Control Arm reinforcements - opinions

[L92 OLDS]

I've been running Nitto drag radials at the track and approaching sub 13 second quarter mile times with the Olds. Future plans will likely put the car in the 11's so I'm a little worried about the rear control arm mounts. I've done some reading but I'm still a little confused as to whether the uppers, lowers or both control arm mounting locations are weak on a G body.

Has anyone on this forum damaged their chassis from launching too hard? If so, was it the lowers or uppers? I'm considering the Hotchkis or UMI reinforcement bars but don't understand the physics and how these design's address the issue. I would rather not add the weight if welding in gussets would be enough. Here's a link to the UMI bars.....Opinions anyone? Make me a believer.

http://umiperformance.com/catalog/index ... cts_id=350

 

[Fox80]

I have those on my car, and I will say they fit nice and will probably help with the flex and twist these frames suffer from but IMO they will not stop you from ripping off the mounts on the lower arms. That can happen easily in these cars. You need to at least gusset the lower mount to the frame and the upper outer plate portion of the upper arm mount. A bar bent for driveshaft clearance across the lowers is best, this is how we would do Pro Stock dirt cars with this frame the lower mount would not last a week ion these frames without serious reinforcement. These are some pics to give you some ideas

 

[L92 OLDS]

I have those on my car, and I will say they fit nice and will probably help with the flex and twist these frames suffer from but IMO they will not stop you from ripping off the mounts on the lower arms. That can happen easily in these cars. You need to at least gusset the lower mount to the frame and the upper outer plate portion of the upper arm mount. A bar bent for driveshaft clearance across the lowers is best, this is how we would do Pro Stock dirt cars with this frame the lower mount would not last a week ion these frames without serious reinforcement. These are some pics to give you some ideas

Thanks for the info. Do you know where I can buy the pre-fabbed brackets to weld onto the lower mounts?

 

[81msw79]

http://jtracecraft.com/G-body_180100.html

LINK FOR BRACES.

 

[Fox80]

<SPAN class=skimlinks-unlinked>http://jtracecraft.com/G-body_180100.html</SPAN>

LINK FOR BRACES.

That is it, sorry I thought I posted that as well, I always like the unwelded kits as the fit to the frame when they weld it into an assembley is not the best, mock them up on the car tack the pieces together then fully weld it and then weld it to the frame

 

[L92 OLDS]

Thanks guys....I appreciate it

 

[Clone TIE Pilot]

Reinforcing the frames and bodies on these cars is always a good idea. GM deleted alot of structural bracing in the 80s G bodies to save costs. Don't listien to the people who claim you don't need bracing, quite a few on the net who are like that.

 

[online170]

The UMI and Hotchkis brace is a beefier version of the old factory braces that used to be on the A-body chassis. They looked very similar to the rad support braces in our G Bodies.

From a physics point of view, the bolt holding each control arm goes through the frame and holds the control arm. The Threaded end of the bolt is not connected to anything so the bolt is essentially a cantilever with a very weak sheet metal brace (the control arm mount). When the control arm moves around so does the bolt. The brace adds a rigid member to the cantilever end of both bolts, and make the bolt a bit more rigid. So when you launch, the mounting points of the control arm are more likely to stay in place rather than flex. The upper control arms are mounted slightly better, but ultimately that cross member is just a piece of sheet metal. From the looks of it, I think the brace MAINLY helps the lower control arm, and not so much the upper.

In terms of weight, the benefits of the rigidity far outweigh the detriment of the insignificant weight gain. You could also make it yourself out of crushed tubing if you were so inclined.


As the others have said, anything you can do to make the chassis more rigid is beneficial, especially with a sub 1.7 second 60ft time that you will definitely get when running in the 11's.


Just to give you an idea. My previous car was a 1971 Buick Skylark with the Buick 350. All big block buicks got this chassis brace from the factory. But my 350 car produced enough torque (with a bit of carb and timing adjustment) to flex the frame. My car sat lower on one side than the other after i was done tuning and thrashing it a few times. It ran 15's in the 1/4, but it snapped your neck on launch from torque. This is likely to kind of damage you will see . The lower control arm mount is definitely the weaker of the two. But the upper mount is also a weak link.



Hope that helps. (Attached is a picture of the A-body Factory Version just to give you an idea).

 

[Fox80]

^^^^^^^ Very well put, and I agree the lower mount is what the real flaw in these frames. As you launch the rotational force of the rear end wants to drive the lower arm forward in the car and it will move easily,that is why the box kit you can buy focuses on giving the mount support on the front to help with this. I still believe the mount I showed that connects each side together and something the joins the upper shock mount area together is nearly as important to keep everything square

 

[lilbowtie]

You got a lot of good advice - I've been in the 10's w/ stock suspension w/boxed arms but all you need to see is one incident to know you are vulnerable. Any bracing you do to those mounts is a plus - there is a lot of on line parts to buy and if you are a good fabricator you know what to do.