Pickup truck or any leaf spring rear suspension

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PackardV8
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Pickup truck or any leaf spring rear suspension

Post by PackardV8 » Fri Jan 10, 2014 1:06 pm

Doing a custom 1955 Studebaker pickup.

As with most older trucks, the rear leaf springs are a tall, narrow stack of a dozen leaves suspended beneath the rear axle tube and the spring hangers below the frame rails.

Most current trucks use three wide leaves above the rear axle tube, with the front mount and rear shackle higher on the outside of the frame.

1. What does the wider spring stance and higher mount do to the roll center?

2. Apart from the esthetics of getting the spring pack up out of the line of sight, what handling changes are to be expected? It widens the spring mounting points, thus reducing the leverage of the body on the spring, so does it reduce roll?

3. We're considering just removing all but the top three stock leaves and using air bags if the truck ever needed more load carrying. It's mostly a street rod now, but being able to fine tune the ride and handling with air pressure is an attractive idea. Any experiences here?

FWIW, many older trucks use two rear shocks mounted toward the center of the axle at a 45-degree angle forward. They don't do much for axle control. Current light truck design is to place the shocks vertically as far outboard as possible. (1969 Saab technology strikes again!) On the initial build, I kept the two heavy duty OEM shocks and added two more of the same outboard and vertical. Despite being told it wouldn't work, would be too stiff, this was a huge improvement and felt like a different truck.

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Re: Pickup truck or any leaf spring rear suspension

Post by Brian P » Fri Jan 10, 2014 5:19 pm

Let's discuss each effect separately.

First, narrow spring stance on the frame versus wide stance (relative to the track width).

It helps to analyze the extremes. Case 1, all springing dead-center in the vehicle; case 2, the springs for each side centered on each wheel. Obviously both are impossible in reality but let's just analyze the physics.

In case 1, when you hit a bump with the left wheel or the right wheel, the spring gets a half-size hit. In case 2, the left and right springing is essentially independent. So if the springs are close together, a one-wheel bump affects both springs, and if the springs are set wider apart, it's closer to the left and right sides acting separately.

But ... case 1, the springing contributes no roll stiffness; case 2, it contributes quite a bit. So setting the springs further apart contributes more roll stiffness for a given spring rate.

Next, spring attachment height versus roll center height. The roll center height is at the height of the leaf springs. So, setting the springs higher raises the roll center. It's not raising the roll stiffness but it's reducing the moment-arm that creates the roll moment in the first place. Not quite the same effect but "less body roll for a given spring stiffness" would still be an accurate description.

Combine both of these effects together and the net result is that the later model design will have greater roll stiffness for a given spring rate, or equivalently, the same roll stiffness with lighter spring rates. In other words, better ride, and better handling qualities with regards to body roll.

Other effects ... You want leaf springs to bend up-and-down. You don't want them to bend side-to-side (sideways axle deflection). Wider leaf springs give more lateral stiffness. So they'll locate the axle side-to-side better.

Leaf springs don't deflect straight up and down - they swing in something of an arc. Putting the springs further apart on the axle (new truck) minimizes the potential "axle steering" effects. Putting them closer together (old truck) magnifies them.

You'll probably find that the older truck has an extremely high spring rate (and rides very rough). "You can make any terrible suspension design work if you don't let it move ..."

I suspect that if you remove a bunch of the leaves with the older suspension design, the handling will get a bit floppy, air bags or not (the air bags don't contribute any axle-location duties). Does that truck use Hotchkiss suspension (like most newer designs - the leaf springs are also responsible for taking up axle torque reaction and it has an open drive shaft with U-joints at each end) or does it use a "torque arm" so that the leaf springs don't absorb torque reactions?

At a certain point, if you are going to go air bags, it might be worth going all the way and just locate the axle with a linkage (many possible choices) and use an antiroll bar for roll stiffness (air bags have terrible roll stiffness on their own) and forget about the leaf springs completely.

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Re: Pickup truck or any leaf spring rear suspension

Post by PackardV8 » Fri Jan 10, 2014 9:13 pm

Thanks for the very clear explanation. This pretty much confirms my conjecture.

The Studebaker is 4000#, so it's closer to a current small truck, such as a Dodge Dakota. I'm going to the U-Pik yard to measure spring lengths and hope to find a close match in a 3-leaf package.

Moving the springs to the top of the axle will require new spring locating pads on the top. What pinion angle would you recommend for a 122" wheelbase with a two-piece driveline?
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Re: Pickup truck or any leaf spring rear suspension

Post by Brian P » Fri Jan 10, 2014 10:42 pm

Oh boy, that's a tough question.

A two-piece drive shaft with 3 universal joints introduces a lot of potential variables. Are any of them CV joints or double-Cardan joints?

For a normal drive shaft in which conventional (non-constant-velocity) joints are used at each end, the input shaft has to be parallel to the output shaft for the nonuniform rotations to cancel out (and the joints have to be phased correctly).

I tend to suspect that your vehicle has no CV or double-Cardan joints but the center hanger bearing is positioned so that the front U-joint is nearly straight (thus doesn't contribute much to nonuniform rotation) which means you can treat the rear two joints as a normal drive shaft (output shaft parallel to input shaft). So, whatever the down-angle of your crankshaft/transmission is should match the pinion angle ... at nominal ride height and with whatever axle wrap corresponds to a normal forward driving torque! I have no idea of knowing how much that axle wrap is, but having the pinion a degree or two "flatter" than the down-angle of the crankshaft/transmission/front piece of the drive shaft at nominal ride height likely won't be too far wrong.

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Re: Pickup truck or any leaf spring rear suspension

Post by Krooser » Fri Apr 04, 2014 2:08 am

I'm late to the party but I'd use a Chevy truck arm suspension with coils or airbags….
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Re: Pickup truck or any leaf spring rear suspension

Post by PackardV8 » Fri Apr 04, 2014 10:25 am

I grew up driving a '60 Chevy pickup with the truck arm suspension.

After having used the truck arms with coils for so many years, GM and most other light/medium trucks use Hotchkiss leaf sprung. Then, Dodge has just gone back to coils. As previously mentioned, "Any suspension can be made to work, as long as you don't let it."

Back to the air bag idea, turns out on my Studebaker there's not quite enough room between the outside of the frame and the backing plate for the standard Firestone bags used in most of the kits. I thought I'd measured carefully, but in a worst case, fully loaded, full deflection, rubber meets the moving parts. And I've learned the worst case is always worse than you ever imagined ;>)

FWIW, I used fiberglass leaf springs on the front axle and they work wonderfully well. I'd considered the same for the rear, but without the air bags to handle loads, I'd be stuck with a single rate spring.
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Re: Pickup truck or any leaf spring rear suspension

Post by Speedbump » Fri Apr 04, 2014 11:05 am

Have you thought about the shockwave 7000 series? They're 4" OD which is the same as many coilovers and they will work on a solid axle rear. I had the same issues on my 55 Chevy PU rear. I chickened out and used the coil overs but the Art Morrison folks said I could switch to the 7000's if I ever decided. They are pretty pricey.
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Re: Pickup truck or any leaf spring rear suspension

Post by Brian P » Fri Apr 04, 2014 4:01 pm

The truck-arm suspension is a simple design but in this day and age, the truck arms slice right through what would otherwise be the most collision-protected place to put the fuel tank. The GM trucks that originally had the truck-arm suspension had the gas tank inside the cab, and *that* would not go over too well today!

Also, the length of those truck arms mean it's a heavy design. And a particular problem with GM's original implementation was that the panhard rod was too short. As long as the ride height remained close to design, it doesn't matter much, but deviate, and it will get side-kick and axle steering effects on bumps ...

Pickup trucks kept leaf springs because (1) it's cheap, (2) it's "good enough".

Friend of mine has a 2012 Ram 1500 with the coil spring rear suspension. Given the parts count, this is surely more expensive to build than leaf springs. But the truck rides very well. With the number of people that use those trucks for personal transportation, they are probably better off that way. I know Ram has an option air-assist arrangement so that they can have good ride quality and high load capacity.

If you have a choice in the matter, use a Satchell link arrangement ... an upside down and inverted-front-to-back triangulated 4-link.

http://www.ratrodnation.com/forum/rat-r ... 1/#msg5001

The regular triangulated 4-link design has roll oversteer (bad). The parallel-4-link-with-panhard design either doesn't have anti-squat/anti-lift because the upper and lower links have to be parallel (bad), or it binds because it is an over-constrained design (bad). The 3-link-with-panhard (Mustang S197) is OK if you can get the lower links level, but if they go down towards the back, i.e. chassis-end is higher than axle-end, it will have roll oversteer (bad). The bad thing about Satchell is that the inner ends of the angled lower links have to attach to the chassis in an inconvenient location ... but on a truck frame, you ought to have room to be able to do it.

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Re: Pickup truck or any leaf spring rear suspension

Post by Bubstr » Thu Apr 17, 2014 1:39 pm

PackardV8 wrote:Doing a custom 1955 Studebaker pickup.

As with most older trucks, the rear leaf springs are a tall, narrow stack of a dozen leaves suspended beneath the rear axle tube and the spring hangers below the frame rails.

Most current trucks use three wide leaves above the rear axle tube, with the front mount and rear shackle higher on the outside of the frame.

1. What does the wider spring stance and higher mount do to the roll center?

2. Apart from the esthetics of getting the spring pack up out of the line of sight, what handling changes are to be expected? It widens the spring mounting points, thus reducing the leverage of the body on the spring, so does it reduce roll?

3. We're considering just removing all but the top three stock leaves and using air bags if the truck ever needed more load carrying. It's mostly a street rod now, but being able to fine tune the ride and handling with air pressure is an attractive idea. Any experiences here?

FWIW, many older trucks use two rear shocks mounted toward the center of the axle at a 45-degree angle forward. They don't do much for axle control. Current light truck design is to place the shocks vertically as far outboard as possible. (1969 Saab technology strikes again!) On the initial build, I kept the two heavy duty OEM shocks and added two more of the same outboard and vertical. Despite being told it wouldn't work, would be too stiff, this was a huge improvement and felt like a different truck.

jack vines

Let's boil this down to basics.

First a spring is a spring is a spring. Leaf, coil or torsion bar, they all get their rate the same. Mass and length. A thick narrow spring coild end up with same rate as wide and thin one. Each diameter or cross sectional area will have a deflection number that is multiplied by the length.

1. The farther away from center of mass or closer to wheel contact patch, will have more real spring rate at the contact patch. It's a leverage thing. Moving the spring outboard will increase roll stiffness and spring rate at the wheel. It will not change the roll center, unless you change ride height. This answers question 2 also.

3. Less spring rate at a outboard location would work. Unless this custom truck is going to be a car hauler or toter home. I'd be comfortable with that. More leafs is insurance for leaf breakage.

OEM shocks are valved to potentual weight and shock placement. Your original shocks, may have been a little under valved for your purpose. Shocks have come a long way since 1955.

If your thinking on other suspensions don't be overly concerned about rear roll steer. Rear acceleration or deceleration steer is defiantly bad, but roll steer can often be your friend instead of enemy. It goes straight when your on the gas or off and helps you go around the corner by loading the outside front wheel. Roll steer in front is bad, rear is not so bad. Every dirt racer couldn't do with out it.

One last thing. The truck arm suspension is 30s tech. For a truck that sits high, you can get relitivly close to a acceptable Anti squat percentage, but lower that truck or car and you have a bad case of wheel hop. Ask anyone in NASCAR. You can control it with shocks but why build it into your car unless mandated by rules.
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Re: Pickup truck or any leaf spring rear suspension

Post by Krooser » Fri May 02, 2014 10:07 pm

I have used air bags on top of a modified mono-leaf spring deal on my '48 F-1. I used two extra half leafs in front of the axle centerline to help prevent spring wrap under hard acceleration. Rode great.

The truck arm suspension is very popular with the traditional hot rod crowd. Rather than fab my own set-up I bought a kit from CPP and acquired a couple nice used original arms. While truck arms may be 30's tech, leaf springs were used on buckboards in the 19th century...
Last place in the B-main is better than anyplace in the grandstands...

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Re: Pickup truck or any leaf spring rear suspension

Post by lefty o » Fri May 02, 2014 10:52 pm

done right a truck arm suspension will ride pretty good, handle fairly well, and hook hard on the dragstrip. those that get wheel hop from them need to do some tuning. i'd either go with a truck arm, or a full 4 link and coil overs. removing existing leaves from your spring packs just undermines what is already an underengineered set up for anything other than load hauling at low speeds.

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Re: Pickup truck or any leaf spring rear suspension

Post by numboltz » Sun Mar 01, 2015 12:07 am

...More leafs is insurance for leaf breakage.

Also, when the industry moved from lots of 1 1/2" leaves to less 2 1/2" leaves the broken-
spring rate went way down. Ford uses 3" on some 1/2 ton 4x4s to further reduce the
cracking through the center bolt hole. IMO, a good reason to update the springs. For
a given spring rate, the industry went to fewer, wider and longer leaves to decrease
internal friction and breakage and increase flexibility [as in ride.]

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Re: Pickup truck or any leaf spring rear suspension

Post by quickd100 » Sun Mar 01, 2015 11:26 am

I drag raced the truck in my signature for years on the original factory leafs. I clamped the front segments, removed the clamps on the rear segments, made up a pinion snubber. Worked great on track and on the street, hooked like no ones business and would launch lifting the front wheels 6-8" off the ground. The motor at the time was a cheap 440 that made about 400hp at the wheels. After upgrading to a 528 and then later a 605 Hemi the leafs decided to try and kill me every pass down the track. I finally tore the leafs out and fabbed up a triangulated 4-link, best chassis change I made. Much nicer ride on the street too.Dave
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Re: Pickup truck or any leaf spring rear suspension

Post by pdq67 » Sat Mar 14, 2015 9:56 pm

Jack,

You mentioned f/g leafs..

Are the Chevy Astro Van f/g rear leafs usable?

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Re: Pickup truck or any leaf spring rear suspension

Post by PackardV8 » Sun Mar 15, 2015 8:24 am

pdq67 wrote:Jack, You mentioned f/g leafs..Are the Chevy Astro Van f/g rear leafs usable?pdq67
Depends on who you ask. I'm running shortened 4WD Astro Van springs on the front of my Studebaker truck and so far, so good.
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