Anti-Roll Bar

sep 2019 - mar 2020 | mechanical engineering, fabrication

As part of Northwestern FSAE, I designed, analyzed, and fabricated an anti-roll bar based on new stiffness targets for the 2020 car.

While I wasn't able to see it in use due to COVID restrictions, the anti-roll bar functioned as intended during the competition, meeting the new stiffness target while introducing a range of adjustability.
Isometric view of Anti-Roll Bar assembly
What is an Anti-Roll Bar?
ARB (blue) connected the rest of the suspension assembly

As the name suggests, the anti-roll bar (ARB) prevents a car from rolling over. It does this by acting like a restoring spring between the two tires, so when one leaves the ground during a turn, the ARB pulls it back down, maintaining tire contact and increasing grip. By adjusting the stiffness of the setup, we can correct for over/understeer based on a driver's style.

The ARB itself is an assembly of three parts: two blades and one torsion rod. The two blades connect to the torsion rod, and they can be turned to adjust stiffness depending on the orientation of the cross-sections. The blade ends are connected to rockers, which are connected to the tires, and the torsion rod are held by plastic bushings attached to the frame of the car.

Top-down view of ARB assembly, with a blade connected to each end of the torsion rod

Design

x-dimension view of blade

y-dimension view of blade

The design of the blade followed the previous year's design with updated dimensions for the new stiffness targets. I tried to make the CAD model more equation-driven, making it easier to adjust. This made testing different x-y cross-section dimensions and running stiffness calculations fairly straightforward.

Stiffness calculations, set up to test different blade and torsion bar dimensions

The torsion rod was a simple construction of steel pipe, designed to be welded together. It had a specific length of 26.41 in, as was determined by the suspension geometry.

Torsion bar angled view, with connection points indicated

The three parts are assembled together to make the ARB. Depending on the orientation of the blades, the assembly can be "softer" or "stiffer".

Top view of "stiff" assembly

Top view of "soft" assembly

Analysis

While my calculations met the target stiffness, I wanted to validate them with FEA displacement simulations to get a better representation of the actual stiffness. I also ran some stress analysis to ensure the design was safe.

Fixtures:

  • Bushing attachment points as fixed hinges
  • One blade end ("inner tire") is fixed

Bearing load:

  • 100 lb force applied to opposite blade ("outer tire")

FEA simulation setup in SOLIDWORKS, with fixtures in green and load in purple

Stiff assembly displacement to calculate simulated stiffness

Stiff assembly stress analysis, with high stress areas due to nature of fixtures. Since we will be using plastic bushings, this should not be a pressing issue

Soft assembly displacement analysis with 30lbf

Soft assembly stress analysis

In an effort to further reduce weight, I was curious about tapering the blades. In theory this made sense: the bending moment maximum is at the end connected to the torsion rod. In practice (without too much consideration)? Take a look at the FEA yourself:

That's uhhh a pretty big displacement there

Tapered blade FEA stress study and sure the stress max is in the right place, but those aren't the colors you want to see...

Fabrication

This was what I was most excited about: fabricating the two blades out of a 1" diameter titanium blank on the lathe and mill. The nature of titanium means a few extra precautions have to be taken to reduce the risk of work hardening and to help cool off the cutting zone, mainly using enough coolant, new tools, and lower spindle speeds with higher feed rates.
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