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Comment by almost_usual | Hacker News Reader

> It won’t void your warranty.

Wow, that is truly surprising. I’d be more worried about the driver if I were Rivian? What stops someone from driving through a creek and submerging their truck? Damaging their undercarriage on a rock? Sliding into a tree? etc.

> The Rivian was designed, and is, an incredibly competent off road vehicle.

It looks like it doesn’t have locking differentials but that’s usually overkill.

> It literally has an Off-Road drive mode with 4 different setups within it

I’ll need to read more about the “Off Road” drive modes. It sounds similar to the Tacoma’s “Crawl Control”.

> a built in air compressor in the truck need for adjusting your tire pressure when you take it off road.

An air compressor is nice but you can pick up a Tsunami Air Compressor and a tire deflator for like $130.

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It doesn't have differentials at all ...

Each wheel has it's own motor.

I see so slips are impossible to prevent, this seems like a big limitation for serious off roading.

https://www.rivianforums.com/forum/threads/quad-motor-vs-loc...

The Tacoma Off Road Crawl Control is somewhat similar but uses ABS to prevent slips. It does come with a rear locker though.

I'm not an expert here, but what stops four wheels from all rotating at the same speed with independent motors?

From the link:

> If the axles are not physically tied, there is no way for any of the four wheels to know how much traction it actually has until it breaks free. Then, of course, it knows "too much" torque and it can apply traction control. But the damage is done. It has broken free, and any negative consequences have already happened prior to the computer being able to make any further calculations.

> Again, if physically locked together with large gears and drive shafts, there is no possible way for one wheel to break free in that manner, unless of course something is broken.

All four wheels rotating at the same speed is the normal case: straight line driving on good pavement.

The real question is can all the wheels go to zero traction at the same moment and thus the computer thinks all is well. I think the odds of that are low enough to not worry about.

Traction control systems also use accelerometers and gyroscopes, so the scenario you describe is not possible.

Well, yes, but you can also sense which ones are turning slower and just increase the torque to those. Locking diffs simply provide torque to the wheels with grip, which is exactly what one motor per wheel can do too. There’s no difference in the real world, and in fact performance is even better because there’s no turning circle penalty from having a locked diff.

> Well, yes, but you can also sense which ones are turning slower and just increase the torque to those.

From the link:

> Again, if physically locked together with large gears and drive shafts, there is no possible way for one wheel to break free in that manner, unless of course something is broken.

Some other interesting tests by an R1T owner.

https://www.rivianforums.com/forum/threads/quad-motor-vs-loc...

I mean, doesn't the set of computer controlled individual motors completely and totally contain all scenarios presented by a locked differential?

Specifically, wouldn't one line of code that says "never ever ever turn this wheel faster than this other wheel" be totally and completely indistinguishable from having the wheels on the same axle, as far as the physical world is concerned?

That's not how electric motors work.

You can measure the speed and adjust the power going to the motor if you go over or under that speed. The lower latency that this cycle has, the better, but it can never go to zero like with physically connected wheels.

Pedantry: The speed of sound in steel is about 5100m/s, so a solid 2m axel will still take about 4µs for interactions to propagate from one end to the other.

Maybe so but what does that have to do with anything?

A physical axle can have both wheels break free too can't it?

Comparing apples to apples the only scenario you're concerned with is where one wheel breaks traction and the other doesn't. Why is a logically driven exact speed match between wheels inferior to a physically linked speed match between the wheels. Explain the difference practically speaking.

You said "totally and completely indistinguishable".

Practically, electric motors can't generally transfer torque from one wheel to another like you can with locked hubs. (The only exception would be where your battery is underrated to provide sufficient power to both motor to maximize their torque)

It isn't surprising that off road is covered in warranty. They may put limits in, but look at truck ads: most of them are showing people towing a heavy load, hauling big things, or driving off road - often more than one of the above. Sure that isn't how people actually use them (most of the time), but that is how they sell them. I haven't looked at Rivian's ads, but if they show their trucks doing something in an ad, then a lawyer will have no problem convincing the courts that because it was in the ads the warranty fine print was a misprint and that is acceptable use.

Off-road cars have limitations. I'll bet the Rivian just like other 4wds will have a maximum wading depth for water crossings.

If the door seals are good there's not a maximum wading depth, rather a maximum time paddle boating in water before seals give way | river current moves the 4x4 too far.

See: https://www.youtube.com/watch?v=LC5ld79joIA

Note that the Land Cruiser is bouyant in the water and not touching ground.

How did the driver turn??

Back wheels spinning in water create weak driving force forward.

Front wheels can turn and provide rudimentary front rudder effect wrt force from rear wheels pushing forward.

If 4xwheel drive was engaged the front wheels wouls also be driven and provide a "clawing forwards" directional force.

It's not the ideal boat, but it floats and works up to a point (as seen).

With the engine weight forward it's entirely possible the front wheels were occassionally touching down when over the pan of the crossing.

When the vehichle went to the left of the crossing it's unlikely the front wheels had any contact as the crossing (unflooded) is typically higher than the river bed on either side.

Looks to me like mostly momentum and current carried him to a spot where his front tires touched bottom at which point he was able to spin around. He was pretty adrift until he hit that spot.

https://www.youtube.com/watch?v=T21njN2HJgY

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