Polystrand GT-Lite CRX: Part 3 – Simulating the Suspension


This is a view looking forward from the left rear fenderwell. It’s a little hard to see, but just to the left of where the puple cable comes through the frame rail you can see where the forward mounting points for the old trailing arms still reside. The cage-like structure on the right houses the fuel cell and the battery. In the center of the shot, just under the purple cable, is a very strong part of the unibody, and that spot is about the only place we can tuck the mounting bracket for our new trailing link.

Since we have the computer model of the chassis, and we have a computer model of the IRS system as well, it’s time to enlist the aid of another expert in the Rassini camp – Principal Engineer Jim Zeimet. Jim can take my model of the suspension along with the model of the chassis that Chris generated, and put them together in Rassini’s ADAMS software. ADAMS, short for Automated Dynamic Analysis of Mechanical Systems, is the simulation software that we talked a little about in our last installment. We located the trailing link bracket as far forward and inward as we could in the model, and Jim performed his magic.


Here’s a top view of the computer model, showing one lateral link on each side, and the left side trailing arm extending forward and inward. The question is – is this rigid enough?
The dashed line shows how the geometry is similar to an A-arm, which pivots on an imaginary axis.

After creating the unified model and applying simulated loads at the tire contact patch, Jim determined that it was impossible to reach the level of rigidity required with only one lateral link on each side – it was going to require a second link mounted to the front of the spindle mount to keep the wheel stable under cornering loads. Fortunately, we had discussed this early in the design phase, and had made allowances to add another link in just this case. Jim added the links to the model, and another run showed that we had the wheel control we wanted.


Here’s an animation from the ADAMS software showing how the single trailing lateral link setup deflects under cornering loads. Note that for animation purposes, the magnitude of the deflection is exaggerated for clarity. It really wouldn’t deflect that far, but the actual numbers were still too much.

Another advantage of the ADAMS software is that it allowed Jim to simulate the performance of the roll stabilizer mechanism we wanted to employ. Jim simulated the suspension loads the CRX's tires would see both vertically and laterally, and we were able to see that the linkage between the upper arms worked as designed – locking the pivots of the composite upper control arms and forcing them to bend, and thus act like secondary springs that add to the roll stiffness of the car. If we just used standard links for the upper control arms, the suspension would lock up under cornering conditions, and the spring rate would instantly go to infinity, causing drastic oversteer – or, more likely, the rate would spike and then something would break – either result being undesirable!


Here you can see we’ve added the second lateral link on the left side, on the forward side of the subframe.

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