There is obviously a threshold, that where the unsprung part of the transmission is heavier than the motor itself. That's why brakes are in the wheel and not inboard working through the axle (which would have to be massively beefed up to handle the enormous braking power). Do they reach that threshold? Probably not, but it's worth noting that they are getting closer and that as a consequence application-specific preference shifts.

Perhaps the an in hub motor could be integrated with braking and a KERS system.

Maximising regenerative braking is always a goal with BEV.

If there was a design tradeoff that allowed to somehow massively sacrifice efficiency for very low low specific mass and extreme heat tolerance, we might see the discs replaced with in-wheel "harvesting brakes" in addition to inboard high efficiency cruise motors.

But #1 I doubt that kind of tradeoff exists, at least not in sufficient magnitude, and #2 it would probably be quite uneconomic to exploit, lots of lower hanging fruits to pick first.

Off topic, but there are a few cars with inboard brakes. The Hummer H1 is one.

Because it has portals at the knuckle and finding a way to put brakes on that without massive rims would result in some janky packaging compromise. Also with the vehicle weight, wheel and tire size they run having the brakes benefit from the portal reduction sure doesn't hurt.

Then why did wheels grow from 15' to 21' on most cars in the past 25 years? I'm pretty sure that hurts unsprung weight significantly...

Bigger wheels reduce the need for suspension to keep the wheel in contact with the road, so there may be a tradeoff that makes sense being made in this case.

It's pretty much a wash. The reason wheels are bigger is for better handling as well as space for larger disc brakes. The combined total weight of brake disc, tire, hub and rim is roughly a constant due to the use of lighter materials.

It's not a wash. A common cast aluminum 20-inch wheel weighs a whole lot more than a common cast 16-inch wheel. Also, only performance vehicles get fitted with larger brakes. Many vehicles have discs that would fit fine under a much smaller wheel because they are designed with the smaller wheel as standard, and the larger wheel as an option.

Also, a larger rotor weighs more than a smaller one.

Combined, a large wheel and tire will require more energy to move and to stop, and will add as 75 pounds or more to the weight of the vehicle. This has negative impact on handling and fuel economy.

The only reason why the manufacturers add a large wheel option is because people want them.

The alternative to a cast alloy wheel is not another smaller cast alloy wheel but a steel one.

Larger wheel vs larger rim is not the same thing. A larger wheel has a larger weight but smaller rolling resistance. A larger rim on the same sized wheel has better handling characteristics and more room for larger discs, whether it is on a performance vehicle or a regular one doesn't really matter, plenty of mid-sized sedans of today are yesterdays performance vehicles.

There are outside exceptions (any SUV, ridiculously sized wheels on many American cars and people driving trucks when a normal passenger vehicle would be more appropriate). Those do have worse handling and fuel economy, but they're the exception, not the rule.

I'm going to defend my "only" as at 2kg per wheel it's about 10% of the weight of tires and wheels (presumably disc brakes are also unsprung. so total increase would be less than 10%) of a (stock) BMW 3 series with 16" aluminum alloy wheels[1].

[edit]

Disc brakes add about 9.5kg on the front[2] (maybe half that on the rear?), so a single disc brake rotor weighs more than the entire weight of all 4 motors.

Obviously this is using Magnax's numbers for very large motors and assuming they scale down, which is silly, but clearly there is some weight at which it would make sense, particularly for a non-high-performance vehicle (1 motor per wheel has lots of advantages for handling, but AFAICT this is not unique to in-wheel motors).

1: https://www.tirerack.com/wheels/tech/techpage.jsp?techid=108

2: http://www.dbausa.com/brake-discs-is-lighter-better/

I remember this being mentioned in car related articles once in a while, would love to learn more. Any sources you can recommend?