most countries Directly connect the motor to a bank of resistors (maybe use the same system in an electric car that is used for AC anyway so you don't waste the heat) or into supercaps, and if you want really enormous braking power apply the power in reverse. You will need brakes though for the last few km/h and for parking brake.

calciphus 9y ago

Put in-wheel motors in the rear wheels and strong breaks in the front. You don't have to drive all four wheels.

kbenson 9y ago
Although the article does mention that they plan to put it in all wheels of some types of vehicle. A typical passenger car would have two such motors, while a heavy-duty or high-performance vehicle could have a motor in each of its wheels. So the question of how reliable braking is achieved is still open.

dsfyu404ed 9y ago

> I doubt an electric motor can provide enough braking power to stop any car at highway speeds in the distance required by most countries This really isn't a problem. A hub motor should have no problem providing more stopping force than the tire/road interface can manage. Heat is no problem either. Either dump the excess energy into a battery or just design a motor housing that can cool itself well enough. You have way more area and mass in a hub motor than in a brake rotor. You'd almost have to try to screw it up. Maximum stopping distance laws are a complete joke. Anything can stop fast the first time. It's the 5th, 6th, 7th time that's a problem. Electric motors actually have the advantage in that situation because they fade more linearly than a hydraulic system.

hinkley 9y ago
My research the last time I bought a new set of tires has convinced me to get new tires when I've only gone through half the rated mileage. The fall-off on braking performance on worn tires is crazy bad.
tlb 9y ago
Hitting the brakes in a 2500 kg car at 100 mph generates a megawatt of braking energy (100 * 1607/3600 * 2500 * 9.8 * 0.9 = 984 kW). Plausibly sized hub motors can't handle this even momentarily. Brake rotors have less thermal mass, but can get red-hot without damage.
- deleted 9y ago
  [deleted]
- gandreani 9y ago
  Another comment in this thread mentioned that the limiting factor is the MOSFET/Switch in the controller, instead of the motor itself
  - abakker 9y ago
    How many actuations is the motor good for? The mosfets? Brakes are great, work for a very long time, and have lots of redundancy, they are also easy to fix without diagnosing issues with mosfets, windings, switches, and wires.

extrapickles 9y ago

Some military vehicles have the brakes installed on the output shafts of the differential (see the hmmwv), but this would defeat the advantages of using hub motors in the first place. I suspect you can only install these motors on the rear wheels so the front wheels can have normal brakes.

gandreani 9y ago
Yes, putting brakes on shafts of differentials is probably a pain to design for. Finding space would probably be impossible for most applications of this since the vehicles would probably use Forward Wheel Drive

londons_explore 9y ago

With a PMDC electric motor, instant braking power is usually limited by the maximum current rating of ones mosfets/switches, as long as the vehicle is going fast enough. Megawatts of braking for 1 or 2 seconds is very much doable, if designed for that.

gandreani 9y ago
Really? I'm no expert by any means, but I'm very much interested in this field. Could you point to any readings, examples on how to design a motor for such a load?

mschuster91 9y ago

> I doubt an electric motor can provide enough braking power to stop any car at highway speeds in the distance required by most countries Directly connect the motor to a bank of resistors (maybe use the same system in an electric car that is used for AC anyway so you don't waste the heat) or into supercaps, and if you want really enormous braking power apply the power in reverse. You will need brakes though for the last few km/h and for parking brake.