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Page 41
TORQUE TUBE REDUX

(Modified from original illustration in the Millikens' "Race Car Vehicle Dynamics")

While the crude schematic above certainly does not represent the torque tube rear suspension, it is...as you'll see...a logical derivative.

The torque tube rear suspension was...at one time...used almost universally in production cars. Millions of Fords, Chevrolets, and other makes travelled billions of miles with this suspension. While there is some isolated usage today, suspension engineers...in their search for improved ride quality...had essentially eliminated the torque tube suspension from consideration by the mid fifties. (This page does not consider recent designs...also called torque tubes...used with independent rear suspensions.)

The torque tube suspension can be considered as one in a family of "no-link" suspensions. We commonly speak of "3link" and "4link" suspensions and, when we do so, we are not including the lateral locating link called the "Panhard" link. Using this same counting rule, we see that the torque tube suspension has no links, for a suspension link is rotatable at BOTH ends. The truck arm and ladder bar suspensions would be included in this family.

The "tube" of the torque tube suspension is solidly attached to the front of the housing containing the ring and pinion gears and extends forward, completely surrounding the driveshaft. At its front, there is attached a hollow ball. Within this hollow ball is the single driveshaft U-joint, located at the rear of the transmission tailshaft housing. The ball is itself encompassed by a socket solidly attached to the transmission. The exterior surface of the ball and the interior surface of the socket act as bearing surfaces. These bearing surfaces absorb all loads associated with rear axle travel and those forces associated with the car's forward acceleration and braking.

As indicated above, the torque tube had its merits. It was simple, strong, and reliable. Its only obvious fault was its relatively high unsprung weight which, as was indicated, affected ride quality. So, where are traits like simple, strong, and reliable highly esteemed? NASCAR, of course! The truck arm, or NASCAR, rear suspension is very similar, in its performance, to the old torque tube design. Of course, if you brought those 2 forward pivots together and substituted the ball and socket from an old Ford or Chevy, it would be even more obvious. The ladder bar is simply a truck arm with no attempt to converge at the front.

But, dragracing has its own unique requirements. Perhaps most important is the goal of achieving and maintaining equal rear tire loading throughout the run. The torque tube, truck arm, and ladder bar suspensions all have a common problem: Driveshaft torque unloads the right rear and loads the left rear. Chassis twist is commonly seen as a different problem, but it is really a part...or, more correctly, a symptom...of the same problem. If the rear tires remain equally loaded, it follows that the front tires MUST remain equally loaded (assuming equal loading before launch). So, eliminate the loading problem and the chassis twist problem is also eliminated.

We'll now take a closer look at the picture above, but, before we do, I must emphasize the fact that this is a SCHEMATIC! This should be obvious when you see a pinion shaft centered in the pumpkin, but I want to be certain that noone attempts, for instance, to scale the picture to see what diameter tubing he should use. Most significantly, I have not included the diagonal bracing necessary to strengthen the "arm." If you can find a picture of an old Ford torque tube suspension, you'll see the kind of bracing required. And, although the arm has the appearance of a single ladder bar, it must be recognized that the loads are DOUBLE those normally encountered by such a single ladder bar. It would be extremely unwise, therefore, to assume that the same size tubing and other components used in a ladder bar car could be used in this application. I would particularly point out that loads on Heim joints which act to bend the threaded shaft are generally to be avoided. Such loads are very high in this application. A far better choice would be a bushing with a rubber sleeve. Note that the centerline of the bushing should be perpendicular to the dashed line shown in plan view. This minimizes the side loading. Since, in this design, the Panhard must absorb the moment produced by the offset, it is important that it be sufficiently strong.

The suspension pictured above is an attempt to retain the valued features of the torque tube while adding the dragracing requirements. When properly implemented, it will maintain equal rear tire loading throughout the run.

With only a single attachment to the chassis (apart, of course, from the Panhard), it is obvious that the binding inherent in the ladder bar...and, to a lesser extent, in the truck arm...is eliminated. The suspension is truly "streetable."

Some portion of the total weight transfer, during launch, is going to be carried through the arm into the rear axle assembly. Since the weight transfer is proportional to the driveshaft torque, this vertical load...when applied at the proper offset from the car's centerline...will perfectly cancel the tendency for the driveshaft torque to upset the rear tire loading.

Although many reference books show the roll axis of a torque tube suspension coincident with the car's centerline in plan view, it is actually on a line which passes through the front roll center and a point midway between the ball and the chassis end of the Panhard and is, therefore, on an angle with the centerline. The situation is best understood when it is realized that a triangle, comprised of the chassis, the Panhard, and the axle assembly, exists in both the original torque tube suspension and this derivation. Rear axle roll, then, is restricted to rotation about one side of the triangle. While the above illustration shows the chassis mounting for the Panhard rod on the right (US passenger) side of the car, the preferred mounting would be on the left (US driver) side. This would minimize the angular deviation of the major roll axis from the car's centerline. With the offset of the front pivot, the situation would actually be improved over the production torque tube design.

The spreadsheet provides the necessary setup information.


horizontal distance forward
from axle centerline to pivot =

desired percent antisquat =

effective rear tire radius =

axle ratio =

center of gravity height =

wheelbase =

ANSWERS:

Offset From Car's Centerline:

Height of Pivot:

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