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Page 52
LEAF SPRING TORQUE ARM

This page involves the use of leaf springs, mounted in their "normal" location. This would be typical of the Fords, Chevrolets, and Plymouths of the fifties. However, in this case, shackles will be used in BOTH the rear and front spring eyes. In other words, though a leaf spring is usually called upon to carry horizontal loads, these springs...with shackles at both ends...can only carry vertical loads.

So, why not use coil springs? Well, there are a couple of reasons. First, there are those who compete in classes where they are required to retain the spring type as originally supplied. Also, the "lowly" leaf spring supplies a surprisingly beneficial function!

Since the leaf spring does not pivot at it's connection with the axle housing, the front half of the spring extends forward as an "arm," similar to a ladder bar or the arm of a torque arm suspension. It occurred to me, then, that the leaf spring suspension could be easily converted to a torque arm suspension, merely by the inclusion of the aformentioned shackles, one or two links, and a Panhard rod.

By saying "one or two links," a decision is implied. That decision would be similar to the one made between a 3link and a 4link. Actually, the decision is not similar, but is EXACTLY the same. Those two forward shackles are short, vertical links. Whether you add a single link or two links determines whether the final suspension is a 3link or a 4link. In either case, the instant center location...and consequent amount of antisquat...is found by the intersection of the link lines, as viewed from the side. The degree of adjustability is up to the fabricator.

If equal rear tire loading and no twisting of the chassis is desired, a single link is the proper solution. Since that single link will be in compression during launch, it would normally be offset to the left (US driver side). This would mean, however, that the axle housing would tend to rotate about the rear mount of that link. To cancel that effect, the link would normally be rotated counter-clockwise in plan view (looking down from above). This would provide a side force, which, when reacting against the Panhard rod. would provide a cancelling couple. This side force, acting at the height of the rear pivot point of the link, would also provide a moment which would help to cancel the driveshaft torque.

The spreadsheet provides the offset and the link plan angle. The "angle," "distance," and "height" require explanation. "Angle" is link angle when viewed from the side, with a positive angle existing when the rear pivot point is higher than the front pivot point. "Distance" is the plan distance from the link rear pivot location to the Panhard rod. "Height" is the vertical distance to the link rear pivot."Angle" is input in degrees and "distance" and "height" are in inches.

Of course, if all that's desired is the equivalent of an adjustable 4link, this spreadsheet is of no value. Simply find your links intersection point for the desired percent antisquat and you're done.

Finally, remember that a single link is carrying more than double the load of two links! Saving a couple of pounds of car weight is never to be valued above safety. Also, track and tire irregularities can easily cause high loading on those fabbed front shackles. Don't count on link placement and angles to completely cancel out high side loads.

axle ratio =

tire radius (inches)=

angle (see notes) =

distance (see notes) =

height (see notes) =

ANSWERS:

offset =

link angle in plan view =

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