Front Suspension - Main & Rebound Spring Interaction

The ‘Traditional’ Morgan front suspension spring characteristics

(NB All references to ‘Morgan’ in this refers to the Traditional Model, not the Aero 8 series)

 

Basics. Attached to both ends of the cross frame there is a nearly vertical king pin (pillar) along which the hubs slide. So this is a ‘sliding pillar’ suspension if you are earth bound and a ‘sliding hub’ if you are in the car at the time of the observation.  The hub has within it two bushes, either made of bronze or a suitable polymer (plastic) and these slide (most of the time) on the steel, stainless steel or best of all on a hard chrome plated king pin.

 

Above and below the hub are uniformly wound coil springs. The spring above is called the ‘main spring’ and the spring below is called the ‘rebound spring’. For all models except the current (2010) ‘4/4 Sport’ and the ‘Roadster’ (V6), the hub is clamped firmly between these two springs even at static ride height; this is only true for cars that are still standard in this area or where the springs have not crept or broken unnoticed over the years. It is interesting to note that the current ‘Plus 4’ still has the hub clamped hard between these two springs.

 

We all know that the front suspension of these cars is considered to be stiff and that the chassis is rather flexible, both compared with ‘modern’ small sports cars. That is how light cars were in the 1950’s, the then new Morgan four wheelers were no different. This combination of flexible chassis and stiff suspension does tend to give good handling on smooth roads, indeed modern racing ‘karts’ or ‘go carts’ as we used to call them still show amazing handling & grip. Over the years the desire for better ride comfort has become more important to most car owners, and certainly modern sports car designs achieve good ride comfort with excellent handling and road holding.

 

So how does the Morgan front suspension really work IF the hub is clamped between the two springs with the rebound spring being compressed at static ride height, as all models before those noted above are? So at static (or normal) ride height the hub is clamped between the main spring above and the rebound spring below. The various models and years of Morgan have different rate springs but the initial static condition and the general suspension characteristic are still similar.

 

Consider when a front wheel hits a bump. The hub will slide up the king pin that is attached to the cross frame that is attached to the chassis and the chassis will twist a little. We will mainly consider the coil spring based suspension in this analysis, this is not to say that the flexible chassis is not important since it is indeed an integral part of the Morgan’s suspension system.

 

So as the hub moves upwards over a bump the rebound spring (below the hub) extends (ie becomes less compressed) and the main spring (above the hub) compresses more. Then after about 1 inch, depending on the model and year, the rebound spring goes free and all the load is taken in the main spring. Then after about another 1½ inches the top of the hub (top bush tube) contacts the little rubber sleeve (bump stop) inside the dust cover or the telescopic damper bump stop comes into play. At this point the hub really has to stop moving and one will feel and hear a thud and the passenger will wince (or is that whinge) again. At this stage the chassis flexes a little more to absorb this suddenly increased load, then things all return to normal – until the next bump.

 

Most modern cars have either non-uniformly wound ‘rising rate’ steel coil springs or the more advanced (such as some Citroens, Mercedes Benz, Range Rover, Allegro/Maxi etc) have air springs; even remember early true Minis had rubber springs, both systems providing rising rate. All these systems tend to be designed so that the springs get stiffer the more they are deflected, try a bicycle pump with your finger over the hole and feel the increase in stiffness towards full compression. This characteristic gives a softer ride over road surface irregularities yet will still deflect over bigger bumps and minimise uncomfortable bump stop impacts as the spring rate eventually gets very high (stiff). So what is the combined spring characteristic from these two opposing springs in the Morgan? The Morgan front suspension does not have this increasing or rising rate characteristic, neither is it linear, so what is it?

 

Back to basics then: a uniformly wound coil spring will have a linear spring rate. Let us consider one around the same magnitude of stiffness as the Morgan main spring. Conventionally, even in the wicked metric world of ours, we often express spring rates (stiffnesses) in Imperial units, so the main springs range from around 95 lb/inch (1987 4/4) to 140 lb/inch (‘Plus 8’ and ‘Plus 4’) as std, depending on model and year. The rebound spring that sits below the hub is typically around 275 lb/inch, but again these change between models and years.  So a 95 lb/inch uniformly wound spring will deflect 1 inch for every 95 lbs applied, whether you start from its free length or partly compressed. Eventually when fully compressed and all the coils touch each other at the same time then the spring goes ‘coil bound’ and solid.

 

Current cars in 2010;

‘4/4 Sport’ main spring of 140 lb/inch, rebound spring free of the hub.

‘Roadster’ V6 main spring of 110 lb/inch, rebound spring free of the hub – car was initially intended as a touring model with the ‘Plus 4’ remaining as the sporty model.

 

Aside: Spring rate theory was discovered and documented by Robert Hooke who lived between 18 July 1635 – 3 March 1703, so read his work if you need more maths than is covered here.

 

Now we have two springs in this suspension. A pair of springs can either be in ‘series’ or ‘parallel’. How are the two in the Morgan front suspension set up?

 

·         ‘Series’ is when both springs experience the same load (force) but different deflections depending on their rates. The combined rate of these two springs is the reciprocal of the sum of the reciprocals of the two rates.

 

·         ‘Parallel’ is when both springs experience the same deflection and but different loads depending on their rates. The combined rate of these two springs is the sum of the springs, thus the rates are just added together.

 

So although there is in ‘our’ case one spring above and one spring below the hub, these springs are actually in parallel, not series. They see the same deflection but different loads, that is until the hub moves up far enough for the lower rebound spring to go free and only the main spring remains in play.

 

Thus although there is one spring either side of the hub, the initial spring rate is the sum of the rebound spring rate plus the main spring rate. This is not intuitive, but check out a school maths book if you have forgotten or are not sure. Or if still not convinced get your ‘Meccano’ out of the loft and build a test rig, that’s what I had to do to convince a friend (he is still a friend, but a lot quieter now on the subject of springs!).

 

So if the rebound spring is around 275 lb/inch and the main spring is around 140 lb/inch then the initial spring rate is 250 + 140 = 390 lb/inch. Then after about 1 inch (depending on model, year etc) the rate drops to 140 lb/inch when the rebound spring is fully extended, that is the front suspension spring rate then gets softer! I warned you that it was not like modern suspensions!

 

 
 

So this is why in these wonderful cars we tend to feel every surface irregularity. The initial very high combined spring stiffness does not allow the hub to deflect much at all over small bumps, yet with the bigger bumps or potholes there is enough input to allow the hub to become free of the stiffening effect of the rebound spring and thus deflect more to cope with the input. The stiction in the bushes as the hubs try to slide up the king pins over small surface irregularities will also add to the subjectively harsh ride. Both can be dealt with successfully, if you so wish.

 

This high initial spring stiffness is not a newly discovered characteristic of the Morgan suspension, far from it. Indeed many Morgan race engineers, technicians and drivers in at least the UK and USA already knew this, but I do not think it ever spread into the general Morgan owners’ mind sets, then why should it if you are more interested in using the car as it was made.

 

Morgan racers often use a main spring rate of around 180 to 220 lb/inch and run a lower ride height that also results in the hub being clear of the rebound spring. This achieves a more controllable front suspension characteristic that does not have a suddenly reducing front spring rate part way through a fast corner that can upset the car’s balance.

 

http://www.britishv8.org/Articles/Morgan-Plus-8-Road-Test.htm    has an interesting line: “Trying hard on Welsh mountain passes the Morgan felt less safe than the Gilbern and I would think that a competition driver would lose time when the back-end bounced upwards and sideways and by the front-end suddenly going softish”. They spotted it then!

 

 

Also some drivers of older Morgans no longer complain about hard ride, why? Well there are a few possible explanations, besides driving slower on smoother roads (maybe not)!

 

1.      The rebound springs have a relatively short life and often break. Thus the hub is no longer constrained by the rebound spring so the rate goes linear and softer by relying solely on the main spring rate from static ride height to full bump. So their spring rate has dropped from say around 390 lb/inch to closer to 140 lb/inch over the years.

2.      Also over decades the main springs can creep, thus they get shorter though not softer. Thus in time the ride height drops and the hub again becomes free of the rebound spring and so their spring rate too has dropped from say around 390 lb/inch to around 140 lb/inch over the years.

 

As some of you will well know, I have trialled 175 lb/inch and now 125 lb/inch main springs with no rebound springs on my 1987 4/4 2 seater. Initially as standard I had a main spring of around 95 lb/inch and a rebound spring of around 275 lb/inch. Thus I have reduced the initial spring rate from around 370 lb/inch to 125 lb/inch, but then the rate at around full bump has of course actually increased from the original 95 lb/inch to 125 lb/inch. This gives a much more compliant suspension and thus better initial ride. However I now control the number and severity of bump stop impacts with more bump damping on the adjustable damper, this is essential.

 

Why or how MMC ended up with this unique reducing rate front suspension is a matter of conjecture. Stiff front springs on a three wheeler certainly will help a three wheeler by providing  a high roll stiffness when the rear single wheel has zero roll stiffness. But why it was continued until a few years ago is unknown. It took a suspension expert (oh no, NOT me I hasten to add!) to demonstrate to MMC the decreasing spring rate was not necessary and that a linear bump rate would give better ride and handling.

 

Effect on roll stiffness. The reduction in the initial spring rate from around 390 lb/inch to around 100 lb/inch significantly reduces the roll stiffness, which is not good news. However if the rebound spring length is such that it is just touching the underside of the hub at static ride height, then although the bump rate is just the main spring rate now the rebound rate is the sum of the main spring rate PLUS the rebound spring rate, so quite stiff. Hence the roll stiffness is now only slightly less than with the rebound spring compressed at static ride height, yet the ride quality has increased significantly.

 

One of the first exponents of this was the great Maurice Owen of Morgan Motor Company (Director and Development Engineer) who suggested that by just cutting the rebound spring so it was just touching the underside of the hub would improve the ride and have no detrimental effect on the handling and road holding; sadly the reason modification seemed to have been lost at Morgan and by those who carried out this modification. Many ‘Plus 8’ owners do this modification and enjoy the result. NB the rebound spring must be just free of the hub to achieve this, if the rebound spring is made just a bit shorter and yet is still compressed at static ride height the initial spring rate goes up as the shortened rebound spring is now even stiffer. Inevitably the ride height will increase when the rebound spring is cut, so beware of this. Of course now MMC should have a vast range of different spring rates and lengths available to suit the different models and whether thrust plates or thrust races were fitted.

 

  

What we need are rising rate main springs……………………………..one day maybe ;-)

  

Peter J Ballard. BSc (Vehicle Engineering). CEng. MIMechE.
 
PS A PDF of this description is also attached.
 
PPS Some early models used a rubber rebound snubber as opposed to a rebound spring.
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Peter Ballard,
Jan 5, 2011, 2:10 AM
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