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Polystrand GT-Lite CRX: Part 2 - Designing an Independent Rear Suspension From Scratch & A Composites Primer

by Jonathan Spiegel

When we last left off, the GT-Lite Project CRX was being loaded onto the big orange transporter and headed to the Detroit area and into the capable hands of SANLUIS Rassini. We’re about to take a trip into a place where absolute state-of-the-art hardware and software is used daily in the design and development of suspension systems used worldwide by most of the major OEMs. It’s a privilege to be able to work with these folks, and, as you’ll soon see, these guys also are very hands-on and fun to work with, too. Our team at Rassini includes Aftermarket Business Unit Manager Tony Berlingieri, (aka: “Goggles” Paisan), Engineering Technician Chris Galea, and Principal Engineer Jim Zeimet.

One of the first things we did was put the car on the scales to see where we were starting. Since one of the goals of the program is to develop a lightweight suspension system, we need to know how much we weighed to begin with. With the existing engine and drivetrain in this car, the minimum allowed weight according to SCCA rules is 1895 lbs. With that in mind, we loaded 175 lbs of sandbags into the driver’s seat, and rolled her up onto the plates. We clocked in at 1880 lbs, with very little fuel in the tank. With more fuel and/or a heavier driver, we’d be right on target. Good to know. We weighed the car on a front to rear incline as well, so that we could determine the height and location of the Center of Gravity (CG).

 

This is the FARO unit, ready for action. The handpiece, which looks like some kind of futuristic laser pistol (because that’s kind of what it really is) transmits a visible laser line which accurately determines the position of whatever you point it at. If the laser has a line-of-sight to a surface, it can determine the position of that surface in 3-D space with an accuracy of about .001.” If you can see it, the FARO can measure it!

Keeping in mind that this car was very competitive, we’d like to know where all the magic came from. While driving talent and horsepower certainly factored into the success of this CRX, this car is known for its exceptional handling. If you’ve read Mike’s Ultimate Guide to Suspension and Handling here on MotoIQ, you already know that proper suspension geometry is critical to handling performance. That being said, you have a few options available to analyze existing geometry. You can get out your plumb bobs, levels, tape measures, calipers, and graph paper, and start locating all the critical suspension mounting points, documenting wheel offsets, and lay out everything with detailed diagrams. It’s certainly an effective method, albeit not necessarily easy. We’re lucky – at SANLUIS Rassini they just get out the FARO FaroArm laser scanner.

 

Chris Galea at SANLUIS Rassini measuring the front suspension geometry with the FARO.

Basically, the unit creates a 3-D model of everything it sees, and it is important for the operator to not only know what to measure, but also to understand what information we want. It’s important to measure all the pivot points accurately and then tell the software what points are important. Since the scanner doesn’t see the actual center of a pivot or a spherical bearing, Chris has to tell the software to determine where those points are based on the geometry of the surrounding surface.

 

Here you can see the laser line that the FARO arm projects. We’re not only interested in the suspension geometry itself, but the frame and chassis geometry as well, so we know where we can and can’t place pickup points or reinforcements.
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Comments
8695Beaters
8695Beaterslink
Wednesday, December 17, 2014 5:49 AM
This is EXACTLY how I did our SAE's car control arm geometry. SolidWorks models are the tits! That design is very interesting, I'm really looking forward to seeing more of it, as I'm curious as to how it works. I'm also curious as to how toe is adjusted, since it looks like the lower arm/spring only has one DOF and wouldn't be able to pivot about the vertical axis to allow for toe change. Finally, any issues with the camber bolts slipping? Looks like they're only held in by the friction of the bolts. We looked at using this method for camber adjustment in our SAE car, but ultimately went with shims on the uprights because we were worried of slippage.

Looks awesome, I'm really looking forward to seeing more of this car!!!
jeffball610
jeffball610link
Wednesday, December 17, 2014 7:54 AM
Thumbs up. Very interesting stuff. Can't wait to see how the final product works out.
buzzboy
buzzboylink
Wednesday, December 17, 2014 12:20 PM
I do love composites. I've grown up in the surfboard manufacturing industry so I've seen many of the huge jumps in the composite industry over the past ~10 years. There is some really cool stuff on the market right now.
Jonathan Spiegel
Jonathan Spiegellink
Wednesday, December 17, 2014 12:46 PM
The outer end of the lower spring floats on a slipper assembly. The toe can be adjusted by varying the lengths of the lower lateral links. Shouldn't be any problem with the eccentric bolts slipping, since they are pretty large and we can get sufficient torque on them - if they do creep, we'll add a simple locking mechanism for the eccentrics.
BillyJ
BillyJlink
Friday, December 19, 2014 12:55 PM
Jonathan Spiegel
Jonathan Spiegellink
Monday, December 22, 2014 7:23 AM
The Wards article is a little unclear - Rassini is making this investment long term to develop composite suspension technology for the OEM markets, and we already have some interesting projects in the works in that respect. As far as the GT-Lite car itself, they are providing substantial engineering support and have spent substantial time and $$ in getting us to where we are so quickly! This car gives us all (Rassini, Polystrand, and PPG) an opportunity to generate data and get performance feedback in a much shorter timeframe than we can achieve in an OEM development project - and it's an exciting way to accelerate the development cycle!
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