Any benefit to boxing upper rear control arms? - Chevy Impala SS Forum
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post #1 of 10 (permalink) Old 01-15-2019, 12:02 PM Thread Starter
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Any benefit to boxing upper rear control arms?

Reason I ask is I'll be swapping the axle soon...
This is on a wagon w/ Hotchkiss lowers and sway bar already installed.

Should I or is it better for them to flex some?
Did a search but didn't really find anything about the uppers.
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post #2 of 10 (permalink) Old 01-15-2019, 12:20 PM
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Are you going to autocross the car or otherwise race it where each 10th is going to matter?


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post #3 of 10 (permalink) Old 01-15-2019, 12:41 PM Thread Starter
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Not really but some "mods" do make things better even on street.
I did box stockers on 5.0 stang many years ago and it was night/day.

Was just wondering about my wagon...
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post #4 of 10 (permalink) Old 01-15-2019, 03:59 PM
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There may be some slight advantage,but boxing the lowers makes a huge difference. Uppers are so much shorter,and do less of the work. Many have left them alone...

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post #5 of 10 (permalink) Old 01-15-2019, 05:28 PM
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I remember reading a thread back in the day. Maybe it was Navy Lifer, but I am not 100% sure. But it was stated that it can cause a slight bind in certain hard driving situations if the rear uppers do not have some flex or a roto-joint/Johnny joint or some similar means to allow a bit of movement.

Now I could be wrong or only partially right. But for this reason that I always use uppers with a flexible joint.

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post #6 of 10 (permalink) Old 01-15-2019, 06:00 PM
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Nope, you're pretty much right on top of it there. A good deal on this around the turn of the century when early adopters installed LCA/UCA as sets with those neat urethane bushings to stiffen up things, and the tops were getting chewed up by the connection point. It generated a lot of forum chatter on too stiff of bushings causing torsional binding of suspension from its full travel arc. The fix ends up losing the urethanes and using either just flexible rubber again (but new at least and not all sponged out), or a swivel bearing with a ball (heim joint is an example). Have not searched but johnny jt. may be aimilar.

I've had extended Metcos with urethanes since '99, and yes they're chewed a bit at the brackets. If I raced then they'd get changed to low maint. rubber. Under 'mildly curious' I always wondered that since most all extendeds are adjustable uppers, why not just leave the jamb nut lose and let the axle bracket rotate free on the threaded mount stud. There'd be zero torsional resistance that way. EDIT: I imagine without constant lube the threads would gall until the stud snacpped = not good.

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Last edited by 96 Black; 01-16-2019 at 06:56 AM. Reason: Just thinking
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post #7 of 10 (permalink) Old 01-15-2019, 07:40 PM
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dick miller sells one of the best upper control arm. great for setting pinion angle to get the car to hook better and preload to get the car to launch straight. i wouldnt recommend it for autocross. drag racing or street. first pic is what you dont want vs the others. last picture is dickmillerracing product. study them carefully.
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post #8 of 10 (permalink) Old 01-15-2019, 09:35 PM
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IMHO, since you already have boxed (Hotchkis) lowers and sway bar I would, and did, go with Hotchkis uppers also. Rubber bushings.

I don't autoX in any formal setting but do drive canyons aggressively and also 1/4" mi 100's of times.


Along with just driving I have not noticed any unusual wear on the upper bushings or felt any effects of binding, YMMV


I also have the Dick Miller Triangulation Braces which I feel help keeping the rear end straight

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post #9 of 10 (permalink) Old 01-16-2019, 08:46 AM Thread Starter
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I have an extra set from parts wagon, guess I'll box 'em and see.
Can always just swap them back out...
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post #10 of 10 (permalink) Old 01-16-2019, 11:43 AM
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The UCA's serve a dual purpose, to maintain pinion angle and also to center the rear. Because of this, the bushings are being loaded from both ends in certain conditions and this can lead to bind. Poly bushings don't like being loaded from multiple angles and by running a stiff boxed/tubular arm and poly bushings you will likely run into bind issues.

Also, when extending the control arms to center the rear axle in the wheel well, most brands DON'T offset the clevis which is another issue. As you extend the control arm, you are changing the angle at which the clevis meets the bushing and it will no longer be square unless you offset the clevis. In other words you will already be loading the bushing awkwardly, leading the premature wear and more bind.


Take this with a grain of salt, but what I did was use a Johnny Joint at one end and build my own offset clevis at the other. I then used rubber bushings in the axle housing. The rubber bushing at one end may not be ideal for a race car because of deflection, but I feel that with tubular/boxed upper and lower arms I reduced deflection enough that leaving a LITTLE room for articulation would not be a bad idea to reduce bind and thus improve ride quality and traction on uneven surfaces. You could also run a spherical bushings/heim joints at both ends of the arm and have NO bind but now you are talking about a lot of vibration transfer and also kind of a PITA to be replacing spherical joints in the axle housing itself when they wear.

I would use a Johnny Joint or similar spherical at one end of the lowers and poly at the other to maintain some articulation and reduce bind as opposed to poly at both ends.

if there was one thing I'd have done differently it would be to build the clevis a little more chamfered at the ends to give me more room for a 3" over the axle pipe, wasn't planning on running tailpipes at the time LOL We'll see how that works out when I get to that stage of the build.

ca1 by Kris A, on Flickr
ca2 by Kris A, on Flickr
ca3 by Kris A, on Flickr
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ca5 by Kris A, on Flickr
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