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


An oblique view from the front left of the system, showing the trailing link assembly and the additional lateral link.

While Jim was analyzing our suspension loads and geometry, I got busy analyzing the flexible bits – the composite springs and upper arms. Based on the geometry, I had some target spring rates in mind. One of the advantages of being able to locate the roll center of a suspension in an ideal position (along with keeping the center of gravity low) is that the vehicle doesn’t have a tendency to roll as much under hard cornering, which means we can run less spring rate. In a production car, often times lowering the suspension with the stock geometry causes the roll centers to move further away from the center of gravity. This results in a larger roll couple (see Mike’s Ultimate Guide again) and increased body roll. In order to counteract the body roll, you usually add spring rate or bigger stabilizer bars. With optimized geometry, we get to run a softer setup, which makes the car easier to drive at the limit, and more forgiving when (not if) we exceed the limit. A softer setup is also easier on the tires, which means we generate less tire heat, resulting in longer life and less chance of the tires “going off” in a longer race. It also means we’ll be able to run a softer compound, for more grip. In the GT-Lite class, we’ll gain one more benefit from this, albeit it may be a small one. Since we’re allowed some limited aerodynamic enhancements – a wing and a front splitter – a softer setup will mean that at high speeds, the suspension will compress some under the downforce generated, lowering the ride height a little and reducing drag. Granted, it may not be much, but in a class where horsepower is tightly limited, every little bit can make a difference.


…and the resultant analysis – a big improvement. The wheels deflect vertically, but no drastic toe change.

You’ll notice that the composite springs and arms in our design have an unusual profile to them – some of you may wonder why they are tapered the way they are. We do this for a couple of reasons. First and foremost, we use this to control the stress on the parts. It would be easy to make the parts as a rectangular bar with a constant cross section, but, as you’ll see, this concentrates the stress in a smaller area. If we design the part with the proper taper, we can actually reduce the maximum stress and distribute the total stress over a much larger area, which means the part will be much more durable.


Here’s a shot of the left half of the IRS for reference. We’re going to dissect this a little bit and take a look at the stabilizer mechanism.
I’ve removed the rear subframe plate in this view so you can better see how the linkage is connected to the upper control arm pivots.

There’s another bonus to designing a proper stress profile – it actually saves weight. For the main springs in our design, we were actually able to achieve a weight savings of almost 10 percent. Since the spring is fixed to the chassis at one end and attached to the spindle mount at the other end, only a portion of it is considered unsprung, but the end that is thinner and lighter is at the outboard (spindle) end, so we get a little extra benefit.


Here’s a top view. The top mounting plate for the pivot mechanism has been made transparent so you can see how the links connect to the center pivot – if this looks familiar to you, that’s because it is extremely similar to a Watts linkage. Visualize both wheels moving up together, and you’ll see that the center pivot rotates freely in a clockwise direction, and counterclockwise as the wheels move down together. If one wheel tries to move up or down independently of the other, you’ll realize that the center pivot can no longer rotate freely. This resistance will counteract body roll, and, if the load is high enough, the upper arms will begin to deflect. In addition to deflecting and adding to the overall wheel rate, the effective length of the arm shortens as it bends, giving us extra camber gain.

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