Frame / Springs: Do leaf springs twist when a solid axle car hits a bump?

[garyash]

I was working on the chassis of the Indy car replica project this week, got to staring at the solid front and rear axles, the leaf springs front and rear, and the shackles. It occurred to me that when one wheel hits a bump, the wheel moves up and that side of the axle is raised. In order for this to happen, something has to twist - or have a lot of free play. My spring eyes have solid bronze bushings, so there is no give there. The shackle faces are butted tight against the spring eyes, so there shouldn't be much free play there and we don't want the bolt to be pivoting in the shackle bar. Even with rubber-lined or polyurethane bushings, there can't be much give. Is it that the leaf springs have to twist in order to allow vertical wheel motion at only one end of the axle? There can't be "slop" built into the axle/spring/chassis system or cars would wander all over the road. The chassis isn't supposed to flex, either, though it does and that's why we get fatigue cracks after a lot of years. If a wheel moved up 3", the axle tilt would be about 3 degrees. The same thing happens when the body leans as the car turns.

I've been driving for almost 55 years and never thought about this. What am I missing? Go ahead, 'splain it to me!

[jclary]

Well Gary, now that GO PRO and other "mountable" cameras are so readily available, I nominate you to provide the evidence.

[Alan]

Something has to give and it will be the weakest link.

[PackardV8]

The historic racing axiom - "Any suspension can be made to work, as long as you don't let it."

Yes, Hotchkiss parallel leaf springs and the bushings in which they are mounted do twist when one side goes up and the other doesn't.

No, if the spring and/or bushings are sufficiently stiff, they will resist twisting and a bump and rise on one side can lift both wheels. In a corner loading, if there's no give, the tires take all the side load and slide much sooner.

If the frame and springs are sufficiently stiff, the right front tire hitting a bump would lift the right rear tire. (Think of a hollow core door lying flat. Pick up one corner; stiff, no give, so three corners come off the ground.)

Maybe, think of an anti-roll bar. The stiffer the bar, the less the chassis will roll in side load, but the more the outer tire is loaded. Put a stiff rear bar and oversteer happens. A really stiff front bar and understeer increases. Balance the two, with enough tire width and the car can corner flatter and faster.

Also, some Hotchkiss leaf spring cars use a Panhard rod to resist side loads. Some use both a Panhard rod and an anti-roll bar so the springs and bushings don't have to control all the forces.

Those of us who've tried to race a Sunbeam Tiger know the tradeoffs. Your suspension was designed as a race-only application for very smooth tracks. On a bumpy track, with a stiff suspension, the rear is constantly being lifted as one wheel and the other goes over the bumps. Under power, traction is being lost/gained and oversteer a constant possibility.

jack vines

[garyash]

Jack is well onto it and covers a lot of the explanation. However, let's consider the slow speed case where we can actually see one wheel lift and the axle tilt - or even at high speed with the 1950 Studebaker Desert Explorer cars and trucks (see https://www.youtube.com/watch?v=tXyfv7yzCw4). A wheel on the rear axle of the car goes up, the axle tilts. How can the spring shackles and front eyebolt allow this?

Here's a table top experiment that I did. Cut a strip of paper about an inch wide from the long side of a piece of printer paper to make an 11"x1" spring leaf. Hold it by the ends and let the middle sag down a bit like a car's leaf spring at rest, then try to force one long side down the way a spring would twist if the wheel on the opposite side went up or down. You may need a helper to provide a third hand. The piece of paper wants to form itself into a cone. As it does so, the two short ends are no longer parallel. I don't think the eye bolts and shackle are supposed to let the ends of the spring do this, but the spring leaf would develop wrinkles if the short ends have to remain parallel. Something has to give in the process of wheel motion. Maybe John Clary has the right idea: I need a GoPro camera to take pictures of this while I jack up my M5 truck on one corner. The Indy car doesn't have enough weight on the chassis just yet.

Aarrghh! My brain hurts!

[jclary]

...Aarrghh! My brain hurts!

Maybe so, but, ain't this fun! Really, not too long ago, even if you were thinking about this...who, and how many folks could you have shared it with? Now, with the internet, and this forum, we can "instantly" ponder these things. And...not just among us car nuts, but anyone who happens to stumble across this posting, world wide, and thanks to language translation software, across the linguistic spectrum.

I'm now seventy years old, have logged many miles atop "leaf springs," and until you posed this question, have never given it a wisp of a thought. In the vastness of the universe, the answer is probably of no real importance or consequence...but now, I'll probably never hit a pothole, or speed bump, without thinking about it. GOOD GRIEF!

[PlainBrownR2]

Maybe this will help.....

Your average leaf spring, well more specifically, the ends of the leaf spring, are constructed in the same way that a door hinge is constructed. If you look at the ends of a leaf spring, they have those eyelets or eyes. The leaf spring shackle bushings, are inserted into those eyelets. There is an outer metal tube, which the leaf spring rides on, and in the center of the shackle bushing, there is a metal tube, which the bolt would pass through. The bolt attaches the leaf spring, shackle bushing, and the shackle on the frame. Now the leaf springs themselves have a natural form or arc that they remain at when at rest. It keeps the axle and the tire planted on the ground, and supports the axle and the front or the rear of the car, wherever the leaf spring is mounted at. The axle is also mounted at a spot on the arc that will permit the leaf spring to travel up and down, but reduce side to side, or front to back motion.

Now, when the axle hits a bump, pothole, or that raccoon you just ran over, the tire and the axle lift up. The spring has a little give in it, so that it can absorb the energy, but because of the design of the spring, it lets it travel only so far. The action of the leafs in the spring behave a bit like when you bend a phone book. It will let you bend it, but the laminations of the pages keep the phone book from folding completely over, and return the phone book to its natural state once you stop bending it. Now what happens at the eyelets? Well, because of the up and down motion, the leaf spring pivots on the outer metal portion of the bushing like a door hinge, and due to the stiffness of the spring, lets it return to its natural arc position.

The spring also has additional help from multiple leafs, as well as U-bolts, spring clamps around the leafs, things to help keep its arc.

Now to add further confusion to leaf spring design. If you wanna see some leaf springs twisting, throw in acceleration or deceleration. The rear axle wants to naturally twist up or down, which torques and distorts the leaf spring and tries to wrap the leaf spring around the axle, where the rear axle is mounted at, until the action stops, and then the leaf spring returns to its natural state. That's where the beauty of traction bars comes from, to stop that motion ...................

The long and the short of it, the leafs springs shouldn't twist, but due to the design, they do flex, and the spring eyes pivot at the ends.

[52 Ragtop]

IF you are going fast enough. you will NOT be thinking of any of that! LOL

Jim

[PlainBrownR2]

IF you are going fast enough. you will NOT be thinking of any of that! LOL

Jim

It's just like flying an airplane. You're not thinking about any of the mechanics and how it's all working together at that point, as you're attention is on keeping it in the air. I.E., whatever it's doing, it needs to keep doing it. All is working well, that is until whatever part quits doing what it should be doing, then it's troubleshooting time. As they say, out of sight, out of mind!

All of this is also why I stopped driving my '64 Commander after 12 years of daily service. The rearmost passenger shackle bushing in the frame is only being held in place by the outer frame rail, because the inner frame rail is all but gone around the mounting point. The suspension system back there is compromise, because it's rusted through. I'm not welding that back up, as I could see that as a dangerous failure point in the future. Nothing worse than tooling in the left lane on the freeway at 70 mph, and suddenly being thrown sideways, or endangering a passerby in the next lane, because the rear leaf spring is now not part of the frame anymore!

[jnormanh]

because of the up and down motion, the leaf spring pivots on the outer metal portion of the bushing like a door hinge

I don't think that is what happens. If the spring did actually move in relation to the outer metal shell, then I wouldn't have found so many rusted in so badly that they had to be cut out. The inner metal sleeve is clamped in position by the bolt through the spring hanger, and the outer is a tight push fit in the spring eye. And after a few years they are rusted in tight, because they do not move.

What gives is the rubber between the two sleeves.

The reason people replace them is because the rubber eventually wears out, tears or shreds. I've never heard of anyone replacing them because the spring eye or outer sleeve was worn out. Have you?

However in Gary's case, I believe he has solid bronze spring eye bushings, so the spring eye moves on the bushing, or the bushing moves on the eye bolt. That's 1930s race technology, not applicable to passenger cars.

[junior]

I guess all things being equal, lets say you have a solid axle, twin parallel mono elliptical springs and solid bushings and one wheel hits a bump...I don`t see any alternative other than the wheel that hits the bump for it`s spring to twist to a degree, and the spring on the opposite end of the axle to bend to a lesser degree assuming the axle acts as a rigid beam and there is a solid junction where the axle attaches to the springs...what other choice do the springs have? Just supposing, but never really ever given it much thought before. Some interesting info in this link...

http://www.suspensionspecialists.com/tech0004.html#7

cheers, junior.

[doug]

Thanks Junior, lots of information to ponder when messing with springs.

[Hallabutt]

I guess that you did not use the Fafner spring shackle bearings as were used on the stock President and the Pierce Arrow. Did the Indy cars use them?

[garyash]

Junior's link has good info on springs. As jnormanh says, perhaps some of the twist is taken up in the rubber "tube" that was used in many of the later Studebaker cars and trucks. However, even today, many large trucks operate with bronze bushings. The early 1930s cars and before used bronze bushings, and as Hallabut mentions, even the Fafnir ball bearing shackles. While Studebaker claims to have tested them thoroughly, they weren't used for many years. Rex Miltenberger says that they rusted up when not used. More importantly, they failed because of fretting wear. This was because the balls only rolled back and forth a short distance, mostly staying in the same spot, and just touching at one point on each ball. Balls and the pins they rested on don't like that kind of motion. Rex says he has replaced the balls with roller bearings and re-machined pins successfully, for as much as any 1930s Studebaker gets driven. See the attached PDF of the Fafnir patent for details.

The race cars didn't use the Fafnir bearings. The rear shackles are one piece, were castings or forgings. An overhung bolt ran through them. The front shackles used forged links. Both fronts and rears had bronze bushings and through bolts, drilled down the center and crosswise, and equipped with grease fittings. Today, the off-road guys also use greaseable bolts with urethane bushings so that the plastic doesn't squeak. Urethane bushings feel hard when you hold them, but under pressure and with confined space, they will deform and flow like hydraulic fluid, then return to the original shape. I was able to buy bronze bushings with spiral-cut grease grooves for the spring eyes. I just need to have some grease holes drilled in some aircraft style 3/4" AN bolts. These have long unthreaded shanks of exact diameter and are available in length increments of 1/16".
Fafnir patent US1798864A.pdf