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View Full Version : Suspension Tuning Without Rear Sway Bar. Need Advice.



Later-A-body
07-18-2017, 07:47 PM
As some people might have read, I have been finishing up the re-restoration of my 1973 Pontiac Grand Am. During the build I followed through on incorporating a rear coil-over suspension. Apparently the rear sway bar was not mounted when the coil-over suspension was designed, which has lead to me finding out the rear bar will not fit anymore. I was somewhat irritated with this at first, but then remembered that many people such as Doug Norrrdin at Global West advocate for no rear sway bar. I did some reading and internet searches on the subject as well. I would like to get some opinions from some of our well versed suspension gurus as to how to go about tuning this set up. The car weighs approximately 4000 lbs. with me in it. Drivetrain consists of LS7 power connected to a Tremec Magnum T-56. Rear suspension consists of GW tubular LCA, and GW adjustable UCA. The front consists of GW LCA, and Speedtech UCA with matching Speedtech Extreme 2.0 spindles. Tires are 200 treadwear Nitto NT05's, 275/35/19 front, and 305/35/19 rear. Shocks are QA1 double adjustable front and rear. Spring rates (for now) are 600 lbs. front, 200 lbs. rear. Front sway bar is a factory '78 Trans Am WS6 solid 1.25". I will attend some autocross events and track days with this car, but it is not a race car. Thanks.

raustinss
07-19-2017, 01:18 AM
Ron Sutton to the rescue

sik87gn
07-19-2017, 06:32 AM
I looked at your build, nice work. I'm not sure I understand why your rear bar will not longer work if the shocks are mounted behind the axle? My only assumption is you are trying to run an aftermarket axle/chassis mount bar that has multiple locating points for tuning? If that is the case, from the thread pics it looks like you still have the option of a factory style/location rear sway bar b/t the lower arms no? Here are a couple options for factory style mount sway bars...

1.125"

https://www.summitracing.com/parts/sum-720209

https://www.summitracing.com/parts/hel-5803

1"

http://www.jegs.com/i/ADDCO/020/939/10002/-1?CAWELAID=1710658953&CAGPSPN=pla&CAAGID=44693592161&CATCI=pla-223041270311&CATARGETID=230006180039219144&cadevice=c&gclid=EAIaIQobChMIzqm_y8aV1QIVlEoNCh2lQw2fEAQYAiAB EgJbi_D_BwE

And, I'd have to guess there are other options out there that the circle track crowd has used through the years!

That said, I'd certainly give it a drive first and see how it feels. And, a quick call to Ron Sutton is always good advice!

Later-A-body
07-19-2017, 07:51 AM
I only planned to run the factory sway bar. On the '73-'77 A-body, the bar mounts underneath the LCA, unlike the other GM cars where it mounts inboard. The shocks now sit in the path of the bar, not allowing the bar to mount. Yes I agree that Ron would be a great help. I used his services for my new brake system front and rear.

07-19-2017, 12:48 PM
Triangulated 4 links are bind prone and will fight itself at some point. A suspension should be allowed to articulate through it's range of motion as freely as possible so it can comfortably follow the front suspension around the corners. Imagine taking a bar and bolting the two lower arms to each other, that adds to that fight and as I've been told has about 30% effectiveness in doing what a sway bar is supposed to do. If you run a rear sway bar try to find one that attaches the links to the frame, a much more effective design.

My car is a G body, same basic design as yours only a little smaller. It weighs 3550 without me and has a little more weight bias in the rear because it's a wagon. I run 700 lb springs in the front and 350s out back. 200 sounds a little soft for your weight challenged bigger car. Even with a heavier rate spring I can dial it down on the shocks and have a very comfortable ride. I found when I had a rear sway bar I had a lot of oversteer and kicked the rear out in autoX curves. I tried a lighter rate bar and still had the issue so I removed the rear bar and the oversteer went away, the car handled a lot better and went faster.

I believe Ron's experience shows that a softer rear spring works well, a mid range rear spring rate slows the car down and then a heavy rate spring gets the handling capability back, he's mentioned that story several times. I hope he can chime in here to explain the details. Lance with his "Barney" Monte SS here on the forum runs a very hefty rear spring, near twice the rate of mine, and no sway bar and the car works really well. Hopefully this info helps in some way.

07-19-2017, 12:51 PM
BTW I wanna see pics of it! :)

Later-A-body
07-19-2017, 04:56 PM
Ben, Thanks for posting and for the advice. Sounds like I will be able to tune the car as you have with some trial and error. I am familiar with the "Barney SS" Monte Carlo and have read many of his posts. I am amazed that the ride can still be acceptable with that much spring rate. I want the car to be comfortable and still be able to be somewhat competitive, but I know the car won't be winning any events and I'm okay with that as long as it handles properly and I have fun. It is a big car with a lot of power, but I really want to get it "dialed" when I have a chance to spend the time driving it in different scenarios. Also what pic? The finished car, or the rear suspension?

pitts64
07-19-2017, 06:26 PM
I prefer jounce bumpers 1" away from the frame..