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.

> 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.

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.

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.

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

That design doesn't look like it has any springs anyway, so the whole thing is a giant unsprung mass, :p

Every time the idea comes up, "unsprung mass" is thrown like a pavlovian reflex. I'm curious about what they have in mind for solving the constraints. They aren't the less experienced people around.

They probably have nothing in mind. This isn't something they're planning to make so much as something that would be cool if the technology existed. The fact that in-wheel motors are put on the same level as "detecting wear and damage", "micro-channels containing 'healing chemistries'", and "supercapacitors out of carbon fiber panels that can be used to form the body of the car" gives an indication of how likely this whole affair is.

This is a concept. It's a bunch of designers sitting in a room thinking "wouldn't this be cool?"

I'd be surprised if any engineer had any input on this process.

Large outrunner motors like Emrax aren't much heavier than typical cast alloy wheels. Assuming wheel-motor eliminates disc brake components (sounds scary, right?) assembly will probably be even lighter.

http://emrax.com/products/emrax-268/

At least on a performance vehicle, that's still far too heavy IMO. 40ish lbs? The (cheap) wheels on my Miata are 11 lbs, without any exotic materials or crazy engineering. Almost quadrupling the unsprung mass is a hard sell performance-wise.

Which gen Miata? IIRC NA had something like 5.5x14" wheels which are tiny. But if we look at bigger production car wheels even modest 8x18" could weigh > 30lbs. Second, (unsprung mass/sprung mass) ratio is the thing that counts, not the mass itself. So, while 40 lbs wheel is terrible for the Miata, it might be quite ok for the vehicle twice Miata's weight.

Thanks a ton for that

Looking at those wheels, and all the fancy material science speculation, they probably expect to use the wheel structure as the motor. If the wheels themselves are acting as coils, then there isn't as much extra unsprung mass.

The wheels themselves can not act as coils because of eddy currents.

and the body can't act as a giant capacitor because it would guarantee the car would explode spectacularly if it ever hit something with enough force to compromise the body panels :-)

I think some designer pooped, err popped, out this 'look' and said "Ohh, I love how futuristic this looks!" let's have a thought shower on how many amazing technologies it might contain without constraining ourselves to reality.

Generally such exercises are useful for helping engineers think outside their generally conservative and self imposed parameters. Even when the actual concept is not really a viable design.

> Having 4 individual motors would be a first (I think) for a production car

It looks like the Mercedes SLS Electric Drive already had it back in 2013: https://en.wikipedia.org/wiki/Mercedes-Benz_SLS_AMG#SLS_AMG_...

Doesn't look like they were integrated into the wheels, though.

Also, I take a slight issue with calling a vehicle that cost over half a million and sold less than 100 units a "production" car. :P

I think 100 kWh batteries and dual-motors should become standard in all EVs within the next 10 years. The second motor should be of huge benefit in snow for only like a $1,000 more (10 years from now).

Four motors will probably offer diminishing returns for "passenger cars", but sure I could see them become standard in higher-end cars, mainly because of that cool factor as well as the performance/acceleration one.

For buses, trucks, semis, tractors, and whatnot, four or more motors will likely be a requirement.

Why do you think 100kWh batteries will become standard within 10 years? Most cars could do fine with 200mi which would really require about 50-60kWh max - assuming a standard sedan and not a sports car.

Tesla is really on the vanguard here. I wouldn't expect the highest-end Tesla configuration to be standard until at least 10 years later unless China or other countries (current US administration/congress will definitely not) really push the ball forward from a regulation/support standpoint.

My money is on electric motors proximally mounted to the wheels which are in turn connected with constant velocity joints. This also opens up the avenue for four wheel steering and increased maneuverability if all four wheels could be turned arbitrarily while power is modulated to each wheel individually.

Unsprung weight would make this a terrible car to drive - it would handle like a truck at best. The motors should be mounted where differentials currently go. Four motors, with an axle out from each.

Reading this, I'm reminded how little I understand why so many car people aren't excited about electric cars: all sorts of engineering problems to re-think and ideas to try out.