Suspension troubleshooting - where to begin

[KansasPaul]

I've inherited a 1990 GT convertible. 48K original miles, parked it's entire life in a garage and not driven much in the last 15 years. My Dad always keep up with required maintenance and the car is visually in excellent condition. My issue is with the driving dynamics of the car - this is where I could use some help. The car has never been abused. When driving it, to me it feels like there is a hinge in the middle of the chassis. The tires are mismatched with one brand (manufactured in 1998) and another brand on the back (couldn't find a date code on the tires) - obviously those need to be replaced. The shocks were replaced years ago but they still appear new with no signs of leaking. The quad shocks are original and likely toasted at this point. I noticed chassis shift (axle movement) during upshifts under power - hence the belief that the quad shocks are bad. Front end does not feel precise and has a slight pull to the right. I realize alignment is in order and I've rolled around under the car but I'm not seeing anything that is obviously in need of attention. Before I go out and buy a pile of suspension parts I'd like to get some thoughts on where to start. I realize that at 33 years of age bushings are going to be hardened but honestly I'm not seeing a lot of cracked rubber bits. Do I just bite the bullet and buy all new bushings, end links, etc. or are there specific bits prone to failure on these cars?

Also, being a convertible there is obvious cowl shake - I'll likely install subframe connectors at some point but what other add-ons help with cowl shake? Do tower braces do anything on these cars? What about K-member braces?

I appreciate input from those who have gone down a similar road.

 

[Noobz347]

You probably need:

All bushings front and rear: https://lmr.com/products/79-93-Mustang-Urethane-Suspension-Bushings
(somebody chime in if you know these kits are missing something)

New upper control arms or new OEM style UCA bushings: https://www.maximummotorsports.com/Rear-Upper-Control-Arms-1979-2004-Mustang-P544.aspx
These are important because they have the OEM style rubber bushings in them. The Prothane in the UCAs can cause some suspension bind that is not good on the street.

Lower control arms it's your call. Most folks swap out the LCAs but it'll work fine with the Poly Bushings.

New shocks, struts, and quads (your call on performance level. If it's a cruiser, just get OEM style replacements. It's still the best ride on a 4-link (Monroe?)).


Oh... And the tool: https://www.maximummotorsports.com/Rear-Upper-Control-Arm-Bushing-Tool-P190.aspx

 

[Bullitt347]

Yes on sub-frame connectors.
Yes on Strut tower brace.
Yes on a 4 point lower chassis brace.
With a vert' you really can not overdo it on chassis support, especially up front.

 

[Mustang5L5]

If you are feeling axle movement, it’s likely not quads but the lower control arms. The quads are doing their best to control the axle but the LCAs are what should be really preventing movement.

To be honest, all the bushings are probably toast. I just went through this on my 2000 converible. Bad bushings and balljoint boots in the front arms, bad steering rack bushings, bad sway bar bushings, and much more.

 

[limp]

When driving it, to me it feels like there is a hinge in the middle of the chassis.

This reminded me of my 73 Z28 I had in the mid/late 70's. I used to say the same thing when I drove it years ago.... never did figure out why...

 

[AeroCoupe]

Replace the front lower control arms (same coat and less hassle than rebuilding the existing ones), change the front sway bar mount bushings, new front seat bar end links, Maximum Motorsport caster camber plates, replace the rear lower control arms with quality units like the Maximum Motorsports units, replace the rear upper control arms with stock replacements, replace the rear upper control arm bushings in the rear end, ditch the quad shocks (no longer needed with the improved rear lower control arms), and replace the front and rear springs. Would consult Maximum on spring choices.

You said the struts and shocks are relatively new so make sure you know what you have on it as the guys at MM will want to know.

As for chassis stiffening you have already heard it from others and I will echo their comments in that more is better. You will want full length weld in subframes. Bolt in units are a joke so don’t waste your time. MM has good ones as does Global West. I run GW’s as I like how they run along side the factory subframe and not blow them. Ground clearance is not lost which is a big deal on my cars as they are lowered.

Another option would be the Kenny Brown subframe system:

Extreme Matrix Chassis Support System 1979-1993 Mustang GT

Extreme Matrix Chassis Support System vastly improves your ride quality and handling and strengthens the unibody, reducing twist/flex. Lightweight, heavy-duty, low profile design. Great for high horsepower cars. Includes Kenny’s exclusive Double-Cross™ Subframe Connectors, Jacking Rails &...

kennybrown.com

Global makes a version of this:

Mustang Competition Subframe Connectors part # 921

Specifically built for Mustangs, Global West subframes are the answer for reducing flex in the chassis. The subframes weld to the strongest areas of the chassis, (bulkhead). They are full length meaning the subframe extends from the forward bulkhead located at the firewall down by your feet to...

www.globalwest.net

It’s a lot but the car is 33 years old and the Fox chassis is not stiff to begin with and then they decided to offer verts.

 

[KansasPaul]

Hey guys, thank you so much for the input. I'll start to put together a list. I'm going to presume that the interior and carpet needs to come out to weld in the subframe connectors?

I expected to spend some money on this car - and I should have time to invest in the improvements. Driving dynamics are more important to me than power upgrades. Once I get the chassis sorted I'll put in a slightly deeper set of rear gears, maybe headers and sort out some minor electrical gremlins.

 

[nickyb]

Nice to see a Fox kept in the family,.
Good luck.

 

[Mustang5L5]

I pulled my seats and carpet out when I welddd my subframes. I’ve seen some put a bag of ice on the carpet in the spot they were welding.

To be honest I don’t recall the floor getting too hot when I did mine, but I wasn’t really checking

 

[Warhorse Racing]

I would recommend replacing all of the rubber bushings. They are all most likely past their prime. A complete polyurethane bushing kit is an option, but there are drawbacks. There's a lot of performance hiding in the spaces between the hard parts of your suspension. Choosing the correct bushings in the correct location is key. Here are some things to consider:

Installing forward-offset A-arm bushings (Steeda poly or Global West Del-A-Lum) in the OEM A-arms will increase caster (that helps with corner entry and straight-line stability).

The poly rear UCA/LCA bushings that come in most complete bushing kits aren't ideal. And there's a big debate about using poly bushings in the UCAs. I've autocrossed 79-04 Mustangs with 3-piece poly bushings in the UCAs/LCAs for years and they perform extremely well. I would avoid 1-piece or 2-piece poly bushings in the UCAs/LCAs. Steeda and J&M offer rear UCA/LCA kits that have 3-piece poly bushings.

The upper differential housing bushing is another area where you can improve the handling of the car. A spherical UDH bushing (J&M) is a good option, but there will be some increased NVH.

Be sure to change the upper strut mount bushings, the steering rack bushings, sway bar bushings and end link bushings.

I know it's unpopular, but I'm a fan of quad-shocks. I use them on my SRA autocross cars. Having driven cars with them and without them (even cars with aftermarket LCAs), I prefer to have them on my cars.

When it comes to chassis bracing, I've used the Stifflers FIT System on my 2000 GT convertible (autocross car) and I recently purchased the Kenny Brown Extreme Matrix Braces for another project. In my opinion, the Stifflers kit is a better design (primarily because the Web Braces have mounting brackets that slip over the ends of the tubes to better allow for differences between cars).

You might not need the full FIT System for a street car. But I think a Fox convertible would benefit from full-length subframe connectors, jacking rails, a lower chassis brace, and a strut tower brace.

Remember to verify that the lower chassis brace you're considering will work with the OEM convertible bracing. The Stifflers LCB didn't fit with the OEM X-brace on my 2000 GT convertible.

If you choose to lower the car, don't lower it too much. That's a common mistake people make with 79-04 Mustangs. Lowering the car too much will make it handle worse. If you replace the shocks & struts, make sure they aren't super stiff (that will make the car handle worse). You might also want to install a larger rear sway bar (25mm) to help get the car's handling closer to neutral. Don't add a larger front sway bar unless you can compensate for the added understeer.

Good luck with the project!

 

[KansasPaul]

I would recommend replacing all of the rubber bushings. They are all most likely past their prime. A complete polyurethane bushing kit is an option, but there are drawbacks. There's a lot of performance hiding in the spaces between the hard parts of your suspension. Choosing the correct bushings in the correct location is key. Here are some things to consider:

Installing forward-offset A-arm bushings (Steeda poly or Global West Del-A-Lum) in the OEM A-arms will increase caster (that helps with corner entry and straight-line stability).

The poly rear UCA/LCA bushings that come in most complete bushing kits aren't ideal. And there's a big debate about using poly bushings in the UCAs. I've autocrossed 79-04 Mustangs with 3-piece poly bushings in the UCAs/LCAs for years and they perform extremely well. I would avoid 1-piece or 2-piece poly bushings in the UCAs/LCAs. Steeda and J&M offer rear UCA/LCA kits that have 3-piece poly bushings.

The upper differential housing bushing is another area where you can improve the handling of the car. A spherical UDH bushing (J&M) is a good option, but there will be some increased NVH.

Be sure to change the upper strut mount bushings, the steering rack bushings, sway bar bushings and end link bushings.

I know it's unpopular, but I'm a fan of quad-shocks. I use them on my SRA autocross cars. Having driven cars with them and without them (even cars with aftermarket LCAs), I prefer to have them on my cars.

When it comes to chassis bracing, I've used the Stifflers FIT System on my 2000 GT convertible (autocross car) and I recently purchased the Kenny Brown Extreme Matrix Braces for another project. In my opinion, the Stifflers kit is a better design (primarily because the Web Braces have mounting brackets that slip over the ends of the tubes to better allow for differences between cars).

You might not need the full FIT System for a street car. But I think a Fox convertible would benefit from full-length subframe connectors, jacking rails, a lower chassis brace, and a strut tower brace.

Remember to verify that the lower chassis brace you're considering will work with the OEM convertible bracing. The Stifflers LCB didn't fit with the OEM X-brace on my 2000 GT convertible.

If you choose to lower the car, don't lower it too much. That's a common mistake people make with 79-04 Mustangs. Lowering the car too much will make it handle worse. If you replace the shocks & struts, make sure they aren't super stiff (that will make the car handle worse). You might also want to install a larger rear sway bar (25mm) to help get the car's handling closer to neutral. Don't add a larger front sway bar unless you can compensate for the added understeer.

Good luck with the project!

This is very helpful. Thank you.

 

[limp]

If you choose to lower the car, don't lower it too much. That's a common mistake people make with 79-04 Mustangs. Lowering the car too much will make it handle worse. If you replace the shocks & struts, make sure they aren't super stiff (that will make the car handle worse). You might also want to install a larger rear sway bar (25mm) to help get the car's handling closer to neutral. Don't add a larger front sway bar unless you can compensate for the added understeer.

In your opinion how much is too low? Would 1" be OK?

 

[limp]

Based on my past experience with bushings, I use Poly everywhere except in the front lower control arms.... I have had squeak problems doing those in Poly....
New to fox bodies, but I did think the rear lower control arms looked a bit " weak" and looked like they could flex alot, so I replaced mine with new steeda LCA... My car will be a street cruiser grocery getter

 

[Mustang5L5]

1" is fine. It's when you go beyond that the control arms start to angle up, and your suspension travel arcs start getting a bit wonky. That's why the MM k-member has two sets of A-arm mounts with 1 set being 1" higher to help correct that.


Like i mentioned before, i just went through this on my 2000 convertible in that all the rubber bushings are pretty much trashed. I only wanted to maintain the car as a daily cruiser, so I didn't want to go crazy on my parts selection, but I picked

Front:
Moog front A-arms
Poly sway bar endlinks front
poly steering rack bushings
Max motorsports caster/camber plates (may not be needed if you don't go too low, but I wanted to get it out of the way)
Bilstien strut
Eiback Pro Kit springs
poly spring isolators

Rear:
Eibach Pro Kit springs
Poly spring isolators
Bilstein shocks
fresh Gabriel quad shocks

Still evaluating if i should change the control arms in the rear but I likely will. For those, i will probably buy a set of the MM sport lower control arms to maintain vehicle comfort, and their stock OEM upper arms. I would also swap out the upper diff bushings at the same time as well. Exhaust heat at the rear of the car just kills all these bushings



I would also recommend Subframe Connectors. Strut tower brace is up to you, but I installed one on my hatch and noticed a difference. I would expect it to have a much more significant impact on a convertible.

 

[Warhorse Racing]

In your opinion how much is too low? Would 1" be OK?

How low you go depends on how you intend to use the car. 79-04 Mustangs using stock-style springs (not coil-overs) handle better when they sit higher than most people think looks cool. I generally don't lower my autocross cars more than 1". My 1992 GT uses Ford Performance C springs. I also use poly spring isolators. They ended up being a little too low up front for autocross, so I added Steeda spring spacers to raise the front of the car .5". That setup performed much better under autocross conditions. Keep in mind that the C springs are advertised as dropping the front .875". I've also made weight reduction mods to the front of my car. Based on my experience, I would hesitate to buy springs that drop the front of a Fox 1".

If you're building a car for performance driving, your goal is to move the limit of traction as far OUT as possible. Lowering the car too much means you can run out of suspension travel when the suspension articulates. Going too low up front makes the car more likely to understeer when driven hard.

Going too low, installing super-stiff shocks & struts, and using super-stiff sway bars moves the limit of traction further IN. A car with those mods will look cool and feel "sporty" driving around normally, but the limit of traction will come up pretty quickly when you push the car.