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When designing electric vehicles, the smallest components can make the biggest difference. Bearings—those deceptively simple, rolling mechanical elements—are among the most critical for drivetrain performance. In this teardown analysis, Munro & Associates Lead Engineer Paul Turnbull compares the bearing strategies used by Volkswagen and Tesla in their electric drive units. Each manufacturer aims to minimize friction and maximize durability, but their methods vary significantly. This post explores their engineering tradeoffs, machining choices, and how those decisions influence EV performance.
How EV Drivetrains Work
Before diving into bearings, let’s set the stage. An electric vehicle’s drive unit contains an electric motor, gearbox, differential, and drive shafts. The motor generates torque that must be efficiently transferred through the gearbox to the wheels. To reduce energy loss, every component—especially the bearings and gears—must operate with minimal friction and high precision.
Volkswagen’s system uses a lay shaft configuration with a gear reduction of approximately 9:1. This configuration delivers torque efficiently to the differential, enabling wheel speed differentiation for cornering. Tesla’s design, while similar in layout, opts for different bearing and housing solutions to optimize for cost and performance.
Bearing Basics: Ball vs. Tapered Roller
Bearings reduce friction between moving parts by using rolling elements—balls or rollers—between an inner race (attached to the shaft) and an outer race (fixed to the housing). There are three main bearing types relevant here:
- Ball Bearings: Lower rolling resistance, better for high-speed and light-load applications.
- Tapered Roller Bearings: Handle axial (thrust) and radial loads better but generate slightly more rolling friction.
- Journal Bearings: Not used in this teardown, but worth noting—they operate using a film of fluid instead of rolling elements.
The choice between ball and tapered roller bearings involves a tradeoff: ball bearings are more efficient under low load, but roller bearings offer better load handling at the expense of slightly higher losses.
VW’s Approach: Precision and Durability
Volkswagen takes a meticulous, precision-engineered route. The company’s use of tapered roller bearings in their EV lay shafts offers a robust solution for handling high torque and axial loads from helical gears. This choice improves reliability during demanding driving conditions.
To reduce oil churning losses—a common inefficiency in traditional internal combustion engine gearboxes—VW positions the lay shaft outside of the oil bath. Instead, it receives a controlled oil trickle, maintaining lubrication without energy-wasting splash.
What truly sets VW apart, however, is its precision machining. Tolerances are tightly controlled through:
- Polished bearing races
- Accurate shim placement
- Consistent center distances
This results in what Turnbull describes as “the lowest loss drivetrain” seen at Munro’s teardown facility. Despite using higher-friction tapered bearings, VW compensates through impeccable manufacturing accuracy, delivering a drive unit with remarkably low overall losses.
Tesla’s Approach: Cost-Conscious Innovation
Tesla takes a different route—particularly evident when comparing the Model S Plaid and Cybertruck drivetrains.
On the Plaid, Tesla uses ball bearings on the lay shaft. These offer lower spinning losses under light load but do not handle axial loads as well. To accommodate this, Tesla uses more robust bearing assemblies and a press-fit steel sleeve within the aluminum housing. This method ensures tight tolerances and a precise gear center distance, although it’s more expensive to manufacture.
However, with the Cybertruck, Tesla made a notable change:
- Slip fit replaced the press-fit, reducing machining cost and assembly complexity.
- To prevent the outer bearing race from spinning (a risk with slip fits), Tesla added a spring-loaded pin that locks into a small notch on the bearing race, securing it in place.
This innovation is clever—maintaining low friction with ball bearings while avoiding the cost and complexity of press-fit machining. It reflects Tesla’s ongoing commitment to engineering cost-effective, scalable solutions in EV design.
Tradeoff Analysis: Engineering at the Margins
Both Volkswagen and Tesla achieve excellent drivetrain performance—but through very different engineering philosophies.
- Volkswagen prioritizes precision machining and durability with tapered roller bearings. This suits applications where load handling and component longevity are critical.
- Tesla emphasizes manufacturing efficiency, leaning on slip-fit assemblies and ball bearings to reduce friction and cost. Tesla’s innovation lies in mechanical workarounds like the spring pin locking system.
Despite using higher-friction bearings, VW achieves low total drivetrain losses thanks to extremely tight tolerances. Tesla, on the other hand, relies on clever mechanical shortcuts to achieve similar performance with less expensive components.
These decisions reveal what’s at stake: in a drivetrain with peak efficiencies over 96%, squeezing out every last watt of loss—from bearing friction to oil drag—can have measurable impacts on range and longevity.
Key Takeaways for Engineers and EV Designers
- Bearing Type Matters: Know the tradeoffs—ball bearings reduce friction, tapered rollers improve load capacity.
- Assembly Choices Influence Cost and Reliability: Press-fit assemblies are precise but expensive; slip-fit methods reduce cost but require locking mechanisms to prevent wear.
- Machining Tolerance Impacts System Losses: Tight tolerances enable low-friction operation even with higher-resistance components.
- Design Reflects Philosophy: VW optimizes for long-term durability through precision; Tesla balances performance with scalability and cost-saving ingenuity.
Final Thoughts: Lessons from the Teardown Bench
This teardown comparison between Volkswagen and Tesla illustrates the nuanced decisions behind EV drivetrain engineering. Each bearing, race, and housing component reflects hours of design deliberation—balancing efficiency, cost, durability, and manufacturability.
At Munro & Associates, we believe these insights deserve a spotlight. When you break open the gearbox, you uncover not just parts, but the philosophy of the brand itself. For engineers, students, and investors alike, these details paint a richer picture of what makes an EV perform at its best.
🔍 Want More EV Engineering Insights?
Stay tuned to Munro Live and check out Munro & Associates for more expert teardown reviews, cost breakdowns, and lean design analysis. And for in-depth consulting or B2B teardown services, make sure you visit leandesign.com.