Morgan Front Suspension Layout
Figure 1
Morgan front suspension is usually described as sliding pillar, but it is not. It is actually better described as sliding axle, because the front axles slide up and down fixed pillars (king pins). Figure 1 shows the layout used on the front suspension of my 1972 4/4 model. It evolved from the original front suspension that H.F.S. Morgan designed for his first three wheel car in 1909. The main improvements since 1909 are the inclusion of shock absorbers, steering dampers and "one shot lubrication", although the "one shot lubrication" has been dropped on Morgan's latest vehicles. Morgan Aero and three wheel models do not have the above layout.
Sliding Pillar Suspension Layout
Figure 2
in In 1895 an engineer named John Henry Knight from Farnham, Surrey, produced one of Britain's first petrol motor vehicles and it was most probably the first motor car with independent front suspension. Some accounts of it claim it was of sliding pillar design. However, the French company Decauville produced the first true production car with independent front suspension in 1898 and it was of the sliding pillar type. Figure 2 compares the 1989 Decauville suspension system with the original Morgan three wheeler design.
Decauville front suspension consisted of a transverse tubular structure with housings containing bushes at each end. A vertical tube is fixed to the centre of the transverse tube. A semi elliptical leaf spring is transversely mounted on top of the vertical tube. King pins are bolted to either end of leaf spring and pass through the bushes at the ends of the transverse tube. A guide shroud at the top of the king pins may have also contained bushes, to allowed the pins to swivel on the end of the leaf spring. The axle was bolted to the bottom end of the king pin. When a wheel encountered a bump, the leaf spring deflected and the king pin (pillar) would slide through the bushes at the end of the transverse tube. Hence the name "sliding pillar"
The three wheel Morgan front suspension design of 1909 consisted of two parallel transverse tubes with king pins fixed at either side. Sliding axles were sandwiched between coil springs, which allowed the axle to slide up and down the king pin. The larger main spring above the axle compressed and absorbed the shock when a wheel encountered a bump, whilst the smaller spring under the axle took care of the rebound.
Wheel alignment
Figure 3
Wheel alignment, also referred to as tracking, can affect tyre wear and straight line stability. Track is measured by comparing the distance at axle height, between the rear and front, of the left and right wheel rims. The term "Toe in" is used to describe wheels that converge towards the front and "Toe out", when they diverge. "Toe in" increases straight line stability and tends to cause understeer, whilst "Toe out" reduces straight line stability and is prone to oversteer.
As rear wheel drive cars tend to oversteer, they usually have "Toe in", whilst front wheel drive cars have "Toe out", because they understeer. Unfortunately "Toe in" or "Toe out" increase tyre wear. Whilst a garage will adjust the front wheel alignment within the recommended parameters, observing tyre wear and making fine adjustments is often the best way to get the optimum setting.
Sliding Pillar Suspension Geometry
Figure 4
Camber angle refers to the angle the wheels make with vertical, when viewed from the front or rear of the car. If the top of the wheels are angled outwards, they are said to possess positive camber. When they are angled inwards at the top, the wheels possess negative camber. In the days before power steering, most cars had positive camber on the front wheels, because it lightens the steering. Negative camber improves road holding when cornering fast, so is good for track days. Converting a Morgan from positive to negative camber on the front wheels, is relatively straight forward. The bottom of the king pin is bolted to a plate at the end of the front suspension cross tube. Replacement plates are available which will move the bottom of the king pin outboard 3/4 inch.
Kingpin angle is also known as the swivel angle, which is the kingpin inclination angle offset from vertical when observing the car from the front. It has important effects on a cars handling. When a wheel is steered around an inclined king pin, the axle hub is lifted. The weight of the car tries to return the axle to its former position, which creates self-aligning torque.
Castor angle is the kingpin inclination angle offset from vertical, when observing the car from the side. It also exerts self-aligning torque, because the tyre contact trails behind the swivel axis on the road. Larger castor angles are required with negative scrub geometry. Whilst the kingpin angle produces the effect of self-centring the steering at low speed, castor angle has greater effect at high speed.
Scrub Radius
Figure 5
The distance between the centre of the tyre contact patch and the centre line of the projected swivel angle intersection point on the road, is called the "kingpin offset" See Figure 4 . It is also known as the "scrub radius", because a tyre does not roll around its centre line when turning, but scrubs. If the swivel angle intercepts the road inside the centre line of the tyre contact patch, it is called "positive scrub radius". A swivel angle that intercepts the road outside the contact patch centre line is called "negative scrub radius"
Most Morgans possess positive scrub radius, which is chosen for better steering feedback at low speed. The disadvantage, is that it cannot be used with diagonally split, dual circuit braking systems. If half of the diagonal split braking system fails, only one front brake, along with the diagonally opposite rear brake, will be applied. A car with positive scrub geometry, will tend to turn around the braking front wheel. Under the same circumstances, negative scrub geometry produces torque in the opposite direction. This gives the driver a better chance of braking in a straight line. Negative scrub also reduces the chance of spinning in the event of a front wheel blow out.
