Watt’s Linkage

A Watt’s Linkage was designed and fabricated as part of the 1970-71 conversion to a race car. The attached Concept Sketch, drawn in 1971, shows many design details. The vertical link consists of two identical 1/8 in. steel plates, arranged so the three attached pivot bolts are in double shear. Two brackets cut from large ¼ in. wall rectangular tubing are welded to the axle housing, and a ¼ inch steel crossplate is bolted to the welded brackets and contains the axle pivot. A ½ inch diameter AN bolt is the axle pivot and goes through the link plates and an Oilite bushing in the crossplate. A tab on one of the link plates contacts a flat on the head of the pivot bolt, causing it to rotate within the Oilite bushing as the axle moves. The crossplate can be repositioned to relocate the axle pivot. The two connecting links are ¾ in. OD x 0.058 in. wall steel tubing with welded tube inserts for 3/8 in. left and right hand rod ends.

Watt's Link Sketch

Watt’s Linkage Concept Sketch Drawn in 1971.

The following photos show the Watt’s linkage from the rear (with the Morgan on the trailer and the fuel tank and inner panels removed), and from the top. The pivot locations and the diagonal support of the lower chassis pivot are shown. The forward reaching Traction Bars above the axle are also shown.

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The Geometry of the Watt’s linkage is shown in the following sketch and described below.

Watt's Link Geometry

The axle pivot is located at the height of the Rear Roll Center so the axle movement as guided by the Watt’s Linkage does not conflict with the movement as guided by the springs. The Roll Center is first identified on the chassis using the spring mounts, and then transferred to the axle based upon the chassis ground clearance at the axle line.

The upper part of the sketch identifies the Roll Center at 3.54 inches above the bottom of the chassis at the axle line. This is based upon the location of the spring mounts, which is where the lateral loads are transferred to the chassis. At the rear, the shackle is the type that can transmit lateral load so it can be considered an extension of the chassis. Thus, the spring mount on the shackle is the location of the lateral load on the chassis. A nominal Roll Center Height of 3.5 inches above the bottom of the chassis was used in the design.

The lower part of the sketch shows the location of the axle and connecting link pivots, using a nominal 5 inch ground clearance and 3.5 in Roll Center height above the bottom of the chassis. The vertical link is 8 inches between end pivots with the axle pivot in the center. The locations of the upper and lower chassis pivots are shown with respect to the sides and bottom of the frame.

The previous photos show the upper chassis pivot on a bracket that is bolted to the upper flange of the frame and welded to a rear roll bar support, and the lower pivot on a bracket that is bolted to the lower flange of the frame and laterally supported by a ¾ inch steel tube diagonally across the bottom plane of the frame.

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