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

by Jonathan Spiegel

At the end of Part 2 of the Polystrand GT-Lite Project, we had finalized the preliminary design for the independent rear suspension (IRS), generated the CAD model, and sent our CRX off to the bodyshop sans rear suspension. Thanks to Rassini, we had our virtual computer model of the chassis courtesy of Chris Galea’s work with the FARO unit, and Tony Berlingieri prepared the car by removing the old rear suspension and a lot of the now unnecessary supporting structure. Additionally, before the car left on the flatbed, Tony added some framework to which we’ll install the new IRS unit when the car comes back.

 

The big empty hole. This is what we were left with after the rear suspension was surgically excised. We still have a few things left to do before she leaves for the body shop.

Before we set the final design in stone, however, we needed to do everything we could to insure that our design not only fit into the vehicle, but that the components would do what we want them to do under load. Of particular concern was the location of the trailing links that attach to the chassis (the links that go from the rear spindle mounts forward to the chassis). We were hoping that we could get away with only one lateral link on each side (the links that go from the center of the subframe out to the rear of the spindle mounts). The ability to do this depends upon the angle of the trailing link from the centerline of the vehicle. If we can get enough angle on those arms, we’ll have enough lateral rigidity so that we don’t need the additional linkage, saving us some unsprung weight.

 

A view from the bottom. Tony’s busy cleaning things up and preparing to add some structure to mount the new IRS.
This car’s been around the block a few times. Tony patched a few holes and prepared some clean surfaces to weld in some bracing.

Think of the lateral link/trailing link assembly as a large A-arm or wishbone, and you’ll get the idea. The problem we have is that the fuel cell in the car is right in the middle of where we would like to place the trailing link mounts. We’d like the links to be long enough so that we don’t have too much toe change when the suspension goes through its travel, but we definitely don’t want to move the fuel cell, because it’s in a great location – low and in the center of the car – which means that as the fuel load decreases during a race, it will not have a big effect on the handling balance.

 

Since we won’t be using coilovers anymore for the rear suspension, we removed the heavy coil towers and cross-brace from the car. We’re adding some of that weight back with the extra framing, but it will be lower in the chassis, bringing the center of gravity down.
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Comments
Crousti
Croustilink
Thursday, January 29, 2015 2:48 AM
This is a work of art.
Where
Wherelink
Thursday, January 29, 2015 7:06 AM
Truly "outside of the box" thinking.

I'm still using the C4 Corvette composite springs in my design, but have been worried that I'd have to augment spring rate for aero loads. I've never even considered trying to have custom composite springs made. Is this even possible (i.e. affordable) for the average joe?
azmidlyf
azmidlyflink
Thursday, January 29, 2015 8:43 AM
Very versatile design. The lower spring sliding through the outer mount is interesting. What keeps the upper part of the spindle from flexing inward?
slowpoke
slowpokelink
Thursday, January 29, 2015 10:36 AM
a) does the lower outer spring perch move in the same arch as the end of the spring or are you going to have a stress concentration where the deflected end of the spring impacts and edge of the perch

b) are those blades going to experience andy shock loading and how are you modeling them?
Jonathan Spiegel
Jonathan Spiegellink
Friday, January 30, 2015 9:21 AM
@Where - custom composite springs are still pricey, because of the tooling involved, but new technologies, like thermoplastics, should help bring costs more in line in the next decade.

@azmidlyf - the upper arms are actually very stiff, and the highest loads, those on the outside wheel under hard cornering, actually load the arm in tension, resisting flex. The design actually induces flex through the linkage however, which is how we control the roll stiffness.

@slowpoke - the lower outer perch hangs from a pivot (it's known as a shackle), so that the perch itself remains parallel the surface of the spring. We also have a high density plastic pad that will sit in between the spring and shackle, to reduce friction and prevent any wear. Much of the high shock loads are absorbed by the tire carcass - the models were created in Solidworks and we use both the Solidworks Simulation package and ANSYS as well to evaluate the designs.
ginsu
ginsulink
Friday, January 30, 2015 9:24 PM
I assume you are going to mount a damper to the rod end on the top of the spindle, correct?
Jonathan Spiegel
Jonathan Spiegellink
Saturday, January 31, 2015 5:04 PM
ginsu - that's right. Those are the mounting tabs for the lower shock eye mounted near that upper rod end.
Reedicus
Reedicuslink
Saturday, January 31, 2015 8:15 PM
Awesome project, super interesting. It looks like you aren't the only ones playing around with this, ZF has a similar concept. Can't wait to see how it works out on the track!

http://blog.caranddriver.com/we-sit-down-with-zfs-head-of-communications-to-see-where-the-company-is-heading-beyond-lightweight-suspensions-and-eight-speed-plus-transmissions/
Jonathan Spiegel
Jonathan Spiegellink
Monday, February 02, 2015 9:30 AM
The concept of a transverse leaf has been around for a long time, as we all know, as in the Corvette. The ZF system goes a little bit further by using the spring as a locating member as well as the spring. This concept has been explored many times by several manufacturers, but it seems there has never been any real impetus to develop a system fully until now, with the new CAFE standards driving the huge lightweighting initiative we now see in the industry.
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