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Comment by stingraycharles | Hacker News Reader

Could you elaborate a bit more? Do I understand correctly that in today’s world of energy efficiency, the car’s mass distribution has to be carefully tuned for maximum efficiency, and hub motors throw that off balance?

Intuitively, I always figured that having mass on an object that’s very low to the ground is a good thing, but I can imagine it’s something different when it’s attached to a wheel.

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The purpose of the car's suspension - the springs and shocks - is to decouple mass from the wheels. This allows the wheels to travel up and down over bumps and dips in the road surface quickly without large forces being exerted, which in turn makes for a smooth ride and good traction between tire and pavement. The more weight you move from the 'sprung' part of the vehicle - everything that's suspended by the springs - to the 'unspring' part - directly connected to the wheels - the more this is compromised. You can think of the extreme case where you take all the vehicle's weight and move it to the wheels: then it's like you don't have suspension at all and you're bumping and jarring across every imperfection in the pavement. Moving a lesser amount of mass to the wheels is basically a lesser version of that. (Likewise anything you can do to reduce unsprung weight - lighter brakes, wheels, tires, etc. - tends to have an outsized effect on handling. (And on performance in the case of rotational weight, but that's a separate issue.))

This made me think of that peculiar vehicle where the wheel is massive and the driver sits inside. Can't remember what it's called now but I seem to remember a spoof version in South Park.

"Tom Slick and his Thunderbolt Grease-slapper".

Thanks for the explanation!

Something funny I've noticed with a few buses and large trucks is that the 'suspension' is built into the driver's seat

Seat suspension is for added driver comfort and does not replace regular suspension.

Apart from the mass distribution, if you look at a car travelling on an uneven road from the reference point of the car itself, you will see the wheels moving up and down very quickly to keep contact with the ground. My understanding is that adding mass to the wheels will slow them down and thus increase the time they don't touch the ground, reducing the grip.

F=ma, so if you increase the mass you either decrease the acceleration (they slow down) or increase the force (they hit the bumps harder). In practice it's both. The vertical acceleration when you hit a bump is essentially determined just by your speed and the size of the bump; it can't be slowed down. So instead what happens is the bump will exert a greater force on the tire (which will deform it more, reducing grip and probably meaning you'll need to run higher pressure, which also tends to reduce grip). On the other side, when you leave the bump, the force is determined by the spring rate of the suspension and the amount of compression, so for the same suspension, a heavier wheel will move back to the ground slower, which yes, will reduce grip. You could increase the spring rate to compensate, but that would exacerbate the first problem, and also wouldn't help in the situation where you hit a dip from steady state rather than exiting a bump.

Think about going over a bump. This gives the wheel a strong upward motion. The suspension has the task to discouple that motion from the car. Which is easier, as much as the wheel mass is low compared to the cars mass. Thus the aim is to keep the wheel as light as possible.

It might be worth mentioning that “because unsprung weight” argument is a chassis design argument, not a powertrain one. It applies to all engine types, including electric, gasoline, diesel, and steam engines.

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