Tri-motor vs Quad-motor or Hub-motor

[lancethibault]

The CT and GMC Hummer are going the tri-motor route as the top available option. The Rivian and Lordstown are taking the Quad-motor or what Lordstown is calling the Hub-motor route. It looks like Ford is sticking with a dual motor for now.

Wondering if there will be a Quad-motor route someday for most electric trucks (and cars). I'm not advocating one way or the other. I don't know jack about auto engineering, but a motor for each wheel seems to make some sense...I guess from a symmetry point of view. I guess 2 also makes sense from that perspective; one for the front and one for the back. I'm not really sure what the 3rd motor is specifically used for in the CT or Hummer. Outside the argument that 4 motors vs 3 motors or 2 motors inherently increases the odds of something failing (from a purely numbers game) what do you think the pros and cons would be? We could assume more power and speed with more motors, but that not necessarily true (see Lordstown).

I guess I'm really wondering what the cons are for 4 motors? Has anything been said by Tesla or GMC why they opted for 3 instead of 4? (I did look. I didn't find anything).

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[alan auerbach]

The CT and GMC Hummer are going the tri-motor route as the top available option. The Rivian and Lordstown are taking the Quad-motor or what Lordstown is calling the Hub-motor route. It looks like Ford is sticking with a dual motor for now.

Wondering if there will be a Quad-motor route someday for most electric trucks (and cars). I'm not advocating one way or the other. I don't know jack about auto engineering, but a motor for each wheel seems to make some sense...I guess from a symmetry point of view. I guess 2 also makes sense from that perspective; one for the front and one for the back. I'm not really sure what the 3rd motor is specifically used for in the CT or Hummer. Outside the argument that 4 motors vs 3 motors or 2 motors inherently increases the odds of something failing (from a purely numbers game) what do you think the pros and cons would be? We could assume more power and speed with more motors, but that not necessarily true (see Lordstown).

I guess I'm really wondering what the cons are for 4 motors? Has anything been said by Tesla or GMC why they opted for 3 instead of 4? (I did look. I didn't find anything).

I agree that two-to-four in-wheel motors seems ideal (fewer parts, and the possibility of limited slip and differential steering), so I'm wondering if it's ever been done.

 

[ldjessee]

From what I understand, since the motor is unsprung weight, it gets beaten up pretty badly do to shocks from potholes and the like that are survivable at lower speeds, but once you get to highway speeds, will eventually destroy a hub motor.

That is what the downside to hub motors is, atleast as it has been explained to me.

 

[showmemo]

Probably why the Lordstown truck's top speed it 80MPH.

 

[Crissa]

The third motor is like adding another gear into a transmission; the efficiency, speed, and power of an electric motor varies by how you wind the motor and what initial sprocket (or none) you place on it. Each motor has a top speed, so you use different motors with different top speeds, then use one or more at a time to change your available torque. This is all done electronically, so the wasted energy is minimal.

Unsprung weight is a problem, but modern hub motors aren't that heavy, compared to old steel wheels.

-Crissa

This is why the Zero uses a single motor but jas a low top speed and the Livewire and Energica use a small single-gear transmission to have a higher top speed. Zero tends to beat them off the line. And why Exro's Coildriver motor is very interesting.

 

[alan auerbach]

From what I understand, since the motor is unsprung weight, it gets beaten up pretty badly do to shocks from potholes and the like that are survivable at lower speeds, but once you get to highway speeds, will eventually destroy a hub motor.

That is what the downside to hub motors is, atleast as it has been explained to me.

I'm out of my depth here but picturing each wheel getting a 30" shaft that pivots at the inboard end -- where both the wheel's motor and its brake are mounted. That would reduce the unsprung weight compared with the conventional design.

 

[ajdelange]

There seem to be two questions here with the first relating to advantages (if any) of 4 motors over 3 and the second being with regard to advantages (if any) of hub motors over more conventional mounting.

Obviously the big disadvantage of a 4th motor is that you need a 4th inverter/rectifier and a more sophisricted control system that can handle 4 motors. Plus you need, in the conventional mount, another set of reduction gearing but you don't need the reduction gearing to be differential. This means that you have more flexibility in torque vectoring and can achieve it without losses (i.e. w/o differential braking or slipping cones or clutches in a LSD). Four motors means that each can be rated for lower peak power/torque and they can, thus be smaller and lighter and less expensive but there are 4 of them. Or, if kept the same size, the vehicle has, at the extra cost of an extra motor, better 0 - 60 time, higher top speed...

Clearly Rivian and Tesla have each extensively studied the 3 vs 4 motor question and arrived at separate conclusions from which I conclude that neither approach has a clear advantage over the other and that differences in the systems overall are pretty nuanced,

Lordstown is aiming at a different market. These trucks are for working folk, not soccer moms. They do not have to go far at high speed nor provide their passengers great comfort. They are more likely to be found in locations where advanced torque vectoring is required and in places where halfshafts may get in the way of obstructions under the vehicle. Obviously hub motors eliminate gear boxes (differential or otherwise). But what I don't know about hub motors if 5 - 10 times more that what I don't know about PM and IM motors.

 

[Crissa]

I think the Lordstown truck is made to sound good on paper, not to be good in practice. :/

Sometimes skimping on features is good, other times it's just to get fancy politicians to take up projects while the execs stuff their pockets.

-Crissa

 

[HaulingAss]

The CT and GMC Hummer are going the tri-motor route as the top available option. The Rivian and Lordstown are taking the Quad-motor or what Lordstown is calling the Hub-motor route. It looks like Ford is sticking with a dual motor for now.

Wondering if there will be a Quad-motor route someday for most electric trucks (and cars). I'm not advocating one way or the other. I don't know jack about auto engineering, but a motor for each wheel seems to make some sense...I guess from a symmetry point of view. I guess 2 also makes sense from that perspective; one for the front and one for the back. I'm not really sure what the 3rd motor is specifically used for in the CT or Hummer. Outside the argument that 4 motors vs 3 motors or 2 motors inherently increases the odds of something failing (from a purely numbers game) what do you think the pros and cons would be? We could assume more power and speed with more motors, but that not necessarily true (see Lordstown).

I guess I'm really wondering what the cons are for 4 motors? Has anything been said by Tesla or GMC why they opted for 3 instead of 4? (I did look. I didn't find anything).

Hub motors are almost certainly the future of almost all electric vehicles.

That's not to say the current state of the art hub motors are better than chassis mounted motors at this time. Hopefully Lordstown has advanced and will continue to advance the state-of-the-art in hub motors to the point where they actually make good sense. I'm pretty sure that point is still a number of years away.

The choice of two, three or four motors is a similar situation - eventually almost all four wheeled vehicles will have four (in-hub) motors. What currently makes the most sense considering performance, cost and efficiency will change over time. But it's not a huge defining difference - they all work well, there are just various advantages and disadvantages of each method.

 

[SSonnentag]

Hub motors also moves the weight outward to the corners of the vehicle instead of keeping it central. In addition to being unsprung weight, hub motors likely hurt handling for this reason too.

 

[ajdelange]

That would increase the rotational moment of inertia thus making the truck less susceptible to yaw from, e.g. trailer sway. This is a bad thing?

 

[Timoj]

Hub motors also moves the weight outward to the corners of the vehicle instead of keeping it central. In addition to being unsprung weight, hub motors likely hurt handling for this reason too.

But.... hub motors reduce overall weight. The issues around unsprung mass are really only an issue for top end racing performance. For a work truck the slightly more unsprung mass can easily be accounted for with competent engineering. The unsprung mass of two hub motors is going to be less than a heavy duty diff and axles. How much does a Dana weigh?

 

[rr6013]

Lordstown and Rivian are going for bragging rights. GM and Tesla are aiming at sales. Simple economics, fewer failure modes and balancing vehicle dynamics are just three win-win areas favoring GM and Tesla economy of scale approach.