Hot Rods Tranverse Springs Tech Info.

Near vertical was conveyed to me from Eaton spring, Less than 45Deg. is better!

 

Ned, amazing explanation !
How much suspension travel can we get before the shackles crash into the perch if set static 7-8 off vertical? And the same if static at 45?
Where's the most favorable conditions - between 45 90 vertical or some place else ?

 

I know it depends on the arch height to answer some of that correctly, to even geuess. Again enter my bitch and ramblings about the arch height missing every place you care to look. About installed vs free arch heights

 

Not sure I understood all of your explanation Ned, but thanks all the same and I'm sure it will have an impact on other who take the time to read your posts and like wise those of 31Vicky.

So if spring behind a front axle as we have just discussed has a minor effect, does one assume the spring located ahead of the axle with a front axle and behind a diff at the rear end does also has different characteristics, like we see on many mid 30's and onwards Ford suspension setups...

 

Thanks 31.

A front spring might have a lateral displacement of around ¾" through its travel; a rear spring might have more. That means you'd need an absolute minimum of ⅜" clearance between the outside of the spring eye and the perch stem. With stock perches that already dictates a shackle angle closer to 45°. Aftermarket perches tend to follow the same pattern. It may be possible to modify the adjustable kind to bring the eye out further from the stem. But arguably that's a lot of trouble to go to for a very subtle change.

The arch height isn't a factor in itself but it will determine the spring's deflection path. As I said before, it is probably not possible to plot the deflection path using math alone; at least not the kind of math normal people might use (I'm the only person I know, including an office full of architects and several consulting structural engineers, who uses trigonometry every day, and I'm by no means any kind of theoretical mathematician. I think the math needed to draw a leaf spring deflection path is the kind which looks like ornate wallpaper to me – especially a multiple-arched rear spring.)

The way to do it is probably empirical, e.g. bolt the spring to a jig of some kind and stick a marker in one eye. Then jack up the spring so the marker draws the path on a piece of cardboard.

Yes. Spring behind will give a softer wheel rate, and likewise spring ahead will give a firmer wheel rate. And of course vice versa for rear axles. That's consistent with spring-ahead setups tending to use a longer, i.e. softer, spring. The example I gave was a bit extreme, moving the spring a relatively large distance on a short unsplit Model A wishbone, so the effect wasn't all that minor there.

The geometric roll self-correction I mentioned before: I plotted a typical shackle instant centre migration path over about 3½° of roll, but I haven't had a chance to draw it up in an understandable way. The path looks a lot like an early rear spring, upside down, by sheer coincidence.

 

Ned you are a legend and I for one, am glad you took the time to take an interest in this thread.

 

That geometric roll self-correction:

An early Ford transverse leaf spring setup without a lateral locating device doesn't simply roll about a roll centre like a conventional suspension system. The shackles allow up to about 3½° of self-correcting geometric roll which is not resisted by the springs. This must have been very useful on some of the bad roads Fords might have seen when new, because it would allow almost 7° of angular misalignment between the front and rear axles without twisting the frame. That's equal to about 3" bumps at diagonally opposite wheels, or a 6" bump at one wheel. It made the very light T and A frames practical under such adverse conditions.

That was all about going slowly in a straight line on very bad roads, not about going around corners fast. If anyone has done any analysis of what this means at the theoretical limit of lateral acceleration and written it down, I haven't seen it.

The first principle in all this is that in order for this arrangement to be self-correcting, as it demonstrably is, the system has to reach equilibrium when the line of the resultant force acting at the centre of gravity passes through the shackle instant centre. At rest that is straight down: so push a parked Model A over sideways, and it will rock back upright. But the force isn't straight down when cornering. In the diagram the car is shown cornering at about 0.2g, resulting in a geometric roll of ½° imparted by the shackles, before the spring is even a factor.

You'll see that as the shackles move, the instant centre they describe moves as well, tracing a predictable path with an inverted bell shape. That means that the geometric roll angle at any given lateral acceleration is likewise predictable. (Obviously there are variables like CG height, amount of axle drop, etc.) Because the path rises higher than the axle at both ends there is a geometric roll angle even for the theoretical limit of lateral acceleration, i.e. the point where it's no good adding tyre adhesion because the car will fall over anyway. In the example that'll be around 1.6g: and that's assuming there is enough clearance for the shackles to adopt the corresponding positions.

We know that early Fords don't roll a mere 2.1° at 1.6g, and the diagram is consistent with a lot of elastic roll being added on top of the geometric roll. Because A<B, we can see that the spring is loaded asymmetrically, and that is going to result in a roll moment. In different terms, the resultant force acting at the CG is offset by distance C from the elastic roll centre, which is at the middle of the spring at the middle of the installed arch height. In practice, the spring resists most of the total roll motion, but not all of it.

But here is the thing: the greater the roll stiffness, the smaller the roll angle at any given lateral acceleration, the smaller the component of total roll represented by elastic roll. So, while geometric roll isn't really a factor on a softly-sprung car which would roll alarmingly if it were capable of pulling more than 0.25g, it starts to become a factor once we get more ambitious.

So what happens when we introduce a lateral locating device, like a Panhard bar? Now the car is constrained to roll about the roll centre as defined by the lateral locating device. The spring still wants to rotate about the shackle instant centre, and the only way the bind can be resolved is by flexing the spring. In other words, the geometric roll is converted into a certain amount of additional elastic roll, which is effectively resisted by the spring.

In the case of a Panhard bar the roll centre will be slightly below the elastic roll centre defined by the spring, and a fair distance away from the shackle instant centre. But let's say we have a lateral locating device which places the roll centre at the shackle instant centre? That's possible with a Mumford linkage, for instance, at least initially (i.e. at 0° roll.) Then there would be no bind between the roll constrained by the roll centre and the geometric roll due to the shackles, and no geometric roll converted into elastic roll. So, if we want to make the most effective use of the spring in resisting roll, the distance between the roll centre and the shackle instant centre seems to be an important factor. And if we're putting our roll centre somewhere in the first 12" above the road, we'll want to put the shackle instant centre as far away from it as we can, like 12' below the road. And that corresponds to shackles nearer vertical than diagonal.

Splitting hairs? Perhaps. Bottom line, I'd say, is that as soon as there is a lateral locating device the shackle angle is more to be determined by practicalities like eye-to-perch clearance than anything else.

 

Thanks again Ned,
For as simple as they appear there's a lot going on there. Then enter the common modifications we love and hold so dearly.

Most, certainly not all but most Hot rods utilize split bones or radius rods up front. The natural consequence of doing that adds roll resistance because it turns the front axle into anti body roll bar and the bones or radius rods into sway bar links that load the frame at a rising rate as roll increases. Some situations and combinations of parts offer more roll resistance than others. The increasing load on the frame is a load transfer and must go into the spring on one side, come off the spring on the other at some point. Factoring that in and thinking of the single spring in a condition that is effectively two opposed 1/4 elliptical springs is likely to crash some software and brain cells.

In its free form on a bench the transverse spring is just another leaf spring.
Once it's stretched and hung on shackles it becomes something else.
Then when it goes into the crossmember and U bolts sinched down tight it becomes more like two 1/4 elliptical springs.
Going down the road experiencing the dynamics of its job it's a really busy situation.

There are a few ford design changes as the years progressed.
In 1935 the ford switch to spring in front changed the travel limits of the earlier spring over shackles. Still mounted in tension though.
A few years later the springs were changed and the shackles were lengthened. Those springs worked from a design that is more flat than arched and shackles closer to vertical . At the same design change the addition of sway bars and panhard bars was necessary because the axle was free to WAG off the spring's longer shackles when those shackles stopped offering any lateral shift resistance.
Then enter the suicide axle spring behind modifications which again have their own unique opertunity to increase ones understanding of suspension and transferring weight directly to the frame at 2 additional points behind the crossmember.

 

The Ford factory in the 30s had a different design issue than cornering. Like mentioned, the roads were bad and the suspension at the time was designed for them. What about building a rod with ball wishbones like original and using anti sway bars to control the body roll? And another subject, instead of a front panhard bar, how about a ball bearing mounted solid to the center of the axle and it travels vertically in a slotted bracket bolted were the front u-bolts are? Would that design act like a Watts link? Lets hear some theory's and comments on above? Would any of this work better than common hot rod suspension today?

 

More great info about these humble springs.
I just wonder how much of this Ford spring design was intentional or are we just lucky how it worked out!!!
I had assumed it was somehow a carry over from the horse and buggy days...

 

I've wondered that myself. I think the overall idea of a transverse leaf spring does indeed come from horse-drawn vehicles, though I believe the norm there would have been full-elliptic springs, with the pivot for the forecarriage in the middle of the lower pack. I'm not sure if Henry thought up the arrangement of shackles on his own, however. A lot of the prehistory of the American automobile is about mud-piercing "high-wheelers" which followed the lines of the horse-drawn buggy or light Victoria. It would be interesting to see instances of transverse semi-elliptic springs among those.

 

I've never given transverse springs all that much thought prior to this thread and never considered how they may have been designed or figured out by the Ford people.
Looking at the whole idea now, obviously the design must have been based on economy of materials used, possibly weight being a consideration, but it also occurs to me that it could be possible be that Ford saw something in those very early car springs which I'd say were copied from horse drawn vehicles that had opposing arches which must have given the impression of built in tension/energy which he has tried to replicate.

 

I have no intention of making this thread my life's work, but in doing a little searching on the internet, I have discovered Renault seems to have had almost identical transverse spring suspension system used in their 1901 milk cart.
there are limited pic's but using ones imagination it looks quite like a Model T transverse spring set up including axles.

 

That's the old "I want to make 'A' but to do that I need 'B' which depends on 'C' which, oops, depends on 'A'.

 

Don't think that 1901 truck is a real deal, as those are Model T Ford axles underneath it.

 

The Renault truck was made in 1901 yet Ford did not produce its first car until 1903, so this is why I was wondering if Ford had some how acquired copy right from some other maker to produce this kind of suspension.
Maybe Ford was not the designer of this suspension style, but utilised it and made it famous and obviously improved on it over the years.
I am starting think there must have been some kind of French connection there.
Look at later years with the French flathead engine blocks.
Just thinking out loud here...

Maybe this is evidence of a connection that was officialised in later years but arrangements must have been in place much ealier.

https://en.wikipedia.org/wiki/Ford_SAF

 

Ok,
Something that has bugged me for years ( if I may ask on this thread ) is, why do most modern hot rods have more leaves in the rear spring pack, compared to the front.
I often see only 5 - 6 leaves in the front spring, yet always seem to be 9 - 11 in the rear spring pack.
Surely with the weight of the motor and braking forces, the front spring should be the ‘heavier’ one ?
Any comments would be appreciated.
Thanks

 

Good question Clem, I see we are both doing the graveyard shift.
A question I had pondered myself and thought that a front spring mostly had its weight carrying capacity determined by engine weight while rear spring was likely to more exposed to variable loads and more affected by passengers and cargo.
That is just my take on it.
Possibly a heavier rear spring may help with stability but that is just a stab in the dark, I'd like to see what the audience has to say...

 

Those wheels don't look right for 1901 to me. Perhaps Model T parts were used by way of approximation just to get the exhibit vehicle on wheels?

 

Because the typical pre-WW2 car had quite a lot of engine setback, and because the typical longitudinal semi-elliptic arrangement tended to a wider rear spring base than front resulting in a rearward roll stiffness bias, the typical handling failure of the day was excessive oversteer. If you look at the earlier suspension innovations, most are clearly oriented to reducing oversteer. I'd even say that the two immediate advantages of early ifs were the opportunity to improve ride by increased polar moment of inertia due to pushing the weight of the engine forwards between the front wheels, and the fact that virtually every practical ifs has worse camber recovery characteristics than a beam axle, inducing compensatory understeer and thus eliminating oversteer.

In light of that it should make more sense to use a stiffer spring in the front and a softer spring in the rear on a beam-axle hot rod.

 

From what I could see it was a recreation that was displayed at a Renault light commercial vehicle exhibition in 2010.
If you look at 1901 Renault vehicles, the underpinnings are more buggy like, as you would expect of that era.

 

I'm certainly open any ideas or opinions that may help us get to the bottom of where and how these transverse spring can into existence.
What you suggest RichB is quite plausible as I have a buddy building an even earlier Renault vehicle 1896 and he is cheating with parts that he can find rather than what was actually used.
I guess I may have been suckered by the fact that Ford was active over there, albeit slightly later.

Good point also Ned, I missed that fact.

 

A lot of it has to do with the arch height. But since is missing or arbitrary at best that's what you get. Also has a lot to do with the length of the spring Since the rear springs are generally much bigger than the front they will act softer.

 

Is the Ford style transverse spring as we know it, the only setup that has the preload characteristic???

 

No, but they will be off topic and different but still requires preload on the spring.
FSM illusionation using the spreader to preload the spring so installation is possible.
The attachment is without shackles.
It's a bear of a job to do.

 

I suspect that the thing which causes all the mystification around preload is the failure to understand that the preload in a preloaded spring diminishes as the spring is loaded. The preload doesn't magically stay in the spring and change its characteristics for ever and ever.

Let's say you have a spring preloaded to 100lbs. If you put a 75lb load on it, the preload will diminish to 25lbs. If you put a 100lb load on it, the preload diminishes to 0lbs. If you put a 150lb load on it, there is no preload, and the spring will act exactly the same as if there had never been any preload.

A preloaded spring is solid and of no use as a spring to any load less than the preload. There is no such thing as a spring which is preloaded in use. At least it would be excruciatingly uncomfortable if used in a vehicle suspension.

It's no different for a transverse leaf spring. We've seen how the angularity of the shackles cause the spring to behave in a very non-linear way, but that has nothing to do with preload.

 

That is very very very true.^^^



If a person is on the 4th floor of a building, the third floor is of little concern,,, unless that third floor is wrong or has a deficiency.
If a person stands on the 4th wrung of a ladder, they are no longer concerned about the 3rd wrung.

The manufacturing process will establish the leaves of the spring in configuration shape A
Assembling those leaves into a pack will establish the spring in configuration shape B.
This is the free state.
Stretching/pre loading and then Installing the spring in the shackles will change from the free state and testablish the spring in configuration shape C .
Letting the working load onto the spring will establish its static configuration shape D.
From this position D is where the magic happens and it begins to do its job as a spring and experiences its dynamics and all the aspirations of a well trained and thought out spring that moves and works.

With all that being said, configuration Shape A ultimately determines configuration shape D. Shape A must be correct to reach Shape B, and shape B must be correct to insure the correct relationships of preload tension in shape C. A,B & C must be correct before the Shape D can Exist and from here, D , it begin to do its job.

Once the spring is in Shape D one could say AB&C are irrelevant or of little consequence, that AB&C don't matter any more or that they vanish,,, that would be the incorrect use of those words. Because there is no, nor can there ever be a correct Shape D unless C,B,&A were correct first.

 

I don't suppose that a preload figure exists in stock Ford specifications that indicates a number relating to preload.
I still am not sure if I have my head around preload in way that I can describe it.
In layman terms is preload to a spring a little like a person eating a meal before starting work, so there is a base from which to provide an energy source???

 

No, that analogy doesn't do it. It doesn't really represent potential energy in the spring at all. I'm trying to think of a better one.

I think a simpler example might be more informative.

Here we've got a spring with a pad of sorts on top, held down by a green bracket thing. Suppose that we've had to compress the spring a bit to get it in under the bracket thing. Suppose also that there is something preventing the spring from buckling and pulling the pad askew, because that's not important for this illustration. We get the spring in under the bracket thing, and now the spring is exerting an upward force on the underside of the bracket thing. That force is the preload.

Now we start putting a downward force like a weight on the pad. The most obvious thing we see is that the pad does not move until the downward force equals the initial preload. I say "initial" because, as we add downward force on the pad, the upward force the spring exerts on the underside of the bracket thing diminishes by the same amount.

At last we reach a point where the pad is just touching the bracket thing, and adding the tiniest downward force will cause there to be a small gap between the top of the pad and the underside of the bracket thing. Now we can either keep the bracket thing where it is or take it away; it won't affect the spring at all.

And if we measure the height of the top of the pad above the bottom of the spring, we'll find that it is exactly what it would be if we'd simply applied that exact force to the spring in its free, uncompressed state, and the green bracket thing had never even existed.

It's perhaps not entirely intuitive how a transverse leaf spring, subject as it is to the shackle angularity shenanigans I demonstrated before, is exactly the same, but if you look really hard you'll see that the principle is precisely analogous. In those exercises we saw how the force on the spring depends on the angle on the shackles – but at any given static load is quite independent of the length of the shackles. We need to preload a transverse leaf spring because the shackles are only 1½" long, and their swing radius does not reach to the free eye-to-eye length of the spring. But now suppose that the shackles were three feet long. Now their swing radius would easily reach the free eye-to-eye length of the spring. And if we were to load the spring so that it is shaped exactly as before, with the shackles at exactly the same angle as before, we'll find that it is carrying exactly the same load as before, without it ever having been preloaded.

Preload is purely a practical thing; it's all about the practicalities of installation. It doesn't affect the dynamics of harmonic elasticity at all.

Here's perhaps a better analogy, sticking with the eating of meals. It's like buying undersize clothes in anticipation of losing weight. You have to squeeze yourself into them, but once you've lost the weight the clothes aren't undersize. But the clothes' degree of undersize doesn't cause you to lose weight, or even affect your weight at all, except in a highly questionable psychological way. That psychological way might be enough to render this analogy fraught with danger of misunderstanding, though.

 

It's really very very simple.
If you break down each step of it - it's couldn't be simpler.
Those steps together create an completed example.

The thing is the each step needs to begin and end at a precise location.
When all the precise locations and perfect steps are all added together its perfect in result.

When you look at the perfect result of all the perfect steps and think you can go in alter anyone of those perfect locations and still have a perfect result well you can't and it will show deficits in its ability to do its job. You can not change 1 thing and make it work, but you may be able to change 2 things and get ti to work correctly. For example you can not decrease the perch distance in an aftermarket axle and use a stock spring but you could change the that perch distance and the length of the spring- 2 things. Or you can not reduce the arch height to achieve ride height but you can reduce the arch and shorten the spring. Not 1 but 2 changes and precise changes.

The correct position for the spring to begin working perfectly is one of being stretched eye to eye. The way you stretch or increase the EYE to EYE distance is to flatten the arch.

Drum brake springs are stretched, if they are not stretched enough at installation they will not work correctly. Easy to understand right? It wants to pull back. Stretching a coil spring is easy to understand, but stretching a leaf spring seems to be hard to grasp for some reason. It's empirically evident by the many photos of bad examples here.