Momentum - November 2020 - 5

STUDENT GENERATION
150 bonus points for teams that brought a
4WD vehicle to competition in 2020. We were
already well into our design cycle, and the
performance benefits of 4WD seemed
questionable. But 150 points sounded like an
offer too good to refuse. Plus, it gave us a year
to prototype-if the 4WD system was a
catastrophic failure, we could just rip it off and
race as a RWD vehicle. And at the end of the
day, we were Michigan Baja Racing-five-time
Mike Schmidt Iron Team champions known for
our crazy design shenanigans. We couldn't just
pass this one up.

4WD DESIGN
Every year, we break down our car's
performance into three broad categories:
maneuverability & handling, acceleration &
speed, and durability. This year was no
Michigan Baja Racing's first ever 4WD car, MBR31, prior to its official unveiling.
different, but when the 4WD announcement
came along, we shifted our focus to designing a
The last major components of the 4WD system were the
4WD system that retained maximum performance in each of
front half-shafts. We decided to use constant-velocity (CV)
these categories. It was already September 2019, though-just
joints from commercial UTVs to save manufacturing time and
seven short months and we'd be racing in Arizona. After
focus on other components of the car. But we still had one trick
weighing our options, we moved forward with a chain-driven
up our sleeve: by eliminating the plunge built into the CV cup
system that would achieve our goals while integrating minimal
and implementing a form of slip shaft to allow for suspension
frame changes. This would provide more time for testing to
travel, we greatly increased the articulation range of the front
evaluate the durability and performance prior to the first of
shafts. This allowed for a larger steering angle and would bring
the three Baja SAE competitions scheduled for 2020.
us within 20% of the 2019 vehicle's impressively tight turning
The first question to address was how to connect the chain
radius.
into the existing driveline. We decided on mounting it at the
Detailed analysis using hand calculations and finite element
final drive output for a few reasons. Most importantly, this
models was performed for each 4WD component to ensure
resulted in minimal gearbox modifications-we just had to beef
that the system would not fail under the cyclic loading of a
up a few bearings, and we were set. Additionally, it eliminated
four-hour endurance race. Our load cases were grounded in
the need for further reduction through sprockets or a
copious amounts of strain gauge data from testing on old
differential in the front. Next, we thought about how we could
vehicles, but we still favored a more conservative safety factor.
implement a failsafe mechanism to make the vehicle faultAfter all, success in Baja SAE comes from methodical
tolerant. This 4WD system was going to race unproven, so if
iteration-and the seniors had to leave something to keep us
something went awry, we wanted our car to keep rolling. We
busy next year.
chose to integrate shear pins into the intermediate shaft
connecting our two chain runs. By placing the pins as far
ON-TRACK TESTING
upstream as possible, if a component further down the line
Students worked late into the night to ready the car for our
seized, the pins would shear to decouple the front and rear.
first day of testing, excited to see 4WD in action. Unlike our
The elephant in the room was how we would implement the
previous cars, which were largely iterations on an old design,
functionality typically achieved with a front differential. We
this design was new and unproven. While the 4WD system had
could have used a differential, but that would have been hard
been designed to work with our original frame, everything else
to package and increased the weight of the car considerably.
from the brakes to the skid plate had been altered in some way
Rather, we chose to use sprags-akin to a one-way bearing. In
to accommodate it. Our front suspension and steering
corners, this would allow for unequal tire rotation and reduce
kinematics had been reworked with dozens of iterations to
understeer. And by under-driving the front by a few percent
maximize steering angle. Even driver ergonomics and pedal
through a slight sprocket mismatch, the front wheels would
placement had to be revisited. I remember people taking bets
only engage when the rear tires started to spin, creating a sort
about how long the vehicle would last before breaking-overof "on-demand 4WD" effect. Finally, in the spirit of fault
under was about 20 minutes, the same amount of time our
tolerance, if the 4WD system seized, the shear pins would
2019 car lasted before shearing a rear driveshaft.
break and the front wheels could continue spinning due to the
It was another chilly Michigan morning as we met up and
sprags.

MOMENTUM

November 2020 5

Momentum - November 2020

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