Hot Rods Tranverse Springs Tech Info.

This one can not be right. Needed some of that thought.
If it were right the shackles would be more horizontal.




I don't know what spring it is, I don't know the perch spacing, but anyone should be able to tell the that either the spring is too long or the perches are too close together just by looking

 

And that my friends is why we are here today talking about transverse springs.
Thanks 31 Vicky and others for your constructive comments and observations thru out this thread.
Here is a front axle diagram which really works the same for a rear transverse spring set up.

 

This is from Speedway Motors site.

Solid Axle - How to Measure For a Spring
By Speedway Tech Team
9/16/2016
Tags: Tech, Chassis and Suspension
There are two different methods you can use to determine which spring length you need:

Measuring For A Spring

Method A: If you are using an axle with vertical perch bolts holes, you simply measure center to center of the spring perch bosses (taken from the top side of the axle) and deduct 5”. This will be the length you need on a spring-over application.

Method B: If you already have your axle, perches and shackles, you can assemble them and situate the shackles so they are horizontal. You can then measure from the centerline of one shackle pin to the opposite one as shown to the right. This will give you the length of spring you need.

 

Yeah, however, some suppliers (P&J IIRC) say deduct 3" not 5". I need to figure out which is right, but I'd tend to believe P&J over Speedway...

 

That information is very correct,
but also incomplete because it never mentions the arch and the spring information is void of arch information and free vs loaded length.

I'm sure you can see how we can have 4 springs with identical eye to eye measurements but all have diffent main leaf length. I can take a 20 foot long spring bar roll some eyes and get you 29" eye to eye, or 34 inch spring bar roll some eyes and get you 29" eye to eye. Go ahead and order you spring off of the Eye to eye measurement alone and see what you get,,, that's the only way you know what you're getting. When you unwrap it. What I'm saying is clearly evident by the bad example pictures in this thread

 

Whilst we're on the subject. I need to lower the front on my Model A and I'm thinking about removing a few leaves from the front spring. Any tips? Should I remove the upper ones or every second one? Any tips with the U-Bolt?

Sent from my Pixel XL using The H.A.M.B. mobile app

 

Leaf springs became popular because they are cheap to manufacture, strong and easily adjustable to suit specific loads and requirements. By altering the length, width and thickness of the leaf, along with the number of leaves, it is easily possible to change the amount of resistance, which alters the ride and handling qualities of the vehicle to which it is attached. Leaf springs are also used because the spring acts as an additional link for the suspension, much like a control arm.

For the purpose of hot rods, many cars built in the 1920s through the early 1940s (particularly Fords) used a single transverse leaf spring on each axle, shaped in the form of a yoke, which formed the upper link on the solid beam dropped front suspension, and live rear axle that linked the left and right wheels. In these cars, the transverse springs were also designed to act as a damper, dissipating energy to prevent excessive bouncing. The design was simple and strong, allowing for sizeable wheel travel. This made it ideal for coping with rough, uneven roads, common in the first half of the 20th century.

US-market Ford vehicles used transverse springs all the way until 1948, while several other manufacturers continued to use them afterwards, notably Chevrolet and European automakers Fiat and Triumph. However, their ideas were slightly different than Ford. In this case, a single, transverse-mounted spring represented a cost- and weight-effective method of utilizing a fully independent rear suspension. This is because the spring, with a solid center mount, actually operates like two independent "quarter elliptic" springs, aiding wheel control and supporting the lower links. Many early leaf springs required periodic maintenance by spraying penetrating oil between the leaves. In more recent years, the advent of springs with fewer leaves, low-friction inter-leaf plates and synthetic rubber inserts have rendered this procedure obsolete. Although originally manufactured from high carbon steel, today, most leaf springs are made from alloy steels like silico manganese and, increasingly, composite or plastic materials. Since 1980, Corvettes and several other cars have used composite transverse leaf springs, which have demonstrated remarkable longevity with few reports of fatigue.

 

ChoppaScott, it is trial and error and as 31 Hemi said most information for the man in the street is incomplete and leaves you scratching your head.
I've removed the two small ones at the top and given it a try.
Usually a closed car has two more leaves than an open car so take a look at an open car spring for the same Model A and see what it may offer you in leaf choices.

 

Trying to lower the rear end of particularly a Model A or T comes with some limitations.
Since the chassis in stock form has no kick up over the diff axles.
So, the more the chassis is lowered toward the ground the suspension travel distance is diminished and eventually you bottom out on the slightest of bumps in the road or with little or no extra load weight added.

 

So again we come across the need to choose wisely the rear mounting point for the transverse spring.
So up front the main consideration was caster and at the rear it in most case will be suspension travel.
Below is an example of an attempt to lower the rearend by placing the spring behind the diff, but no consideration has been given to suspension travel.
It also is not a good example of my favourite attachment method but done with proper thought should be OK.

 

Not to cast stones; but there is a lot of stuff that needs attention in the picture fiftyv8 posted above.

 

No problem, all constructive criticism is welcome.
It's just a friendly discussion that may draw facts out that are helpful to others.

 

That is being nice Rich.

Fifty, this post of yours reads like the build pictured is close but needs attention to details. The group of words used "proper thought" really encompasses volumes of principals and calculation being applied and they've all been ignored evident in that photo. It's a total fail on the rear suspension. It's also not a good way to lower a model A chassis. Sorry if that ruffles someone's feathers but if this is to be an educational thread the truth trumps everything else.

On the rear end of these hot rods using a transverse spring behind the axle - the hardest thing to figure in is the pinion angle. The spring hangers getting welded to the axle housing must be located width wise according to preload tension needed for the spring chosen, rotationally located to pinion angle, and far enough behind the differential so it clears the crossmember, strong enough to hold. It can be quite daunting to get all of that correct let alone hit ride height, hit ride quality, hit wheelbase, apply the principals and hit the calculations and do ALL of that ALL while in mid air.

When the spring perches must be narrowed for what ever clearance reason presents, a '40 front spring is used in the rear. They are very low arch and require a dipped crossmember. Altering a stock 40 rear crossmember is how the ride height is established.

If there's a quick change involved, a model a style crossmember and spring used then the hangers still need to be at 49-1/2 and that causes major clearance issues at the tires when spring behind. One can not simply move the hangers in order to accommodate the clearance issue and use the spring,,,, they must have a custom spring made, not just the main leaf to get it done right.

 

RichB and 31Vicky, I hear you both.
That is why this thread is running.
I fully declare that I am no tech expert or engineer, but I do know from experience how much I have learned over the years playing with hot rods and even more scary is what I have seen produced by others in their projects.
I don't expect to reach those arm chair experts out there, or ostriches with their heads stuck in the sand or where ever else, but giving this topic oxygen is generally enough for me to highlight and help others to see reason why they should take a few moments more to consider pro's and con's of their next move during their project build.
It may also provoke them to understand that they maybe further out of their depth than they realised and should take the opportunity to ask a friend or a question on the forum before they do too much that will need to be done over...

I am posting examples to reflect the part of the specific portion of what I want to discuss, warts and all with no excuses for failings, but it does show how many failings exist without having to look too far.
You are actually right in the true sense that showing stuff that is incorrect could be a danger of misrepresenting what I am trying to achieve.

I had hoped more folks that I would have considered expert on such a topic would have fired up and provided some real chestnuts of wisdom, but there has been only a small number of comments in relation to the great number of views we've had.

I'm almost tuckered out now any ways and had considered walking away from this thread since I am no expert and have almost exhausted what I wanted to say.
There are only a couple more pic's I had considered posting and that was going to be it from me until I find something else of interest to post.

You are dead right when you suggest there are many points to consider regarding setting up a rearend.
My best friend was a blacksmith spring maker during his working life and made numerous custom springs for local hot rodders thru out his career and I tagged along a good number of times on weekends to look at special spring needs and suspension setups and learned a lot from the experience.

Most of the guys out there are not engineers etc I would think and so technical stuff would probably go over their heads as it would mine, but some technical talk can't do any harm when mixed in with other views.
Also adding pic's generally seems to provoke thought and so that is what I have attempted to do...

 

Russ, I've been following this thread with interest. I haven't contributed that much because the contributions I've got are firstly mainly theoretical and secondly related to my sometimes slightly weird investigations. I've learned a lot more than I have taught here: thanks.

But there is something which can still do with some clarification. @31Vicky with a hemi has mentioned preload a few times thus far. Preload is one of those things people know a bit about but when you ask them what it is, they can't tell you. May I try a definition? Preload is the condition where the spring element in a spring assembly or system is under stress when the assembly or system as a whole is not under stress. Would you agree?

In the case of a coil-over, the coil spring is stressed against the extension stop integral with the damper when the unit is assembled. But once the unit is installed and subject to the vehicle's weight, the spring is clear of the extension stop and preload is moot. Similarly, a torsion bar setup might incorporate an extension stop against which the lower control arm etc. bears when the vehicle is off the ground. As soon as the vehicle is on its wheels there is nothing touching the extension stop, and again preload becomes irrelevant. In neither of these cases does preload influence the spring rate at all, though it might seem to if you try to calculate the spring rate directly from static deflection, because the static deflection is physically limited by the presence of an extension stop.

Yet time and again we'll hear how torsion bar wheel rate depends on preload. It doesn't: torsion bar spring rate is perhaps the simplest and most linear kind of wheel rate calculation there is. If you "tighten" the bar all you're doing is raising the car – and not actually changing the torsion in the bar.

But now: leaf springs – not only leaf springs but specifically early Ford transverse leaf springs, and in particular the rear ones. This is perhaps the only instance where there is anything we might rationally call "preload" in a spring assembly at ride height – because only here does the assembly consist of several spring elements which may be stressed against each other. The ends of each leaf in the pack exerts a force on the upper surface of the leaf below it, the overall total of which equals the force required to press all the leaves together at the centre bolt.

I've never actually bolted an early Ford transverse spring together, so I don't know what kind of force that'd be. Is it significant? Is it likely materially to throw out the leaf spring rate calculations available in various places online? It surely would in absolute terms; and I can't think that the math would be very onerous (it is most certainly a serial-spring problem. But the real question is this: would the overall spring rate not nevertheless equal the sum total of all the respective leaf rates, despite the static deflection being influenced by the cumulative preloads? Serial-spring systems can be infuriatingly counterintuitive: but this is not a question for a lazy Sunday afternoon.)

Coincidentally I'd recently read something about a new development, IIRC in off-road racing, where coil spring assemblies are so assembled that secondary coils actually pull the vehicle down, against the primary coils which hold the vehicle up, and the implications of that for wheel rates etc. That was interesting because I'd been chasing a line of thought around counterbalancing upward- and downward-acting airbags in a similar way. It didn't go anywhere in the end, except that the observation that varying pressure in the upward-acting bag might potentially lead to the desirable situation where a higher ride height coincides with a lower spring rate is now safely filed away. But that just by the way.

 

I going to attemp it, I'm not the best at explaining such a multi factorial and complex issue.

Let's differentiate between the terms "spring rate" and the "load rate"
(In the back of your mind remember my earlier ramblings about the arch too)

Henry springs, in free form are progressive, that's to say the first inch of deflection is so easy. The rate is very low compared to the next inch, then again into the next inch.

The term "Spring rate" is used to say how much weight is needed to deflect the spring 1".
The term "load rate" is used to say how much a spring can carry at is installed loaded height.
The two are related yet different.

Think along- each part builds on the next. You will need them all
If we take any progressive spring and stretch it (not compress it) what happens?
It wants to pull back to its original state, right?
If we take a progressive spring and compress it hat happens? It also wants to return to its state but at what force?? Well that force greatly depends on how far it was compressed doesn't it?


So first let's take Henry's progressive leaf spring on a bench and stretch it about 3 inches.
Next let's attach the shackles and the release the external manipulation forces we used to stretch it.
Now the spring is pulling back on the shackles, hard enough to keep them at 180 degrees.
But how hard, what's the value of the pulling force? Well unless we know what the arch height is or was or supposed to be we've got no fucking clue do we ?? Did we take leafs out? Did we reverse the eyes and make the spring longer thus reducing the stretch and force?
That's why all that stuff matters.

Right now, the stretching force, (aka preload) is fighting the arch's need to rise and return to its original free state, it's doing so via the shackles. Draw a big force arrow pointing up under the spring.

Ok so right now the spring has its pre loaded tension , so let's slip the axle assembly under the car with its 180 shackles and bolt it in Sinching the u bolts tight.

Now let the full weight of the complete car down on the spring.
That downward force will try to compress the arch. Remember the stretching forces is trying to raise the arch and the weight of the car is trying to lower it. As the arch compresses the eyes must move further apart. To do so the shackles must move downward away from 180.

These 2 opposing forces, tension within the spring and weight on the spring have a story to tell you and they use the shackles to communicate. When the pre load tension force and the weight compression force is balanced the shackles will be exactly in the middle of 180* pulling force and 90* straight down weight force, or the 45*.

If they are 44* the tension is greater than the weight, if they are 46* the weight is more than the tension. Within a few degrees is fine but that 1degree tells you the story of the imbalance. If the shackles are at 45 + on bare chassis that's a problem.

 

All well said and thanks to both of you for your contributions.

 

All preload works like this graph.

You keep adding load, but until you reach point X there is no movement in the spring. Once you're past point X the spring deflects in accordance with its spring rate, and preload becomes irrelevant. The preloaded condition does not alter the spring rate or any other characteristic of the spring once the load on the spring is to the right of point X.

Leaving aside any effects of inter-leaf stress, it is exactly the same with a transverse leaf spring. Because of the (incidental) geometric characteristics of the shackles, the spring needs to be spread in order to fit it. With no load on it, the spring will pull the shackles horizontal and coplanar (ignoring the weight of the spring and shackles themselves) because the entire assembly is like a three-link chain in tension. Now you start adding load. You add a little. The spring and shackles don't move. You add a bit more. The spring and shackles still don't move. You add a bit more, and still no change. But the next time you add a bit of load the spring and shackles start to move all of a sudden. And once this happens, there is no longer any preload. Once the shackles are at even the tiniest angle to one another, there is no preload. The spring acts exactly as it would if the arrangement of shackles had allowed the spring to be installed in an unspread state.

I'm not sure if the inter-leaf stressing imparts falling-rate characteristics: I haven't been able to think that one through. It certainly doesn't impart rising-rate characteristics. Rising-rate leaf springs are possible, but they are nothing like early Ford springs. Rising rate leaf springs are arranged that there is a gap between the ends of one leaf and the surface of the next-longest leaf, which gap closes up when a certain deflection is reached, bringing the shorter leaf into play. What does indeed happen with a stock early Ford is that elastic roll resistance only develops as you run out of geometric roll self-correction, and that acts as a rising-rate system. It can be manipulated by playing with roll centres and shackle instant centres, as I was on about above.

Given an understanding of the shackle dynamics v. the way the force exerted by a leaf spring is usually tangential to its deflection path, there is nothing to suggest that the spring rate of a transverse leaf spring cannot be calculated from the length, width, leaf thickness, and number of leaves alone. That is not to say that things like the arch height won't bear on your physical installation, for practical reasons.

 

I'm not sure how the preload is irrelevant, there is no way to get where the spring needs to be with out the pre load. I'll agree that once the shackles are holding the spring that the external forces needed to load the spring are no longer needed because the shackles are now holding the pull of the spring- VS any forces uses to compress the arch.
So again, without preload stretching of the EYE to Eye distance to get the shackles mounted the spring shouldn't be able to be mounted and really function as designed.

Calculating the spring rate,,,
On a progressive rate leaf spring I'm sure it's possible to calcultae the rate. So let's remember the spring being progressive could be calculated and plotted on a graph starting at zero rate with zero compression. The spring is measured in free resting form to begin and because it really takes no measurable effort for the first 1/4" and really the first inch is quite easy but each fractional distance increase inturn increases the force needed to get it to move further. The term "spring rate" is the weight needed to compress the arch 1". That being said, why oh why is the starting arch height omitted from our transverse springs ??? Why oh why do different manufactures have different arch heights ???

When we are assembling these things the EYE to EYE distance must be stretched to meet the shackles located in the fixed perches. Considering the arch height is needed for spring rate knowledge it's missing and different across the board. Stretching the E To E distance must in turn lower the arch but how far depends on where your spring started. That means the rates are different, they have to be.

Now if we take the very same progressive rate spring and stretch the Eye to Eye to 8 different widths at 1/8" variations we will have 8 chances to physically measure the Weight to get that loaded arch to move 1". We will have 8 different spring rates that were physically proven. That's because of the 8 different preload forces needed.
Again I can't see how it's irrelevant. Now in this 8 example experiment we measure the arch movement, however the eye to eye grows underneath dynamic shackles that will lower along with the arch.

So if we take a model a rear spring designed to be spread out 8" and mounted up at 49-1/2" and find we need to mount it at 46" to clear obsticals then we are really looking to screw the pooch and put a turd on the road with no care in the world about spring rates. But bitch about rough rides, unsprung weight, squirlly handling and all sorts of other shit.

Everyone has their own farms and their own pigs out there. They can do as they see fit. Some aren't quite sure how to care for that pig, understand how the pig works and bitch about their pig. The thing is all the pigs are the same.

 

So 31Vicky, being a rule of thumb man and not a technical kind of guy, I was interested to realise that spring rate for a Ford transverse spring should not be measured when it is at rest, hence not preloaded, but the 1" compression should be measured when the spring is actually in the preloaded position lets call it.

I am right in my thinking?

One would imagine that a spring rate reading at rest compared to a spring rate reading in specified preload would be two different animals.
Correct me if I am wrong, I am basically thinking out loud here...

 

That thinking is correct.


I'll try a little bit more clarity to build upon that.

There's conventional leaf spring knowledge that's about all leaf springs,,,, that knowledge would apply to any spring that you can take out of the box install it. This would be your pickup trucks and other parallel leaf set ups. In this application There's a solid mount, slam a bolt, there's one shackle and slam 2 bolts, attach to axle and set the car down.
In this situation you're simply supporting a weight/load and the range of compression is targeted to get the ride height based off its free state. It's SO simple.

So take that conventional pick up truck spring knowledge and put ALL of it away, hide it someplace you can't get to any of it when working with any transverse spring. It's all different when it comes to these so don't try to apply it. The "spring rate" as its being thrown around begins at the free state and is easily predicted or confirmed as weight is applied.

I do not know of any other suspension spring in any other application that needs to be stretched like these Henry style transverse springs in order to be installed. Arbitrary lengthening of the spring or adjusting hanger/perch width changes the amount of stretch and changing that amount stretch changes the spring rate. Removing leaves changes the rate of course, but removing leaves combined with additional vehicle weight the also lets the spring's arch get flatter thus lengthening the EYE to EYE measurements. This creates the same situation as arbitrary lengthening of the spring.

Now let's say the spring is too stiff, what you'd want to do is remove leafs, but what do you do about the spring's natural consequence of growing in the EYE to EYE length. The proper thing to do is get a shorter main leaf made, the difficult part is that there no way to calculate this that I know of. Its a full mock up at full weight and then measure how much shorter the spring main leaf should be. It's a pain in the ass and sometimes it's 2,3,4 guesses and expensive so most guys don't bother with it but only complain about their suspensions ride quality. Oh and in the midst of all that trying to hit the ride height you want,,, ride height and wheels will make or break a car.

I had more than a few lengthy conversations with Dick Spardo about these things. It takes a while to wrap your head around it all and see how the relationships interact between themselves. It's nearly impossible to grasp it until you realize the conventional leaf spring thinking does not apply here. It could be said that conventional spring thinking should thrown out but that tends impart that it's wrong. It is not wrong and good stuff to hang onto, you just have to put the conventional spring thinking and theory away and do not try to apply it to a transverse leaf spring mounted in tension before the spring begins to do its job holding up the vehicle. You have to start over with the thinking, start from scratch because it's quite different.

 

Interesting comments, as it lines up with a couple of guys I've seen using Model A type rear springs with a quick change and custom perch eye centers.
Boy they struggled with height and preload as well as main leaf length.
Their concern was not having the U bolts hitting the quick change housing if they got it wrong.
Obviously, a front transverse spring once setup correctly pretty much stays that way, but when it comes to a rear transverse spring you seem to be torn between getting a decent ride comfort, ride height and capacity for carrying a variable load.

 

Here's a few examples of what the spring should look like @ bare chassis stage with the right spring length, perch distance, and preload tension.
The spring's pictured here and the shackle attitudes are right on.
There could be an opinion about other components shown but this is exactly what the spring should be doing. The spring is pulling hard to keep the shackles at that position just slightly off of 180* while suppuring the weight of the frame.

 

Yep it can be a M#€^*R F?&$3R.
Here's some examples

Read the description of what he did to achieve ride height,,, look at the results,
Think about how the described manipulation of the spring increased the eye to eye measurements and now the spring has no room to work with the shackles crashing into the perched. Shackles against the perch turn the spring into a solid bar.

The proper solution to simply get ride height adjustment here would be to get a shorter main leaf or leave the spring alone and raise the crossmember for ride height.

And in the grand scheme of a build maybe things do wind up like this to get everything exactly how you want it,,, I get that, been there and done that myself. You need things in place to establish relationships and measure correctly.
So the last step is to measure the arch height you want, the eye to eye distance needed and have a leaf made so you can finish it correctly.



The poster said this was a swap meet find, axle with spring.
Super high arch and jambed up at the perch is not going to work.

The reversed eyes make this one ride lower but that process of reversing geometricly makes the eyes further apart.

This spring appears a little short but we can't see if it has full weight or if chassis blocked up


This spring is a little long, probably because of the reversed eyes.
Suspension travel stops when the shackle hits the perch.
The visual check on the shackles of spring rate to load balance is telling that the spring rate is too low, these are not 45* shackles. The spring will move and the shackles crash into the perches and create a solid bar out of the spring.
Or
This car has an exceptionally heavy engine like a early hemi dressed with its original cast iron parts and a blower. Then the spring needs another leaf to increase the rate and hold the load.


Not sure if this is a traditional lift kit or what the hell happened.
The spring is damn sure to long, reversed eyes and de arched or leaves removed.

 

Looking at some of those examples of poor engineering I don't think I will be able to sleep tonight...

 

Yep, those last examples are all lacking the correct amounts of preload tension.
So, i can't see how it's irrelevant. The preload must be correct before it begins to carry the real load.

It's not so much the engineering, it's more of a piss poor execution that ignores the details of "function" in favor of "Form".
It's supposed to be "Form follows function" and that really means the function is first then what it looks like is purely a secondary consequence.
Any one of those examples can be corrected and a perfect condition of having cake and eating it too will be had.

Used to be we asked grandma if we could go swimming, she'd say "oh yes of course honey- you sure can. Just don't get wet "

 

31 Vicky, thanks for your explanations and examples which sure makes a clear and strong point about the proper use and execution when it comes to using a transverse spring.
They sure are tricky little suckers and looks so unassuming in many ways...

You will note on my chassis video (link below), I have made that exact preload mistake which is quite clear, however upon realising the error, I have since corrected it by using a new shorter main leaf.
It seems like many springs we see in front end sales pictures the manufacturer/seller had either made a mistake or just did not know enough about the function of these springs. Sadly, on occasions we accept such products without question.
This in fact was the motivating force that lead me to starting this thread.

Grandma ain't never wrong.

 

One question I do have, is;
When using a spring say behind the front axle for example, where it is say hooked up between the radius rods.
Will that spring be capable of doing the same amount of work as the same spring sitting on top of the axle or does one need to factor in some kind of lever effect???

 

The spring doesn't care, the perches and the bones do though.
The spring will see the same weight and do the same job.

The bones are being asked to support the sprung weight with loads in a different direction than their original job- what to just control the location of the axle without any sprung weight bearing on them. Radius rods are built differently, like a truss and there is usually a bat wing bracket to carry the weight not the rods. The perches are a different story. Spring over or spring in front have a substantial forged part supporting the weight. Spring over perches have a pretty wide tapered collar to spread and hold the forces they see. Spring behind perches are usually kinda scary. A DOM tube welded to a threaded doohickey with that threaded doohickey bearing forces of compression, tension and shear as well as the weld doing the same. I've seen more spring behind perches fail than I have Henry spring over perches fail.

 

There will be a motion ratio at play. It'll be small, but it will have an effect.

Unsplit, the axle-wishbone assembly pivots about the rear ball of the wishbone. The principle is the same, mutatis mutandis, for split wishbones. Let's say that the longitudinal distance from the axle centreline to the ball is 34" (stock Model A, approximately) and you're moving the spring back 4". Then, your motion ratio will be 34/(34-4), or 1.13:1.

That means the effective spring rate (wheel rate) as installed will be about 78% of what it would have been with the spring installed over the axle. The wheel rate is the spring rate divided by the square of the motion ratio.

I italicize "as installed" because as installed over the axle the spring is already subject to a motion ratio, which might not be obvious just from looking at it, due to the angular action of the shackles. @31Vicky with a hemi mentioned a progressive spring rate above, and I couldn't see it because there is nothing in the spring itself to give it a progressive rate, but as installed between angled shackles, the effect at the ends of the shackles are indeed those of a rising-rate spring.

So, with a spring-behind setup you'd have to multiply that 1.13 (or whatever) by the motion ratio imparted by the shackle action, and then square the product to get to the factor by which to adjust your spring rate.

I did a quick vector analysis to get my head around the effect of the shackles. A few things are obvious in a typical installation:

• The top end of the shackle will impart an inward force on the axle, putting the middle part of the axle in compression, more greatly so at the top than at the bottom;
• The bottom end of the shackle will impart a usually outward force on the end of the spring, at right angles to the spring's deflection path; and
• The spring's deflection path is irregular and probably only absolutely ascertainable by empirical means, but it is nevertheless likely to be roughly elliptical and indeed near-circular over the 3" or so either side of ride height. I am therefore confident that my educated guess of 15° off vertical at ride height is realistic enough to go on.

First, I looked at a typical setup with the shackles at 45°:

I've assumed a 600lb vertical load at the perch. Remarkably, the lateral force on the perch is as great as the vertical force, and the tension in the shackle is considerably more, at 849lbs. The force acting effectively on the spring is 735lbs, with a corresponding reduction in instant deflection at the spring eye, representing a motion ratio of 1.23:1. This means that the wheel rate in bounce will be less than ⅔ of the spring rate of the assembled spring lying on the bench.


At 30° all the forces are comparatively smaller. The motion ratio is now 1.1:1. The wheel rate is therefore 81% of the spring rate.


At 15° an interesting thing happens. There is no tension imparted to the spring, because the shackle is tangential to the spring's deflection path. The motion ratio is now 1.03:1 (94%).


With the shackle vertical there is no compression imparted to the axle. There is a very small compression imparted to the spring. The motion ratio is now on the other side of parity, at 0.97:1 (106%).

We'll find that a motion ratio of 1:1 will occur somewhere around 7-8° from vertical. A couple of things coming out of this:

• The closer the shackle to vertical, the less pronounced the rising-rate characteristics.
• The closer the motion ratio to 1:1, the smaller the sum total of forces acting on the system.

There is a lot to be said for rising-rate springing. On the other hand reducing the forces acting on the system allows all components either to be lighter or to have a greater surfeit of robustness, and to make for an elegant design. You'd just have to adjust the spring rate downwards by the appropriate amount. @31Vicky with a hemi is quite correct: a spring which gives a decent ride with shackles at 45° will give a far harsher ride with shackles near vertical.

But apart from this effect there is nothing magical about the figure of 45°. The fact that the spring was at one time attached to a spreader has nothing to do with it. Substantially diagonal shackles play a role in geometric roll self-correction, to which I've referred before and which I'll explain in more detail if you like. Once there is a positive lateral locating device in the system, however, I still maintain that you're better off with the shackles at the abovementioned 7-8° off vertical, subject to the following caveats:

• The spring rate is adjusted to suit, and
• There is enough clearance around the spring eye throughout its movement.