Parabolic Springs: Are they really worth it? What do they do? How do they do it? Series Land Rovers are hung on Leaf Springs, and Parabolics are vaunted as the ultimate upgrade for them. I found out there wasn't much though to tell me how they worked or why they were any good. So, I figured it out, and this article is the result.
Only thing is, I cant help myself and revise these things every time I do some changes to the site! Consequently, the piece has sort of grown and evolved and got a bit inflated. So THIS TIME around..... I decided to try and prune it a bit, and just answer the question of Paras on Series Landies!
A couple of things before I begin, though; the article was one of the first I wrote for Tef's-tQ, and has generated a lot of feed-back; not least, I say with more than a little glee, from a certain Paul Heystee, of Automotive b.v. who were other wise known as TIConsole.... which he founded, mainly to market the parabolic springs he introduced to the Land Rovering world!
And his comments were quite heartening. As I said, when I came to investigate Parabolic springs as a possible upgrade for Wheezil, what I could find out about them was pretty poor. There were plenty of magazine articles that said that they were great, or that they were a bit disappointing, or went into great detail about how to fit them, but other than the one line statement, that they had a unique tapering profile, and were subject to rigorous heat treatment processes, that was about it!
So, I went back to fundamentals; dug out me first year Mech Eng lecture notes, and even my A-Level physics books, on things like damped harmonic motion, engineers beam theory and all that kind of stuff, read everything I could find in books, magazines and on the 'net, and tried to figure out what the heck these things did, how they did it, and why that might be of some benefit!
This article was then the product of that research and a LOT of (admittedly highly) educated guess work!
And hearteningly, the correspondence I have had with Mr Heystee and others 'in the business' have corroborated a lot of what I made guesses about. which was a bit of a relief!
Any way; none of this answers any of your questions; which are probably;
- Are they any good?
- Are they worth the money?
- Should I buy some?
- If so, which ones?
So, I am going to start at the very end, and say, I cant tell you the answer to those questions, only give you some better idea of what they might do for you, so you can decide for yourself.
I also have to qualify that I have not tested all the different makes and types of spring, and when I use the term 'Parabolic Spring', I am talking in general terms of the type, as described by the many people selling them. I recognise and accept that there is a vast difference between the many brands and the different springs those brands offer. And hopefully, this article will explain why some brands might be better than others.
However, hopefully what will become clear from my ramblings on the topic, is that far more important than the choice of spring you might choose, be it a plain leaf or any brand of parabolic; is finding a 'set-up' of spring and matching damper, that will suit your purposes.
And here, I REALLY do not believe that parabolic springs should be considered on their own; but if you are considering fitting them; then you would be very well advised to acquire a matching set of gas dampers to go with them, so I have devoted a fair bit of this article to looking at damping, and gas dampers and the benefits they offer; for the simple reason, that the biggest single difference between a plain and parabolic leaf spring is that the parabolic attempts to remove all of the inherent internal damping that a plain leaf normally provides.
But, to begin with, we need to have a look at suspension, what its for and how it does it.
The Leaf Spring Suspension 'System'
This is probably the biggest bit of pruning I have done on the article, so you'll have to forgive me if its a bit terse. The stuff that I cut out formed the basis for the article All About Bump's, but in typical fashion, when I came to edit it to shape, I ended up re-writing most of it!
Any way; for the moment; the important thing to recognise is that the suspension system is just that, a system, not just a spring, which is just one element in that system, and a pretty insignificant one for most purposes, EXCEPT in the case of the leaf spring, but that will become clear, hopefully in a minute!
Suspension, basically lets the wheels go up and down in relation to the vehicle body as it goes over a bump in the road or surface. The technical term for this is 'compliance', and it makes the 'ride' a bit more comfortable for any passengers in the vehicle, but it also has a few implications on the vehicles handling and stability.
In the system, there are a few important features. Obviously, we have to have a wheel and a vehicle body, and they have to be connected in some manner that allows them to move in relation to each other. This is known as 'location'
Then we have to have something to control the movement between the two, and keep things in 'equilibrium', ie; when a wheel hits a bump, having compliance that lets the wheel move up, is all well and good, but be helpful if it went back down again afterwards! So, we have a spring.
Last thing on the list is a thing called 'Damping'. Without too much heavy duty physics, things move because of forces. Things have 'weight' or mass. When they move, they have kinetic energy which is proportional to their mass and their speed. When they change speed, that kinetic energy must change, and as energy cannot be created or destroyed, has to go somewhere as something else. And basically, the 'Damping' is the bit of the suspension system that takes the bump energy from the wheel and gets rid of it!
OK. Looking at our real world Series Land Rover; we have a simple twin rail chassis, and two solid 'live' axles. Between each axle and the chassis are two semi-elliptical leaf springs, and two telescopic dampers.
Except I have left the dampers out of the sketch, which, the pedants will tell me looks nothing like a Land Rover chassis, because its flat, not curved, and the spring eyes curve the wrong way at the back, and there's only one leaf in the spring 'pack'.......
Any way, you get the idea of what it looks like, and how the bits are arranged. Very important, the leaf spring, in THIS arrangement performs just about ALL the functions of 'the system'.
We have a chassis, and under it, axles, and joining the two, providing the 'location' is the spring itself, which is what provides the compliance, the only thing missing is the damper, which a multi-leaf spring can provide, without any further devices from the friction of the leafs in the pack rubbing against one another as they flatten out as the spring is compressed.
There are hundreds of alternative suspension arrangements, using all manner of different types or forms of springs; they all need some means of locating the wheel in relation to the chassis, body or frame of the vehicle; they all need some means of providing spring compliance and some form of damping the motion.
Beauty of the twin semi-elliptic live axle arrangement is that the spring itself can do nearly all of it. In engineering terms, it is a very elegant solution to the problem of providing suspension, which I guess it ought to, having probably a couple of millennia of development behind it!
The arrangement is one of the oldest there is, dating back at least to the medieval coaches if not before, while the leaf spring itself, is even older. Turn one through 90 degrees and thread a string through the bush eyes and you have a stone age hunting bow!
It is not without inherent compromises though, and more modern and sophisticated arrangements have been devised to over come a lot of it's short comings. Those, however are now beyond the scope of this piece and have been expunged, mercilessly! And I am going to stick to the subject of Land Rover leafs and the Parabolic alternative..... I promise!
And the main differences between a 'plain-leaf' and a 'Parabolic-leaf' come down to two things; the spring's rate linearity, and, as I've already mentioned, it's internal friction damping.
The damping is the more important difference, but I'm going to start with the rate linearity issue, because, it has bearing on what I need to say about damping later.
We are dealing here with forces and distances. Take your Landy, and sat on the drive, its own weight compresses some of the available suspension travel. Load it up with whatever, people, logs, bags of cement, and the suspension will compress some more, wont it? More load you put into it, the more the suspension will compress under that load.
Now, the spring 'rate' is the thing that governs how much the suspension will compress for a given load; a stiffer spring, or higher 'rate' will take more load to make it compress the same amount, a softer spring or lower rate, will compress further for a given load.
However, different springs have different 'linearity', ie. how far they compress for a certain force changes, depending on how much they are already compressed by.
Rising rate springs give proportionally less compression, the more load is put on them, reducing rate springs give proportionally more compression, the more load that they are put under.
So, a linear rate spring will be just as hard or soft with the car heavily loaded as it will lightly loaded.
A rising rate spring will be harder, the more load that's put in the car, and a reducing rate spring, effectively softer.
Keeping the load the same, and looking at the suspension forces; a linear rate spring will deflect a little for small forces and a lot for bigger ones.
A rising rate spring, however will proportionally be softer for small bumps, but harder for very big ones.
While a reducing rate spring will be the other way around, and hard on small bumps but effectively softer for big ones.
Which, by the sounds of it, suggests that rising rate springs would be pretty good; they'd be nice and soft for small bumps but hard enough to cope with very big ones, and fairly reasonable in the middle, while reducing rate springs sound horrendous, and likely to be really harsh on little bumps, not giving much compliance at all, but then wallowing out or smacking against the bump stops for big ones.
Which is not far off the mark. And, now I'm going to tell you, leaf springs tend to have a reducing rate!
Normally, coil springs tend to have a pretty linear rate, which is one of their advantages, but, it all comes down to the operating ranges and actual 'set up', and the reducing rate of the leaf spring doesn't have to be a huge handicap. In some circumstances it can be an advantage, and ultimately, the actual 'rate' of any spring can be fiddled with to some degree.
In the case of coil springs, you can do some curious things by way of changing the pitch and diameter of the windings, to get a spring that doesn't have a strict linear rate, and can in fact have a rate that goes from rising, to linear to reducing, or vica versa.
And in the case of a leaf spring, you can do something similar, messing around with the shape and or using multi-leaf 'stacks'. But before looking at those, I think we need to look at a single leaf, and figure out some of what is going on.
Single Leaf Springing
If you go back to my sketch of a Land Rover chassis and suspension, I drew the springs as a single leaf. One strip of metal, with a fixed curve in it. Real Land rovers, and most other leaf sprung vehicles, however have leaf springs with a multitude of leafs in them all clamped together.
But, for a minute, lets just look at a single leaf. what we have is a strip of steel. and for a minute, I'm going to draw it flat, with no bow or ellipse to it what so ever.
It has a length, a width and a thickness, and the width and thickness are uniform along the length of the strip.
And at either end, but of little significance are the eyes that we will use to attach the spring to the Land Rover. I've only drawn one on the end of the flat strip, so you can see the section at the end, but, really they don't matter, its only the strip in the middle that is doing any 'springing'.
OK, now beneath my flat strip, I have drawn the same strip, with an eye on each end, bent into a curve. I'm taking a bit of artistic licence here, but.... its for simplification.
An ellipse is a squashed circle. And a semi-ellipse is a squashed semi circle. Our leaf spring, is going to get squashed, so it really doesn't matter very much whether we start with part of a circle, or part of a squashed circle, it would get more squashed any way, and this keeps things simpler to explain, OK?
Right, first curve I have drawn has a certain bow or radius, second one, beneath that a bigger one. Both strips are actually the same length, the only difference is the degree of bow.
Now, I have added to each picture the centre line of the strip, along it's thickness. Reason for this is because when you bend a flat strip into an arc, the length along the outside gets longer, and the length along the inside gets shorter, but the length of the centre line stays the same.
When our strip is being a spring, and we haven't got there quite yet; that's what's doing the springy thing! The outside of the strip is put under tension; the molecules of the metal being pulled away from one another, while the inside of the strip is put under compression, the molecules of the metal being pushed closer together.
A bit mind boggling, but at that molecular scale, its the forces between those molecules, that are providing our spring force, as the ones being pushed closer together try and push themselves back out, and the ones being pulled apart try and bunch back up. Tiny tiny particles, held together or apart by minute forces, but together, in their millions exerting the sort of force that could hurl a brick half a mile or so!
Any way, our flat strip bends giving us spring force, as the top and bottom of the strip are squashed or stretched, and it has a reducing rate, and the important things controlling how much the rate changes are the thickness and the length of the strip.
The longer the strip, then for any given deflection, so the radius of bow needed to allow it will be less, so over the same range of travel, the more 'proportional' the rate will be. But, the thicker the strip, so the more stretch and squash of molecules there will be for a given bow, so the less proportional the rate will be.
So, the ratio of thickness to length controls the degree of proportionality, while the basic 'rate' or stiffness of the spring is controlled by the thickness and the width.
Pretty simple, hugh? Good, 'cos then you understand it better than me!
OK, well that is for a flat strip. Why bend it into a curve to begin with? Does that reverse the effect so that instead of having a reducing rate, we get a rising rate?
No. But your on the right lines.
The springing is coming from bending the metal and forcing those molecules closer together or further apart inside the strip. Reason that hey 'spring' is because they don't like like being squashed or stretched, and the metal is solid, so they cant do much about it.
If we heated the metal up, until it was soft, almost a liquid, then they wouldn't care so much, they would just shrug and move into a more comfortable alignment, which is what forging metal is all about, and what heat treatments like annealing are for.
Our strip is flat, basically because that's the shape it has been formed, and in which the molecules are happily aligned. If we heated it up and formed it into a curve, before we allowed the molecules to align themselves, then the strip would naturally want to try and hold that a curve when we tried bending it, either straighter or into a tighter curve.
And, the strip would still behave, as though it was flat, and try and give us a reducing rate. However, the curve of the strip, going from curved towards straight, rather than straight towards curved, would mean that the geometry would to some degree counteract the strips natural reducing rate, and what we would actually get is an effective rate that still reducing, didn't reduce as much.
How much more proportional the rate would be, ultimately would be back to the thickness to length ratio, but now also effected by the ratio of length to 'bow', while the basic rate would still be controlled by the width to thickness.
BUT! It gets even more convoluted, because our 'bow' isn't just the radius of a simple curve or part circle. Our curve is an ellipse, or squashed circle!
Basically, the radius of the bow changes along the length of the spring.
My little sketch shows an exaggerated and simplified example, with just two radii, a small one at the ends, and a big one in the middle.
In reality, they could be the other way around, and the radius could be get tighter towards the middle and wider towards the ends.
But we now have not just the ratio of length to thickness, and length to bow, we also have the ratio of radius change within the bow, effecting the proportionality of the spring rate!
OK, so are we ready to combine string theory and relativity and discover the universal principle of the universe?!
No. thought not. Gets heavy doesn't it!
And what the heck does it matter?
Well, it doesn't matter that much, other than to explain what is controlling the rate linearity in a plain leaf spring, and why we get this tendency towards a reducing rate.
So lets look at.......
The Parabolic Taper
Well, look at the sketch, and it's quite simple.
Drawn in the profile of the last sketch, instead of the strip being the same thickness from start to finish, the parabolic taper is thicker in the middle than it is at the ends. Or vica versa, if you really wanted, I suppose!
That is what all the fuss is about?!
THAT is the revolution in suspension design? An ordinary spring hammered a bit thinner at the ends?!
Pretty much! And, its not even that novel an idea. Remember me saying a leaf-spring is basically an old fashioned hunting bow turned horizontal?
Well, the Welsh bow, or 'Long Bow' renowned of Agincourt, has a similar taper, as do many other bows
The implications are quite far reaching. Thing is, looking at the 'Plain' leaf, where the strip is the same thickness all the way along, we've seen how as a spring, it has a basic rate and a rate proportionality, controlled by the width, the thickness and the length of the spring.
Changing the shape from a straight strip to a bow or ellipse has changed the proportionality of the spring rate. Playing about with compound curves to the bow can modify that basic level of proportionality a bit further. And by tapering the strip into a parabola, we can play with the proportionality even more.
There is one more way we can mess with the spring rates and proportionality, however, and that is..........Multi-Leaf Leaf Springs
OK; I will get to the bit about damping, in a minute. For the moment, we are concerned with how going to a multi-leaf 'stack' can help us to modify the behaviour of the spring by way of its basic rate and it's proportionality of rate.
And it can get even more complicated and confusing. However, trying to keep things simple.
In the sketch I've drawn two identical plain leafs; for simplicity they are a simple single curve; and in the top pic, you can see that they don't fit together.
At least, not if they are 'unrestrained', because outside curve of the one is bigger than the inside curve of the other.
To make them 'fit', like in the bottom pic, you would either have to make one with a bigger curve than the other, OR force the the one inside the other, by squashing the upper one and stretching the lower one.
This is where it gets interesting. If we make one leaf bigger than the other, so that they fit together without being stretched or squashed, then the two springs are not going to have the same rate proportionality.
If we stretch one and squash the other, then they will still have the same rate proportionality, but, they will be 'pre-loaded', and starting from different points in their range when you come to put any load on the resultant 'compound' spring.
Make sense? Basically, whichever way you put together the 'stack' either with similar pre-loaded leafs, or dissimilar unrestrained leafs, the rate proportionality of the stack as a whole is going to follow something that is derived from the individual rates and proportionalities of each individual leaf, depending on where it is in it's range.
Which brings me to the idea of the 'helper' leaf. Look at the sketch to the left, and we have the same multi leaf spring as before, except we have an extra leaf hung below the pack, with a much flatter bow to it.
This means that it's not going to get bent until the leaves above it are all flattened enough to take up the gap and start bearing on the ends of the 'helper', right?
Pretty simple, its a two stage spring, and the 'helper' will stiffen up the stack after so much travel is used up. Up till then it doesn't do very much at all.
However, imagine a 'stack' where all the leaves were arranged like that, and there was a small amount of 'take up' before each one came into operation. The effect would be that your proportionality would increase in steps, each time a helper leaf came to bear.
So, cutting to the chase, we now have a myriad more possibilities by way of permutations and combinations to put together a compound leaf spring from a whole bunch of different leafs with different shapes, lengths and thicknesses, arranged in a compound to give us a basic rate and rate proportionality tuned to the application we want to put it to.
What I haven't mentioned is the idea of a compound or multi-leaf parabolic spring; and the answer is yes; you can compound parabolic leaves the same as you could plain ones, and make things even more complicated by way of controlling your rate and linearity! In fact, if you wanted to get REALLY daft, quite possible to combine plain and parabolic leafs in a 'stack'.
However, plain leaves are the same thickness all the way along their length, parabolic leaves taper. So, if you made a conventional stack of parabolic leaves, the shape of each leaf would have to follow not just the ellipse of its neighbour, but also the profile of its taper. Also, when the stack was flattened by deflection, not only would the leaves rub against each other, but as they did, the taper would act like a wedge and try and push the leaves apart and change the shape of the ellipse.
This starts to make things rather complicated, and the anomalies and impediments that these phenomenon create tends to make it a bit counter productive; so a different kind of 'stack' is used.
Separated 'Stack' Leaf spring
Right, well this looks something more like we expect a parabolic spring to look like, and as I have drawn it, I have indeed drawn a separated stack of tapered, parabolic leafs.
As you can see, where they all clamp together, each leaf is held a small distance away from its neighbour by a spacer plate, so that the don't actually touch.
However to work together the do have to bear on each other at some point, but they only do that at the ends of the leaf, and as shown, the middle leaf, in this case, bears on the top leaf 'naturally' due to it's curve intersecting with that of it's neighbour towards the end.
Curve on the bottom leaf however is a lot wider, and it wouldn't naturally bear on the middle leaf until the spring had some load on it, it has the sort of shape of a 'helper' leaf in a plain stack.
However, to take up the gap and make the upper leaf bear on it from the start, the ends have been cranked a little bit. They needn't be, the manufacturer could have taken up the gap some other way, and TIC springs are notable for using low friction PTFE slip blocks between the leaves where they bear.
Anyway; two important points; first, separating the leaves gives more scope for using more complicated and dissimilar spring forms in the stack, which includes, the use of parabolic tapers, but is not solely confined to using parabolic tapers.
Second, separating the leaves, so they don't bear on each other over the greater proportion of their length hugely reduces the inherent internal damping you normally get from the friction of the leaves rubbing on one another.
Which gets us to the point I said at the beginning was the important bit.
Right, back at the beginning, I said that damping was probably more important than the spring. I also said that what made the twin elliptical leaf spring suspension arrangement so elegant was that the spring itself provided all the necessary functions of the system, by way of location, compliance and damping, by virtue of the friction of the leafs rubbing on each other.
I also said that damping was basically, the way of getting rid of bump energy, because the forces that the bump made moved things, and that movement gave us kinetic energy we'd like to 'dissipate' somewhere, so it doesn't throw the car, driver or passengers about.
Now, little history, it wasn't until the 'light' motor carriage that the importance of damping really became all that apparent. The amount of 'shock' energy generated by a bump depends on the speed that the bump is hit, and how 'sharp' the bump is. Before the motorcar, wheeled vehicles vary rarely did more than about thirty miles an hour, about the speed of a modern moped, or a Series Land Rover!
Except for trains. And the thing about trains, is that they run on rails. Rails tend to be pretty smooth, so trains don't tend to have very many big bumps to deal with! So even suspension, didn't seem all that important. But any way, other thing, is that trains are pretty heavy, and as far as suspension goes, heavy is good.
The heavier a vehicle is, so the more effort that's needed to make it change direction, which is what bumps try and do, they try and make the vehicle stop going straight and level, and start going up. So, bumps only started to become a problem, when vehicles started getting both lighter, and faster. Which they did with the advent of the high speed internal combustion engine.
Anyway, in the early days, having recognised the importance of suspension, designers turned to the tried and trusted technology of the day, the humble cart spring, like what we have been talking about all along.
And those designers found that the arrangement worked pretty well, and that was that. No one consciously recognised that the leaf spring was damping as well as springing in the suspension.
How separate damping arrangements came into being is a bit quirky, and more through trial and error than anything, but it did. The friction damper had been around for a long time, and had been used on horse drawn buggies amongst other things, and it is probably derived of the friction clamps that were used to hold the 'set' on control levers on steam engines and things.
Any way; early days of motor racing, trying to make the cars a bit more stable at speeds, the navigator/mechanics did a lot of funny things, which may or may not have effected the damping of the suspension, but obviously did.
Most famously, the 'Bentley Boys' wrapped the springs of their Bentley tourers with tightly wrapped chamois leathers soaked in linseed oil...... and there is some connection to cricket in there some-where, in a very English Edwardian manner.......
I think that the idea sort of followed the logic that some-one had noticed that the car soaked up the bumps better after some-one had spilled engine oil on the leaf springs. This would obviously have reduced the inherent damping in the leaf stack, but probably more important, a phenomenon known as 'sticktion' which is a problem with friction damping, I'll explain in a minute, for now, basically very high initial damping rate, and not a lot after that.
So, following the Bentley Boys logic, they probably discovered that oiling the leaves gave an improvement, but that it only lasted a short while, as the oil would eventually leak out or be worn away, or would collect all sorts of road muck until it became a kind of paste and made things worse than to begin with.
Wrapping the leaf stack in an oiled chamois, then would have been a sensible idea. It would have offered some protection against road muck; it would also have held the oil in place, and also acted as a sort of crude oil reservoir.
Thing is it worked. And obviously some-one had the notion to ask why, and people's interest in damping was piqued, and a lot more mechanics and engineers started looking at suspension a bit more closely.
And I mention this, because I have frequently read posts on the Land Rover forums, from people offering the advice of oiling Land Rover leaf springs, or even the Bentley Boys chamois leather wrap. And in all seriousness, too!
In fact, I've even heard some modern refinements on the ideas, like oiling the leaves with waxoil, or packing them with grease and then wrapping them in old supermarket carrier bags!
'Lore' you see is perceived wisdom, and these things filter down through the years, and old technologies are often 'rediscovered'; and sometimes such old 'tricks' are worth knowing; unfortunately, very often, the tricks are recalled, but the reasons, if they were ever known, aren't!
So, I'm NOT going to ridicule these ideas...... at least; not straight away. They DO have some validity; and if any-one that offers such witches cauldron advice can explain the science behind it, they might be worth listening to. Other wise, take anything they have to say with a large pinch of salt, and weigh it up against what I'm going to talk about!
Ok, getting to the meat of the topic. I have mentioned friction damping a fair bit, and I have just introduced an oft encountered problem with friction damping, that of sticktion.
Friction is what we get when things rub against each other. Its also what allows us to grip things. But we are interested not in gripping, but rubbing. We have a suspension system, we have an axle moving in relation to a wheel, and we'd like something to resist that motion and get rid of some energy for us.
Friction, will quite conveniently do this for us. Just like rubbing your hands together on a cold day, friction can turn motion into heat. So if we take our suspension motion, get it to rub two things together, we can get them to warm the air around them, and effectively dump our unwanted energy to the atmosphere for us.
Pretty simple, and conveniently, as we have mentioned much before, in a plain multi-leaf 'stack' we have already have numerous chunks of metal that will rub together and give us some friction, when the spring is compressed, or springs back.
Worth noting that. We have suspension motion in two directions. When the wheel goes up; the spring is 'compressed'. When it comes back down, it 'rebounds'. And we can damp the motion in either or both directions, and where different damping rates are applied in the different directions, and that is often useful, we describe one as compression damping, and the other as rebound damping.
OK. Leaf spring. Simple one, just two leaves. as we said before, they each have a length, a width and a thickness. We are only interested in the damping we are going to get from them rubbing together, and that is going to come from the area of the two leaves that are in contact and rubbing.
So, wider and longer the two leaves, the more damping friction we'll get. But, it will also be dependent on how tightly the two leaves are clamped together, and how 'grippy' they are.
If the leaves are oiled, or polished really smooth, or not held together very tight, then they will move more easily. tighten them up, or make them more grippy, (like they have a load of rust between then) and they'll give a lot more damping friction.
Add more leaves, and you effectively increase the rubbing area, and you'll get more friction, so more damping.
Right. That's the 'internal' damping of a leaf spring. And, obviously, when you mess around with the design of your leaf, you have a fair amount of scope to 'tune' the spring to give however much damping you want, by adding more leaves, making them wider or clamping the stack tighter.
But, with so many other facets of the spring, by way of its rate and rate linearity also controlled by the design of the spring, trying to tune the spring exactly as you'd like it for your application might make things a bit difficult.
So having a separate damper, that you can set up only to give damping force, might be quite useful, and let you concentrate on making a spring that gives you the rate and rate proportionality you want.