FSAE Differential Mount
Steven Tompkins
Overview of completed prototype.
Photograph of flange, prohibiting lateral movement of differential
Summary
This project has been completed in conjunction with Hope College’s FSAE team. FSAE is an
international competition where students design, fabricate, and test one-fourth scale formula one
type cars. The goal for this project was to provide a mount for the differential onto the frame of the
car. The concept of a differential mount is not new, but every car’s frame is unique and this is
where the challenge lies in designing the differential mount. The differential that the team has
purchased includes bearings. These bearings are the only components of the differential that can
be captured.
This mount was designed for a performance car and thus several performance-related
requirements were identified. Because the frame of the car is steel, there should be steel
components to ease the degree of difficulty of the welding process. The remaining fabricated
components should be made from aluminum in order to reduce the weight of the mount. Related
to the weight of the mount is the amount of space the mount occupies. The available area where
the mount will be located has approximate dimensions of twelve by fifteen by six inches. The
mount should be large enough that there are no concerns about its durability yet
overcompensating will result in unnecessary weight. The design should also be simple enough to
fabricate in the time allowed.
All concepts considered included two aluminum plates (0.375 inch-thickness) with mirrored
dimensions whose purpose is to capture the differential’s bearings. These concepts also shared
three female threaded rods to be fastened in between the aluminum plates. The purpose of these
rods is to keep the aluminum plates parallel and spaced correctly. Another component shared
amongst the concepts is the steel tab (1.50 by 1.25 by 0.375 inches). These tabs will be welded to
the frame on one end and either fastened to the aluminum plate with 0.5 inch diameter bolt or
fastened to a turnbuckle with a 0.5 inch diameter bolt. Concepts utilizing a turnbuckle will
incorporate one turnbuckle for each aluminum plate. The turnbuckles will allow for the position of
the differential to be altered towards the front of the car and in turn will allow for the tension in the
drivetrain to be adjusted. Because the differential will ideally be mounted six inches from the rear
of the car, the deciding factor in choosing whether or not to employ the use of turnbuckles was
finding a turnbuckle short enough in length while having a high enough load limit.
The final design consists of two aluminum plates (6.25 by 6.00 by 0.375 inches), two turnbuckles (6
3/8 inch closed length, 130 pound limit), three female threaded rods (6 inch length, ¼-28 thread),
and eight steel tabs (1.625 by 1.50 by 0.375 inches). The aluminum plates have two ½-inch
clearance holes; one centered on the width of the plate and the other 0.50 inches from the bottom
and 0.5 inches from the edge closest to the front of the car. They also have three ¼ -inch
clearance holes spaced around a 3.936-inch diameter (0.312-inch deep) bore, concentric with a
3.930-inch through-bore. The smaller through-bore provides a “stop” for the bearings so they do
not push all of the way through the plates. The plates will be press-fit onto the bearings so that the
“stops” face outside of the car. The steel tabs include a 1.00-inch diameter half circle cut at one
end centered on the 1.50-inch edge in order to fit on the car’s 1.00-inch diameter steel tube frame
for welding.
Finite element analysis of the fabricated components revealed a low factor of safety of twenty-five.
This factor of safety occurred in the aluminum plate when the two ½-inch clearance holes were
restrained and a one-hundred twenty pound force was located on the inside of the 3.936-inch bore,
directed towards the front of the car.