Home Ford Revisiting Mustang FOX/SN-95 solid rear axle issues for HPDE

Revisiting Mustang FOX/SN-95 solid rear axle issues for HPDE

2017-12-09

For FOX/SN-95 Mustang driving enthusiasts who are interested in suspension and handling modifications for HPDE events, the solid rear axle suspension has always been the single worst issue of the car. In our opinion (having owned and tracked a dozen of them), Ford should have moved onto an independent rear suspension by the end of the nineties. And indeed that was the plan: an all-new Mustang based on the MN-12 platform (Thunderbird/ Mark VIII) with it’s short/long arm independent front and rear suspensions was in development but was cancelled due to cost and declining sales… much later the same idea was explored again with the state-of-the-art DEW-98 chassis but was again cancelled. More about the lost history of the SN-95 Mustang IRS here, and the mis-steps of the missing S197 2005 Mustang IRS here.

But here we are in 2017…. and the Mustang finally got it’s independent rear suspension across the board for 2015 and the new suspension is a rousing success. But what happens to those driving enthusiasts who might want to take up the HPDE hobby with a FOX (1979-1993) or SN-95 (1994-2004) solid-axle Mustang? Is there a band-aid for the solid axle, or should you just transplant a independent rear suspension from the 1999-2004 SVT Cobra? The answers are yes (of course) and yes (absolutely, although it will cost you).

Let’s start with the solid-axle Mustang. After all these years, a FOX (good) or SN-95 (better) Mustang can still be a viable starting point, with a very low initial cost and room to grow as your skills develop. Here’s one of our authors, at Nelson Ledges in 1991. The Mustang perfectly fit what we’d call the “80/20 rule” then – with some preparation it was as potentially fast as 80% of the cars you’d encounter at these types of events, with room to grow thru further modification. Of course it’s driver skill that is most important, but as that develops you’ll quickly find the inherent weakness of the solid rear axle.  

The Solid Rear Axle Issue

Let’s say that you’ve already done all the basics to take the stock car to the next step: springs, shocks, swaybars, wider tires and wheels, and performance tires. Now your driving skills are starting to improve, and you start doing really well on the track, held back less by the car’s original traits.

But behind the scenes your solid axle suspension is being stressed more than it was designed for. Then at one event, while in a particularly hard turn, you hear a metallic grinding noise from the rear of the car. Maybe you even get black-flagged when the corner workers spot a couple inches of your axle hanging out one side of the car during certain turns (yes, we’ve seen it happen)! Or perhaps you just happen to look over your tires during the event and notice some large chunks of sidewall missing – maybe even the inside lip of the right rear wheel has been ground off! Your weekend has just been ruined, you can’t continue. In fact you can’t continue doing events like this at all without a solution. What to do?

17×9″ Mustang Cobra R wheel with BFG 275/40-17 rubber

If you are very lucky, this is all the damage you’ll have – at first. If not, you’ll notice much larger chunks of rubber missing, along with some of the wheel rim! At worst, you will experience a disastrous sudden deflation out on the track.

And eventually an even worse problem will surface: the wheel and tire will eventually impact the exhaust pipe… rubbing it away. Here’s the same car after a couple more events:

In a twisted way this is a good thing… before Ford ever so slightly realigned the exhaust pipes for 1996, the wheel would have hit the inside of the frame and done even more damage. With the stock 8″ wheels, the problem is less noticeable but still a major issue. With 9″ wheels, which the car is inherently not designed for, it will be a major stop-dead issue for the track events. And don’t even think about “hammering” out the issue. 

The problem is that the architecture of the solid axle suspension is so poor, and the bushings in the control arms wear so quickly, that the entire rear axle is actually moving as much as 4″ in either direction side to side! All solid axle FOX and SN-95 solid axle Mustangs share the exact same issue when stressed. And as the bushings are squeezed and the axle moves, the geometry of the rear suspension is also affected – the entire axle wants to move in the wrong direction to your turn. We’ve actually observed this on track, as an instructor watching Mustangs from the passenger seat of a student’s car. 

You may remember that when Ford raced a prototype 1995 Mustang Cobra R at Nelson Ledges, the press reported the exact same issue as we’ve seen here. Ford decided to leave it alone since racers would address the issue (with the same solution as we propose below), and since an IRS was already under development – in fact this likely accelerated it.

Everything we say from this point forward applies equally to the FOX and SN-95 Mustangs, since they are the same platform and all parts are interchangeable.

The Solution

There are two solutions to this issue:

  1. Obtain an IRS from the 1999-2004 SVT Cobra. Issue resolved (and many other benefits to be found)
  2. Install a panhard rod, and optionally a torque arm.

A panhard rod is the minimum solution, the quickest solution, and it solves the immediate issues with the least impact to your chosen spring/shock rates or ride harshness.

A word on replacing control arms: replacing control arms will not solve this problem, although they might delay it slightly, and in fact could make it even worse. Most control arms for solid axle Mustangs are designed for straight-line racing and most of those use plastic bushings which allow no axial movement (a requirement of this suspension architecture) and will likely cause binding of control arm movement – making a bad problem even worse in turns. If you need to replace the control arms (after all your Mustang may now be as old as 39 years), select ones with stock bushings (or better yet Ford’s Police-spec upper control arms with their higher quality bushings) and then install a panhard rod. 

Why didn’t Ford install a panhard rod (or even a torque arm) from the factory? It’s not possible given the position of the gas tank to the rear of the axle – any kind of crash would likely spear the gas tank, resulting in Pinto-esque issues. When Ford finally added a torque arm and panhard rod to the Mustang in 2005, the gas tank was moved forward to a safe position under the rear passenger seat.

When you choose a panhard rod, look for strength. The panhard rod is designed to control side-to-side movement, and the axle and the forces acting upon it are massive. All unsprung mass, BTW.

Here’s an example: a straightforward and strong panhard rod we purchased back in the day for our 1994 SVT Cobra.

On the left side, the panhard is located by a vertical brace welded onto the frame (dark gray in this picture, located 1.4 of the way over from the right)

Two points here:

  1. The vertical brace may possibly impact your exhaust system. Only a few brands allow use of stock pipes. You will probably need to do some exhaust pipe re-bending, and that’s a royal PITA. Visit your local exhaust shop.
  2. The brace is under a lot of stress – cheap designs tend to rip off the frame (the best known brand of panhard on a friend’s car did just that, twice – until we jury-rigged some massive custom bracing and welded it in place).

When buying a panhard:

  • Good designs have another brace bracing the vertical brace to the opposite frame (this brace also has to be welded in – and you have to protect some gas lines while you are welding). In the picture below, this is the brace leading upwards at 45 degrees away from the vertical brace.
  • Poor designs have this brace going only halfway up – stopping at and bolting into the spare tire well. This is absurd – the spare tire well might be a good intermediate point to add a tiny bit of strength but it is not strong enough on it’s own! Be sure you get a brace that goes side-to-side.

Before you buy, if you can, do a test fit underneath your own car to check for exhaust system clearance. “Your mileage will vary”.

The long arm has a bushing at each end, and it should be lubricated periodically. It needs to allow free movement throughout it’s arc. It should also be adjustable in length, to accommodate different ride heights. Note the brace below, which bolts on next to the lower shock mount. This is another area which you should weld on carefully instead of relying on a bolt. Note the clearances all around, and that you still have your choice of rear sway bars. These days there are many fewer choices for vendors… however our product recommendation (having been involved with buying and installing several) is  Maximum Motorsports. It’s strong, light, well designed, does the job well, and clears most 96 and up exhaust systems. Strength and design are the first considerations, clearance is secondary. MM just happens to have both!

Now there is yet another solid axle problem you’ll encounter very soon.

The solid axle tends to push it’s lubricant into the right axle tube as the fluid heats up. The (standard) vent then expels it – all over the right rear wheel, tire, and brakes. At first you’ll notice some very fine misting of oil on the back bumper. Later on, as your handling improves (and the lubricant therefore gets even hotter), you’ll notice large gobs of oil everywhere underneath and outside the car. You may even spin off the track. Just what you need!

Solution: an F-150 vent, and a line to carry the vented fluid away

In this example, the owner has just added a short section of tubing for the fluid to accumulate in. The line ends just behind the gas tank. In practice, the lubricant never gets hot enough to spurt out the end of this line because the length of it is enough to cool the lubricant sufficiently. 

No, this is not SCCA legit – and it’s not the best total solution. There should be a collection container for the fluid. You don’t need to spurt this onto the track.

So here are two problems resolved (not solved, but just re-compromised).

A torque arm, in our opinion, is overkill (and requires entirely different shock and spring rates)… by the time you buy it., install it, get matching spring and shock rates, and tune it – you may we;l have spent as much as you would getting an IRS and it stil doens’t solve the issue of the massive sprung weight. In fact it’s just a band-aid itself, since it it attempts to prohibit the movement of the axle. An IRS is the only permanent solution – so let’s talk IRS instead.

SVT Independet Rear Suspension

The IRS suspension designed for the 1999-2004 SVT Cobra will be the single best suspension possible for the FOX/SN-95 Mustang. The geometry is vastly improved (even adding anti-dive) over any form of solid axle (without or without panhard and torque arm), and unsprung weight is vastly reduced. The IRS assembly weighs a little bit more than the solid rear axle, but this helps offset the nose-heavy weight balance inherent in the FOX/SN-95 platform. SVT engineers worked hard to develop this suspension, with extensive track testing, and those of us who have met and talked to those engineers understand the extent of their good work. And those of who have tracked Mustangs with this suspension (versus uninformed naysayers who have not) regard it very highly. It also opens up an entire new world of abilities along with new avenues for tuning. When it originally came out, it was the ultimate rear suspension for a Mustang, and is still regarded as such for the the FOX/SN-95 Mustangs.

Better yet, it’s an easy bolt-in and a variety of spring rates are still available to suit it’s use in track events. We’ve owned 1999 and 2003 SVT Cobras that were both heavily tracked, and we modified both with different spring and shock rates. We also transplanted an IRS unit into a 2000 Mustang in the year 2005.

People even bolt them into Mustang SVOs, which when combined with a DOHC cylinder head approximates the ideas that SVO was experimenting with for the very late 1980’s SVOs (a DOHC cylinder head was planned for the 1987-1988 timeframe).

We started tracking out 1999 Cobra in stock form… but then as Mustang owners tend to do, we took it to the next step. We worked with SVT engineering late in the year 1999 to select springs and shock rates that approximated the upcoming 2000 SVT Cobra R – which is the ultimate expression of the IRS with the best bushing rates overall. We started with 750 pound springs for the IRS, and eventually moved to 1000 pound springs – rates you could never use in a sold axle car but which worked perfectly in the IRS (in fact the class-winning Multimatic Cobras used as high as 1500 pounds depending on the track!) And in our 2003 SVT Cobra, we just used Cobra R springs and shocks.

Unfortunately for us, SVT was supposed to release the complete IRS suspension in their parts catalog but couldn’t keep it in production past 2004 as they would have needed to. So one will need to be located and purchased and there is still market for these. That market is small, so jump on the opportunity when you get the chance. Be sure to buy the entire unit, including rotors, shocks and springs (because their rates are entirely different from solid axle-stock and those stock parts will not work). The calipers are same as SN-95 disc calipers, even tough the rotors are vented instead of solid.

Installing the IRS is very easy because it is literally a bolt-in. The pinion is in the same place, so the same driveshaft is used unmodified. The brake lines are identical in any cars that already have rear disc brakes. The forward mounts bolt into the lower control arm body mounts and the upper mounts bolt into any car that has “quad shock” mounts (and can be easily adapted to earlier cars). The only other change that will be needed is to use the IRS-specific exhaust system, which winds under the IRS and then around the gas tank. Even that bolts into place.

We literally could have completed this swap in a couple of hours, and as it was we did it in a driveway with only 2 floor jacks and a few sockets. And while we were over-prepared with a spring compressor, we found that we could just set the springs in place and raise the IRS into place with a floor jack (something we wish we’d thought of when we were changing springs in our own SVT IRS Cobras). The unit we installed above did not come with rotors, so they are not shown here.

Summary

In summary, we’ve covered the issues with the solid rear axle, the necessary panhard rod modification, and the ultimate rear suspension – the SVT IRS – for the FOX/SN-95 Mustangs. Following this path will provide the solution (partial and ultimate) to the issues inherent in the archaic solid rear axle.

Related Articles