Fox Thunderbird Rear Control Arm Upgrade and Pinion Angle Check

This article assumes you have a basic understanding that Fox chassis "stamped type" upper and lower rear control arms are inferior to aftermarket boxed units. The article also assumes you appreciate the snappier suspension made possible by eliminating control arm "flex" or "twist" while carving a turn or simply driving hard. I'll reiterate that I am not a racer, so I cannot dissect and explain the advantages of aftermarket control arms for specific racing applications although you can be sure that a more rigid rear four link suspension will make for a sportier ride.

The 83-88 Fox chassis Thunderbirds and Cougars have a great deal in common with 79-93 Mustangs with some minor differences that can enable or negate interchangability of suspension and braking components. The varied and numerous manufacturers of fox chassis suspension components for Mustangs, make the 83-88 Thunderbirds and Cougars an inexpensive option for a serviceable and easy-to-upgrade Ford "sports" project. However, most of the manufacturers of Mustang suspension and braking components do not "tend-to" the 83-88 Fox chassis Thunderbird/Cougar market. This means, product literature and part descriptions will not usually spec 83-88 Thunderbird/Cougar interchangability even though the product might work perfectly on the bird, the cat, AND the pony. The performance camp of 83-88 Thunderbird/Cougar enthusiasts, largely served online by COOL*CATS ::*The 1983-88 Mercury Cougar and Fox T-Bird/Cougar Forums - Powered by vBulletin, have long been used to banding together to isolate and determine what aftermarket Mustang parts will interchange. I referenced these sites on many occasions before taking on this upgrade.

In the following pictorial, I will show you the steps I went through to properly install Lakewood rear control arms on my 1985 Thunderbird.


These are the Lakewood units I picked up. The lowers (boxed units with red urethane) are PN 20150. They are spec'd for 79-98 Mustang, although they measured just a 1/4" shorter center-to-center when compared to the factory Thunderbird lower control arm. The shorter distance requires a verification of acceptable pinion angle, a topic I cover at the end of this article. The upper control arms (tubular units with yellow urethane) are PN 20154, they are adjustable and are spec'd for 80-85 Thunderbird by Lakewood. PN 20150 run $164.00/pr and PN 20154 run $250/pr per Summit Racing.


I supported the rear of the car on the subframe just in front of the rear wheels


Removed both rear wheels with an impact wrench


Began removing the rear sway bar


Sway bar removed


Using a jack, I supported the rear axle by lifting it about 2 inches


Next, I unbolted the axle side of each "quad shock"


And unbolted the axle end of each shock


Next, I lowered the rear axle


And unbolted the axle end of each upper control arm


I lowered the axle further with the jack


With the shocks unbolted and the axle lowered, I could pull out the rear coil springs easily since they were now completely uncompressed


At this point, I dropped the exhaust in order to gain access to the lower control arm bolts


I removed the bolt(s) holding each lower control arm to the chassis


Then I removed the bolt(s) holding each lower control arm to the axle.


Lower control arms removed


I began prepping the new lower control arms with the included grease for the urethane bushings


The urethane bushings slipped in easily, then I pressed in the metal inserts by hand


And finished them off with a vise


As I mentioned in the opening picture, I used 79-98 Mustang lower control arms on my 85 Thunderbird. There is a slight difference in length, the factory Thunderbird arm being about 1/4" longer than the Mustang. As you can see, the difference is hardly detectable in this photo, although I would still go through the pinion angle verification process once both the upper and lower control arms were installed. I address this in the final steps of this article.


With both lower control arms assembled, I installed them using an 80 ft/lb torque spec for the chassis attachment bolt...


And the same 80 ft/lb torque spec for the axle attachment bolt.


With the new lower control arms installed, I finished pulling out the upper control arms


A piece of cake!


The Lakewood upper control arms compared to the factory upper control arms.


In this shot, I am temporarily setting up the adjustable upper control arms to match the center-to-center length of the factory upper control arms.


There was no need to snug the jam nuts on the upper control arms at this point since I'd be setting the pinion angle (relationship of rear axle pinion to transmission output shaft) once the arms were installed. Adjustable upper control arms are primarily designed for adjusting the pinion angle on race cars and or any car with a modified rear suspension. While there is a good amount debate on the subject, it is generally accepted that an "at rest" pinion angle of -2 degrees is desireable. This allows the pinion angle to be at or near zero degrees when the drivetrain is under load. A pinion angle of zero means the output shaft is in a virtual straight line back to the pinion.


Before installing the new upper control arms, I needed to set the coil springs in place. I purchased some new urethane spring bushings since the old ones were wasted.


Here I am setting the coils in place, obviously the rear end was still dropped but on the jack pad.


I set each upper control arm in place.



And torqued down the attaching fasteners on each arm to 80 ft/lbs.


Next, I raised the rear end on the jack and reattached the shocks.


And reattached the quad shocks as well.


Next, I reattached the sway bar.


And bolted the wheels back up.


At this point the car was ready for a pinion angle check and adjustment. This can be done a number of ways but I'll show you how I did it using various online resources. The car needs to be off the ground with the suspension loaded. No jackstands under the rear axle since the rear end needs to rotate. I used Rhino brand plastic ramps from Auto Zone. The car does not need to be perfectly level, nor does the operation need to be done on a perfectly level surface, just get the car in a relatively flat position with the suspension loaded.


This an angle finder, a necessary tool of the measurement and pinion angle adjustment


I highlighted the flat surface on the pinion mounting plate where I could use the angle finder to determine at what angle the rear end was situated "at rest"


Using the angle finder, I found the rear end was tilted up at +5 degrees


Here I did the same on a flat surface at the transmission output shaft which read -5 degrees.


I added the two numbers together to get my pinion angle... zero degrees.


Zero degrees at rest is not desireable so I cinched each upper control arm down by loosening the jam nuts and giving the turnbuckles two complete turns to make the overall length of each upper control arm shorter thus tipping the nose of the rear end downward a couple degrees.


Two complete turns of each upper control arm turn buckle tilted the pinion downward an additional 2 degrees for a total upward tilt of 3 degrees. Adding that number to the transmission output shaft downward tilt of -5 degrees gave me a pinion angle of -2 degrees. At this point I snugged down the jam nuts and toasted myself to a project well done. A test drive proved a much more responsive and agile rear suspension on an already good handling car.

Mustang Rear Lower Control Arms and Shortened Wheelbases

One of the concerns of using fox Mustang rear lower control arms on an 83-88 Thunderbird/Cougar is the pontential for shortening the wheelbase since factory Mustang lowers are a tad shorter (in my case about 1/4"). The picture above shows an aftermarket Mustang lower control arm that moved the rear axle forward a fair amount (left) in another install. Notice the position of the tire in the wheel well. Also pictured (right) is my Thunderbird with Lakewood PN 20150, the lowers installed in this article. Notice the wheel position is normal.

As an option, Carriage House Engineering had developed upper and lower rear control arms specifically designed for 83-88 Thunderbirds and Cougars.

Good writeup...just one thing. Poly bushings on both ends of the lowers and in the uppers will greatly increase bind. Although these types of arms will vastly improve straight line acceleration, it will cause severe problems during turns. There is some real nice info on this site that explains it: Late Model Mustang Suspension Basics

Nice article, and great write up. Have you driven the car yet? The reason I am asking is because of the 2 degrees negative pinion angle. I have found that 2 degrees negative is great for a leaf spring equipped vehicle that goes through movement because of the spring wrap, but a more rigid Fox 4 link might not experience any movement on the arms at all especially with the poly bushings. If there is no vibration, then my post is moot.

Hmmm. No, I've only noticed a really improved driving experience since putting these in. But I am new to these cars so my baseline is relatively low. Regarding the pinion angle, I did a fair amount of research on Fox Mustang sites and found that the -2 was a good target angle.

In fact, I drove the car for week after installing the arms with the uppers set to the factory length (pinon angle of zero degrees). During hard turns, I heard a consistent rattle in the rear. Once I got under there and adjusted the angle to -2, the rattle went away.

I'm really enjoying the easy upgrades on this platform while keeping the "gentlemen's musclecar" mystique.

If Im not mistaken fox mustang arms will work for pre 87 birds but not 87/88 tbirds. It might just be turbo coupes im not 100% on that.

Ive never heard any positive about poly bushings in STOCK control arms im not sure about aftermarket ones though. bind was a big thing, and everything Ive read as with most internet gargle was everyone says it produces bind without any proof. Could it be a myth most surely.

Nice to see someone trying out new stuff though its the only way to learn.

Debunking myths is one of my favorite parts of writing articles on FordMuscle. While most "myths" in our hobby are grounded in some truth... the mistake is people taking them as absolute, as if every application and every version of an aftermarket component is the same.

Other than saying my Bird rides great now, I can't speak to the "binding" from urethane. Maybe I'll notice it in 3 months when the lube starts drying out.

But in this article, I did "debunk" one myth and that's that an aftermarket Mustang rear lower control arm will move the rear axle 1" forward in the wheel well in 83-88 Thunderbirds/Cougars. While this may happen with some aftermarket control arms, it didn't happen with the Lakewood lowers.

Originally Posted by Mikelonis

But in this article, I did "debunk" one myth and that's that an aftermarket Mustang rear lower control arm will move the rear axle 1" forward in the wheel well in 83-88 Thunderbirds/Cougars. While this may happen with some aftermarket control arms, it didn't happen with the Lakewood lowers.

Arnt the earlier bird arms different then the 87/88?

What specifically would be different about the lower arms?

Originally Posted by Mikelonis

What specifically would be different about the lower arms?

I cant seem to find any pictures from my old 88 turbo coupe... I dont recall Im thinking it was the length. a Friend of mine had installed aftermarket control arms on his 85 and at carlisle we were comparing and Im pretty sure the arms I had (stock) were longer.

The latter ones went to IRS, maybe the 88+ is IRS? I'm not really an expert on T-Birds but I know they switched to IRS after the Fox platform.

As for the arms binding, this usually occurs in cornering. The problem is that because the arms are angled, as they move in an arc, they swing inward (for the lowers) and outward (for the uppers). Poly bushing don't allow this twisting action necessary, so binding occurs. For the uppers, because of their extreme angle and short length, it's even worse. That's why most companies don't make uppers for the Foxes, unless the arms are for drag race only, as the only thing appropriate is the stockers. As for the lowers, thats why the spherical/poly combo is popular. However, for a daily driver that isn't a corner carver, a good combo would be MM arms w/ either the stock uppers or uppers w/ a spherical joint (such as Edelbrock or Currie).