Kubs

I am looking into making my own adjustable upper control arms and was wondering if 6061 T6 Aluminum is strong enough? Im made the cross shaft out of the T6 and then Im going to put heim joints on the end and use adjustable sleeves. Do you guys think T6 is strong enough for this?







Just looking for some other opinions. I did some FEA on it but wasnt sure what load to use for it.

StealthLT4

Brian Cunningham might have some idea. I know he has been making CAD drawings of the entire C4 suspension, maybe he has an idea of the loads exerted on the arm.

korvette4u

when using any aluminium in a stress application as in "A" arms.. the best material is 70 (70,000psi tensil strength) series aluminium.. less prong to stress cracking then 6061 t-1 series.. if in doubt refer to "american engineering & Welding specification" handbook for proper application guides..

another concern would be if the construction doesnt conform to "orginal manufacture equipment" your efforts are in vien... insurance will not cover modified equipment with out GOVERNMENT Quality Control and inspection!! think twice about this!!

Z06X

Well, your comparing a billet piece versus a cast piece. If you create an identical piece, the billet will undoubtedly be stronger. Here, your altering the design entirely, so it's going to be up to you to calculate the loads involved and establish if the piece will work reliably. If you could get some data and then do some software strain analysis that'd be the way. Your placing that piece in shear and dividing it by two arms, so getting some numbers of the shear strength of that material then a guess of the loads involved would help.

That or you can do it the old school way and simply bolt it up and do some trial testing, increasing loads and doing some inspections.

BrianCunningham

I'm thinking T6 would be fine

are those the stock dimensions?

here's my adjustables



compare to the stockers they're a lot smaller, but they're also a lot stiffer.

Mr. Peabody

Brian, did you replace just the uppers or uppers & lowers? Those look pretty nice. Did you do this for autocross purposes?

ch@0s

I love it when you talk dirty

rodj

Will have to crash test your car

Kubs

Yes I did some testing but was unsure as to what loads the tire puts on the a arm under cornering with a r comp tire. I think it will be fine seeing as the lower one takes most of the lateral load.

Brian,

The dimensions are very close to stock. I dont have the money for those SPC ones, but they are the ones I am replicating. I am using heim joints instead of the bushings, but the setup is similar. I got the T6 material from my school and made the cross piece in my CNC class, Tom at Banski hooked me up with some heim joints, and I am making the ball joint plates at work. All in all I think I have less than $100 in for both arms.

I dont have a state inspection that checks the suspension. Only a sniffer test.


Thanks for all the input guys. It sounds like this should be ok from the comments. I agree that something like 7075 would have been better but this was the best aluminum my CNC class professor wanted to buy!

Churchkey

Lateral loading during cornering is normally not the major concern with upper A arms as the A arm is either in compression or tension. IE: at work 90* from the vehicle longitudinal center line.

IMHO the issue of major concern is increased loading during HEAVY BRAKE application on corner entry with the major forces being applied to the suspension components that are positioned at the outside radius of the corner. The forward leg on the upper arm is in compression, the rear leg of the arm is in tension while the spindle acting as a lever (moment arm) pivoting on the lower ball joint is atempting to twist the entire upper arm assembly off the frame.

I have seen the adjustable leg A arms fail on oval track cars.

Good luck with your projects.

Aardwolf

black88z51

7000 series is better then the 6000, it's stiffer and stronger. That's why when mountain bike frame makers started building frames with 7000 series a few years ago it was a step forward in the technology as a more rigid bike frame is better. As far as I know 6000 series is still readily available, and is popular for machining and easy to work with and will bend more before snapping.

Kubs

That is true. I plan to keep an eye on them for stress cracks or any bending while they are on the car. If they do fail it will be on the end where the heim joints sit, and if that happens I plan to drill into the cross shaft and use a bolt on the end instead. I left about 3/4" for threading on the end just in case.

Yeah 7000 series was not in the budget for my school so I went with 6000.

LT1inaMGB

T6 is not an alloy, it is a heat treat condition. The alloy is the 4 digit number before the heat treat like 6061, 7075, 3003 etc. The alloy is what you should be looking for first then the properties that the particular heat treat provides

astock165

7075 is a better choice on paper because it does have higher strength but practically is it more expensive as you've found out and much care must be taken if any anodizing is done to the finished part. If the parts are not anodized correctly it can actually weaken the part.

A general rule of thumb for a starting point in your FEA can be assuming a worst case scenario: that the full cornering load of the car is put on that one component. Yes, this is likely overkill but should also tell you what ballpark you're playing in.

So, what can your pull in a corner? 1 g? 1-1/4 g? < 1 g? Whatever that number is will give you a starting point. I think for most C4s with fuel and driver you're looking @ 3000 to 3500 lb.

Apply that force and run the analysis to see what kind of stesses are present and adjust from there.

Keep in mind though that in a case like this you're likely going to design to more criteria than just strength. Things like ease of assembly, mating up to standard components or hardware (like in your case the rod ends) come into play as well. You might end up finishing your design to these practical considerations and then running the analysis to make sure it will stand up.

FEA is a great tool but like any other it's only as good as the info that is put in. So, be mindful of the constraints used to make sure they're realistic.

There's no better test than real world application. You're in a good position because your car is street driven as well as tracked. Install your stuff and go for a little spin on a not so smooth local road. If it survives that, drive it for a while and see how it stands up. Then take it to the track and be observant the first few sessions out to see how it feels. After the event a tear down and inspection is a great way to get info about what's really going on. "Autopsies" on new designs can be very enlightening.

ghoffman

Well, you must consider many things that nobody has mentioned here. First of all, the only real source for your material properties should be referenced from Mil-Hdbk-5 for metallics. Realistically, the biggest loads that a Corvette control arm will see are the impact loads due to hitting a curb or pot hole. That is pretty hard to characterize exactly, so you are going to have to make some assumptions. I think that having the entire car on one wheel, times 1.5x, is reasonable in my experience (for the wheel loads, not for each individual suspension link). We use that criterion with our Penske shock parts, like our lower "T" bar shock mounts, upper clevis mounts, etc, and they are neither excessively heavy, nor has anyone ever even one yielded them yet.

To limit this discussion to 6061 and 7075, 6061 is the better choice here. Since this is a clean sheet design, you can increase the sections to use it and 6061 does not suffer from stress corrosion cracking to anywhere the extent that 7075 has (depending on the heat treatment). Alloy 7075 can have serious susceptibility to stress corrosion cracking at most common heat treatments that will show up in areas that have built in prestress like the corners of a clevis that does not fit perfectly. In addition, 7075 is considered not weldable even though if you take a TIG to it, it will weld, look good, but the material properties in that area are terrible. Alloy 6061 is cheap, easy to machine, and easy to anodize.

If you want to do an FEA, what are you going to use for you pass-fail criterion? Your output will be in Von Mises stress, which is the tensile equivalent to the octahedral shear stress. That is nice, but are you just going to plug in the FEA the tensile yield value that you look up? If you want to make it last a long time and want to consider fatigue, I think the modified Goodman criterion is a more reasonable value. If you not not familiar with it, it is more appropriate for things that see some positive stress value and cycle +/- about that positive value, like an aircraft spar, or car suspension, that is supporting the entire weight of the aircraft then sees bumps about that positive value. Again, Mil-Hdbk-5 will provide you with the allowables for whatever stress criterion that you deem appropriate. Good luck, but if I may, at TRW and L-M we had zillions of resources, and we still had the attitude that 1 test is worth 100 opinions.

astock165

Thanks for the clarification on the 7075 material properties and the 1.5 g starting point for design.

because we all know what opinions are like? And that everybody has one

Crepitus

The easy way to buy your self some pice of mind, Do some side by side destructive load testing. Your custom fab part next to the OEM. Maby fab some up for the local short oval guys too.

BrianCunningham

just the uppers

the lowers got these, greaseable del-alum bushings

they're available to the tops as well, if you want to keep the stockers

I wanted to added adjustment to autoxing.

Kubs

These are the kind of answers I was hoping to get. Thanks guys! I did not consider any cycle stresses but I figured I would look them over after and during each event.

My idea of testing these was pretty much exactly what you described. I will be taking them off and looking at them after each event to make sure they are still good.

In my FEA I used a load of the corner weight (~950lbs) times the 1.5g's it can pull in a corner (estimate worst case like you said). I dont recall the number off hand but the deflection was low and the stress was below the ultimate yield strength by a safety factor of over 2.