Project Update for August 31, 2012: Before someone asks why we went to a Watts, hopefully I can explain this. I'll back up a bit and talk about the Fox and SN95 Mustang suspensions, the stock S197 suspensions, and then the differences between a Panhard rod and Watts link. Then I will cover the dedicated test day we did in the Mustang yesterday, to get the brand new Whiteline Watts link and bars dialed in, and then our last minute preparations and trailer loading for the 2012 Solo Nationals next week.
Why is a Watts Better?
There are several ways to laterally locate a solid rear axle assembly relative to the chassis in a RWD car like the S197 Mustang. The earlier Fox and SN95 generation Mustangs with solid axles used an opposing 4 link rear suspension that had the upper two control arms canted in at a severe angle, which made their intersecting arcs of travel automatically bind. The upper arms sort of worked to locate the axle, at least with a 4 to 5" window of lateral displacement (with R compounds we would commonly see that much movement!). If you put poly bushings in all control arm locations it would instantly bind up the rear suspension and eventually rip the mounts out of the car. Entire companies existed for a time replacing this car's terrible rear suspension with 3 links, Torque arms, Watts links and Panhard rod solutions. The IRS that came in the 1999 model year Mustang Cobra was a Big Hot Mess adapted from an earlier MN12 chassis (Thunderbird/Lincoln MarkVIII), with new toe links. It never really worked all that well, added weight tot he car (like IRS often does), and had pretty bad axle hop during straight line acceleration. And yes, a solid axle rear suspension is actually lighter than most IRS systems, but just more unsprung weight.
Left: The Fox and SN95 solid axle Mustangs has this opposed 4 link. Right: 1999 Cobra introduced this parts-bin IRS
Things changed for the better on the 20056 Mustang "S197" chassis. The two most common laterally locating solutions for solid axles in use today are a Watts linkage and Panhard rod. Both of these do essentially the same thing, but in different ways and with different levels of complexity and costs. They provide a linkage which connects the axle to the frame and gives positive lateral locations, while still allowing fluid up-down movement of the axle. The locating device must be free to move in other directions so it does not interfere with the intended motions of the axle. When Ford began the S197 chassis design they used a Panhard rod and a 3-link, with 2 lower control arms and a single upper arm that are pointing directly fore-aft. Much improved, but not perfect. The rear suspension in the Crown Vic of the day was actually more advanced, as it had a 3-link and a Watts.
The S197 Mustang had an all-new rear suspension design for 2005, with a 3 link and a Panhard rod
A Panhard rod is commonly used by factories that still build solid axle RWD cars because it is simple and cheap. A Panhard rod normally runs across the entire width of the car, with one end pivoted on the axle housing and the other end is pivoted on the frame. Ideally the Panhard rod should be horizontal with the car at the resting ride height position. A Panhard rod does not provide true vertical motion at the end attached to the axle because it arcs about the end pivoted on the frame and there is a slight sideways motion of the axle through suspension travel - this is the big limitation. If the bar is long, if it is horizontal at normal ride height, and if the axle vertical motion is small, then the sideways motion is minimized and not too important, which is the case for most racing cars.
On a road car with soft suspension and a fairly large amount of wheel travel, a Panhard rod has downsides. The lateral axle movement allowed by the rod gives a rear-axle steering effect when the wheels rise and fall over larger bump travel. The bar can flex, the rubber pivot bushings can deflect, and when the static ride height is lowered the axle will displace laterally at rest (our car's axle was moved 3/4" to one side) - this is why all lowered cars need an adjustable length Panhard rod, to re-center the axle. Aftermarket Panhard rods usually replace the rubber pivot bushings with Polyurethane, Metal rod ends or some sort of plastic rod ends. Metal rod ends have lots of noise and accelerated wear, and as we have seen the Delrin sphericals seem to suffer the same fate:
noise and wear.
Left: The black "football" shaped part pivots at the axle cover. Right: The chassis "tower" that the left side Watts link mounts to.
A lateral locating device without these problems is the Watts linkage, which is much more common to see on a race car with a solid axle RWD set-up. This design is more complex (2x as many arms and a complicated pivoting axle bracket called a "football") and takes up more space than a Panhard rod, so it is not always the best factory solution - more cost, more difficult to package. The geometry is inherently better, though, and it should be considered on any competition vehicle if at all possible. A Watts linkage consists of the two parallel lateral links plus a vertical link connecting them. If the horizontal links are parallel and equal length they will have the same amount of lateral motion as the ends attached to the vertical link. Because each lateral motion is in a different direction, this rotates the vertical link (football) through some angle, but the center of the vertical link is not affected. This center point travels in a true vertical path, and that is where the axle is attached - thus the axle goes strictly up and down, without the lateral displacement associated with the arc of a Panhard rod.
A Watts linkage can consist of tubing for the lateral links, using spherical rod ends for pivots or polyurethane bushings. On a true race car you use rod ends but for a quieter, street-worthy set-up polyurethane is much preferred. The vertical links should be designed to carry bending loads and the center football pivot is usually a rigid bearing on a post so it can rotate but not pivot laterally, often connected to the rear axle differential cover (many Watts kits come with a new cast aluminum diff cover that has rigid holes cast into it for a bracket to mount the football). The attachment between the Watts linkage and the axle is difficult and it must be designed strong enough to carry the side loads in cornering. I've seen some that clamp onto the cover bolts, but the styles that have an all-new rigid cover are preferred.
The improvement a proper Watts Link makes over the Panhard design is the rear roll center is fixed at the football, and does not move up and down with ride height changes due to braking, acceleration, and road undulation. This creates more consistent tire loading, more predictable handling, and especially improved transitional response. So in short, you definitely want a Watts Link system if you are competing in any form of motorsports that involves changing lateral loads, like autocrossing or road course racing. The more changes in direction (slaloms or esses), the more a Watts link is desired.
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