Polystrand GT-Lite CRX – Part 2



The FaroArm is a non-contact Coordinate Measuring System, or CMM. By scanning all the critical points of the chassis with a laser mounted on an articulating boom assembly, this sophisticated system can create a virtual model of the suspension system, accurately locating all of the points in space to within a thousandth of an inch. Once all of these points are located and documented, this data can be evaluated using any number of methods, from plotting the points on graph paper and connecting the dots, to uploading the data into more sophisticated software.


Another shot of Chris scanning the front suspension.

Remember, we’re at SANLUIS Rassini, so they use ADAMS. ADAMS, which is an acronym for Automated Dynamic Analysis of Mechanical Systems, is simulation software that is used by most major OEMs to create models of vehicle chassis systems and evaluate them by applying virtual loads and cycling the suspension system throughout its range of travel. Not only can this software tell you things like how much toe change you’ll have at full jounce on the right rear corner, but it can also account for things like bushing deflection and tire sidewall stiffness.


Chris scanning the rear frame.

Once we scanned the chassis and created the model, the real work began. First, we had to decide where we wanted to start – at the front or the rear. While we have some pretty neat ideas for both the front and rear suspensions, we also have a pretty tight deadline. We promised to have the car ready to display at an automotive composites conference over in Troy, Michigan – in about eight weeks! Oh, and did I mention we wanted to repaint the car before then as well? After a brief discussion, we decided to design and build a complete IRS module for the back of the car. As much as I hated to see the really trick Mumford linkage come out of the rear of the car, there’s lots of room under the back of the CRX to get creative, and we won’t have to work around the engine and drivetrain package. We’re taking a risk here as well – currently, the SCCA GT Lite rules only allow IRS in FWD cars if they were originally equipped as such. If our CRX had been of 1989-1991 vintage, we’d have been okay. Being a 1986 model, it originally sported a beam rear axle. The rules do, however, allow RWD cars to convert to IRS at a 2.5% weight penalty. It made sense that I could petition the SCCA Club Racing Board to allow us to use an IRS by choice as well, even if it meant a weight penalty. I figured it was only fair to allow a FWD to use an IRS by choice if a RWD car was allowed to convert. A quick letter went off to the SCCA CRB with fingers crossed, and we were off to the races, so to speak.


Here is a rendering of the surfaces that were scanned. You’ll notice it’s not a complete image, since we only need dimensions for the important points. You can see how sensitive the FARO is – anything that the laser sees, it measures, even if we don’t necessarily need it. This is why an experienced operator like Chris is critical to the process.

After the flight back to Colorado, I immediately dug in to creating the IRS design. In our timeline, we decided I would have about a week to complete the design phase if we were to make our deadline. Having the virtual computer model of the chassis was a huge help – I was able to tell exactly what the design envelope was. Knowing where the roll center of the original rear suspension was also important – we knew the car worked very well, and roll center location is probably one of the most important factors to control, so I tried to nail that point in space and work from there. Volumes have been written on optimizing suspension geometry (and I suspect Mr. Kojima has read at least as many of those books as I have, if not more). There’s not much new under the sun, and my experience with what works and what doesn’t is pretty well summed up in Mike’s aforementioned “Ultimate Guide.” I don’t need to regurgitate any of that, since all you have to do is click on the link and read it yourself. If you haven’t, now would be a good time!

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