Polystrand GT-Lite CRX – Part 2

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Hopefully, I can tickle your gray matter a little bit here. The two big takeaways from the previous page are “straight lines” and “springs,” and if you haven’t figured it out already, we’re going to be making straight, flat springs. These designs are nothing new – leaf springs, cantilever springs, semi-elliptic or quarter-elliptic springs, whatever you want to call them, are all variations of a simple beam spring, and they’ve been around since the horse-drawn carriages of the 17^th century. This type of spring is easy to design, and allows us to take advantage of the properties of our materials. We’ve already determined what our suspension geometry is going to look like, and we’re going to control that geometry using linkages – things like control arms, radius rods, trailing arms, etc… Nothing new there, it works, and it works well. What we’re doing differently is using a different material to store our energy.

Here’s the beam axle and linkage that came out. We’ll need the spindles, hubs, and rotors, and the rest goes into the parts archives.

Time for a brief physics lesson – springs are not just a method of holding a vehicle up, but they are also energy storage devices. They take kinetic energy – the energy of motion, store it as potential energy, and then release it again as kinetic energy. As a suspension moves up and down, this conversion process is constantly occurring. A suspension system not only controls geometry, but it manages energy as well. Suspension springs, in a sense, store and release energy, and shock absorbers (dampers) help control the rate of this conversion by converting kinetic energy into heat. Class dismissed.

This is the IRS geometry layout. The line across the bottom is the ground plane reference. The ends of that line represent the center of the tire contact patch. The bullseye in the middle, where the dashed line drawn from the left contact patch to the virtual intersection of the control arms crosses the vehicle centerline, is the roll center.

Back to our suspension design. We already have our geometry laid out, and we know what type of springs we’re going to use, so it’s time to put everything together. Since it’s a racecar, we want to make sure we have plenty of adjustability so that we can tune for different tracks and conditions as well. All that, and we need to make sure that it fits under the car. After developing the 2-D sketch of the geometry, it was time to bring the design to life. Thought was given to welding a framework and bracketry into the rear of the car, but, since we were probably going to have plenty of development to do, I decided to design the system as a modular subframe that we could bolt in and out as needed, in case we had to make any drastic changes or needed to do a complete r&r at the track due to damage. I also wanted to try a new approach to increasing roll stiffness. While the car originally came with a big adjustable rear bar, we will be using a material that not only has high strength but can also withstand deflection. With that in mind, I wanted to design a linkage that not only defined geometry, but also functioned as a roll stiffening mechanism.

Here’s the next step from the geometry sketch. This is a near final layout of the plate we’ll be using to create the suspension “box.” There’s a few other things going on here, but you can see where all the basic points fall into place.

Just like I started with a clean sheet of paper for the suspension geometry design (or a clean computer screen, anyway), I decided to start with a clean, flat sheet of steel for the rear suspension. Actually, it ended up being two sheets. Since this is a front-wheel drive car, the rear suspension sees primarily lateral (cornering) loads, minimal braking load compared to the front, and no drivetrain loads. By creating a module with a box section, it was easy to make the suspension geometry highly adjustable, since everything pretty much stays in the same planes. It was very easy to lay out all of the pivot points that the geometry required – all I really had to do was transfer the dimensions from the two-dimensional drawing directly onto the “faces” of the box.

This is the plate, brought to life from a 2-D drawing.

Here’s the first iteration of the actual solid model. The left side is almost complete in this view. You can see some of the adjustable features, along with the thermoplastic composite materials, featured in red. The lower tapered parts are the primary springs – the boxes they mount in slide in and out to adjust spring rate, and have eccentrics to adjust ride height. The upper arms are connected with a bellcrank linkage (similar to a Watts link) that allow the arms to move freely in two-wheel bump – but in roll, the linkage locks the pivots, and forces the upper composite arms to flex, which provides the increased roll stiffness. We’ll examine this more closely in the next installment.

As you can see from the rendered model, the linkages are pretty standard race-car fare – DOM tubing with spherical bearing rod-ends. I’m using a simple trailing arm to control the fore-aft loads (basically, the braking forces). The parts in red are the actual thermoplastic composite springs – the lower ones are the primary springs, and the upper ones function as the upper control arms as well as being a “secondary” spring that essentially act as the stabilizer (or anti-roll) bar when the car experiences body roll. We’ll examine the linkage that makes it work next time. There’s lots of adjustability here – all the linkages have multiple mounting choices, the primary springs are adjustable for rate and height, and the upper arms have eccentrics for quick and easy camber adjustment. I also designed the spindle mounts so that the hub position can be easily changed.