Unsprung components need to rotate in order to transfer energy, ...
True for the unsprung parts that rotate but not for the brake assemblies etc. They do not rotate.
...the lighter that component is, the less energy it will take to turn it.
The power delivered to the wheel goes mostly to accelerate the vehicle or move it up a grade(at least that's what we design for) and is stored as the vehicle's kinnetic (m*v*v/2) and potential (m*g*h) energies that I mentioned in my last post. But some goes to warm bearings, some goes to warm the tire friction patch and sidewalls, some goes to warm the air (drag) and some goes to the kinnectic rotational energy of the wheel/tire assembly (I*omega*omegea/2) and half shafts, gears and rotors (brake and motor). The part that goes to the wheel/tire depends not only on the mass but on how it is distributed. The farther from the axle, the more a given mass adds to I. Thus a 22" wheel inherently has a bigger I than a 20" wheel of the same weight. Even were the 22" wheel were the same weight as the 20" it would require more energy in coming up to speed. BUT a portion of that rotational energy is recoverable by regenerative braking just as their translational kinnetic and potential energies are. And the key word in that sentence is "portion". If you put an extra 100 lbs in the trunk an drive it up a hill and then back down you will not recover all the potential energy you had to put into it and if you use bigger wheels you will not recover all the kinetic energy you had to put into their rotation to get them up to driving speed. Thus a larger wheel is inherently less efficient than a smaller one from this perspective and it has nothing to do with the fact that the wheels are unsprung weight.
You add 50lbs to your wheel setup you’ll feel it much more vs having a 50lbs child in the vehicle.
Accelerating along a smooth bit of straight road I would expect, with a larger wheel, less available acceleration for exactly the reasons given in the last paragraph. At 50 mph a child contains a fair amount of kinnetic energy and thus the system (vehicle and child) contains more kinnetic (translational) than it would were the child not in it. Similarly a child free vehicle with 22" wheels contains more rotational kinnetic energy than one with 20" wheels (if the wheels weigh the same) and perhaps a bit more translational as well if the wheels are heavier. As more energy has had to be put in acceleration at the same available power would have been more sluggish. Were I to add 50 lbs to, say the brake caliper, I would not notice any difference relative to the 50 lb child simply because the weight is unsprung.
If you like to understand it further google “unsprung weight” you’ll even find a wiki page on it.
I used smooth straight road in the last paragraph because when the road starts to make demands of the suspension there are effects related to the suspension design, including the ratio if sprung to unsprung weight that could indirectly effect consumption. This sort of thing is mentioned in that article the thrust of which is that there are, surprise, surprise, trades between ride comfort and handling that depend on the unsprung to sprung ratio.
Sport car manufactures spend loads of money to reduce unsprung weight and consumers also spend to reduce unsprung weight. Someone with a $60K car spend $10K on wheels to reduce 10lbs.
Most consumers are not engineers and get taken in by various marketing ploys. Most consumers are not engineers. If you read the article you referenced you should take away from it that suspension design is pretty complicated. If the notion can be spread among the consumer base that unsprung = bad then a manufacturer can sell more cars by advertising that he has lowered the unsprung weight. Most consumers won't understand the trades or even appreciate that too little unsprung weight can lead us into parts of the trade space where we don't want to be in some applications.
Go to Tesla.com if you select 22” wheels on your model X performance it drops from 305 range to 270 range. You are telling me for every 50lbs passenger you put in the vehicle you are going to lose 10% range?
I am telling you that a 22" wheel has a rotational inertia that is
at least 21% greater than a 20" wheel merely by virtue of its greater diameter and probably greater than that because it probably weighs more. Thus more power is required for the same acceleration. This may be why you think having the 22" wheels makes you feel as if you have 50 lbs extra load on board. I'm a little skeptical about this though because I don't have any numbers on the kinnetic energy stored in a rotating tire of either size but fully expect that it is small relative to that stored in the translational energy of the 2500 kg vehicle. But then the translational kinetic energy stored in a 50 pound (22 kg) kid is very small relative to the kinetic energy of the car.
I am also telling you that the energy losses associated with recovery of wheel rotational kinetic energy are higher with 22" wheels than with 20" because of the physics. Again, I doubt that this is responsible for the 8.8% increase in Wh/mi because, again, I expect rotational kinnetic energy in the wheels is a small fraction of the total system kinetic energy.
Bottom line here is the 22” wheels put a great strain on the vehicle, reduces range, wears out brakes, effects bushing, shocks, causes more vibration throughout the vehicle loosing things, more road noise and much more.
High unsprung weight can put stresses on brakes and clearly shocks but we don't use the brakes in our Teslas. High unsprung weight reduces vibrations (low pass filter effect) and road noise (at least theoretically).
I think the real bottom line here is that the increased energy demand with the 22" wheels comes in small part from the increased rotational inertia of the larger wheel but probably mostly from the fact that different tires are used - tires that have more rolling resistance and thus dissipate more heat in the friction patch and sidewalls. The fact that there is a little more unsprung weight would seem to have little to do with it.
I have both the 20s and 22s and find myself always having the 22s on. For me the looks outweighs the disadvantages lol.[/QUOTE]
