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- #16
You sure you had the front differential locked up? It's a mechanical locking diff, it's impossible for the two front wheels to spin at different speeds when it's locked in the tri-motor Hummer EV. Brakes aren't needed at all.Hummer's front diff is locking because it's only a single motor and absolutely will just spin one loose tire if it doesn't use the brakes to lock the spinning wheel up. Rivian should make enough torque at each wheel independently to not need anything which will transfer power from one wheel to another.
I expect the issue is the person driving the truck just wasn't giving it enough pedal. You'd fail with the beefiest off-road vehicle if you weren't giving it enough throttle under certain conditions. The hard part with the Rivian is the lack of audible feedback that an ICE has.
The Hummer EV requires 45 psi in those giant tires because of its heft.
https://www.aam.com/media/story/aam-s-tracrite-differentials-help-new-gmc-hummer-ev-conquer-the-road#:~:text=The GMC HUMMER EV features,turn at the same speed.
Speed/throttle is the nemesis to a lot of rock crawling. So much in fact that a good driver in an ICE vehicle two-foots over rocks, presses down on the brake pedal with their left foot to create drag and slow throttle modulation to overcome the brake friction and slowly roll over rocks as to not jerk the truck and driveline.
This is possible with something like a Jeep Rubicon that has way, way more mechanical advantage than say the Rivian. In my calculations, a Jeep Rubicon has over 31K of effective ft/lb of torque that can be applies to all four wheels simultaneous in the Diesel.
Richard Farquhar - Vice President of Propulsion - Rivian:
"
Charged: You said that there are symmetrical drive units, so you have four identical sets of motors and drives?
Richard Farquhar: That’s correct – we have four identical motors, left and right, front and rear. Every wheel has the exact same amount of torque and power available to it to maximize performance. We achieve over 14,000 Newton-meters of grounded torque at the wheels combined and 125 miles per hour maximum speed. With the fixed-ratio single-speed gearbox there is no need to change gears, no need for twin speeds. This maximizes efficiency in terms of losses from gear meshing while achieving all of our performance targets. "
14,000 nM is 10,325 ft/lb. That's 2,581 per wheel. Compared to a Jeep Rubicon which can do over 31,000 per wheel.
The mechanical advantage of something like a Diesel Rubicon that has its Engine Torque multiplied by the fluid Torque Converter multiplied by transmissions 1st gear, multiplied by the transfer case, multiplied by the differentials, gives insane torque figures at the wheels. Far far higher than Rivians small motors can accomplish.
But Rivian is more worried about efficiency than climbing over rocks. Rivian motors are actually quite small and aren't geared to put down a ton of TQ at low speed per wheel. Hence why this Rivian having a hard time with this moderate obstacle was readily apparent. This was also readily evident shown by on a cool mountain trail, the Rivian cooling fans kicked into overdrive on this obstacle as the wheels with traction touching the ground were being fed incredible amounts of power.
The Rivian motors could have been getting so hot from the near zero high current torque requests that they were being de-rated too. Rivian is also a quite heavy vehicle, it's no small matter to lift 7K lb up an incline with no momentum. Weight is the enemy of off-roading.
I believe what I am showing above will bare itself out more as the Rivian is off-roaded more and more. Inclines with rocks are going to be its nemesis.
The Rivian will most likely be limited to 3, maybe 3+ difficulty trail on the top end:
https://www.of4wd.com/education/trail-ratings/
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