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Chromoloy Adjustable Rear Control Arms


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This first set requires that the bolt going through the strut housing be removed to adjust the camber and toe. The next set will be on the car adjustable. The first set was for me, and I wanted to keep it as simple and light as possible. I don't make frequent adjustments to the rear, and I don't expect that many people do once they find a setting they are happy with. As it is, I can adjust the rear fairly easily and quickly.

 

These control arms also don't have provisions for the rear sway bar; I don't use one. If and when I make these available to others, I will make sway bar provisions optional.

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I did a little testing of the control arms yesterday. I drove the car ~ 75 miles to the course, and ran an autocross practice day yesterday (got about 30 runs). I am happy to say that the conrol arms are still intact.

 

I did pretty well against most of the competition. The only cars that ran the course quicker than I were some AP ZO6 corvettes that were brought in on trailers and driven by national level drivers, and a factory Five cobra. I feel pretty good about this considering that I was running on 2.5 year old street driven Kumho V700 tires, and the vettes were on new Hoosiers. I need new tires. :D

 

Only time will tell if there is any fatigue issues. Until then, I guess I'll just drive it like I stole it until the wheels fall off. :wink:

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Just wondering why it seems to be the done thing to have suspension parts made from chromoly? The little bit of checking I've done on chromolly indicates that the welds require post treatment and that chromoly tends to break under impact rather than bend.

 

Anyway, I made my adjustables out of structural steel that does not require any special welding, etc. Will have to do some pics of them sometime for my gallery :)

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260DET I completely agree with you, and don't think chromemoly is a good material for control arms UNLESS it is for an all out race application, where strength to weight ratio's are key. (4130 excels here) "Chromemoly" has become a buzz word in the car industry of late, much like "Titanium", "Aircraft Grade", "Billit", etc...

 

Not to knock 74_5.0L-Z's work at all! I am just generalizing about public perception having little touch with reality sometimes. ;)

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Guys,

 

I am a mechanical engineer and also did a little homework before designing and building the control arms. AISI 4130 steel is known for its weldability and toughness when welded properly. Weldability of steel is dependant on several variables, but the most important that I've been able to determine is carbon content. Low carbon steels have good weldability. 4130 is a low carbon steel (0.30%).

 

From steel textbooks, and welding-advisers. com:

 

"The concept of Carbon Equivalent was developed in an effort to reduce to a single number the influence of the contribution of the various alloying elements on the difficulties encountered in Welding-alloy-steel, therefore making the problem more tractable.

 

One of the accepted empiric formulas equates the carbon equivalent to the sum of the percentage of each element divided by a certain factor as follows: Carbon Equivalent

CE = %C + %Mn/6 + %Ni/15 + %Cr/5 + %Mo/4 + %V/5.

 

The usage of this formula is intended to provide a rule of thumb for deciding if and what special provisions should be implemented for Welding-alloy-steel: for CE equal to or less than 0.40, no provisions are required. For CE more than 0.40 but less than 0.60 some preheating should be provided before welding. For CE more than 0.60 both preheating and postheating should be applied. "

 

The chemical compositions for some common alloy steels are given below:

 

 

C Mn Ni Cr Mo

Min. Max. Min. Max. Min. Max. Min. Max. Min. Max.

4130 0.28 0.33 0.40 0.60 - - 0.80 1.10 0.15 0.25

4140 0.38 0.43 0.75 1.00 - 0.18 0.80 1.10 0.15 0.25

5046 0.43 0.48 0.75 1.00 - - 0.20 0.35 - -

 

Therefore, the carbon equivalent of 4130 is given by:

 

CE = 0.30 + 0.50/6 + 0/15 + 0.90/5 + 0.20/4 + 0/5 =0.66

 

Based on this, I anticipated a need to preheat and post treat the welds.

 

 

So I researched preheat and postheat requirements and found that preheat and post-heat requirements are dependent upon material thickness and the type of filler rod used. For 4130 material less than 16mm thick the metal should be warmer than 50 F before welding, and for thicknesses greater than 40mm the preheat temperature should be 300 F. The preheating is performed to slow the rate at which the weldment cools after welding is complete (this applies to large weldments that may not be thoroughly heated during the welding process). Cool sections of the weldment act as a heat sink that causes rapid cooling of the HAZ (heat affected zone).

 

The following article summarized much of the information on welding technique and rod selection that I applied towards the construction of my control arms:

 

http://www.project-ch701.net/ch701_misc/FAQ4130N.pdf

 

 

The tubing was cleaned, meticulously fitted, and welded using ER70S-2 filler rod in still air. The ambient temperature was ~85 F(the tubing was hotter than 100F as it sat in the sun prior to welding), and the whole piece was brought up to temperature and allowed to air cool. It also didn't hurt that the man doing the welding is an aircraft certified welder with 30 years experience including welding on the space shuttle and space station components.

 

I didn't want to get on a soapbox. I was only offering these control arms as possibility, and expected a bit of a warmer response.

 

By the way, don't try to weld tool steel. It has too high a carbon content and get very brittle :wink:

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Guest JAMIE T

74_5.0_Z, Welcome to the world of trying to please the general public. It has been my experiance that you will not impress most engineers with any with anything you make yourself. They will pick it apart. And like I've stated in other posts of similar matters, I pose the question to the naysayers... "What are YOU doing to further our hobby?" Without folks like us(I will include Mike Kelly, Myself, you, Ross, Mike/SCCA, and others in this), the classic Zcar would just die as a 30 year old rusty hulk.

 

Congradulations on building yourself a great pair of control arms. Unless you have ALOT of spare time, limit how many you will sell. Ask me how I know. When I was doing this sort of work on the side, I maybe got about 4-5 hours sleep per night.

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Guest JAMIE T

When I was building adjustable arms(steel, on car adjustable) with Mike Kelly, they weighed 7 lb. each. Stock is 10 and this guys are 6.

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First of all the stock control arms are not 10lbs. I had been told this over and over on the internet until I actually weighed one. A bare, 240z fully stock control arm with no bushings weighs exactly 8 pounds. Maybe the weight I have been told are with bushings? (either that or my scale is way out)

 

[soapbox]

 

As far as 'pleasing the general public' goes, we've been over this to death here. 74_5.0L_Z just posted some super information, and I'm happy to see that. It's funny, but you'll notice guys like Ross never get questioned about this stuff... There is a reason. He has a reputation as an engineer for producing a quality part, and for serious R&D work. People that don't demonstrate their qualifications at all often get scrutinized by the general public... I think it was one of the jon's that said a while ago that he wouldn't trust a janitor to perform brain surgery on him. Well duh, that will kill you. So would you trust someone less than qualified to design key suspension components for your car? Perhaps death is less likely than with our lost janitor in scrubs, but you get my point. (hey, maybe he stayed in a holiday inn express last night?) :P

 

For me, once you get into suspension and brakes a large amount of responsibility is on your shoulders to produce a quality & safe part. Not to say those that aren't properly qualified can't make these parts successfully, (hell, look at Mikelly, he made control arms successfully and he's only a secret agent!) ;) but they do have to demonstrate that they've provided some sort of consideration towards testing, strength analysis, SOMETHING that validates their design in it's entirety.

 

Jamie, engineers are designed to be critical of everything. It is our nature for better or for worse. An engineer can make anything sound like a bad idea. :P Considering this in the grand scheme of things, I think it is probably for the best too. False optimism isn't a trait I'd feel comfortable with in an engineer. ;) "Oh yeah, looks good enough... right?" I applaud you for your efforts with Mike to do what you did, I have some notion of the testing you guys did in order to validate your design. I think the mistake that most people make here is to announce a new product they are making without also announcing the pains they are going through in order to test their products safety prior to selling it.

 

[/soapbox]

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What I'm going to do some time is post pics of my non-engineer designed structural steel adjustable rear control arms. :)

 

Give the real engineers a chance to pick them to bits, get some of their own back. :P

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I've had a set of the AZC adjustable sets in my hands, and they are very nicely made indeed. Nice attention to detail, good fit and finish... My only complaint is they came with a couple quality aurora road ends for non-critical parts (sway bar links) and cheap tawian rod ends for the critical parts. (inboard pivot, TC-rod pivot) A nice package none the less... though obviously a part that would be for race only applications.

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