TECHNICAL – SOME BASICS FOR Traditional Morgan Suspensions.
Front - There is still some confusion with respect to Morgan front suspensions, I hope this helps.
Variously described as ‘sliding pillar’, in that the king-pin slides inside the stubaxle/hub assembly; it could equally be called ‘sliding hub’ – it all depends where you are, in the car or on the road.
The stubaxle/hub goes up and down the fixed (to chassis) king-pin for suspension and rotates to provide steering.
OK there is some stiction but well greased it works well enough – such sliding suspension works on many thousands of aircraft landing gear and motorcycle forks!
The king-pin as standard is made from mild steel (sometimes was a stainless steel, a poor choice) and the bushes from bronze. Replacement king-pins are made from 1 inch hydraulic rod that is a high grade steel core, copper plate to bond to the steel, nickel plate to protect the steel from corrosion and layers of ‘hard chrome’ plate ground back to size with a surface finish to hold a layer of oil. There are many after-market suppliers of these king-pins as the 1” hydraulic rod stock is common and not expensive.
The sliding stubaxle/hub has a short rebound spring below it and a longer main spring above it. Many used to think that the rebound spring softened the main spring rate to give a soft initial ride but was overcome by the stiction in the stubaxle/hub on the king-pin, but that is incorrect. The two spring rates add together and that results in a very hard ride that many thought was caused by stiction.
The use of rebound springs is now quite common in sophisticated dampers on such as Mercs and BMWs. The rebound spring is there to provide increased roll stiffness IF SET UP CORRECTLY. Some important points:
1.The stubaxle/hub must just be kissing the rebound spring at laden ride height. This is because the initial spring rate in bump is the addition of the stiffness of the rebound spring to the stiffness of the main spring. Get them compressed together and the initial spring rate is very high resulting in poor ride. If you buy aftermarket main or rebound springs then you have to select a length and rate that suits the weight of car to get the stubaxle/hub just kissing the rebound spring, not easy as there needs to be ‘hundreds’ of spring lengths to suit all variants.
2.The rebound spring must not go coil bound when the inside wheel in a corner goes light as then you loose all suspension compliance and thus loose grip. Jack the car up and see if the rebound spring is coil bound – bad.
3.The ratio of the rebound spring stiffness to the main spring stiffness is important to get the most benefit out of it as it controls the roll centre and when and how it moves in a corner, there is a ‘sweet’ range of ratios.
4.Up until recently the only bump stop was a length of rubber hose inside the shroud but as it is fully constrained it has no compliance (rubber is incompressible), better to use a progressive bump stop on the damper.
The SSL RS (See 'PRODUCTS' at the top of the page) kit has:
A.Built in main spring preload adjuster to compensate for different weight cars.
B.Progressive rebound spring with the ‘sweet’ ratio of [rebound stiffness]:[main spring stiffness].
C.Rate-Riser spring to add stiffness progression in bump.
D.Damper tuned to the spring and car with a progressive bump stop.
Rear – leaf springs from 4 leaf at around 125 lbf/inch (not ‘lb’) up to 7 leaf on early four seaters with rates around 250 lbf/inch. Asymmetric leaf springs are available from SSL that reduce wind up under hard acceleration for both ‘two seaters’ and ‘four seaters’. These are the same rate but with more camber on the four seater springs. Four seater dampers had a long rubber ‘spring aid’ (looks like a long bump stop) on the damper rod to carry the weight of the two rear passengers, leave this out and the differential will pound the underside of the rear seats when laden.
Front to Rear matching.
Rear ride frequency needs to be slightly higher than the front to minimise unpleasant pitch motions. So as the cars are close to 50:50 fore:aft weight balance then if you go too hard on the front then you will get pitch.
More to follow!
SSL runs its own calibrated leaf spring test rig
Myth - leaf springs go soft after many miles, many years or with overloading.
Truth - leaf springs may well sag and lose ride height as the steel yields or creeps over time or use. If the rate has dropped then they are close to fatigue failure as the drop in stiffness will be due to cracks forming!
SSL runs its own calibrated leaf spring test rig where we load the springs up to maximum load the corner or end is likely to see and plot off the load-deflection curve. If the deflection of the leaf spring is known as installed then the load that the spring sees on the car in that state can be read off the curve, this is the sprung corner weight and is essential for dynamic analysis.
SSL then can modify the spring to typically increase the rate for racing by adding leaves then get the spring recambered back to the previous or maybe lower ride height. Springs so tested and some developed include: Morgan, MGB, Jaguar Mk2, Rover P5, ACCobra, Ford Coupe 1941, Land Rover Defender and various 4x4 pickups.
Classic, race, vintage, replica, kit cars and specials
SSL carries out dynamic and static analysis to detemine spring rates to give the car balance for race lap times or road comfort.
SSL designs coil springs, either linear or progressive; then works closely with UK coil spring makers to produce low or medium volume runs.
SSL designs and develops leaf springs for road or race, whether quarter elliptical or semi elliptical, linear or progressive, symmetrical or asymmetrical, then workls closely with high quality UK leaf spring manufacturers.
SSL works very closely with damper/shock absorber manufacturers such as SPAX, Quantum, Gaz.