Wednesday, September 28, 2011

The GR40 Suspension System

Why the stock Mustang suspension doesn't work and how Griggs Racing fixes it

In the beginning Fox bodied and SN-95 Mustangs feel so good. There's the easy V-8 power, the light steering and quick handling. Compared to other street cars the Mustang is ready and nimble, a joy to drive.
But for the ever-learning enthusiast there comes a day when that magic Mustang sunshine dims. Often it's because another, modified Mustang showed you its taillights, or perhaps you took your Mustang to the drag strip or slalom. You begin to notice things, like how the rear end seems so inconsistent. At the strip the car never seems to launch the same, and when you get what feels like that rare perfect launch, the tires blow off just as soon as the car gets rolling. Your front tires grind off their outer front edges long before the rest of the tread shows any real wear. Start whipping around freeway ramps faster and your Mustang begins to feel unsure; you find yourself waiting for the rear end to snap out unexpectedly. Ultimately, your Mustang feels more over-powered and less capable than simply fast.

All products are abused on the track prior to making it to the street.

Those first moments of insecurity about your Mustang's white-knuckled handling at the limit, or its fickle appetite for traction at the strip are not your imagination hitting the rev limiter. Those are the first realizations the Mustang chassis is far from delivering the confident handling its high-output powertrain deserves.

At Griggs Racing we've dissected the Mustang chassis and suspension to identify its shortcomings and engineer cures. Our fix for the Fox and SN95 chassis Mustang is no quick medicine; it's a fundamental change in the suspension geometry that yields a fundamental handling improvement. Our suspension is also adaptable to a huge range of Mustang performance. From the street, to the strip, road course or slalom circuit, our re-engineered GR-40 suspension system provides the stable, consistent, responsive platform you need. It's only shortcoming is it has to be experienced to be believed.

So what are the issues working against you in the Fox and SN95 chassis Mustang? For starters, the unibody structure is lightly built, with insufficient rigidity. High torque and cornering loads deform the structure, causing the suspension to lose precision, doubly so with convertibles. Welding in reinforcing structure is the cure.

Knowing where to reinforce the structure is important, as indiscriminately adding braces wastes money and adds weight without gaining meaningful increases in rigidity. By twisting a Mustang unibody on a frame table, we learned the main problem is in the middle of the car. Ford counts heavily on the rocker panels as the primary structure between the firewall and rear wheelhouses, especially with the '79-'04 cars. This lets the front and rear axle forces to twist the car far too easily.

Naturally, a full roll cage will cure this, but that solution is cumbersome and expensive on street cars. More practically, a dual-plane brace to provide triangulation of the floor pan is required; we do this with our Full Frame Kit.

The mid-car twist also explains why we don't offer g-load and strut tower braces. By strengthening one end of the car they actually increase the mid-car twist.

An even larger concern is found in the rear suspension. Ford uses a 4-link design, but with the upper two control arms angled heavily outward. This means the lower and upper rear control arms are not parallel, so as the suspension moves the upper arms are twisted in their bushings. During performance driving this quickly leads to a near total binding of the rear suspension, called roll bind. With the axle bound, it acts like a giant anti-sway bar, causing the rear roll stiffness to skyrocket and the overloaded rear tires to loose traction and spin. This is why the rear end snaps into uncontrollable fishtailing when cornering, and it is also why the rear tires break loose at the drag strip once the body starts rising from the initial power hit.

2005+ Griggs Racing TorqueArm and Panhard Bar

Ford addresses the roll bind by fitting the upper arms with very soft bushings, a sloppy fix, to say the least. Our cure is to fit a torque arm and either a Panhard bar or Watts link to provide the necessary axle location, then remove the stock upper arms. Roll bind is then impossible, and the tires freely follow the pavement.

You may have noticed we use two locating devices, the Torque-Arm, and either a Panhard Bar or Watts link, to do the job Ford uses just the upper control arms for. This is to separate control of the fore-and-aft engine and braking loads from the lateral suspension loads. More precise suspension control is then possible.

GR40 Watts Link
Additionally, the rear roll center is now defined by the Panhard bar or Watts link instead of the upper control arm angle as Ford had it. Stock the Mustang's rear roll center is far too high, which overloads the outside rear tire and causes oversteer. By lowering the rear roll center with the Panhard bar or Watts link we get the rear tires to carry more of the load so the rear end will stick longer.
More compromised geometry is found in the front suspension, a point made abundantly clear when driving a car with the rear suspension fixed and the front suspension stock. Ford built the Mustang with generous steering axis (king pin) inclination, which requires equal amounts of caster to keep the tires flat to the ground when turned. Unfortunately, Ford gave the Mustang only minimal caster, a condition we reverse with caster plates and redesigned K-member.

GR40 SLA System
Also at the front, Ford's tall ride height comes into play. Lowering the entire car benefits the center of gravity, but causes the front suspension geometry to lower the front roll center well below ground level. Combined with the tall rear roll center, this results in a roll couple (the relationship between the front and rear roll stiffness, of which roll centers play a part), to heavily load the front tires. Imagine trying to drive your Mustang around a corner with the front end squashed below ground level and the rear end raised a yard or so in the air; obviously the car would be trying to turn using just its front tires. That's about what the stock suspension tries to do. Lowering the rear roll center with the Panhard bar or Watts link helps this condition, of course, but we also raise the front roll center, accomplished by relocating the points where the front suspension attaches to the chassis. Moving the suspension pickup points is done by redesigning the K-member, which is the crossmember the front suspension attaches to. Redesigning the K-member also allows us to add more anti-dive to the front suspension help correct the lack of caster.

Ackermann is also a concern on stock Mustangs. Ackermann is the steering geometry that steers the inside front tire more than the outside tire, a necessary condition as the inside tire follows a smaller diameter turn radius. With the Mustang, Ford actually ended up providing reverse Ackermann, meaning the front tires toe-in slightly when turned. We also cure this with our K-member.
So, did Ford really goof terribly on the Mustang? Well, not by accident. By selling a relatively high-powered, inexpensive car into the youth market, Ford wanted a car that signed off in the handling department so soon that only the fool-hardy would get in high-speed trouble with it. In short, Ford designed the Mustang for inexpert drivers.

Unfortunately, dumbing-down the chassis is a common manufacturing tactic in the affordable performance car market. That is why it is so difficult to describe the incredible improvement a complete chassis re-engineering provides; few enthusiasts have experienced the huge thrill of piloting a powerful V-8 machine that starts, sticks and stops as well as it's engine goes. Once you've wheeled a GR40 car, however, you'll be one of the few.