The ride quality of the Ford Excursion has been suspect since day one of the 2000 Excursion all the way to present day (2005 models as of this writing). Ride quality and handling is notably improved with aftermarket shocks, a rear anti-sway bar, and now radius rods. This article discusses in detail the problem with the factory "traction bar" on Excursions and the benefits of a radius rod (replacement for factory traction bars).
Excursion & SuperDuty Chassis Background
The Excursion is built on the SuperDuty chassis (F250, F350, F450, and F550 Ford trucks). The SuperDuty trucks have stiffer, high-capacity leaf springs and helper springs mounted atop the rear axles in order to carry larger payloads than the Excursion. The leaf spring hangers are mounted high on the chassis because of the tall stack of leaf springs used on the F450 and F550 models.
The Excursion was first rolled off of the assembly line as a 2000 model year, 1 full year after the SuperDuty line-up had rolled off of the assembly line. Ford, responding to demand in the SUV market, was able to create a 3/4-ton SUV utilizing many existing components from the SuperDuty line of pickup trucks. However, due to the design of the SUV body, the Excursion's suspension needed to be changed in order to ride at under 7'0" total height (the height of the average garage door on a house, fast food drive-throughs, etc.). Additionally, to make the Excursion more attractive to the SUV consumer, the ride also was made to be softer than that of a SuperDuty pickup. The end result was a modified suspension specifically for the Excursion in the form of a smaller spring pack, no helper springs, and shorter spring height.
To compensate for the high front-end on the SuperDuty models, Ford simply added lift blocks between the leaf springs and the rear axle housing to allow them to sit level with the front end. Different sized blocks are used on the 2WD and 4WD models.
Spring wrap is born
While the blocks perched the Excursion's rear end at a proper altitude relative to the front suspension's ride height, another problem was amplified. That problem is known as spring wrap. Most any light-duty rear-wheel drive vehicle with a solid (or "live") rear axle, and leaf spring rear suspension, will suffer from spring wrap. This problem will occur regardless of whether spacer blocks are used, or whether the axle is located above or below the leaf springs (the Excursion leaf springs are located above the axle). Spring wrap is caused by the torque of the axle when accelerating and/or braking. Inserting lift blocks amplifies spring wrap by allowing the axle to impart more leverage upon the springs simply because it is mounted further away from the springs.
Factory traction bar is inefficient
When the Excursion is stationary, or is moving along at a steady speed on a smooth surface, the Ford snubber-type traction bar does nothing in this condition:
When the Excursion is accelerating, the torque transmitted to the leaf springs causes the spring-wrap condition illustrated here (this sketch depicts an exaggerated view), and allows the axle to move forward. The Ford snubber-type traction bar reduces the amount of spring-wrap in this situation, but is also subject to deflection itself:
When the Excursion is braking, the torque transmitted to the leaf springs causing the spring-wrap condition, allowing the axle to move rearward. The Ford snubber-type traction bar is useless in this condition (again, an exaggerated view):
Leaf springs and their responsibilities
Leaf springs must perform several functions. First, leaf springs suspend the weight of the vehicle with enough resistance and resilience to provide adequate payload capacity and acceptable ride quality. They perform fairly well at this, their main function. Second, the leaf springs position and hold the axle transversely. This is not their main job, but leaf springs perform fairly well at this function, too. Third, leaf springs position and attempt to hold the axle longitudinally. This is their weak spot.
The inefficient factory traction bar
The rear axle is torqued in both direction from acceleration and braking, and that torque is transferred to the leaf springs, especially when 4-wheel-drive low-range is engaged. Ford realized this and included a pair of snubber-type traction bars with the Excursion's weaker leaf springs on the 4WD model. Unfortunately, those traction bars are functional only when accelerating. They do nothing to improve ride quality or braking.
Axle-wrap is amplified due to the Excursion suspension design
While driving your Excursion over a series of bumps, the rear axle actually moves, or tilts, fore and aft a small amount, and the reverberation you feel is from the leaf springs attempting to normalize themselves -- attempting to return the axle to its normal position. With spacer blocks mounted between the axle housing and the leaf springs, the fore and aft movement is amplified because the axle has more leverage to act upon the leaf springs. This is less of a problem on vehicles whose axle housing is bolted directly to the leaf springs (no spacer blocks between them), but is still a problem.
Rear-axle induced steering: a bad thing
When the Excursion is driven over an abnormality in the road surface that affects only the left or right-side wheels, the leaf springs allow axle-wrap to occur, but this time the axle-wrap is not equal from side to side (as it is under acceleration or braking). The weak leaf springs allow the rear axle to momentarily become out of square with the vehicle's centerline. The rear axle then steers the vehicle briefly, and unexpectedly. This is why your Excursion will suddenly shoot one way or the other in a lane on the road when you hit a pothole, bump, etc. This then leads to you overcorrecting and sometimes leaving you wondering why your Excursion is so difficult to keep in a straight line.
Driving on smooth surface:
After hitting an obstruction in the road:
As you can see, the rear-axle induced steering is just that, steering caused by the rear axle going out of square when the axle wraps. This causes your Excursion to change direction which requires steering correction by the driver through the steering wheel.
So, what now? How do you fix this problem? Install a set of LANDYOT radius rods, of course. Read the installation article!