2 or 3 motor vs 4 motor off road capabilities

[JBee]

Engineers would do a quadmotor with smaller high power CF motors, maybe we'll even get axial flux motors finally. Radial flux are so yesteryear already.

The best part is no part only goes as far until it loses all functionality too, so the compromise is to have the least amount of parts for the most amount of functionality. EM just doesn't emphasise that bit as much as the no part thing. Have alook at the cast design, one big part to replace lots of little ones. But with the drivetrain you need each wheel to rotate at a different speed to navigate a corner, so your sort of stuck with free wheeling uncovered wheels or all powered wheels.

You need to consider the whole drivetrain as an assembly of parts where parts can be removed from. Things like differentials, gears, bearings, cables and speed controllers etc.

Overall QM torque vectoring is other worldly if you have experienced it, and the best thing yet for seasoned off roaders.

PS I miss Adjelange on the forum a bit...

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[Tinker71]

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.

Great analysis.

I don't plan on rock crawling so I don't think anything extra is worth the complexity. A 7000 lb. vehicle coming down on a 3" rock knob would likely dent the structural pack. Trying to design a skid plate for this situation is not worth it in my book.

Design for 95% use case because the effort/weight/cost for the last 5% is usually not worth it.

Let the Rivian be superior here. It has the shorter wheelbase. It sure as heck doesn't have a 3500 lb. payload to manage either.

 

[Crissa]

As pointed out, a muddy track would have the same problems of shifting traction as rock-crawling and would be a far more common problem.

Some 4x4s now tend to hit their heat limit (either transmission or brakes) on rocky, muddy roads like that without ever doing any true crawling.

-Crissa

 

[HaulingAss]

As pointed out, a muddy track would have the same problems of shifting traction as rock-crawling and would be a far more common problem.

Some 4x4s now tend to hit their heat limit (either transmission or brakes) on rocky, muddy roads like that without ever doing any true crawling.

-Crissa

Absolutely! As well as climbing a steep, icy hill. Braking the opposite wheel of open differential is a good thing, but it cannot come close to the potential effectiveness of eliminating the mechanical differential and modulating power using electromagnetic forces, rather than friction on a brake disc (as transfered though a mechanical differential).

And for those who think the Rivian is superior, you will be sad when it goes away. Because Rivian is selling for $73-$100K a vehicle that costs them $150-$180K or so to manufacture. It's more like a mirage than a real competitor.

It's easy to make a nice vehicle, it's hard to make them in high volume at a price people want to pay.

 

[Gurule92]

If the marketing department is in charge, there will be a quad motor CYBERTRUCK.
If the engineers, remembering the mantra of "The best part is no part" holds sway, a Dual Motor CYBERTRUCK will be the top spec.

If there are advantages to having more motors then having another one wouldn't fall into best part is no part.

Otherwise why would we have a dual motor and not just a rwd? Because there are benefits to having 2 motors.

We just have to wait and see what those benefits are. One I can think of off the top of my head is regen. More motors = more regen. (Probably)

 

[HaulingAss]

If the marketing department is in charge, there will be a quad motor CYBERTRUCK.
If the engineers, remembering the mantra of "The best part is no part" holds sway, a Dual Motor CYBERTRUCK will be the top spec.

The superiority of independent all wheel drive is not about marketing, it's about snow and ice capability, safety, manueverability, tire longevity, efficiency, and the list goes on. It's just a much more elegant way to apply power to the different wheels when compared to a crude mechanical differential.

When cars were first being developed, would you say if the marketing department is in charge, cars will have four wheels, four brakes, and a rear differential, but if the engineers are in charge, remembering the mantra of "The best part is no part", then cars will only have 3 wheels, one brake and a solid rear axle? Because that eliminates a differential and an entire wheel and three brakes?

The mantra "The best part is no part" should not be misused, or we would not have anything at all.

 

[Crissa]

If there are advantages to having more motors then having another one wouldn't fall into best part is no part.

Otherwise why would we have a dual motor and not just a rwd? Because there are benefits to having 2 motors.

We just have to wait and see what those benefits are. One I can think of off the top of my head is regen. More motors = more regen. (Probably)

Well, the advantage of two motors is mostly 'It's cheaper and more efficient to do AWD this way than a transmission'. Motor or transmission? Which has more moving parts? More moving parts more bad. Which is heavier? More heavy is more bad. Etc. And the parts and heaviness comes with direct costs, too. It's just metal.

Tho regen is usually limited by the battery, not the number of motors. There's a tiny more you can get from the front axle vs the rear, but that's not going to increase at four.

And for those who think the Rivian is superior, you will be sad when it goes away. Because Rivian is selling for $73-$100K a vehicle that costs them $150-$180K or so to manufacture. It's more like a mirage than a real competitor.

It's easy to make a nice vehicle, it's hard to make them in high volume at a price people want to pay.

Hopefully they fix their problems, because it's a cute truck. Too expensive, but cute.

-Crissa

 

[Tinker71]

Absolutely! As well as climbing a steep, icy hill. Braking the opposite wheel of open differential is a good thing, but it cannot come close to the potential effectiveness of eliminating the mechanical differential and modulating power using electromagnetic forces, rather than friction on a brake disc (as transfered though a mechanical differential).

And for those who think the Rivian is superior, you will be sad when it goes away. Because Rivian is selling for $73-$100K a vehicle that costs them $150-$180K or so to manufacture. It's more like a mirage than a real competitor.

It's easy to make a nice vehicle, it's hard to make them in high volume at a price people want to pay.

I think that is an old figure. They have done layoffs and increased volume since then.

 

[Tinker71]

I agree. If you can't go down something in complete control don't try going up. A well balanced RWD vehicle will get you by 80% of the time in moderate snow and mud. Add single wheel braking and traction control and that improves.

A dual motor arrangement is probably optimal for cost and performance for 95% of us 99% of the time. Getting rid of the front to back drive shaft is a must though.

 

[HaulingAss]

I think that is an old figure. They have done layoffs and increased volume since then.

OK, by your reckoning, how much do you think Rivian is losing on each truck they sell? On average.

Anything greater than zero is not sustainable. But I would be interested to see how close to break-even you think Rivian is after almost two years of production. Do you think they can build millions of them?

 

[Tinker71]

OK, by your reckoning, how much do you think Rivian is losing on each truck they sell? On average.

Anything greater than zero is not sustainable. But I would be interested to see how close to break-even you think Rivian is after almost two years of production. Do you think they can build millions of them?

I don't have figures and don't follow them that close. I bet they still lose $5-10 k per vehicle which is not sustainable at all.

Putting old doomsday figures out there will not help them at all. It is not as bad as it was.

I think they are learning and I hope they don't go under. Their commercial vehicle sales are in the mix as well.

 

[HaulingAss]

This isn't about out-performing ICE, it's about using real innovation and hard work to make the most functional trucks possible, at the most affordable prices possible. Yes, they will be better than ICE, much better. Replacing the gas motor of a typical 4x4 truck with a single electric motor, using the same transmission system would cause the efficiency and range in 4-wheel drive mode to plummet due to how inefficiently it transfers power.

I've been wood-cutting in my 2010 F-150 4x4 for the last few days and I can tell you the 4x4 drivetrain seems positively archaic, primarily because all wheels are driven by one motor using a series of shafts and gears and differentials. But there is no differential between the front and rear wheels so the front and rear wheels can bind against each other during turns. This system is chosen because it is simple and has a high load capacity at a relatively low cost. I would not characterize it as "excellent" unless I had nothing better to compare it to. It's the same system I used in my youth, 40 years ago, with the exception that I no longer have to go outside to lock the hubs when I need 4x4 because it has electric locking hubs.

Simply replacing the one gas motor with two electric motors, and using open differentials on the axles would be a huge improvement. Try doing that with gas motors! The next step, and a huge improvement, would be to eliminate the differentials altogether by using a dedicated electric motor for each wheel.

ICE 4x4 trucks are archaic and have serious limitations and disadvantages and expenses compared to a 4x4 truck with two or four electric motors.

 

[Crissa]

No. Well, sure, but...

You literally cannot put four internal combustion engines in a car and even if you did, they wouldn't give you minute positive and negative traction control.

I don't know why this is an argument. An electric does away with many of the moving parts between the ICE and the axle, such as the transmission, drive shaft, etc. All those things were where there's slop and wear and lost energy. Some EVs even get rid of the differential and make the axle no bigger than the wheel itself.

-Crissa

 

[HaulingAss]

I don't have figures and don't follow them that close. I bet they still lose $5-10 k per vehicle which is not sustainable at all.

Putting old doomsday figures out there will not help them at all. It is not as bad as it was.

I think they are learning and I hope they don't go under. Their commercial vehicle sales are in the mix as well.

I didn't use "old doomsday figures", I was using the financials reported for the most recent quarter. They included the costs of selling the vehicles like selling and administrative costs. Also the buildout of their charging network (which is a relatively minor expense so far).

You say "doomsday figures" will not help them. What do you think will help them produce vehicles efficiently enough to compete with Tesla? At similar stages of production, Tesla was losing 10% as much as Rivian when accounting for all corporate costs. That's just looking at the facts objectively.

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