Semi-elliptic Leaf Springs
Traditional Morgans possess a live rear axle with semi-elliptical constant rate leaf spring suspension. A 2018 Roadster developing 280 bhp possesses similar rear suspension to that used on the first production 4/4 models manufactured in 1936, which produced 36 bhp. This type of suspension can be seen on carriages dating back to the sixteenth century. Until the mid 1980's many production cars still used it, but only a few cars manufactured use it today. Even Morgan have abandoned it on the latest V8's and Aero models.
Semi-elliptic suspension is cheaper to make than a coil spring system, because of the simple construction. Leaf springs are self locating and also locate the rear axle. They have few working parts and require no complex linkages, which make them extremely durable and easy to maintain. They also require very little space, which helps give more room in the boot. Leaf springs are self damping to a degree, because of friction between the leaves. This also helps make them stiff, so body roll is low and road holding is good on smooth surfaces. Leaf springs are fairly resistant to lateral movement when cornering. If the car is going to be used at high cornering speeds on track days, any lateral movement of a live axle can be prevented by fitting a Panhard rod.
The greatest disadvantage with leaf spring rear suspension is its high unsprung weight. The wheels, rear axle casing, differential, drive shafts and brake drums all contribute to the unsprung weight. Issac Newton's second law of motion states F=MA, where F = force, M= mass and A= acceleration. When wheels run over bumps, they inevitably suffer from vertical acceleration (A). This, along with high unsprung weight (M), obviously leads to a higher force (F) acting upon the sprung weight of the body above. Combined with stiff springs, this causes discomfort to the vehicles passengers and more importantly, can lead to the rear wheels losing contact with the road. To avoid this behaviour, modern cars are usually built with a relatively heavy body and low unsprung weight, using light weight coil springs and alloy wheels. The wheels on a live axle change camber when one rear wheel rides over a bump. It is also difficult to change the position of the roll center with elliptical springs, whereas designers can choose its position, using a non rigid axle located by independent suspension. Hard acceleration and braking can cause leaf springs to wind up, which results in axle tramp. However fitting an anti tramp bar to a Morgan is fairly straight forward and kits are available from Morgan Agents.
As mentioned above, maintenance is usually straightforward, as there are are no linkages requiring lubrication on the semi-elliptical springs. The bushes on the rear shackle and front spring mounting should be checked for play occasionally. They appear to last a long time and during my rebuild they did not require changing. It looks like a lot of effort is required to re-fit them, so they were the only item left on the chassis during the rebuild. I did however wire brush the springs, bringing them back to bright metal. I then painted them with rust preventative paint. I spray WD 40 on them from time to time so that it penetrates between the leaves. Some people will disagree with this treatment, claiming it may affect the self damping properties. Personally I think rust is the big enemy of leaf springs and I have never felt an adverse effect from lubricating them. Carefully check the suspension leaves and U-bolts for cracks when servicing the car.
If you have to replace the rear springs or bushes, Melvyn Rutter can usually supply them for most Morgan models. Check how many leaves your springs have before ordering new ones. 2 seater cars may have six or five leaves, whilst 4 seater Morgans have seven. Mark the position of the U-bolts on the rear axle before removing it. Also note that the distance of the axle to the bushes at either end of the spring are different. If fitted the wrong way round, the axle will be located in the wrong place. The bush on the longer end of the spring is located on the shackle at the rear end of the car, which raises the roll center slightly.
Armstrong Lever Shock Absorbers
In 1936, the shock absorbers fitted to the original 4/4 were Hartford scissor friction dampers. Shock absorbers have improved greatly since then. Modern Morgans have telescopic dampers on the front and rear suspension.
The purpose of the shock absorber is to control resonance and minimise the force of the unsprung mass (suspension) acting upon the sprung mass (body) as a vehicle travels over a bumpy surface. If the frequency of bumps coincide with the natural frequency of the suspension springs, then the oscillations produced can be dangerous due to the effect of resonance. Ella Fitzgerald could shatter a wine glass, allbeit with the aid of an amplifier and speaker. This is because she could sustain a note at the same frequency of the glass. This is the effect of resonance. If the shock absorbers are badly worn, resonance within the springs can cause the wheels to bounce excessively on bumpy roads. This can result in loss of tyre contact with the road. When certain speeds are reached, poorly balanced wheels produce a similar effect that can be felt through the steering wheel .
My 1972 model came equipped with Armstrong lever, double acting type hydraulic shock absorbers. The main body of these shock absorbers consists of two pistons interconnected via non recuperating valves. A crank shaft with double rocker arms is connected by rods to the pistons. The shaft is actuated by a lever which is connected at the opposite end to a link on one of the suspension spring u-bolts. As the spring deflects, the lever operates the crank, driving a piston to force hydraulic fluid from the compression cylinder to the rebound cylinder. During this compression stroke, the hydraulic fluid flows via one way valves in the cylinders. As the spring rebounds, the one way valves close and hydraulic fluid is forced from the rebound cylinder to the compression cylinder via a restricting taper screw. The taper screw is adjustable, offering different resistance to the the rebound of the spring. On the bump (compression) stroke the damping action of a shock absorber is usually less than on the rebound. This is why the one way piston valves on the compression stroke offer less resistance to the hydraulic fluid than the taper screw valve does on the rebound stroke. Driving on a smooth track will require stiffer settings than over bumpy surfaces. If you intend to use your Morgan for competition and road use, it would be better to fit adjustable tubular shock absorbers. You will need a conversion kit to mount them, but most Morgan agents will help you with this.
Armstrong lever shock absorbers appear to be very reliable. They are found on many other cars built in the nineteen sixties and seventies. However if you pick them up at an auto-jumble, note the angle of the lever, compared with those fitted to Morgans. On many old cars, lever shock absorbers are mounted with their cylinders aligned vertically, with the end cover on top. A nut is often on the top cover of vertically mounted shock absorbers, which would allow topping up the hydraulic fluid with the shock absorber in place. On a Morgan the shock absorber is mounted at an angle as illustrated in Figure 1 and there is no top up nut on the end cover.
If there is any play in the shaft bearings or any other problems, it will be difficult for a home mechanic to put right, unless they have access to a high pressure press. This will be needed to remove the lever from its shaft. The position of the lever on the shaft will have to be marked to ensure the correct alignment during reassembly. Melvyn Rutter can supply uprated versions.
Changing the Shock Absorber Hydraulic Oil
The job of changing the hydraulic oil is made easier by removing the shock absorber. The old hydraulic fluid should be flushed out before replacing it. Undo the two nuts securing the shock absorber to the mounting plate on the chassis, then remove the nut connecting the shock absorber axle link to the u-bolt. Remove the shock absorber and hold it in a vice with the end cover facing upwards. Undo the six philip head screws and remove the cover along with its gasket. Drain the old fluid and remove the taper screw nut, then remove the valve assembly. Be careful to note the order of any shims present. Check the condition of the 'O' rings on the valve nut and replace them if they appear faulty. Check the oil channels are clear in the taper valve nut and control spring assembly. Bump damping can be increased by adding more shims between the taper screw valve nut and the larger spring. Rebound damping is increased by screwing in the nut retaining the smaller spring on its shaft. If you adjust the damping, carefully note the exact settings used on the first shock absorber and repeat it on the other one. It is most probably wise to leave adjustments alone if the shock absorbers were performing well before stripping them.
The advantage of removing the taper screw nut, is that the body of the shock absorber can be flushed with new oil to get any dirt out of the system. Use SAE 20 W motorcycle hydraulic fork oil to refill the shock absorber. When you are happy everything is in order and clean pour some of the hydraulic oil into the taper valve housing and reassemble the valve components making sure any shims and springs are put back in the order they belong. Replace the valve nut and support the body upright in a vice. Fill the body of the shock absorber with hydraulic fluid and pump the arm to get any air out of the system. The level of the oil will drop and you will observe bubbles coming to the surface. Keep pumping the lever until no more air bubbles can be seen rising, then fill the main body leaving 3/8 inch air space at the top. Refit the top cover, ensuring a suitable gasket is used between the body and end cover. You will most probably have to obtain some gasket paper and make your own. After checking that there are no leaks, refit the shock absorber.