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Materials Compared: 4130 Condition N vs 1018 Mild Steel

Equivalent Axial Tension Load in pounds to meet Tensile Strength (i.e. hoop-stress mode breakage.)

This is for 1" Diameter Tubing. Does not take into account yield effects, nor does it take into account SIF as the wall thickness increases.

kafka,

Nice graph. But what's it mean to us? Granted, structural steel courses are a distant memory from 35 years ago.

Wouldn't the worst case in control arm design be the axial compression causing column buckling with a dose of bending moment added in due to the rigid welded joints; or bending alone due to spring loading?

Some Birkins have experienced lower control arm bending due to poorly adjusted coilovers experiencing coil blocking.

Look at Mark R. after-crash photos. The control arms were stronger in axial compression (and tension) than the frame in which several tubes failed in bending. We are better off designing for bending. BTW, traditional race car design preferred the control arms to fail first to save the frame. (That's how I justify my 3/4" dia, 16 ga. ERW control arms on my Locost 7. Going strong after 2 years of abuse.)

4130 has higher Fy than 1018, but how many of us are optimizing the designs sufficiently to realize the benefit vs. cost penalty? I promise I ain't.

JonW, PE in PA, NC, GA, WI

I'll add my 2 cent along with Jon W's

You shouldn't be to concerned about tensile strength of the control arms. What you need to look at is Young's modulus "deflection rate" Basically the same for both types of steel.

Unless you can pick your crashes, High Enough to yield common steel and not enough to effect high strength steel you will not see a difference.

More important is the design, shock attachment, and gusset placement on the control arms.
Add reinforcements & gussets, Dave W

great points all of you. I've been on a numerical analysis kick lately because I had so many questions in my head. You know, the kind that you think about while sleeping, and wake up feeling not well rested :)

This is maybe just me, but I am not so concerned about what happens to my control arms in a crash. I do know what I do not want them to do and that is penetrate my body, anything else is just money so to speak. If I hit a barrier or a car hits me, whether they bend or break is secondary to whether or not I live ;)

But the big question in my head is always will this break while I am driving. It is the worst thing that I can think of - let's say I am driving along on a highway and hit a pothole or a big bump. Or maybe I am pulling a very strong turning maneuver that puts load on the arms. That's when I ask my question - how strong will I need my control arm to be?

Well, I know how much load my spherical joints are rated for (let's say 11ksi, chromoly) so at least I must make the arms that strong. So it is for that reason I present this data.

I think the graph is useful as a way of looking at the data from a perspective of relative estimation. For example, let's say I have a choice of buying some tube for your control arm. But should I buy 0.64" wall thickness tubing or .125" wall thickness tubing? Is it twice as strong? Almost - about 190%. But let's say I am a very paranoid fellow. I want to use 0.25" wall tubing instead of 0.125" Is it still twice as strong? Almost, maybe about 180% stronger. OK, but let's say that I am designing a car and I run an insurance company. I want to use solid bar instead of 0.25" wall thickness tubing. Will it be twice as strong still? No, actually it will only increase in strength by 20%.

Let me put up another graph, it is very interesting - it is how strong the tube is as the diameter increases. Holy cow! You know that tubing is stiffer when the diameter is increased? Sure, but did you know that the equivalent tensile load it can support increases also with diameter? It's pretty sick! A 1.25" tube with the same wall thickness of a 1.0" tube can really absorb a load. A small increase in diameter, but it may make a difference if you need a pushrod tube. I hope you catch my drift.

kafka,

Your analysis and thought process is commendable. I hope you don't take this the wrong way. My job and years of experience force me to look for the weakness in a design and I am just trying to point you in the right direction; to consider the next question in your quest for knowledge. My experience is not in structural analysis, but I have some training and work with some excellent structural engineers. Some knowledge rubs off.

Look more closely at the spherical joints specs. The radial strength is probably 11,000 pounds. The axial strength will be 15% to 10% of that value depending if they a 2 piece or 3 or 4 piece construction. Also, don't forget the cantilever strength of the threaded shaft extending from your tube. Also, check the shear strength of the threaded shaft. Your joints may be the weak link. Check out kinetic vehicles control arm/joint testing.

This is not basic structural design; it's complex stuff. That's why I don't try a complete analysis. I take great pride in being able to look at existing designs and stealing/replicating the best ideas in all of them. I go to hill climbs and only look at the cars in the pits and take lots of photos. Watching them go up the hill is less satisfying than learning something new about their suspensions and other construction.

Your first drive will be a bit scary. But mild steel will bend and change the suspension geometry noticably prior to complete fracture/separation. Get in the habit of doing a pre-flight walk-around inspection during your early drives. Do some "stress testing" in an open parking lot until you gain confidence in the car and its components.

Just remember that these control arms are attached to a flimsy frame.

here is the data regarding diameter and wall thickness

I'll add my $.02 of experience to the mix as well. We used 1020 throughout the frame and suspension of all of our FSAE cars at school. While I was there, we had reduced our lower a-arms down to 0.625" dia 0.049" wall (or was it 0.035"?) with no failures. Mind you, it was only a 469lb car, but it was down to the Young's modulus, sufficient strength and less hassle with post-weld heat treatment.

_________________
Concordia University FSAE - Chassis
00 Impreza OBS - RIP
81 Rabbit 1.6D
95 525i

_________________
Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress

The video is consistent with what everybody is saying. The ERW/DOM was bent further than the 4130.
Everything fails when bent far enough.

Design to minimize potential bending. Try to load tubes in tension/compression with sufficient wall and tube diameter to prevent collapse/buckling from overload.

Use a material that bends rather than breaks or cracks from higher carbon content.
Use a material that doesn't require heat treatment or have special welding requirements.
ERM/DOM checks all those blocks plus it is much less expensive and easier to source in a variety of dimensions than 4130.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

The first challenge is to determine the required strength including safety factors. This is a very difficult task, if at all possible. Then and only then, can you optimize materials and sizes. Or you can just use existing builds as a reference. MV8's response says a lot. The "book" design falls short, especially when moving the spring/shock away from the upright. This is a known design weakness. Lower control arms in a book chassis take most of the load. The upper control arms, much less.

The video clearly overloaded the ERM (what grade?) beyond its rated capability. I think that if the loads applied were below the yield strength for the ERM, both materials would have reacted exactly the same. i.e. the same arc, droop, and the resultant permanent deformation. I am no expert on materials though.

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

The big takeaway for me in the video is that nothing failed catastrophically. As the materials guys say, steel good plasticity.

Do you remember the Car & Driver tests of the 3 Locosts? On one, the control arms bent under braking. They bent, but didn't fail. If that control arm miscalculation had been made with some aluminums or titanium - it would have been "sudden failure" time.

Cheers,

_________________
Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886

I'm at the stage to learn/build my front control arms, and I would like your expertise, on which material should I use. I won't use what the book suggests, 3/4"-16 gauge tube for sure.
1" x1/8" wall thickness, ERW 1018/A513 should be fine?
Onlinemetals sells 12" DOM 1"x1/8" for $35(!!).

_________________
Tibor


'20 Alfa Romeo Stelvio daily
Locost/442E in progress

1x1/8 may be ok depending on what you do with it. Control arm designs vary a great deal.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

A lot of problems with the book design and various versions thereof arise from the location of the coilovers being installed too far away from the upright. This forces an immense amount of bending force mid beam, the weakest place to be. I used 1" .083 wall and did not have an issue. Of course, I was careful as to where the coilover was placed. YMMV of course.

_________________
Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766