Formula SAE Motor Differential Mount

Load-bearing drivetrain mount with an eccentric chain tensioner, from FEA to CNC

Goal

Design a mount that rigidly locates the motor and differential to the frame, integrate a chain tensioning mechanism, and keep mass as low as possible.

Full drivetrain assembly render — motor, differential, mounts, chain, and halfshafts

Render — full drivetrain assembly (motor, differential, mounts, chain, halfshafts)

Chain Tensioning

The team chose an eccentric plate over a turnbuckle. A turnbuckle would have split the mount into two pieces, which makes aligning the plates considerably harder.

Eccentric tensioning plate model, front view

Model — eccentric tensioning plate

Eccentricity: 0.1875 in
27 holes on the mounts against 24 on the plate
3 holes align per configuration — 216 distinct tensioning positions
Plate retained with 3 bolts

Analysis

Assumed a 4 kN chain tensioning force; motor and differential masses came from their datasheets. Free body diagrams of the mount resolved reactions of approximately 3040 N and 960 N at the two supports.

FEA and Redesign

The first FEA iteration showed the two mounts were badly mismatched: the right mount came back at a factor of safety of 1.3 while the left was at 3.1 — one under-designed, one carrying dead weight.

FEA von Mises stress plot, right mount, iteration 1, FOS 1.3 — FEA von Mises — right mount, FOS 1.3

FEA von Mises stress plot, left mount, iteration 1, FOS 3.1 — FEA von Mises — left mount, FOS 3.1

The geometry was reworked into the final design, which significantly reduced mass while bringing both mounts to a factor of safety of at least 1.5.

FOS 1.3 / 3.1 · mismatched FOS 1.5+ · both mounts

Before

First-iteration mounts — right side FOS 1.3, left side FOS 3.1.

After

Final mount geometry — reworked to bring both sides to FOS ≥ 1.5, with reduced mass.

Manufacturing

Programmed toolpaths in Fusion 360 CAM and CNC milled each component in-house, including multi-setup operations on the eccentric tension plates.

Fusion 360 CAM toolpath setup for the eccentric tension plate — Screenshot — Fusion 360 CAM toolpath setup

Freshly machined mount, held in hand next to the mill — Photo — machined mount on the mill

Assembled onto the car with the motor, differential, chain, and halfshafts.

Assembled drivetrain on the bench — motor, differential, mounts, and chain

Photo — assembled drivetrain on the bench