Getting the Handle on your SVO

By: David Sandiford – SVOCA #0038

Taking what is arguably the finest handling Mustang ever produced to the level of a surgical cornering weapon can be an exercise in refinement.

The factory supplied the basics with adjustable shocks, stout brakes, good basic front geometry, and a favorable weight distribution. The aftermarket adds the pieces to make all those components more focused for the enthusiast driver.

The Mustang (Fox) chassis is of unibody construction, with sub-frames hung off both ends supporting the engine/suspension/drivetrain components. The front suspension is a MacPherson strut, with an anti-roll bar. The rear suspension has upper and lower control arms locating a live axle, also with an anti-roll bar.

These basics are noted because unless you want to spend a bucketful of money, the locations of all these components are fixed. The primary objective in achieving handling "feel" is to keep all these locations and their relationships exactly as FoMoCo designed them, while the primary objective in maximizing cornering force is to keep the tires in contact with the ground.

So, where to start?

First order of business is to replace all the bushings with the polyurethane variety. These bushings are much less compressible than the stock (rubber) units. Your front and rear control arms will now rotate, rather than wallow, around the center of their pivot points. And when you compress your front anti-sway bar when bending into a corner, the entire twisting force will be applied to the opposite side a-arm, rather than partially into the frame. See, reduction in variation of location. On an SVO with old, worn out bushings the transformation is awesome. Make sure you have a gas torch to help get the old bushings out, and a big bench vise sure helps in putting the new ones in. I personally don’t run the poly bushings in the upper rear control arms (possible binding problems), but I’ve heard good and bad both ways.

Next up, chassis stiffening. With all the suspension loads going into the outboard sub-frames, chassis twist is a real problem on the Fox chassis. This is particularly noticeable on turn-in. You turn the wheel, the outside wheel plants (thanks to the new bushings), the spring compresses, the K-member flexes, the front sub and body flex, then the rear end does it’s rollup and flex routine…. and eventually you sort of pogo around the corner. You may even hear the tires do short chirps as they load and unload. Your cornering force is definitely not being maximized.

The front-end fix is pretty easy. Install a 4pt G-Load brace. These install between the K-member and the unibody. Usually no new holes required, and installation time of under an hour. Don’t be surprised if that straight G-Load brace seems to take on a weird twist when you begin to tighten it down. It’s not the brace, it’s the K-member/unibody relation. A 4 or 6pt strut tower brace would be a real nice addition too, but I haven’t found one that clears all the SVO specific parts. A Monte-Carlo type cross-brace would probably be better than none at all.

The rest of the chassis stiffening is achieved by the installation of sub-frame connectors. Buy the weld-on type, you don’t really want to drill holes in your chassis, if you can avoid it. Make sure that the sub-frame connectors have the seat crossbars in them too. Once you have welded these in place, your SVO is now (essentially) a full-frame car. Loads into the sub-frames are now resisted by the entire chassis, rather than absorbed by bending into the unibody. If you want to install additional bracing like extreme subs, jacking or convertible rails, or the ultimate, a street cage; Go for it. Stiffer is better.

By now you’re well on your way to that slot-car feel. Your SVO should feel very crisp on turn-in, and very solid in normal driving. Total cash outlay for parts? Should be less than $500.

Front End Basics

Camber

With all the chassis mounts solidly held in place, it’s time to optimize the front-end geometry. This requires the installation of caster/camber plates. The stock units have limited camber adjustment, no caster adjustment, and use rubber onion heads for the upper shock mount. Aftermarket units have far more adjustment range, and typically use spherical bearings to solidly locate the shock.

Camber is measured, looking from the front to the rear of the car, through the center of the wheels in degrees from vertical. Negative (desirable) camber is when the inside top edges of the tires are closer together than the bottom inside edges. (Think of camber as vertical toe-in) What negative camber does is a combination of pre-loading the sidewalls of the tires (to better resist cornering forces) and also present a flatter profile as the tire deflects during cornering. (Same old idea about keeping as much of the tire as possible in contact with ground, maximizing cornering force). There are some other issues with Ackerman angles, bump-steer, and roll-centers that also affect this, but we’re trying to work with original components as much as possible.

A quick digression … My favorite explanation of push and loose conditions. "A car that’s pushing (understeering) will wreck into the fence front end first. A car that’s loose (oversteering) will wreck into the fence back end first." – Benny Parsons.

Bottom line – More negative camber = better turn-in. Unfortunately, more negative camber also = more tire wear. A car with aggressive negative camber settings will very quickly wear out the inside edges of the tires. It may also be too squirrelly if you mainly drive at high speeds. The opposite is true with minimal negative camber settings. A car setup this way will push the front end (understeer). The outside edges of the tires may become prematurely worn.

So what’s the setting range? Here are some suggestions, gratefully plagiarized from a wide variety of owners.

Stock – ¾ deg negative.

Performance and high speed driving – 1~1 ½ deg negative

Weekend AutoX – 1 ½ ~ 2 ½ deg negative

Hardcore AutoX – Push the struts in until you run out of adjustment.

If you occasionally AutoX your SVO daily driver, you might want to look into the camber/caster plates that adjust camber and caster separately, with lock bolts for each function. This way you can drive to the track, readjust for racing, then put them back to the street setting when you’re going home.

While we’re on front-end adjustments, a few words about setting tire toe-in. For street applications, suggested toe in is –0-; that’s right, flat. While this makes the car more susceptible to cross winds, it is also far more willing to change directions, and well worth the trade-off.

If you AutoX, the hot setup is to set the front-end with some toe-out. Definitely not suggested for the daily driver.

Caster

Caster is measured from the side of the car, through the center of the steering axis, in degrees from vertical. Positive caster (desirable) is towards the rear of the car. By angling the top of the steering pivot point towards the rear, the steering is less upset by road deflections fed into one front tire or the other. High-speed stability is improved.

Performance settings are generally anything over 1 deg positive. The road-racers I’ve talked to really like 2~3 deg Positive caster.

Spring Selection

Always a hot-button issue, there are probably as many opinions on spring selection as there are springs to select from. Can you improve the handling of your SVO by changing springs? Absolutely. You can also mess up your ride by installing the wrong springs.

A few basic suggestions.. If you plan on street driving your SVO, use progressive rate springs. While single-rate springs are great on a billiard table smooth racetrack, Main Street usually has potholes, expansion strips, speed bumps, and railroad crossings. The softer initial spring will absorb these impacts enough to save those fillings you’re almost finished paying for, and the higher secondary rate will keep the suspension from bottoming out when you’re feeling frisky.

Second suggestion, don’t buy 5.0 racing rate springs. We don’t have that big lump of V-8 iron sticking past the wheel centers, and stiff rear springs will cause that light rear-end to dance sideways over the slightest bump, particularly under braking.

500lb rate fronts and 300lb rate rears are more than enough for street performance. If your manufacturer does not have SVO specific rates, select the V-6 rate or the lightest 5.0 "sport" spring they have. One last thing, do re-install your spring pads, or the squeaking will drive you nuts.

The next big question is always "How much drop?". The Fox chassis really likes to be lowered about 1". Going beyond this point is more a decision about looks, and how carefully you like to drive up and down driveways. Or how many front air dams you have stored in your rafters.

The Bump-Steer Question

Your steering rack is connected, through the tie-rods, to the steering knuckles on your strut. As the front suspension is compressed and rebounds, the steering knuckles follow the arc whose endpoints are the lower a-arm pivot centerline and the upper shock-strut mount.

Since the steering rack centerline is fixed in place, as the steering knuckle centerpoints travels away from the rack centerline, the effective length from the steering rack to the steering knuckles changes. To put it simply, the toe-in changes. Sometimes you get more; sometimes you get less. Toss in turning radius, body roll, bumps in the middle of a corner, and you have a pretty complicated set of events taking place.

Bump-steer events run the gamut from barely noticeable, to white-knuckle near-death experiences. SVO’s that have been lowered are very prone to the effects of bump-steer.

Now, the really bad news… The only way you can really find out how bump-steer is affecting your setup is to pull the wheels and the springs. Then actually measure the changes in the width between the steering knuckles as the suspension moves through its travel. I don’t know many people that have actually done this. Or, you can find someone who has done this, and is nice enough to share the information. Which is what I did.

Conventional wisdom says that if you are suffering from bump-steer, you should install a set of offset steering rack mounting bushings.

I have a different suggestion to try first. Try installing a set of 90 and up Mustang tie-rod ends. They have more offset than the stock SVO rod ends, and in many cases reduce bump-steer to acceptable levels.  Another alternative to offsets are solid rack bushings.  These replace the (stock) rubber bushings and takes a little more play out of the steering rack. 

The finishing touch on a steering rack would be the addition of a solid steering coupler in place of the factory rag joint.  Look for a unit with automotive components, not something made from a socket universal.

Rear Axle

The last upgrade area is the rear axle assembly. The primary axle locating functions are handled by the lower control arms. The upper control arms maintain pinion angle, resist axle wind-up, and serve to keep the ring and pinion centered down the centerline of the chassis. Axle hop is controlled by the quad shocks or slapper bars (in the case of the early 1984 models).

Ever looked at the rear suspension when the car is up on a rack? Ever thought to yourself "Gee, these components are a little crude?" Well, you were right, they are.

Replacing the original bushings with the poly’s goes a long way in tightening things up. The weak link (so to speak) becomes those stamped steel lower control arms. If you’re looking for that little bit extra, replace the stock lower control arms with tubular aftermarket units.

Available from a wide assortment of manufacturers, look for a set with spherical bearings at the axle end. Tubular lower control arms take a lot more loading before they start to deform. They’ll hook-up better on launch too. Some tubular control arms are now available with an adjustable spring perch, if you want to be able to change the rear ride height of your car. Be sure to check out the condition of your torque boxes when installing upgraded control arms. If they’re tweaked, (normally only a 5.0 problem) get one of the aftermarket repair/upgrade kits, and have it professionally installed.

Tubular upper control arms are a matter of personal preference. I can’t feel the difference between them and the stock units. If you have a pinion alignment problem, there are some adjustable length units now available.

If you’ve followed us up till now, your SVO now has much quicker response, in transition and turn-in. Your wallet is also about $1000 lighter. Drive the modified car back-to-back with a stocker for a real eye-opening experience.

Adjustable Settings

There are a number of adjustable suspension components capable of dialing in your ride for your driving style.

A few words about changing settings from an old racer: Never change more than one thing at a time. If you do, it’s almost impossible to determine what helped, or hurt your setup. I would strongly suggest that you keep a notebook with a running history of settings, observations, and results. It’s always easier to go back to a setup that worked, when you know what that was.

The SVO was delivered from the factory with front and rear adjustable Koni shocks. These units are single adjustable, meaning that changing the setting changes both the compression and rebound stiffness. As near as anybody can tell, all SVO’s were delivered with both front and rear shocks set to full-soft. Adjustability runs from 0-"full soft" to +4 turns-"apparently welded".

The job of the shock absorber is to control the cycling of the spring. Take away your shocks, and you have a 4-wheel pogo stick. Take away your springs, and you have a go-kart. So the trick in setting up the shocks is to get enough dampening action to control excess spring movement, while still allowing the springs to suspend the car. The shock rebound settings also help control body lean, particularly during transitional and turn-in maneuvers.

The rear shocks need to be (fully) compressed prior to adjustment. While they can be adjusted while still on the car, I prefer to completely remove the shocks, unless you’re doing a track thrash. Hold the bottom of the shock, turn the upper shock body counter-clockwise until it hits the stop. Then start counting the clockwise turns. Typical performance driving settings for the rear shocks would be +1~+2 turns. At the full soft setting, there is a lot of inside body hiking on turns, and some rear wallowing on high speed banked turns. At +3 turns, the rear axle tends to jump sideways over bumps and off-camber turns.

The front shocks are adjusted by turning the half-moon shaped top shock end. While there is a Koni adjuster for this (originally supplied with the car), an adjustable wrench works for those of us whose Koni unit has grown legs.

Similar to the rears, first back off the adjustment (counter-clockwise); then count the clockwise turns.

Performance front settings should be +2~+3 turns. Too stiff in the front, and the car will really take off on rippled roads. Too soft, and you will notice a lot of body roll on turn in. I have noticed that, after adding the g-load brace, I could back off the front stiffness about ½ turn.

Tires and Wheels

The 16"x8" –225/50 OEM wheel/tire combination is actually an excellent compromise for the SVO, given the clearance problems of the SVO suspension underneath the Fox Mustang body.

Don’t waste your time trying to fit a 9" rim to your SVO, unless you have a set of fender flares sitting under the workbench, or you like the looks of the Monster Trucks. The 94~98 Mustang offset seems to work well in the 8" width.   Another offset that works are wheels made for the late-model Probe GT.  Just make sure to try before you buy. Some fit, some don’t. If you are making contact with the wheel wells, roll the lips, or have someone do it for you.

A word on bigger diameter rims. The only thing that you are going to change by going to a 17"x8" 245/45 wheel/tire combo is the appearance of your car and the weight of your wallet. Performance wise, the difference is minimal; and certainly not worth the $1500+ expense from a bang-for-the-buck standpoint. If you like/want the look of the modern style rims, go for it. Just don’t kid yourself into thinking that you’re bolting on a slot car performance upgrade.

The 245 width is right on the edge of too much tire for an 8" rim. Under hard cornering loads, you can feel the sidewalls steering the car.  Increasing tire pressure minimizes this phenomenon.  A size of 225 or 235 would actually work better, from a performance standpoint.   

Handling Reference