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Discussion Starter #1
Ok, got a question for everyone that's done any sort of performance driving in their GT....

What's the bad points of the front suspension?


I've been doing research on making a coilover system for the front. It looks like I can adapt a Mustang II IFS to work on the GT, I have to do some measuring of A-arms, but it looks like it'll work. Plus I'll gain ease of finding parts, a 5-bolt hub, and lots of brake options.

I'm just looking for the bad points of the stock suspension so I can engineer them out. The leaf spring is a given, and I've also heard that the front/rear track is pretty bad, I'm thinking about narowing the rear end a little bit more, and widening the front some to even it out. Anything else anyone can point out?


Thanks
 

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Well, if you're swapping a complete Mustang II front end in, then the GT's front end problems aren't an issue anymore, since you'd be replacing all the offending parts. The only potential issue is the front suspension cradle "height" relative to the GT chassis. If it were installed incorrectly, and you compensated by lowering via coil-overs drastically, you'd start to create some more problems based on geometry changes. However, if the spring rates, motion ratios, and front end weight were calculated correctly, the cradle could be modified to fit the GT chassis correctly, and all would (should) be well.

I'd leave the front end as wide as possible, and try to make the rear axle come in at about .50" to 1.00" narrower. This can be acomplished by using wheel spacers, or swapping to a wider Manta rear axle perhaps. The rear suspension of a GT is really just as bad as the front suspension, and must be modified to exhibit decent handling. The trailing arm geometry is bad, the panhard bar geometry, the lack of sway bar, the shock inclination angle reduces their damping effectiveness, etc. At this point, why not completely swap out the rear axle and design a proper rear suspension too? Check out what Travis has been doing for his GT's rear axle, using a Toyota truck rear end (light, cheap, strong, and lots of available parts). He will be revising the suspension mounting points too, to make the car handle better, as he will be running Solo II events and perhaps other forms of racing later on.

Bob
 

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Discussion Starter #3
Sorry, should have mentioned the rear end too, I'm planning on replacing that with a coilover system as well, that was a little more straight forward for me than the front suspension though.

I'm planning on using a Ford 7.5" axle, narrowing it to fit (- 4") just transfering the trailing link and panhard bar mounts over, converting it to coilovers and putting disk brakes on it. Using the Ford axle gives me lots of leeway for power, gear and differential options, and brake choices.

For the rear I was just planning on using a rear coilover suspension kit from Summit Racing, it comes with all the hardware for mounting. I guess the only problem I have with getting the rear correct is getting the placement of the upper mounts in the right spot for best performance.

What I was planning on doing was instead of having the shocks angle out like the stock, have them as vertical as possible with the pivot points going front to rear instead of the stock left to right on the lower mount.

I'll take a look at how Travis is doing it, and see how it comes out. It'll be a while till I'm to that point. Oh, and yes there will be sway bars on both ends, I don't like body roll, and will probably be doing some autocrossing with the car once I get it done.


As for the front, I wasn't planning on lowering it much if any, I'm wanting to run 15-16" tires on the car, and have found out that 205/55R16's will fit on a stock suspension, this would be just about perfect for what I want. Plus widening the front would make it a little easier on wider tires for turning and not hitting the wheel well.


Thanks for all the points, this is what I was wanting to find out.
 

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So if you are going to swap the rear axle over to another brand anyway, and use coil-overs, then just get rid of the Opel suspension links...they kinda suck anyway! Make your own trailing links (three or four link trailing arms, with longer arms and proper geometry), a longer panhard bar with correct geometry, and position the coilovers as near vertical as possible. I wouldn't try to situate the coilovers front to rear, I'd still keep them out back and as far out on the axle as possible. The only reason you usually see shocks that are staggered front to rear is when the original suspension design is lousy and has wheelhop! Very common on leaf-sprung rear axles actually....

That's a tall tire you're going to run, although that is a very popular size. I have 205/45-16's on my father's GT currently, and right now am deciding whether to go with 225/40-16's or 245/35-16's on his car (same rims, 16 x 7.5"). The circumference will be nearly the same as the stock 13's with any of those tires, in fact the latter is a tad shorter than standard. But his car is lowered 2.5" in back and 2.875" in front...wanted the "touring car" look.

Bob
 

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Discussion Starter #5 (Edited)
Ok, this is what I was wanting to find out, I didn't drive the car all that much before I tore into it, so I didn't know about any performance issues with the suspension. I got a few books on Auto suspension today, so I'll be looking those over for some ideas.

Also as for mounting the shocks, what I ment is instead of the shock pivoting side to side (mounting bolt running front to rear) change it so they pivot the other way, but keep them in the rear of the axle. That way it makes it easier for mounting them... the kit comes with a cromemoly tube that they use for the upper mount, and just weld a few tabs on it to mount the shock to. This would also make it a little harder for the axle to go lateral.

I'll have to look at my Eagle Talon a bit, it's got a solid trailing arm system with panhard bar on the rear and it does pretty well, it's almost all I can do to get the rear end to break traction and drift, would be a little different with the power on the rear though.

As for tires, I think the 205/55ZR16's are only 1.5" bigger in overall diameter than the stock 13's, we'll see how it works though... I'm kinda wanting to run BG Goodrich G-Force T/A KDW's, and didn't want to use the 205/40ZR17's that are the same diameter as the 13's... I think 17's would just look too big on the car, and kinda like having a little bit of sidewall.
 

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Unfortunately coil-overs are not in the budget at the moment, which is why I went with the GT spring buckets. I am reworking the trailing arm, panhard bar and shock mounts but I am not going with a four or even three bar link. I just didn't want to cut into the floors which would have been necessary to achieve reasonable geometry on the upper link/links. I will just construct a simple torque arm and connect it to the bottom of the floor(not rigidly). I'm going to go a little shorter than stock and keep the mount as high as possible to improve anti-squat at the risk of introducing some wheel hop. Until I try, I won't know if this is going to be an issue or not. I can always change it later...

-Travis
 

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Discussion Starter #7 (Edited)
Sounds good, have to let me know how it turns out.

blah, nevermind, just re-read over stuff and figured it out :rolleyes:
 

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The biggest problem with the front suspension is the bump steer... which is very bad. The real fix for this is to relocate the steering rack. It needs to move down 3/4 of an inch. To do this you have to cut clearance in the front suspension cross-member, and then weld in re-inforcement where it was cut.

With this fix, the geometry is very good. The German engineers did their homework on the caster and camber gain. OK... it's not very adjustable, and changing leaf springs is a pain, but once it is set-up, it works remarkably well. We've won 5 regional SCCA regional road race championships with the stock geometry.

Coil-over set-ups are problematic. (We've built three different versions trying to get one we like.) The biggest problem is that there is no suitable structrue for the top of the shock/coil spring. The original shock tower is too flemsy... we have cracked two of them under road racing conditions just below the upper "A-Arm" mount. (This should be re-inforced for any serious agressive driving) (I'll try to put up a picture in a couple of days.)

Even with the re-inforcement, the top of the shock tower just bolts to sheet metal fender well. So with a coil-over set-up (that puts all of the forces into the fender well) the fender well flexes... a lot. You can put a "strut" between the two shock towers and this helps the "in-out" flex, but does not cure the shock tower/inner fender flex under hard brakeing. (It gets your heart rate up when you're at 120+ MPH... jam on the brakes for a 60MPH corner, and the fatigued sheet metal starts to rip.)

So, the real fix to all the problems is to fabricate a full custom suspension cross-member that relocates the steering rack to the proper place... use tubular A-Arms with adjustable rod-ends..., and a series of tubes and plates welded in to the "frame horns" and fire-wall reinforcing the top of the shock towers . In reality, this is far too expensive for all except for a couple of fanatics and the 6 Opel GT race cars in the US. (By the way, to do this the best way, Tom Drake cut off the front of his GT4 Race car at the firewall and "tube-framed" the whole front end. His is the best I know of... Maybe Rally Bob has some pictures of Tom Drakes "front-end" that he can post.)

So... what do we do for our race cars??? Well, after trying to come up with a good coil-over set-up we never found enough difference to justify the expense. We just lower the steering rack, reinforce the cross member/shock tower, use a strut brace between the shock towers, and "live with" the $%$R%@$ leaf spring. (There are a few different leaf springs available, and you can have them custom made a lot cheaper than building a coil-over suspension. Rally Bob experimented with a bunch of leaf spring stuff, so he can probably shed more "light" on what is still available.)
 

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Actually, when it comes to bump steer on a lowered GT, the rack would have to be raised up to counter the effects of the tie rod ends pointing up at the steering arms, either that or the steering arms must be drilled out, and modified tie rods with heim joints fitted to the underside of the steering arms rather than to the top of the steering arms. This levels the tie rods and reduces the bump steer, and it can be shimmed to compensate for variances in ride height. Another method is to drill out the steering arms, and machine an insert with a taper (as the original arms had), butg install the machined insert inverted, so the stock tie rods install from the underside.

Bob
 

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Discussion Starter #10
Here's what I was planning on doing for the front suspension, I was going to base it off of a Mustang II IFS suspension... like the one all the hot rod guys put on their rides.

Below is a picture of the stock GT, and what I'm going to use for a design...





Notice the one I'm basing off of has the upper shock mount built into the crossover. I'm planning on just building a whole new crossover that will bolt up to the stock location, judging from the placement of pivot points on the stock suspension it should all fit just fine.

I wasn't planning on using the stock upper shock mount, I could tell right away that it wasn't up to supporting an agressive suspension. This way all of the suspension stresses are on the crossover, and thus transfered to the unibody frame.

I was also thinking of redoing some of the front structural crossmembers like the radiator support, partially for stiffmess, and the other reason is because I cut it out in frustration when I was taking the engine out :rolleyes: (didn't have the room to drop it out the bottom)

Anyways, hope this gives you some ideas of what I'm planning on doing.
 

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Suspension geometry and especially bump steer can fake anybody out. So here's my recommendation before you go cutting and hacking into your car:

First, go buy one of these two books by Allan Staniforth.

Competition Car Suspension: Design, Construction, Tuning
http://service.bfast.com/bfast/click?bfmid=2181&sourceid=39053434&bfpid=185960644X&bfmtype=book

or

Race and Rally Car SourceBook: The Guide to Building or Modifying a Competition Car
http://service.bfast.com/bfast/click?bfmid=2181&sourceid=39053434&bfpid=085429984X&bfmtype=book

These books are very good. They have lots of diagrams and ways to figure out what you need before you start cutting and welding. Read the books at least twice so you'll have a good understanding of what is happening to the suspension geometry.

Next... AND STILL BEFORE YOU START CUTTING AND HACKING... OR ORDERING PARTS.

Buy a software program called "Front Suspension Geometry Pro" We have been using the versions of this program for about 5 years. It is a very good software program that lets you measure and enter your info directly from your car. Then before you cut, weld or buy anything, you can enter alternate dimensions and see the results. (In the case of bump-steer you can see what happens when you move the inner tie rod pivots (rack) up or down.) You can also enter alternate A-Arms, pivot points, outer tie rods... all without spending money on parts or cutting and welding. We use this every time we start a new project and it has not let us down yet. Although it costs $119.95 it will save you far more than the cost of doing your suspension twice. We don't sell this one (wish we did) but you can buy it online at:
http://www.auto-ware.com/software/asgp/asgp.htm

By the way, the Auto-Ware web site shows a bump steer graph (about the middle of their page) that is very bad... the stock Opel GT is worse.
 

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Discussion Starter #12
Thanks for the Info, this is the kind of things I was hoping to find out about. I've still got a little while till I get to doing the suspension, gotta rebuild the engine first.

I was planning on just copying the dimensions of the IFS kits, but I'll prolly be getting the the rally car book and do some reading before I get it all set in steel.

But for the whole thing I was going to buy just the tubular control arms that were set up for coilovers, and then I can get the steel for next to nothing and have all the equipment for cutting/welding.


Thanks for all the info though, I might have to buy that program also
 

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Hey Maddyn I just got back from a deployment to McChord AFB yesterday
I would have loved to hooked up with some Opel People.

i did see a GT parked out side of a Dinner, but i did not get to talk to the owner

Bob and Bob are a wealth of info

The problem i have is there are a lot of books and stuff that tell you what you have and not much info on what your car needs

Still, I just went out and bought Comp car Suspension.

It looks to be a good book.

My favorite is Chassis Engineer by Herb Adams

I never bought the software program since it never said anything about front transverse leaf springs

but i think there is a demo at Performance Trends.com



anyway we just had my GT modified for Coilovers last month

still haven't bought the shocks yet. so any suggestions on shocks would be great

some of the problems i see with the front end with the GT are

Bump steer (already mentioned)
overall track width (need fender flares if the problem is corrected)

weight on the front of the car and overall weight of the car

A-arms of the car are a problem both upper and lower

the non adjustability of the Suspension

Now i am thinking racing if you have a street car you could do a plus 2 and put z-rated tires on it and you will do pretty good

except for the Bump steer

David Ligon
Davegt74
 

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David,

The software is for the geometry for the front end, not to help you select springs. What really counts is all of the suspension and pivot points for the suspension and steering. So, you can just enter some arbitrary number for the springs, and the software will think you have coil-overs.

Once all of the pivot points are entered, you can check the camber gain (as the car rolls) or the bump steer. Once you find out where the faults are, you can see what modifications will make it better. For instance, you can give the software a different longer A-Arm length (from the chassis pivots to the lower ball joint, and then see the resulting effect in camber gain.

Using this approach, you can do lots of "what if" without having to buy the parts or cutting up your car. Then, after you figure out exactly what is needed, you only have to buy/cut once.

By the way, there's nothing inherantly wrong with the traverse leaf spring as it concerns the suspension geometry. The A-Arms don't know if it's a leaf or a coil-over. The real problem is the lack of a variety of different springs to suit different race conditions/tracks... and they are a pain in the butt to change.

For anyone thinking of modifying their front suspension... or fixing the Bump steer, buy the software. It will be money well spent.
 

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http://www.performancetrends.com/

Do you think one of the programs on this web site is the one you are talking about?

one of the things that came up in the coil over conversion was how long the A-Arms should be

we all know we need longer lower A-Arms but how long

what about wheel travel when i got to the nuts and bolts of the mod a decision on wheel travel had to be made

i ended up not changing the lowers A-Arms

maybe later after i give the program a try

i did try the demo and it was of no help

David Ligon
Davegt74
 
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