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Project GD STI: Getting Rid of Chassis Flex with Cusco

by Mike Kojima

 

The GD STI/WRX had what was considered to be a very stiff chassis when it was first released internationally in 2000. The new Impreza platform was built to be much stronger than it predecessor and used much of the experience gleaned in rally competition to build a chassis that was over 100% stiffer than the outgoing GC platform. However, that was 17 years ago, and the design that was once the leading edge in unibody chassis integrity is now something that is just average.  

In chassis tuning, the stiffness of the body structure is everything. We tune a lot of the car's dynamic response by altering the slip angle of the front or rear tires. We do this by tuning the weight transfer to either end of the car by messing with spring rates, anti-sway bars, shock damping, and other mechanical things. A flexing chassis makes these things difficult by making it hard to transfer weight; the adjustments to the suspension are taken up by body flex instead of being applied to the tire contact patch.

A flexing chassis also makes it hard to get shock calibration right and deliver a smooth ride with good control. Think of the chassis as a huge spring with no damping. The chassis flexes in response to road irregularities and rebounds without the control of dampers. This uncontrolled rebound is detected as harshness by the driver and passengers. A stiff chassis diverts this force into the springs and dampers where it can be controlled with tuning. 

We used to feel that our GD STI had pretty decent body structure, but after spending some time in the new VA WRX, even some standout older cars like our EP Civic, and as well as the new ITR Civic, we realized that the GD is just about average in the stiffness department- especially compared to the late model stuff. 

In order to stiffen up our 90's designed chassis, we enlisted the help of Cusco. Cusco produces a more extensive list of body stiffening devices than any other manufacturer for most popular Japanese performance cars. We have had a lot of experience and good results with their chassis braces on our FR-S and EVO IX projects in the past, and we now felt it was time to bring our STI up to speed. 

The STI is a special case; we had adjusted the STI's ride height pretty low, purely for cosmetic reasons, probably too low for ideal, rough road conditions. To prevent from blowing through the travel on harsh, unmaintained LA county pavement, we turned up the high-speed compression damping on our KW 3-way Clubsports to the point where the ride was slightly jiggly- not ideal for best grip. Could we get rid of this by making the chassis stiffer?

Read more about Project GD STI !

 

Of course, a strut tower brace is a good basic addition that many people add for chassis stiffness. The Cusco part is made of a nice, stiff oval section aluminum tube.  

The only issue we have is that it has a bend in it to clear the stock top mount intercooler, but since that is gone on our car, the bend is not needed. We would rather have a triangulated brace with a straight section between the towers, but no Subaru bar is made like that due to the factory top mount. 

At least the Cusco bar has one of the largest stiffness improving cross sections of all the Subaru bars on the market. 

 

Another thing we like about the Cusco bar is that it is full width all the way to the shock tower brackets.  

Many bars neck down here or have a rod end bearing here, which does not couple the bar in torsion well. There is both bending, mostly in compression and twist here, so a rod end is not ideal. We much prefer how the Cusco bar's end construction is more suitable for the types of loads it will see in this area. 

 

For the rear strut tower brace, we chose Cusco's triangulated brace. This brace connects the shock towers and also bolts to the floor. This reduces the tendency for this part to the car to twist in torsion, as it is an open structure. 
 
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Comments
ginsu
ginsulink
Tuesday, September 05, 2017 3:26 AM
The problem with that strut tower brace is that it has only one bolt per side, so it is only loaded in tension and compression. It keeps the towers from moving in/out, but it provides no stiffness in bending. That's why I try to look for, either rigidly mounted strut tower braces or ones that have two offset bolts per side so the brace can resist a bending moment. Although, I don't know of these are available for the Subaru platform.
Dan DeRosia
Dan DeRosialink
Tuesday, September 05, 2017 8:24 AM
@ginsu: Funny, I was thinking that too. I personally have one of the old STI titanium strut tower braces that doesn't have any pivots, ala https://www.jspecauto.com/engine_details/2157/jdm-subaru-02-07-impreza-wrx-sti-front-oem-titanium-strut-tower-bar-gdb (not affiliated just linking for their picures) But obviously the tube is smaller diameter than the Cusco one.

@Mike: Have you done anything where you've tried to measure chassis stiffness? I know how hard it would be to do accurately, and I trust your impressions, but I'm an engineer, so I always think of numbers.
Mike Kojima
Mike Kojimalink
Tuesday, September 05, 2017 8:36 AM
I have done it with targets and laser pointers on opposite ends of the car while jacking up opposing corners of the car.
Supercharged111
Supercharged111link
Wednesday, September 06, 2017 10:21 AM
How much of a change did you observe Mike? I seem to recall a video of jacking the car up with a brace cut in half to show unbraced flex.
Scott Helmer
Scott Helmerlink
Wednesday, September 06, 2017 4:54 PM
Interesting... I guess I might as well only be holding onto my old STi carbon fiber/titanium strut tower brace as a cool wall ornament at this point, huh? :P
Anonymous User
Anonymous Userlink
Tuesday, September 12, 2017 1:34 AM
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