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Discussion Starter · #1 ·
Lift-Throttle Oversteer, rear sway bar

Just wondering...

Does adding a rear sway car only, greatly increase the lift throttle oversteer? This was a serious issue on my Impreza, when upgrading the rear bar, it seemed like any lifting of the throttle would violently swing the rear around.

I'm wondering if it's drastic on the GT or not. From what I can tell, stock, it's a quite neutral car... I wouldn't mind adding a little bit of oversteer (once the winter is gone, of course)
 

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Adding a rear bar only will increase oversteer in the GT, both static and lift-throttle. If you lower the car, the effect is compounded because of the rear trailing arm geometry.

In stock form, the GT is a dedicated understeerer....as bad or worse than most front-drive cars. Sure, oversteer can be induced if you try hard enough, but generally the car pushes like a pig when the car is driven hard through the twisties. The biggest flaw here is the bind of the front spring...it effectively creates an infinite front spring rate. This is why the shackles work so well to make the car more neutral.
 

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Discussion Starter · #3 ·
pardon my ignorance...(I don't know much about leaf spring designs and all), but what are shackles?
 

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Just a way of simultaneously lowering the front end of a GT, and eliminating the bind in the front spring that causes a rough ride and crappy handling. It also is the reason the GT front suspension bushings wear so badly, the binding puts all the load into the bushings, and then they deflect to allow suspension movement. This link has a photo of a set of shackles I designed for a GT, installed on Wayne Torman's GT. Someone has posted the drawing somewhere.....don't recall offhand where though.

http://www.opelgt.com/forums/showthread.php?s=&threadid=398&highlight=shackle

Bob
 

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kwilford said:
Here's a set that Todd Benson machined out a solid piece of steel, just welded at the cross piece.
An important note here: I noticed in your photo that Todd did not add the radius at the 'stepped' area as I had drawn it. Not doing so is an important issue. As the front suspension compresses, and the spring elongates as it is supposed to do, the shackles travel outward in an arc. This design, as shown, will strike the lower a-arm when these shackles swing outward, and probably crack it! You have been warned!! I don't want to see someone get hurt, or their car damaged, so please understand I designed the shackles the way I did for a reason. If you alter the design, you do so at your own risk!
 

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Good point, Bob. I hadn't noticed that problem in Todd's shackle. I suppose he could get the radii machined in to allow the shackle to "swivel". I also wondered about the square shoulder in the lower mount, as it might interfere with the spring-eye bushing.

Have you tried this design with a bit less drop? I am considering getting a set made up, but the 2 inch drop seems a bit radical, especially without lowering the rear suspension, which comes with it's own gremlins. Would this work with less drop, say even 1 1/2 inch, or preferably 1 inch? Although I suppose that you need to lower the spring-eye bolt enough to clear the bottom of the A-arm.
 

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kwilford said:
Would this work with less drop, say even 1 1/2 inch, or preferably 1 inch? Although I suppose that you need to lower the spring-eye bolt enough to clear the bottom of the A-arm.
Bingo, that's the issue. Check out the photo on Wayne's GT, you can see how close it is already. Any higher and it hits the a-arm. Although you could circumvent this by having another leaf added to the front spring by a spring shop to raise it up, then installing the shackles anyway. Roundabout for sure, but it would work.

Bob
 

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I remember seeing the drawings for Bob's shackles once, possibly with a supporting tech article. I can't seem to locate it. Does anyone know where I can find it? I have my 72 GT front suspension apart and this is the perfect time to do shackles.

:p

Moderators note:
Right Here (second article from the top): (and earlier in this thread!)

http://clubs.hemmings.com/clubsites/oana/tech/shackles.pdf
 

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Discussion Starter · #11 · (Edited)
does any vendor produce these things for sale? Or would this be something I'd need to find the designs for, and bring it to a machine shop?

Just curious. I would like to greatly improve the handling of the GT. I know it's capable...but just not quite there yet.

ps. Ok, I think I've gathered exactly what the shackle is and does. Effectively allows the pivot point of the spring/control arm/wheel hub to move in and out, as opposed to just up and down. Gotcha!

And yeah, would this be a better alternative to just new springs? It sounds it. (as a side note, I was thinking of getting those Koni adjustables from OGTS, if that's a bad idea, let me know :) )
 

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Front Leaf Spring Binding? Slide It?

OK, Bob, I see why the shackle has to be a two inch drop, since the lower spring-eye bolt has to clear the A-arm. I don't really want to lower my GT (at least not yet), and in any event, I am not fussy having a shackle hanging down another two inches below the A-arm, looking for all kinds of objects to hit. But I REALLY see the problem with the spring "binding", especially now that I assembled my front suspension this evening. I installed Polyurethane bushings all around, including the spring eye bushing. Even with the proper leaf spring compressor I built, I had quite a time getting the spring-eye bushing to line up to allow the bolt to be inserted. The leaf length and the A-arm spacing wouldn't quite equal each other. I finally had to loosen the A-arm perches, fit the spring-eye bolts, and then tighten the perch bolts. Which instantly caused the bushings to deflect. But I wondered if there was another solution.

Could I build a leaf spring "slider" such as I've posted below (it's a rear leaf spring from "Beyond's" Nova, http://iola.com/71nova/ . He's the fellow who built that really nice red GT for his daughter and posted the photos)? It wouldn't have to be so elaborate, just a short "slot" really, in place of where the spring-eye bolt normally goes. I noticed that when the suspension is unloaded (no weight), the leaf tries to pull the A-arm outboard, and the A-arm bushings are taking the deflection. But when I compressed the suspension to a "normal" position, the A-arm was fairly neutral. So I suspect that as the suspension loads (over a bump), that the leaf will be trying to pull the A-arm inboard.

I was thinking that it could be a slot about 3/4 inch long and the width of the bolt (8 mm) wide, perhaps reinforced with a outer plate, centered where the existing hole is. Then a teflon washer on each side of the A-arm wall, backed up by a steel washer on each side. The spring-eye bushing and inner sleeve would have to be machined a bit narrower, but there is lots of material to make room for the slide washer.

What do you think? Would 3/4 inch be enough, or even too much? It seems that the bushings only deflect at most 3/16 inch each way, so perhaps even a 1/2 inch slot would suffice. The only problem I really see is that the spring isn't "fixed", as it is in the Nova (the rear eye is fixed), so it might want to slide from side-to-side. But I doubt that it would, once it was centered. The other problem would be to ensure that the slider "slides". The Nova's slider looks like it has some kind of horizontal rail above and below the slider, which a roller washer runs in. Hopefully the loading on the GT would't require that elaborate a mechanism, but it could be done.

You know us damn Engineers, always trying to improve on things, even those that worked well enough when the first Engineer was done with it. Of course, he's long since gone by now!
 

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Those sliding leaf-spring mounts are an off-the-shelf item. I bought some years ago, they're too big to readily adapt to a GT.
And there are two types, the 'UHMW polyethylene slider' type and the roller-bearing type. Regardless, I would not trust simple slotting the stock a-arms, even with reinforcements. Especially with an 8mm bolt.....

For my Kadett's front suspension, I cut off the eyelet from the spring entirely, and made a friction block from HDPE (High Density PolyEthylene) with a steel backer, which bolts to the underside of the a-arm. The ends of the spring ride along the HDPE block, and don't bind. BUT, I had to locate the center of the spring so it wouldn't move side-to-side. I did that by first bolting the leaves of the spring together with countersunk bolts from the top (a bitch to drill, spring steel is!). Then, I put vertical slots in the sides of the leaves at the center, and welded steel tabs into the crossmember that those slots ride on, therefore allowing vertical movement but no side-to-side movement. I have no photos of this (pre-digital era for me), but I recently got a scanner. Once I figure it out, I'll scan some old photos I took of this front suspension design.

Oddly, when I've checked for movement of the leaf spring relative to the ride height, compressing the front suspension has always resulted in the spring elongating (as it 'de-arches'), which puts the loading on the a-arms in the 'outward' plane. You can see this when you put the shackles on a GT. Lay on the ground and look at the shackles, then have someone push on the fender from above....you can see which way the shackles pivot...it's amazing how much they really do move.

Bob
 

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Discussion Starter · #14 ·
Bob,
Do you have a set of Shackles I could buy from you? Or do you have the dimensions spec'd out so I could have a shop make them for me? Thanks

Jim
 

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RallyBob, I have a GT with a stock binding front spring and rock hard spring bushings. My idea is making poly bushings with a larger ID ( 1/2 ? ). I was going to keep the stock diameter spring bolts. With the extra clearance the spring should be able to move through its arc without binding on the lower A-arm.
I like the shackle setup but don't want to lower my car.
Any imput would be appreciated.
Lyle
I own a machine shop so making bushings or shackles is no problem.
 

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Was also thinking about making the front spring bushing a tight fit on the bolt and clearance fit on the OD. The bushing would then be able to move inside the spring end.
Lyle
 
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