The Rivian R1t has generated significant buzz, particularly regarding its innovative quad-motor all-wheel-drive system. Heading into an off-road driving experience with the R1T, the expectation was clear: a four-motor system, with individual motors at each wheel, should theoretically deliver unparalleled off-road prowess. The promise of more precise power distribution compared to traditional mechanical systems ignited anticipation for a superior off-road performance. However, real-world testing reveals a more nuanced picture.
During a recent test drive of the R1T across challenging Colorado trails, while the electric truck demonstrated commendable off-road capability, observations revealed a surprising amount of wheel spin. It’s important to clarify that wheel spin, technically defined as the tire’s tangential velocity at the contact patch exceeding the vehicle’s linear velocity relative to the ground, isn’t universally detrimental. In scenarios like mudding, controlled wheel spin is actually beneficial, clearing tire grooves and enhancing forward propulsion.
However, for precise maneuvers required in rock crawling, excessive wheel spin becomes a hindrance. As demonstrated in the accompanying video footage of the R1T tackling rocky trails, instances of wheel spin were evident, complicating precise navigation. The truck’s nose subtly veered as tires lost traction, sliding on slick rocks. Further into the drive, during a steep, rocky ascent, the R1T’s wheels spun, ejecting dirt and rocks – actions that not only failed to aid the climb but also damaged the trail surface.
This raises a pertinent question: would a conventional four-wheel-drive system, equipped with a single motor and traditional front and rear locking differentials, perform more effectively under these specific conditions? Consider a scenario involving a steep, slippery incline. To ascend this grade at a consistent speed, a vehicle requires a specific amount of torque at the wheels. Imagine a Jeep Wrangler Rubicon climbing this incline; maintaining a steady pedal position allows for a constant ascent velocity.
Now, envision the front passenger-side wheel of the Jeep encountering an ice patch, losing traction entirely, while the driver maintains consistent pedal pressure. Will this tire slip? Only if the remaining three tires, mechanically linked to the slipping tire, cannot compensate for the lost traction. In essence, if the tires with grip possess sufficient traction to collectively generate the torque needed for the ascent, the vehicle will maintain its pace smoothly. The wheel on ice, while potentially spinning faster than the others, will be largely dictated by the speed of the axle it’s attached to. In simplified terms, picture the Jeep Wrangler Rubicon’s drivetrain as a single robust axle connecting all four tires. Torque flows through this axle, ensuring all tires rotate at a unified rate. Should one or more tires encounter a low-traction surface, the axle’s rotation remains consistent as long as the gripping tire(s) can generate enough torque to overcome the required wheel torque for the climb.
In contrast, how does the Rivian R1T’s four-motor system respond? Again, picture the R1T ascending the same incline with consistent pedal input. A specific current level is supplied to the motors, producing the necessary wheel torque for a steady 2 mph climb. When the passenger-side tire loses grip, the wheel torque at that wheel diminishes to zero, initiating slip. Does the torque at the other three wheels instantly increase, mirroring a fully-locked traditional system, to maintain vehicle speed? Not instantaneously. The R1T’s electronic control system must rapidly redirect more current to the wheels with traction to sustain momentum.
The complexity escalates when considering current distribution. How much current should be allocated to each wheel? Over-supplying current to a tire with marginal grip can induce further wheel spin. Furthermore, should the slipping wheel continue rotating at a speed corresponding to the vehicle’s intended velocity, to ensure immediate engagement once grip is regained? And if so, how does the system accurately ascertain vehicle speed, particularly when all wheels might be experiencing slip?
These intricate control challenges were discussed with Mason Verbridge, Principle Engineer of Rivian’s Drive System team. Verbridge explained, “Our refresh rate is 100Hz in the inverter. Basically, our torque command can change on the order of 100 times per second. 100 times per second you can say ‘Am I going too fast, too slow.'” He clarified that the R1T’s four-wheel-drive system isn’t designed to directly emulate a traditional locker. “But basically where you set your slip target defines how close to [a locker] you are. Slip targets are surface dependent — what the vehicle expects the mu [friction coefficient] to be will change how much we allow the wheel to slip above whatever we think is vehicle reference speed.”
Verbridge further elaborated on the system’s complexities: “That does get tricky… if all four wheels are spinning, what is vehicle reference? GPS can help you a little bit, but you don’t know how fast is the ground moving beneath me, right? But in the case of one wheel we generally have a good idea from the other three how fast is the vehicle actually going, and we can keep that slip within check.” He acknowledged the inherent limitations: “There’s a PID gain loop there… a control reaction time iteration time in your way. To your point, it’s not gonna be perfect. How quickly can you go ‘I’m slipping. Stop.’ That’s your limit of the technology.” He admitted that while the system is rapid, wheels can still exhibit a “quarter turn at a time” of spin before the control software corrects wheel speed.
Interestingly, Verbridge also indicated Rivian’s exploration of powertrains with fewer motors for future models. He emphasized the genuine advantages of the four-motor system in high-speed off-road driving and its crucial role in distinguishing the R1T in a competitive market. Launching with a fully-featured, top-tier system was, in his view, a strategic move to make a significant impact. While agreeing with this approach, he also suggested that a two-motor setup might represent a more balanced design for the average user, potentially offering benefits in cost, weight, and packaging. Furthermore, a simpler system could potentially offer superior off-road performance in certain niche scenarios and only marginally reduced capabilities in high-speed conditions.
“Everything’s a tradeoff,” Verbridge stated regarding the four-motor system’s overall performance. “We’re kind of evaluating that for ourselves. How much of this can we replicate with two or three or one? Where are the real objectionable limits?”
An independent engineering source further validated these observations:
Fundamentally, you’re absolutely correct – if your only focus is to maximise traction, 3 locked diffs will always win for the reasons your 4 tyres on 1 axle analogy explains neatly. No control system will ever be quick enough to truly mimic the effect of anchoring all the wheels to each other so that a wheel can only spin once you’ve exhausted the traction available from the sum of all 4 tyres. Another significant factor there is that spinning the wheel on some surfaces, like compacted wet mud or wet grass, can polish the surface, further reduce the friction coefficient and exacerbate your traction problems.
However, the “IF” in “if your only focus is to maximise traction” is a huge if, as anyone who’s decided to drive their old-school 4×4 on-road with the diffs locked to see what it’s like can relate to!
Having individual wheel control, through multi-motor electric drive units opens up a world of torque vectoring possibilities to increase agility and disguise the mass of a heavy BEV truck on-road, but for the off-road conditions where you’d traditionally lock the diffs, there will always be some level of trade-off. The system will only ever be as good as the mu estimation and tyre vertical load estimation model, and how it correlates that to torque requests for each wheel.
In conclusion, the Rivian R1T’s four-motor system represents a significant advancement in electric vehicle technology, offering unique advantages, particularly in high-speed off-road scenarios and on-road handling via torque vectoring. However, in specific low-speed, high-traction demand off-road situations, particularly those requiring maximum traction at each wheel, traditional locking differentials may still hold an edge. The R1T’s system, while sophisticated and rapidly responsive, operates within the inherent limitations of electronic control systems. As Rivian and the broader EV industry continue to innovate, the optimal balance between multi-motor complexity and traditional drivetrain simplicity for off-road applications remains an evolving area of development.
