Rear control arm material ?

[30psi 4agte]

what material should i use for the rear control arms in my rear end ?

Steel i hear you all say.....

but what about alloy ??

Im thinking im gonna use alloy - machine grade (dont know exact spec) for my watts link bars and im considering using the same for the control arms.
The bar is solid and 25mm thick.
i know steel is almost always used but does anyone see a reason why i cant run alloy ?

Im thinking the solid alloy bar would be as strong as the hollow steel bar and it is slightly lighter.

[Ben Wilson]

Are you going for rose joints?

If so, something like this or this depending on size would make life a lot easier..

[big_zop]

You should be able to work out which one would be stronger based upon the base material properties and engineering equations (of which i cant remember and my books are at my parents place - just trying to look up).

While im not familiar with your suspension setup you can make the general assumptions that the longitudinal arms will be in bending during acceleration and braking (in the vertical plane - should be aligned with mounts) and during cornering (in the horizontal plane - not aligned with mounts so more critical). If you have access to suspension analysis software, then that would be your best way to analyse the problem but most suspension (theory) books wouldnt cover a beam axle in depth.

How these forces are distributed between the links and how they all interact is way out of my league but i do know that the Orion Falcon front suspension has suspension arms made out of aluminium alloy (not sure what type) that are at least a 40mm square cross section, but it is supporting a 1600kg (minimum) car. And that our Formula SAE vehicle used hollow chrome-moly which is no more than 15mm in dia (not sure on wall thickness).

Bending Info
Tensile Strength including common values of max stress.

Going onto rose joints, they are shunned upon (in racing) when placed in bending but you could just go overkill on size but i dont know how this will go for engineering. Also, the engineering may also ask for this kind of analysis when it isnt a factory part to ensure that it is up to the job. And remember that you will either need to machine it or weld adaptors to it to take the rod ends so if the rod is up to the task then this would more than likely be the point of failure.

I know most of this is overkill but there is so much that can go wrong with suspension its best to do it right once.

[oldeskewltoy]

Hmmmmmmmm the track rod(panhard rod) can EASILY be aluminum... Battle Version and a few other makers make the Panhard from Aluminum.

Trailing arms..... That may be a different story.... EVERY company I know making trailing arms make them from steel( Cusco, Battle Version, TPS, Megan Racing) I'd say you can likely use aluminum.... but get ACCURATE measurements(down to a 1/10th of a mm) to make sure they don't stretch

Btw... all my info is based on the AE86 chassis/parts available

[big_zop]

Oh and probalby a very important aspect is to determine what kind of safety factor you are going to run. The more accurate your measurements - the closer you can take it to 1 - although i probably wouldnt run anything less than 3.

Appropriate factors of safety are based on several considerations. Prime considerations are the accuracy of load, strength, and wear estimates, the consequences of failure, and the cost of overengineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury or death usually can use a safety factor of four or higher (often ten). Non-critical components generally have a design factor of two.

A design factor of 1.0 implies that the design meets but does not exceed the design requirements. A high design factor implies "overengineering" which results in excessive weight and/or cost.

Factor of Safety

[30psi 4agte]

just to clarify abit..... I will be using rose joints on both ends of the rods (they are a 1/2 inch joint with 5/8 thread)

As for engineering .......thats no prob. The car is far from legal anyhow

So all up it will be the stretching / bending factor of the alloy i will need to consider?

surely a solid alloy bar will be less likely to stretch than hollow steel....... Ill try to find out more about the material i have.

BEN: that link is pretty much what im after.... the only difference is mine are solid and i dont use LH thread joints only RH---- which is no biggie to me.

I can make the arms pretty easy and already have the material just need to know if they suit my application or not.

If they are making steering arms out of the stuff they must be pretty damn strong.

[oldcorollas]

perhaps the weak point will be the threads in the alloy, that the rose joints go into?
long term damage to the threads after being mashed by teh awesome powah of the 4AGTE?

the bottom bars are in compression and top bars in tension (under accel)

compression is only a prob when the bar bends, (binding ends etc?) and then trying to transmit large forces thru an elastically bent bar...

i vote threads as being the problem...

[30psi 4agte]

i thought the threads to be the major prob..... but a mate of mine thinks there is no prob at all....

He likened it to a head..... say a exhaust stud .you can do the stud up that tight that it will break the stud before it pulls the thread. He said the same applies for the bar.

[IN 05 NT]

also remember you are running a FULL SPOOL diff,

thats going to put a shit load of forces on the threads, aswell as the bushes and rest you already know, ( i think that alone will fuckem pretty quickly!)

i think there made from steel for a reason,

linden will have all the answers tho, he does all of this stuff.

[oldcorollas]

that is true. but it depends what your alloy is and how the threads are cut also.

it is more like screwing a bolt into an alloy head... do it enough times and the thread will strip out.. whereas a stud will be happier.

i dunno the exact bumbers to use, but..
take torque of engine,
multiply by first gear ratio,
multiply by diff ratio,
divide by the distance between axle centreline,
divide by two (two bars)
and that is the force in newtons the threads have to survive at... not just continuous or once, but cyclic... ie fatigue.

as a totally random guess.. 350Nm * 3 * 4 / 0.1 / 2 = 21000N = 2100kg.. per arm.
or.. 2.1 tonnes per thread.....

i suppose you could gte some of the rod, thread it.. put the rod end in.. and then load it to failure on a tensile tester (or, a hydraulic press which has a pressure/force reading)

edit: usually made from steel cos it is cheaper, and because the threads are stronger. and hollow steel tube will likely be lighter than solid alloy bar..

[takai]

The problem in the end is as OC hit on, its a cyclic fatigue, which aluminum alloys are much more susceptible to than steel.

I would trust it in limited circumstances. Usually if i could monitor/pull down after each meeting/usage. In a similar way i would for CF suspension componenents.

[The Real Roadrunner]

my vote is on hollow moly tube, light stiff and strong as fook. tube+jambnuts+tig+3/4 of a day=big smiles and a rolling car.

cheers
linden

[merc-blue]

Lindens right,
alloy alu is plainly not the right material, i cant think of any specific alloy that would suit.
You can the ChrMo with some serious thin wall thicknesses.
We have bananaed ChrMo ones on just axel tramp.

Do it properly the first time

[Rodger]

My vote is also hollow chrome molly.

A solid bar off equivalent OD and material is not as strong in any direction as a tube. The young and young at heart engineers on this forum should know that from basic principals.

No solid alloy bar that is lighter or even the same weight as the equivalent steel hollow component will be as strong.

Or as an alternative as I am trialling, cut off the OEM bush ends from the OEM steel bars, weld in 5/8 nuts and use 5/8 rose joints.

Regards

Rodger

[oldcorollas]

My vote is also hollow chrome molly.
A solid bar off equivalent OD and material is not as strong in any direction as a tube. The young and young at heart engineers on this forum should know that from basic principals.

thats only for similar weight tho...

a solid 1" bar will be stronger in compression, bending, tension and torsion than a 1" tube with 1mm wall thickness..

a solid bar with same weight as the 1" hollow tube will be worse in bending and torsion.. but same in compression and tension...

thickwalled moly = bulletproof?