Motor-in-wheel vs. motor-in-hub

[Silveredwings]

Given the articles from the thread about these kind of motors, in eCorner thread they seem a logical design choice, but which one would be better?

[Silveredwings]

Pro/Con configuration comparison of motor-in-wheel (Siemens, Porsche) vs. motor-in-hub (Mitsubishi):

MIW pros:
-It's an easily replaceable or upgradable unit.
-The design is likely reusable on other platforms, vastly increasing developmennt ROI.
-Allows for large diameter motor easily giving high torque.
-Potentially easier to cool.

MIW cons:
-Still needs modification of the hub for transferring elec. power and torque.
-More difficult to isolate from damage because of dirt, vibration, and impact.
-Obsoletes a whole cottage industry of aftermarket wheels.
-May interfere with, or must be integrated with design of brakes.

MIH pros:
-Virtually unlimited choice of wheels.
-Easier to isolate from damage.
-Some design work might be transferrable.
-Well suited for mass production.
-Easier to transfer elec. power and torque.

MIH cons:
-Not well suited for one-off custom conversions.
-Hub parts development not universally reusable.

[Silveredwings]

Factors that apply to both:
-Potentially expensive replaceable unit.
-Probably similar unsprung weight (maybe favors MIH).
-Probably equivalent CG.
-Not yet available on new cars let alone aftermarket.
-Once available as an oem part, would significantly reduce manufacturer design changes among car platforms.
-If available aftermarket, would make an EV (or HEV) conversion so much easier.

[rvanengen]

Let's not forget the theft angle... :-) How much money do you want to lightly bolt to the side of your car?

[Silveredwings]

True... I admit it, driving on public roads is a risk. With the already high costs of alloy wheels and high end tires, locking lug nuts have been a normal accoutrement on cars for over 25 years. Wheel-motors would certainly add to the road-going expense, but I think the lug-locks would sufficiently secure wheel-motors (for which there could eventually be a black market). Also, there's only so much I'm willing to do to safeguard my insurance company. If someone wants part of my car badly enough, they'll take the whole thing on a flatbed somewhere and harvest it in peace (in piece? ) with a torch.

That all said, putting motors into the hubs would be my first choice anyway. The major advantage of the wheel-motors is the ease of replacement, but I think AC or brushless DC motors are pretty reliable and shouldn't need replacement much more often than a wheel bearing.

If an EV used regen braking well enough on all 4 wheels, they could conceivably use a modified brake rotor as the basis for a motor. In order to provide brake redundancy (for emergency use only), you could still have drum brakes within the 'hat' portion of the rotors. The vast majority of the time, the braking action of the motors would prevent the smaller drums from overheating and fading (see the QED Mini project they even put the inverter electronics in the wheels with only small unsprung weight gain).

[Bill in Houston]

Toyota made a 4wd hybrid version of a JDM minivan, in which the rear wheels were driven only by electric motors. Was it MIH or something else?

[Lug_Nut]

Pot-hole$
$peed bump$
Curb ra$h

[JanGeo]

Problems is the the unspring weight add more suspension dampning requirements and the electrical connection now has to allow for flexing of the conductors. Generally it is not a good idea to put weight in the wheel (unsprung weight) because on bumpy road conditions it tends to NOT stay on the road providing the control and traction needed for safe operation. Why do you think SUV's get into trouble ?? heavy wheels and 4 wheel drive heavy axles at high speeds with soft suspensions that can't keep the tire on the pavement!!!

[Silveredwings]

Originally Posted by JanGeo

Problems is the the unspring weight add more suspension dampning requirements and the electrical connection now has to allow for flexing of the conductors. Generally it is not a good idea to put weight in the wheel (unsprung weight) because on bumpy road conditions it tends to NOT stay on the road providing the control and traction needed for safe operation. Why do you think SUV's get into trouble ?? heavy wheels and 4 wheel drive heavy axles at high speeds with soft suspensions that can't keep the tire on the pavement!!!

I'm confused (or maybe too tired): Firstly, I'm not sure the unsprung weight (UW) needs to actually increase much at all - as proven with the EV Mini with the MIH configuration. But regardless, if you actually did increase the UW, why would you need to increase the damping of the sprung weight (SW)? If the increase resulted from trading SW for UW, I would think there would be less requirement for damping.

As for the dynamics of keeping heavy wheels in contact with bumpy roads, I agree. In the extreme, you would not want to increase the UW. I'm just not considering making an off-road or high-speed challenger (yet), and there are ways of minimizing UW in MIH. If the amount of increase in the hub weight was offset by the loss of the whole drive-shaft/CV-joint assembly, it might be a wash.

BTW, I think SUVs also get into trouble because of their high SW resulting in a very high CG - a deadly combination when speeding downhill, slamming on the breaks or popping a Firestone, and swerving. They'll almost always roll.

[JanGeo]

When a heavy wheel hits an elevated bump in the road it is forced upwards at essentially the same speed as a light wheel. Pushing the wheel back down is the spring which has a stiffness/spring constant designed to support the "sprung" weight of the vehicle. Since the weight of the vehicle could actually be less with a hub or in wheel motor the spring could even be lighter but if the same it now has to stop a heavier mass of the wheel and push it back down on the ground. The only way to reduce the distance this heavier mass travels from hitting the bump without using stiffer higher spring rate springs is to increase the bound and rebound dampening which would reduce the speed the wheel would reach when the bump is hit. Also keep in mind that the compressed spring applies more force to the sprung vehicle forcing it upwards and the heavier mass wheel applied more force than lighter wheel and for a longer time since it takes longer to stop the wheel from moving upwards and it travels further upwards which compresses the spring more. This means you "feel" the bump more and the tire is off the ground longer. Now if you increase the rebound damping the force exerted on the vehicle is reduce during the downward travel of the wheel because the shock is being extended holding the spring compressed and lets it decompress more slowly again keeping the tire off the ground longer but giving a smoother ride.