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Upper Control Arm Fail

PostPosted: Mon Sep 14, 2020 10:36 pm
by n8thenetninja
Quick info about the car for context:
It has a 6.0l LS with a Texas Speed MS3 cam (238/242deg .600"/.600", 113LSA), and PRC CNC ported LS2 heads.
TH400 transmission, 9" Ford rearend, and 235 drag radials. At high altitude, it usually runs high 11's or low 12's, depending on the air. No nitrous, blowers, or anything crazy.

Early on in the build process, I ditched the stock rear control arms because I ruined them while trying to install new bushings.

As far as I know, UMI is the only company that makes off the shelf rear control arms for H-Bodies, so I got all 4 adjustable.

In the third round of a bonus race before the final points race at Bandimere, I heard a bang at launch and got out of it. This is what I found when I got back to the pits:

That's the drivers side upper control arm. The passenger side has a crack in it also that looks like it was about to go:

Sure glad it happened at the starting line and not the finish line. Probably would have put me in the wall at 100+mph.

There's a few things I find wrong with these UMI parts:
1. They're made of mild steel, not chromoly
2. The bushing pocket metal is very thin
3. The bent design makes it flex in such a way that it constantly stresses one side of the weld going to the bushing pocket on acceleration, and bends it back the other way when braking.

If you have these on a moderately fast drag car, I think it's only a matter of time before they break like mine did.

Re: Upper Control Arm Fail

PostPosted: Mon Sep 14, 2020 10:58 pm
by n8thenetninja
After doing some research, I found some posts here about the "no hop bars" people used to use in the 80's, that move the upper control arm mount on the rearend about 2" up. This has two advantages: you can run a straight control arm, and it eliminates wheel hop and body squat.

The day I broke the car, I signed up to go to the division 5 finals in Topeka, so I needed to get the thing fixed in 2 weeks if I was going to make that race.

Since the parts people used in the 80's are long out of production, I called around the local fab shops and described what I was trying to do. Got turned away at a couple places due to the time constraint, but then I called Overkill Racecars:

This is a place that went under for a while, and has since made a comeback on Facebook. He makes a bunch of Vega parts including a rack and pinon kit that I've been drooling over. He said he was familiar with the concept and had been meaning to draw up a prototype for a while, but had been working on front suspension parts so much he hadn't gotten to it yet. He stayed in his shop late at night prototyping the brackets for me, and welding up new chromoly control arms.

The bracket bolted into the existing bracket on the rearend, and then I welded them in for extra insurance. He also stepped it up to 1/2" bolts, so I drilled the original holes to accept the larger bolt. This is the finished product:

He got the parts to me in time for me to install everything and get to a race to test it all out before Topeka. It worked beautifully. The car plants the tires and hooks. No body squat, and no sign of stress on the parts.

He plans to make this a production part in his catalog, so if you need something like this, definitely hit him up. He also told me that since this is a prototype, he used a plasma cutter. An off the shelf version will be laser cut and have a nicer finish.
He charged me the same price for thick, tig welded chromoly, adjustable control arms as the UMI equivalent is listed for.

IMHO, if you need tubular H-Body parts, ditch UMI, and support this local business. He does excellent work.

Re: Upper Control Arm Fail

PostPosted: Tue Sep 15, 2020 8:13 am
by cosvega76
I made my own years ago, out of some square tubing. Look in my Garage for photos.


Re: Upper Control Arm Fail

PostPosted: Tue Sep 15, 2020 7:02 pm
by Smiley
Spohn and PMT also make Vega upper control arms.

Impractical Customs on facebook makes the no-hops.

For those welding brackets to a new housing
Monzamouse makes new Vega brackets
with extended upper brackets that function as no-hops.

Re: Upper Control Arm Fail

PostPosted: Tue Sep 15, 2020 8:45 pm
by cjbiagi
I have been saying this for a long time, but when you have more solid bushings like poly or Heim joints you end up transferring more load into the surrounding metal. This can be either the arms themselves or the sheetmetal chassis. Instead of having the rubber "give" and absorb some movement and flex it is now transferred somewhere else. Eventually this can lead to metal fatigue. The amount of movement in the metal may be minuscule but over time fatigue and failure can occur. If you have a racecar that undergoes frequent scrutiny maybe you are fine, but on street cars that just get driven this failure can be catastrophic. Heim joints may be better because they can pivot and allow some movement, but frequent checking of components would still be advised.

Re: Upper Control Arm Fail

PostPosted: Tue Sep 15, 2020 11:33 pm
by n8thenetninja
Yeah, the fabricator offered to put Heim joints on both sides of the control arm for more adjustability and rigidity, but I was concerned about the upper mount on the car side. I figured urethane bushing ought to give it a little shock absorption without moving too much, so I went with that. Heim joints on one side help with bind, so that's a benefit in addition to being adjustable. I just don't think they're necessary on both sides for my application.

Of course, if you have worn out rubber bushings, everything will flop around and shock the mounts that way too. There's a middle ground you want. lol

I've welded the seams on the upper and lower mounts, and welded big washers on the sides of the lower mounting points to give it more thickness at the bolt. I'm not sure what else I can do to strengthen the stock mounts up. There isn't really room to do the washer weld thing on the upper mount. That's probably the weakest link in the system right now. I do check things frequently, and in fact, I thought the upper link looked odd the last time I looked up there (about a week before it broke), but it hadn't pulled through the powder coating, so I thought it was undercut on the weld. Turns out it was a stress fracture.

If anyone has any tips to strengthen the mounts on the car more than I already have, I'd love to hear it. I'd be open to welding in a stronger bracket if anyone makes something like that, or has good drawings.

Re: Upper Control Arm Fail

PostPosted: Wed Sep 16, 2020 10:50 pm
by chevyart
look at the project journals of roccos vega. welder from pa(robert). built a beautiful vega and show very nice closeups and pics of the pieces he welded in those areas to strengthen the body pan brackets. i also believe Megavega(don) showed some of his methods to beef these areas up. im no welder so i am going to try and make up brackets for the uppers that go around the factory mounts and get make sandwich between the body pan and upper steel plate under the rear seat( i call it a fitch plate). I have a complete floor pan cut out of vega to build race car and it will make it very easy for me to use this piece to make up my brackets. art

Re: Upper Control Arm Fail

PostPosted: Sun Sep 20, 2020 3:51 pm
by Smiley
Are you running a roll bar/cage ?

Re: Upper Control Arm Fail

PostPosted: Mon Sep 21, 2020 6:03 pm
by n8thenetninja
Well, I made it to and from Topeka for the ET Finals! Didn't make it past the first round, but got four passes, nothing broke, and nothing looks stressed. Best pass was a 10.90 at 123mph. :shock: Amazing what a little air density will do...

I will probably still try and beef up the upper mounts this winter. Doesn't hurt to have a little extra insurance.

Smiley, yes, I'm running a mild steel 8-point roll bar.