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  • John Merton
    February 1, 2011

Sliding Axle Suspension

The Sliding Axle Front Suspension.

From the first three-wheelers until relatively recently all Morgans have used a sliding axle independent front suspension system.  In typical form the stub axle assembly slides up and down on a fixed axle pin (alternatively called a "pillar") located vertically between two tubular cross members which are part of a cruciform section bolted to the front chassis rails. It differs fundamentally to a sliding pillar system in which the stub axle assembly is integral with the pillar which slides up and down through a bush or bushes set in outriggers to the chassis. This Morgan system was always and traditionally described as a sliding axle system until an incorrect reference in an "Autocar" article in 1956 described it as "sliding pillar" an erroneous term which seems to have taken vogue widely since. The sliding axle assembly except for a few early four wheeler cars has two bronze bushes, top and bottom.

Shock absorbtion (ie springing) is provided by a large coil spring at the top which encircles the axle pin and bears on a flange on the stub axle assembly, and except for a few early cars, a rebound spring at the bottom. Damping has generally been via a telescopic shock absorber.

Lubrication. Generally via a grease nipple on the inner side of the sliding axle assembly. From the advent of the Plus 4 in 1951 until the 2000's the system was augmented by a one shot lubricator, operated from a foot-actuated button valve on the firewall which diverted engine oil via the top of the axle pin. The axle pin was grooved and the groove had a hole in it so that this oil supply could effectively reach the inside of the upper bush. With the car at rest, this groove and hole are roughly located centrally inside the top bush. On depressing the plunger valve, oil is fed at engine oil pressure through the hole and distributed round the groove to the inside of the bush. Suspension movement and the working clearances, which are fractionally more than those in the engine's crank and camshaft bearings, for example, are sufficient to ensure a good spread of lubricant. The effectiveness of this system can be gauged by the fact that if used regularly very little wear took place on the upper bush - the same could not be said of the lower bush, however.

The main reason for wear on the lower bush, and this area of the axle pin, is road grime, which when combined with the grease in the area makes a very effective grinding paste.  The use of a cover or gaiter, described elsewhere in this manual, can significantly reduce wear here.

Other design features. During the 4/4 Series 1 production run, a friction steering damper made from a spring steel plate was added to the front suspension. At the inner side it attaches to the chassis rails and at the outer bears on, or is bolted to, a bronze bush taking the thrust on the flange of the axle pin just under the mainspring. Although written works suggest this was a postwar initiative, available evidence indicates that it was probably introduced just before the War.  Its purpose was "to prevent any twisting motion set up in the springs when under compression being transmitted to the stub axles and interfering with the steering" (Series 1 Instruction Book). Early illustrations show the first of these blades to have been attached at their inner sides by turning them up at the ends and then bolting them to either the chassis side rails or to the inner wheel valance. However this approach will obviously constrict movement with the consequential risk of breakage, and it is believed that few, if any at all of the early cars were so set up. The production approach on these early cars was to allow the blade to slide in and out of a sheet steel sheath which was mounted on a small block of wood through bolted to the top chassis rail. A photograph of this set-up appears below.
 

The recommendation was that the bronze thrust washer area should never be greased, only given a light oiling to prevent rust. It should be noted that from some time in the 1950's a relief hole was added to the inside of the sliding axle assembly tube just above the damper blade assembly. This was to allow air to escape thus relieving pressure which would otherwise have inhibited grease being pumped in. Series 1 stub axle assemblies did not have this hole. 

The Morgan front suspension assembly is often criticised for apparently rapid wear, necessitating axle pin and bush replacement fairly frequently.  However, it must be pointed out that this was no better or worse than the standards of the times.

(NOTE  See also under "Wheel Wobble")
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