Leaf spring design

[klahanie]

I’m looking for comments on rear leaf spring design as applied to an on/off road camper.

Consider the Ford F350 srw pick up (PU) versus the srw cab and chasiss (C&C). The cab and forward is common on both trucks but from cab back there are some differences. Same axle and wheels, different frame, suspension, fuel tanks. Let’s look at the rear springs (see table below). The PU has fewer and longer leafs with a softer, more progressive rate. For 2008 MY Ford increased the overall spring length 8” and reports superior ride, better control, less axle wrap etc. Of note the new 2009 F150 spring length is also increased. The C&C: more leaves, stiffer rates, much like larger, higher GVWR trucks.

Naturally, the PU has a better ride empty, as they often are. But many are worked hard on a wide variety of roads surfaces. I’m thinking this design allows more articulation and perhaps better control on pot holed, washboard resource roads driven at "speed".

Now take the same haul road but with a 3000 load. Is the C&C spring design stronger? How about short pitches on rough trails. Stiff springs too restricting?

Assuming the camper in permanently mounted what would you recommend?




2008,9 Ford F350 SRW Rear Spring Specifications
………….…………......……………….Pick up………….Cab and Chasiss

Combined rating @ ground (lbs)……7,000…………………..7,280
# Leaves main and aux…….....…………6…… …………………10
OAL (inches)………………..........………66.3…………………..55.6
Width (inches)…………….......…………….3……………………….3
Deflection rate lbs per in/spring
.Stage1/stage2/stage3…….………349/654/1174………613/1342/na
Rating ea @ pad lbs/spring………….3133……………………..3271
Axle rating @ ground (lbs)….……….7,280……………………7,280


Thanks,

[ntsqd]

For such a simple looking thing, leaf springs are complicated beasts. There are two factors involved in ride quality of leaf springs. Internal friction and spring rate. The latter is obvious and known, but the former wasn't fully appreciated by me until a spring swap and the subsequent mods required by that swap.

Your assumption of greater compliance holds true in my case. I used GM 63" long rear 3/4 ton pick-up springs under the rear of my '84 Toyota Xtra Cab. (Second best mod that I've made to that truck.) I used the three longest leaves in each 5 leaf spring pack. It now articulates far beyond what I'll ever need it to do, but the ride quality has vastly improved - which was my motivation for the swap.

There are thin spacers about the size of the contact area of the spring perch between each of the leaves in these springs, leaving air gaps between the leaves except for where the tip sliders contact the leaf above. I've had to bump the damping valve stack in the rear Bilsteins up one step as a result of this subtle change from traditional leaf springs.
As best as I can tell this design of spring is referred to as a "parabolic" on some forums. The number one complaint that I've read of parabolics is a lack of control. Well, the damper design was counting on a certain amount of spring internal friction to help damp the motion and the parabolic's removed a significant portion of of this without a parallel increase in damping rate. Of course there would be a reduction in control. I do not recall reading of a parabolic install coupled with a re-valving of the dampers.

A common mod seems to be to reduce the load carrying capacity of the leaves to achieve unladen ride quality and add air springs to compensate for the wide range in rear axle weight. The adjustability is appealing, but unless you're willing to swap out dampers or valve stacks every time the camper was added or removed you probably be stuck with the high spring rate - high rear axle weight damping. Not to mention adding another system and it's failure modes. Damping mixed spring types is also a trick, but I doubt that many would go to that length in tuning the rear suspension.

My approach would be to build a custom spring pack, either from the existing or from a vendor. Set up the primary spring with a rate that has the unladen ride quality you're looking for. Then set the second set (the overload springs, if you will) such that they are carrying their share of the laden load while offering a couple inches of non-contact travel when unloaded. There is no formula for this, I'm afraid it's an iterative solve.

[klahanie]

There is no formula for this, I'm afraid it's an iterative solve.That friend, is the story of my life ...


Interesting about leaf pack internal friction and it's affect on dampening.

That your vehicle's articulation and ride quality improved with the installation of longer (and modified) springs is in keeping with the refinment made by Ford. My thinking is the longer PU spring have these inherent advantages over the shorter thicker pack used on the C&C. Which is best for carrying load on rough roads is an unkown to me. I'll talk to a spring shop.

I appreciate your comments.

[charlieaarons]

With respect to non-tapered leaf springs:
The spring rate (stiffness) is proportional to the 1st power of the width, the 2nd power of the thickness and the inverse 3rd power of the length. This applies to individual leaves.
So when comparing one spring pack to another you can use these relationships to estimate thickness.
Tapered leaves are a very complicated calculation, by comparison.

Charlie