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Comment by stoev | Hacker News Reader

Also, an in-wheel electrict motor would:

1. suffer a lot more from wear and tear than a motor mounted to the chassis.

2. restrain the amount of space available for brakes. While it would be reasonable to say that a small city car could forego its rear brakes and use electric motors instead to deccelerate, coupled with brakes at the front, a four wheel drive supercar with four electric motors would have no chance relying simply on the stopping power of those motors and no brakes.

3. be very difficult to connect to a powerful cooling system. Even electric motorcycles with 30-40 bhp rely on water cooling, let alone a car 20 times the weight.

4. need serious protection from water damage.

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> Even electric motorcycles with 30-40 bhp rely on water cooling, let alone a car 20 times the weight.

FYI my Zero SR motorcycle, which has 70+hp, uses passive cooling only.

I think you just showed me what my next major purchase will be. Those look awesome!

A lot more wear- it's seriously nontrivial to make large flexing electrical connectors that will last through the stress cycles of loading and unloading the wheel. It also makes cooling waaaaaay more difficult for high power motors. Liquid is obviously challenging but air is also a huge problem- you run out of space for sinking heat. There's lots of air in that area but it's very, very challenging to prevent it from becoming turbulent and stagnant. Wheels are a big aero drag and it's a very hard choice to add more drag there.

Just to reinforce the brakes message: The braking power on the Bugatti Veyron is in excess of 1.1 megawatts. Braking requires 30% more power than accelerating.

>be very difficult to connect to a powerful cooling system. Even electric motorcycles with 30-40 bhp rely on water cooling, let alone a car 20 times the weight.

This is true- Teslas besides the model 3 and several bikes use liquid cooling. I want to point out that this is not a necessity for motors <200 kW though. Manufacturers are uniformly switching to IPM motors (aka PM synchronous reluctance motors), including Tesla. These motors are much more efficient and allow for cooler high power operation. For instance the Chevy Bolt's 150 kW motor (a weird but decent design) is fully enclosed and cooled pretty much passively.

Previous motors were either inefficient at high power (induction motors) or inefficient at low power (PM motors)- in order to reach low-power efficiency they had to be undersized and required extreme cooling to reach high power.

I’m car illiterate. Why is the braking power proportional to the engine rather than mass or drag or something? Or did i misunderstand?

It's a rule of thumb for "normal" cars.

If anything it takes less power to stop just as fast as you accelerate because of this wonderful thing called the atmosphere.

The problem is that stopping as fast as you accelerate is still too slow to be satisfactory in a lot of cases.

On low end cars that don't have enough power to roast the tires at any speed in any gear you need more braking power than engine power because you need to be able to use 100% of available traction to get from 60-0 for obvious "think of the children" reasons.

Nobody cares if your Yaris doesn't have enough engine to use every available bit of traction to get from 0-60.

Basically "you should be able to brake 30% harder than you accelerate" is a good rule of thumb for economy cars.

When you start talking high horsepower to weight ratios or other vehicular "extremes" rules of thumb don't really work.

Braking power is proportional to mass and velocity. Larger mass moving faster needs bigger brakes.

Electric motors can be used to provide some braking via regeneration, but even so braking generally comes down to "how much heat can you dissipate". A small, slow (relatively speaking) city car won't have as much heat to dissipate, so there are more options when braking.

I'm not sure if that's what the great parent meant but think about two sports cars on the race track: on every lap at the end of the straights, the better accelerating car will be going faster and therefore need better brakes to slow down to make the next corner.

In case people are interested, a prior HN submission, Maker of In-Wheel Electric Car Motors Goes to China[1], had some interesting details on in-wheel motors.

1: https://news.ycombinator.com/item?id=15570079

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