Frame / Springs: Springs and shackles: how do they really work?

[garyash]

I've been working away on the final details of the spring mounts and shackles for my Indy car replica and I started focusing on how much weight/force was transferred from the springs to the frame at various points. So, the front leaf spring eye has a bolt through it that secures it to the front end of the frame. The rear eye of the leaf spring is bolted through the lower end of the shackle. The upper end of the shackle is bolted through the frame so that the shackle can pivot freely. But, I'm thinking, if the shackle can pivot, it can't transfer any upward force to the frame, so all the weight and bump forces must be transmitted through the front spring eye.

I had always made the simple assumption that the half the weight of the car was supported through the front eye and half through the rear eye, but that doesn't seem to be the case. As the car hits a bump, the spring flattens, so the shackle must pull on the bolt through the frame, but it still isn't supporting any weight. This doesn't make sense to me. What am I missing?

[gordr]

Imagine, if you will, a leaf spring, bolted at the front, and with no rear shackle. What's going to happen when you put weight on it? The entire spring, and the axle bolted to it, is going to swing in a big arc on the front bolt, right? Of course the rear shackle bears weight. Draw a diagram, and put some vectors on it.

[Spinneyhill]

Engineering mechanics. The rear shackle arrangement is to allow for "lengthening" as the spring flattens. If the axle is midway between the two chassis mounts, they will take half of the load each. You cannot put more of the load on the front mount unless the axle is mounted closer than mid way OR the shackle seizes up and you apply rotating force to it OR you apply the brakes going backwards. When you rotate the axle (e.g. apply the brakes going forward) the axle mount applies torsion to the spring at the mount. The spring will change shape - ultimately to a lazy S - and the front is pulled down while the rear is pushed up. You also push the spring backwards and apply a significant horizontal force to the mounts.

If my crummy explanation is a bit opaque I can draw a wee diagram that might help.

[Jerry Forrester]

I aint no engineer, just a poor ole dumb southern country boy, born and raised and educated in the government schools of Aladambama. Common sence tells me that if the axle is in the middle of the spring, there's going to be equal weight on the frame at the front and back of the spring. If this is not the case, then why do shackle pins wear at a rate similar, if not more, than the anchor pins? Shackle pins are usually smaller than anchor pins 'cause there's two of 'em and they may wear even more than the anchor pin (when proper maintenance is not maintained) because there's more turning involved at that joint.

[leyrret]

Not an engineer either but here's my take on it. Seems weight distribution of the car would probably have more effect than anything. If you mount a spring on axle, center say a 15 foot plate between the two eyes. Load on each eye would vary as to where you placed the weight. Dead center it should be even(probably some negligible variation with change in spring length). Movement of the weight in either direction from center would basis the weight in that direction. A car heavier in front should have far more weight on front pin than rear. The actual difference would vary with anything that changes position of horizontal chassis center line, front to rear, such as changes in springs , sway bar, shocks etc. as well as axle position on spring.

[jclary]

Well...you have the fancy drawings...you have examples of how it's been done (rather successfully) for decades.

This is an interesting topic, but also a distraction.

Some things are very tempting to "over-think."

Let's not get bogged down in minutia

I have been following your progress with this build with great anticipation for quite some time now. We're not getting any younger. Just build the thing. I wanna see it while I still have enough lucid mentality to appreciate it.

By the way...enjoyed meeting you and looking through the notebook you had at York.

[avantibngrant]

Free body diagram

It is a free body diagram of the axle and spring as an assembly alone you need and assume the spring is acting as a rigid body at a given point witht he static load of the car on it. Sum of the forces in y =0 and sum of forces in x = 0 as well as the moments about any given point = 0 or else the the axle is moving or rotaing about the frame which with the static load of the car it is not. For a simple example shown with the axle at the center of the spring, the vertical force at the rear is the force in the shackle x cosine of the angle and since the axle is in the middle, it is = 1/2 the weight on the axle and equal to the vertical force on the front of the spring. similarily, the horizontal force must equal 0 so force in the shackle x sine theta must equal the horizontal force at the front spring mount. If the axle is toward the fron or rear of the spring, it will ratio up the forces front or rear to reflect the unequal moment arms as the sum of the moments about the axle = 0.
Dynamically it can be analysed similarily by adding the accelerating force on the axle (F=Ma) and treating the spring as rigid for an instant with the resulting geometry at that instant.

Make sense?

Neil

[Warren Webb]

My thought on this is the shackle allows the main leaf to compress to its length otherwise it would bind & cause undue stress on the main leaf resulting in premature fracture. This stress would also put additional stress on the mounting points & the "weakest link in the chain of events would fail.

[wittsend]

This is more of a characteristic of function than weight distribution, but if the rear portion of the spring didn't articulate through the shackle then you would just have a fixed arc. It would pretty much remain rigid until it yielded and collapsed. Not much of a spring if used that way. Maybe that is what "avantibngrant" was trying to say in post #7??? When you start getting into math theory I get lost.

[avantibngrant]

"I had always made the simple assumption that the half the weight of the car was supported through the front eye and half through the rear eye" That is correct essentially as long as the axle is in the middle of the spring. The force in the shackle will be higher because of its angle, but it is the force on the frame you are concerned about I believe. The force the frame will see in the vertical direction will be the same on each side (1/2 the axle force) and the horizontal forces in the frame will be equal and opposite directions front and back. If the shacke ends up straight up and down, there will be no horizontal force on the frame at each end.
I hope that make it clearer what I was saying earlier.

Neil

[garyash]

Of course I'm overthinking this! It's just that I got to the point of construction of having to cut some holes in the side of the chassis and weld in the tube supports for the front spring shackle and mount the plates for the rear hangers. I only want to do this one time, and I want to be sure everything is in the right place and strong enough. I always use the rule: "If in doubt, add more steel!" At about $1/lb, it's cheap insurance.

As it turns out, the center bolts through the springs are typically offset about 1/2 inch from the center, making the front end of the leaf springs about 1" shorter than the rear section. In the case of the 1929 President front axle and 1928 Commander rear axle I'm using, the head of the center bolt locates the fore-aft position of the axles, as well as pinning side-side. There are pockets for the heads in the axles. I'm not exactly sure why the center bolts are slightly offset, but I think it has to do with damping the peak of the natural frequency of vibration of the springs to avoid wheel hop, etc. In the Studebaker Indy cars, the front springs are incredibly stiff at about 480 lb/in while the rears are slightly softer at 240 lb/in. The stiff fronts control body roll in the absence of sway bars. Of the projected 2600 lbs final weight (car, driver, passenger, 1/2 tank of gas), nearly 800 lbs of unsprung weight is attributable to the solid axles plus tires and wheels. I assumed half the weight of the springs is also unsprung weight.

I'm trying to move as fast as I can to get the car to the "roller" stage. Wire wheels are due in a couple of weeks, hubs are in process of being machined, springs are here. I had a set-back when I realized the chassis rails had been made wrong, so I had to cut the rails at the rear kick-up, lift the back section almost 7" at the rear shackle mount, and weld it all back together. There is a beefy fishplate on the back side. That's why I had to make the "fancy drawings". Here are some photos of the springs and cutting/welding the frame rails. The body will eventually hide the fishplates.

[PackardV8]

What's between the frame rails at the rear shackle mounting points?

Reason I ask is I've always been fascinated by a leaf spring trick used by Max Balchowsky of "Old Yeller" fame. Instead of shackle bolts locating the Studebaker rear axle, he used a solid shaft extending across the frame and through both spring shackles. He claimed it functioned like a traction bar to make both springs react evenly. He also claimed a benefit of allowing the springs to float and be self aligning under torque and side thrust. That's exactly opposite of how a Panhard rod functioned, but one cannot argue with the successes of the various Old Yellers.

jack vines

[garyash]

Jack: I'm planning on using the same basic design as the original Indy cars. The back ends of the frame rails have some very solid bars used to attach the rear shackle. A bolt or threaded rod goes through the attachment, then through the spring eye. The spring eyes have bronze bushings and the bolts and shackle assemblies have grease fittings. [My sketch below doesn't show the grease fitting on the bolt, but there will be one, with a small hole down the center of the bolt and a small crosshole to let grease reach the bushing - the off-road 4x4 guys use these still.] The spring eye and its bushing can rotate on the bolt but the bolt shaft is supposed to remain stationary. The ends of the bolt or threaded rod are cross drilled for safety wires to keep the castle nuts in place without having to torque everything too tight. I'm adding a thin bronze washer to keep the shackle from rubbing on the mount. There is also a solid bar that joins the left and right rail together, but it's fixed and isn't part of the suspension. The engine block gets bolted into the chassis without benefit of rubber mounts, so I think that's supposed to be a major contributor to chassis stiffness.

[Dan Timberlake]

I think a 1/2" or even 1" diameter tube or bar extending from frame rail to frame rail may keep the rails separated to a consstent distance, but can't add much bending or torsional stiffness. Being bolted to the web of a channel or one side of a box rail will just wiggle and flex a section a few inches around the bolt heads, if the rods are even asked to keep the rails in line.

With no disrespect to Max Balchowsky, one or two 3 or 4 foot long 3/8" or 7/16" rods fixed at the ends are a towel rack, not a crossmember or significant torsion bar.

[avantibngrant]

A fascintating project. Good luck with it and I hope my little input was some help.
Will you be in in Dover this summer?

Neil