2 or 3 motor vs 4 motor off road capabilities

[TI4Dan]

How effective Is Tesla's off road prowess compared to a Rivian. The Rivian
having 4 motors ( one per wheel ) may have better wheel control then a Tesla
Dual or Tri motor under extreme conditions. Tesla will have to use the braking
system to transfer power to the wheel without traction, no doubt that it works
but what are the long term effects on brakes. Tesla use a open face differential
to transfer the power to both wheels on the same axle. Rivian can apply power
to all it's wheels by modulating the power to the individual motors. I am wonder
the long term use on off road conditions were wheel traction is poor for 1000's
of miles. It would seem to me that the efficiency of the drive system is higher on
a 4 motor then using the brake system to overcome wheel spin. I don't own a modern
four wheel drive system I have no idea of long term ownership abs/traction control
vehicles. Just thinking out loud.

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[Crissa]

You don't need to press return at the end of the lines, computers have been auto-wrapping text for about thirty years now.

Four is clearly better. So is indepent steering. But it's also more expensive, and requires controls we haven't really had to think about yet.

-Crissa

 

[TI4Dan]

You don't need to press return at the end of the lines, computers have been auto-wrapping text for about thirty years now.

Four is clearly better. So is indepent steering. But it's also more expensive, and requires controls we haven't really had to think about yet.

-Crissa
[/QUOTE
you found me out typing not my forte

 

[ajdelange]

I assign 2 degrees of freedom (DOF) to each independent motor. One is for forward torque and one for reverse torque. Thus the Rivian has 8 torque vectoring DOF. I Give the TriMotor CT 6.5 to 7 as it can have 2 on one wheel if the other is locked by and vary torque to the dependent wheel by releasing the brake partially. Thus the Rivian has more DOF but not by much. This allows it to do things like tank turns which the CT won't be able to do. This gives a marketing edge to Rivian but practically speaking not much of one as most people will not be doing tank turns. In fact I wonder if it will be enabled in the trucks they deliver to us unless we agree to waive some portion of the warranty.

Obviously if you reduce thrust from one of the front wheels on a CT by applying its brake you are wasting energy relative to the amount that would be wasted by reducing the torque to a motor electrically but you don't do this sort of thing at cruising speed. You do it under conditions where the Wh/mi are near 1000 anyway so that this loss to the brake is in the noise.

I think we can be sure that the Musk boys gave careful consideration to having 4 motors and concluded that there was no appreciable benefit in the space which the product is going to be used. Rivian did the same and decided there is in their application space.

 

[ldjessee]

I am hoping that once they get more offroad testing done they will offer a 4 motor or locking differentials as an option.

 

[ajdelange]

They already have locking, in fact, variable locking via the friction brake.

Evidently their studies have shown no appreciable engineering advantage to a 4th motor. But you like the idea of a 4th motor. So do I. If there are enough of us that do there may be a marketing advantage to offering a 4th motor and they may do it.

 

[TI4Dan]

I am hoping that once they get more offroad testing done they will offer a 4 motor or locking differentials as an option.

I like your idea of a locking diff or 4 motors, decades ago I almost sunk my truck while launching my boat the boat ramp on one side had gravel under the waterline, everytime I tried to get the trailer out my truck slide farther in the water. The whole back side of the truck was in the water, rear wheels up to the wheel wells under water. I did manage to get it out but it did make me think of a 4 wheel drive. Now I have owned them for years but 4 wheel drives can get you in or out of trouble if not careful, the tri motor may have better off road abilities than a dual motor, I am just speculating.

 

[ldjessee]

There are too many situations where due to off-road conditions a locking differential (where both wheels on an axle) spin at the same rate.

A 4 motor version would actually be better, since then the computer could spin each tire only as much as needed to keep the most traction.

 

[ajdelange]

There are too many situations where due to off-road conditions a locking differential (where both wheels on an axle) spin at the same rate.[/QUOTE] I'm not sure whether this is what you want or don't want but you can have it or not as you desire by differential braking and torque control. Each wheel will be able to measure slip and torque. The only difference between this and a 4th motor is that with the 4th motor you can apply thrust in either direction whereas with braking only in 1. Thus 8 DOF vs 7.

A 4 motor version would actually be better, since then the computer could spin each tire only as much as needed to keep the most traction.

That is exactly what they do with differential braking in 2 motor Teslas. I'd never be able to find them but there are some amazing videos out there of a model X with the wheel on one side on a roller set and the other against an obstacle.

 

[ldjessee]

There are too many situations where due to off-road conditions a locking differential (where both wheels on an axle) spin at the same rate.

I'm not sure whether this is what you want or don't want but you can have it or not as you desire by differential braking and torque control. Each wheel will be able to measure slip and torque. The only difference between this and a 4th motor is that with the 4th motor you can apply thrust in either direction whereas with braking only in 1. Thus 8 DOF vs 7.

That is exactly what they do with differential braking in 2 motor Teslas. I'd never be able to find them but there are some amazing videos out there of a model X with the wheel on one side on a roller set and the other against an obstacle.
[/QUOTE]

But sometimes you want both wheels to turn at the same rate, even if one is slipping. I am not sure how braking one wheel will allow it to provide any traction (even if it is slipping)? Meaning, the current situation needs even limited, fractional traction a slipping wheel provides but also do not need the available torque limited.

Right here at this point, he refers to a situation where even on road you might want locking differentials (limited grip while towing).

I admit that I might be making a mountain out of a mole hill, but I think that there will be some who will want locking differentials on the CyberTruck, even if they are not as useful or needed as they are on ICE offroad trucks, just because the perception of their need is there.

 

[ajdelange]

II am not sure how braking one wheel will allow it to provide any traction (even if it is slipping)?

The only time a wheel does not produce traction is if the slip is 0. So if the slip is finite, T is being transmitted to a wheel and r*T (r < 1) is being absorbed by the brake (1-r)*T is going to the "road".