Off-roading in the Rivian R1t presents a unique experience, blending cutting-edge technology with the unpredictable nature of challenging terrains. While the R1T generally handles off-road conditions well, it’s not without its nuances, particularly concerning its four-motor drive system.
Initially, the expectation might be that the R1T’s quad-motor setup, with individual motors for each wheel, would establish it as the ultimate off-road machine. The precise power distribution offered by four motors seems theoretically superior to traditional four-wheel-drive systems relying on clutches and brakes. However, real-world off-road scenarios reveal a more complex picture, especially when considering wheel spin.
During tests on Colorado’s varied trails, the R1T demonstrated competent off-road capability. Yet, observations indicated a degree of wheel spin that might be considered more pronounced than ideal in certain situations. It’s important to clarify that wheel spin, defined as a tire’s tangential velocity exceeding the vehicle’s linear velocity relative to the ground, isn’t always detrimental.
David Tracy, automotive journalist, discussing the R1T four-motor system's performance.
In scenarios like mudding, wheel spin is actually beneficial. It serves to clear mud from tire treads, enhancing grip and forward momentum. However, for activities like rock crawling, as demonstrated in the R1T tests, excessive wheel spin becomes a hindrance.
As seen in video footage, wheel spin can complicate precise navigation, causing the R1T’s front end to deviate as tires lose traction on slick rocks. Further along the trail, during a steep, rocky ascent, wheel spin becomes evident again, kicking up dirt and rocks. This not only detracts from climbing efficiency but also degrades the trail surface.
This raises a pertinent question: Could a vehicle with a simpler, single-motor setup and traditional front and rear locking differentials outperform the R1T in these specific conditions? Consider a scenario involving a steep, slippery incline.
To ascend such an incline at a steady pace, a vehicle needs a certain amount of torque at the wheels. Imagine a Jeep Wrangler Rubicon climbing this grade; maintaining a constant accelerator pedal position allows for a consistent ascent speed.
Now, if the front passenger-side wheel encounters an ice patch and loses grip, while the driver maintains pedal pressure, will the tire slip? In a traditional system like the Rubicon’s, slippage occurs only if the remaining three tires, mechanically linked to the slipping tire, cannot compensate for the lost traction.
In essence, if the gripping tires can generate enough torque to meet or exceed the total torque required for the ascent, the vehicle proceeds smoothly. The slipping tire’s tangential velocity will likely remain close to the vehicle’s speed, ensuring a controlled climb. In a simplified model, envision the Jeep’s driveline as a solid axle connecting all four tires. Torque is distributed through this axle, ensuring uniform wheel speed. Even if some tires lose traction on ice, the axle maintains motion as long as the remaining tires provide sufficient friction to overcome the necessary wheel torque for the incline. (Note: Yaw might occur depending on tire grip distribution).
How does the R1T’s system respond in the same situation? Again, picture the R1T climbing the icy incline with consistent pedal input. A set current is delivered to the motors, generating the required wheel torque for a 2 MPH ascent.
If the passenger-side tire loses all grip, wheel torque at that point drops to zero, leading to slippage. Unlike a fully-locked traditional system where torque to other wheels instantly increases, the R1T’s electronics must react. The system needs to quickly redirect more current to the wheels with grip to maintain speed.
This introduces complexities in control. How much current should be sent to each wheel? Overcorrection could induce spin in other wheels. Should the slipping wheel continue spinning at vehicle speed to regain grip seamlessly? Determining vehicle speed accurately becomes challenging when all wheels might be experiencing slip.
Mason Verbridge, Principle Engineer of Rivian’s Drive System team, shed light on these intricacies. Rivian’s system operates with a 100Hz refresh rate in the inverter, allowing torque commands to adjust rapidly – approximately 100 times per second – to correct for speed deviations.
Verbridge clarified that the R1T system isn’t designed to mimic a locker directly. Instead, “slip targets” dictate how closely it behaves like one. These targets are surface-dependent, adjusting based on the vehicle’s estimated friction coefficient to control wheel slip relative to the perceived vehicle speed.
“If all four wheels are spinning, what is vehicle reference?” Verbridge pondered. While GPS offers some assistance, precisely gauging ground speed becomes difficult with widespread slip. However, in scenarios with only one wheel slipping, the system can infer vehicle speed from the remaining three, managing slip effectively.
Rivian R1T electric truck navigating a rocky off-road trail, demonstrating wheel articulation.
Verbridge acknowledged inherent limitations. The system’s PID gain loop and control reaction time introduce delays. “It’s not gonna be perfect. How quickly can you go ‘I’m slipping. Stop.’ That’s your limit of the technology.” Although the system is responsive, wheels can still exhibit a brief “quarter turn at a time” spin before correction.
Interestingly, Rivian is exploring fewer-motor configurations for future models. The four-motor system excels in high-speed off-road scenarios and serves as a key differentiator for the R1T, contributing to its market impact. Launching with a fully featured system like the four-motor setup was a strategic move for Rivian to make a strong initial impression. However, a two-motor system might present a more balanced design for everyday users, potentially offering advantages in cost, weight, and packaging. Furthermore, it could offer enhanced off-road performance in specific low-speed scenarios, without significant compromises in high-speed conditions.
“Everything’s a tradeoff,” Verbridge noted regarding the four-motor system’s overall performance. Rivian is actively evaluating the optimal balance, questioning how much of the four-motor system’s benefits can be replicated with fewer motors and identifying the critical performance thresholds.
An engineering source outside Rivian offered additional perspective, stating, “Fundamentally, you’re absolutely correct – if your only focus is to maximise traction, 3 locked diffs will always win… No control system will ever be quick enough to truly mimic the effect of anchoring all the wheels to each other.” The source highlighted that wheel spin on surfaces like wet mud or grass could further reduce friction, exacerbating traction issues.
However, they also emphasized the trade-offs of permanently locked differentials, particularly for on-road driving. Individual wheel control via multi-motor electric drive opens opportunities for torque vectoring, enhancing agility and masking the weight of electric trucks on paved roads. But for traditional off-road locking scenarios, a trade-off is inevitable. The effectiveness of these systems hinges on the accuracy of friction coefficient and tire load estimation models, which directly influence torque distribution to each wheel.
In conclusion, the R1T’s four-motor system offers a sophisticated approach to off-road driving, providing benefits particularly in dynamic, high-speed conditions and torque vectoring for on-road agility. However, in extreme low-speed traction scenarios, the inherent response time of electronic control systems introduces a trade-off compared to traditional locking differentials. Rivian’s ongoing evaluation of motor configurations suggests a commitment to refining the balance between technological sophistication and optimal performance across diverse driving conditions.
