Momentum - April 2020 - 5

Case study: broken brake pedal

Photo of brake pedal failure in the previous design due to a
manufacturing error. Instead of a solid master cylinder mounting
bar being welded into the pedal, two separate bars were welded on
each side. Over time the welds fatigued and failed.
the best it's ever been. The next year, this team member
becomes a person of some influence on the team-let's
say the chief engineer-and another team member takes
over his system. The chief engineer is not particularly
interested in some new member undoing all the good
work he did on the system. So the system stays the same
that year. The year after that, the original designer has
graduated, the previous project engineer never did any
real design work, and while everyone on the team might
be able to put that system together, no one understands
why it's designed the way it is. So they defend it as best
they can at competition, and the second-place car in
skidpad gets two points in suspension design.
This is probably a familiar story to anyone involved in
Formula SAE. Many people would say the reason that
the current team or project engineer doesn't have a
good understanding of his or her system is because the
senior team member was arrogant and refused to allow
his system to be improved. But this is probably not true.
The new project engineer is performing poorly because
members on the team weren't able to communicate
their design process and the engineering principles
behind it. The team captain could be the best of people,
but still fail to recognize what is going wrong. He does
not need to show new members the why, but the how.
When a new member asks why a part is designed a


2004_SG_4-13.indd 5

During a summer test day for the
Missouri S&T Racing Formula
SAE team after Lincoln 2018, our
brake pedal broke while driving.
The pedal had been
manufactured different from its
design, which resulted in a severe
failure of the system. This
happened because a previous
project engineer of the system
was not able to communicate the
fundamentals of the system and
why the brake pedal had been
designed the way it was. Because
this understanding had been lost
at some point in the team's
history, it had to be built up again
from fundamentals, and a
complete pedal redesign resulted
from that process.
The pedal that broke was a
square, 0.049" 4130 tube with a
solid 3/8" steel rod mounted
through the pedal and welded in
place to mount the master
Finite element analysis of the redesigned
cylinders (MC). That MC mounting brake pedal run in NX Nastran with an
bar had been manufactured
850-lb load normal to the pedal face. 850
incorrectly; instead of a solid rod
lb represents our measured maximum
running all the way through the
applied load of 565 lb with a safety factor
pedal, two shorter rods were
of 1.5, which the pedal should not see under
welded on each side of the pedal. normal operation.
This meant that only the cross
section of a small TIG bead was
connecting the pedal to the brakes. After an entire season of driving, one of the
welds finally cracked and the rod bent at the weld, tripping the brake overtravel switch.
This incident prompted thoughtful consideration of the brake pedal as a
whole. Looking to reduce weight while improving strength, we pursued an
aluminum design. However, our team does not have easy access to a CNC and
so machining a solid pedal was not an option. Also, to preserve the existing
packaging, we decided to keep the steel MC mounting bar. Fabricating a
pedal from 6061 similar to the current steel design was considered, but we
didn't have enough confidence in our capability to weld aluminum. Ultimately,
we determined a 7075-T6 plate would offer the strength, rigidity, and weight
we wanted.
With our on-campus waterjet, it would be easy to cut a 2-D profile from plate;
however, a single flat plate does not offer enough width to be mounted on
bearings, or to support the steel master cylinder mounting bar. Because of this,
two plates connected by tubular 6061 spacers were used, but with only a tight
press fit, as 7075 is not weldable. These also support the MC mounting bar,
helping distribute load into the pedal.
Though we were cautious about using a press fit structure on aluminum in a
critical part like this, after testing different press fits we decided to go forward
with a prototype pedal. Manufacturing the prototype went smoothly, and after
testing and inspection we moved forward with the design on the car. The finished
aluminum pedal, with the addition of a carbon faceplate, bearings, and bolts,
weighs just under half a pound-a 50% reduction from the previous 1-lb pedal.
FEA showed that the aluminum pedal was actually stronger than the steel tube.
By Perrin Habecker

April 2020 5

3/19/20 11:25 AM


Momentum - April 2020

Table of Contents for the Digital Edition of Momentum - April 2020

Momentum - April 2020
Start a new SAE Chapter in your life
Focused on design
Taking the classroom to the racetrack
Great moments happen due to the work of many
ONE-ON-ONE – Emily Kerr
Sticking to the schedule
GM deploying electricals of the future in new architecture
Myers Awards winners named
SAE 101: Industry Leadership Award
Dossier: Justin Lange of John Deere
Focusing on the ‘E’ in STEM
Momentum - April 2020 - Momentum - April 2020
Momentum - April 2020 - Cover2
Momentum - April 2020 - 1
Momentum - April 2020 - Start a new SAE Chapter in your life
Momentum - April 2020 - BENEFITS U
Momentum - April 2020 - Focused on design
Momentum - April 2020 - 5
Momentum - April 2020 - 6
Momentum - April 2020 - Taking the classroom to the racetrack
Momentum - April 2020 - 8
Momentum - April 2020 - Great moments happen due to the work of many
Momentum - April 2020 - ONE-ON-ONE – Emily Kerr
Momentum - April 2020 - 11
Momentum - April 2020 - Sticking to the schedule
Momentum - April 2020 - 13
Momentum - April 2020 - BRIEFS
Momentum - April 2020 - GM deploying electricals of the future in new architecture
Momentum - April 2020 - Myers Awards winners named
Momentum - April 2020 - SAE 101: Industry Leadership Award
Momentum - April 2020 - Dossier: Justin Lange of John Deere
Momentum - April 2020 - 19
Momentum - April 2020 - Focusing on the ‘E’ in STEM
Momentum - April 2020 - Cover3
Momentum - April 2020 - Cover4