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Yet another Rear control arm design


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Seems like bad form to me to post on a thread 7 pages long questioning the point of the thread, especially when there are other threads about putting 240SX suspension and SLA in these cars already and the question "why don't people focus on the rear suspension as much as the front" has come up probably 3 times in the last couple years.

I asked a question. You gave me an answer. Sorry I peed in your cheerios.

Edited by ModernS30
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  • 2 years later...

I keep thinking that by putting the bolts through the monoball and screwing into the arm on the inner joints, I essentially have the arms mounted in single shear. It's "double single shear" since the arms have a bolt on each end, but I'm now wondering if I should drill the threaded bungs out, cut the heads off of the bolts and slide them in backwards and use nuts on the outer ends.

Finally back at work on the Z, these control arms were off the car while I was measuring for CV fitment, etc. Went to put them back on and decided to fix these. They were loaded radially on the threads, and I just didn't want to take a chance seeing as how I'll have 13" wide rear tires...

 

I'll get better pics after the paint dries and I get them reinstalled, but here is a quick shot. Only took a couple hours to get them fixed up the way I wanted.

 

Word of warning; if you hold onto a big ass corded 1/2" drill one handed and it grabs the threads, it will try to rip your friggin arm off. LOL If you hold it with two hands it might also rip the control arm out of your vice and slam it on the workbench. wink.gif

post-553-036205700 1351709137_thumb.jpg

Edited by JMortensen
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No bar. I'm going to try some insanely stiff springs and a smaller front bar. If I can't get the balance right then I'll mod the rear arms for a bar. I do have a bar and I have nice brackets to install it to the rear uprights, so it won't be as bad as doing it from scratch if I end up needing it.

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Nice to see your Z might be out and about some day soon!! Like how you placed the fuel cell over to help with corner balancing. Remind me what engine you're using as those rear brakes are a whole lotta stopping force. BTW, nice looking frame rails you got there :icon47: Oh, Bob Sharp's crew set the car up with big-big sway bars and soft springs so I'll be interested in how you like the handling. We don't run a rear sway bar in our 240 w/400lb springs.

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5.3L aluminum V8 from a 2006 GM truck. Cam and springs should put it at ~350 whp-ish and 400 ft/lbs. Yes, thank you for the rails John. Working on the front crossmember to get the ps installed now. Really would like to see it run in 2013. Tired of autoxing a basically stock Miata. The Miata handles pretty damn well for what it is, but it is too slow!!!

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  • 10 years later...

I'm getting ready to rebuild my rear control arms. I need shorter. Too much camber for the slicks I'm running, and I have the camber plate tops flipped around to minimize, still not enough.

Question: I have some 1 1/8" x .120" DOM mild tubing. Looking around, most tubing ends are 1" so that works fine. Could also do some 1 1/8 x .058" wall chromoly. I'm not very proficient with my tig welder yet, but that would be another option. When I dyno'd the car they tied it down with the control arms and it didn't want to start spinning the drum, car was bouncing all over. Not sure I'd feel confident with .058 in that scenario.

Slight revisions I'm considering, would appreciate feedback:

1. Using a monoball holder instead of an outer rod end in the rear
image.png.6677d1c5f3543bb201c87cc11d8a8044.png
2. Welding in 5/8" studs into the strut upright rather than using a long bolt

3. spacing the rod ends away from the strut a little bit. Monoball holder does this automatically, if I did a rod end again would probably use a rod end spacer like so:
image.png.5bfabf0597d4936a8a8fb3afbfd4197b.png

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How much are you looking to shorten the control arms by, Jon?  Even though flipping your existing camber plates didn't yield enough camber change, would switching to a different camber plate (and/or camber plate mounting inside the strut tower) be an option?  That might be cheaper/easier than redesigning the control arms?

 

Looking at your pictures from Nov 8, 2012...it doesn't appear that reverting back to OEM LCAs is a reasonable option, is it?

 

As another option for consideration, what about altering the inner mounting points for the LCA?  (i.e. relocating them further inboard?)

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I don't know how much I'll need to shorten the arms by, but I was thinking exactly the same thing this morning while at my kid's dentist appt. My rear struts are nearly topped out. I could make a spacer that bolts to the existing camber plate and lowers the top of the strut to clear the bottom of the plate, then just slide the top of the strut farther than it is now and maybe get the pos camber I need that way. I do have Ground Control plates, so they're not like the shitty ones that have 3/4" of adjustment that you get with BC coilovers or something like that.

I don't have any of the axles that I was having made back in the day, so I'd need to figure out the CVs. If I did something on the top end I wouldn't have to mess with that. Might have to cut back the plate on top that the roll cage is welded to. 

image.png.47c0d3e6a13e900738508424d8c241fb.png

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On 12/28/2022 at 12:09 PM, JMortensen said:

Question: I have some 1 1/8" x .120" DOM mild tubing. Looking around, most tubing ends are 1" so that works fine. Could also do some 1 1/8 x .058" wall chromoly. I'm not very proficient with my tig welder yet, but that would be another option. When I dyno'd the car they tied it down with the control arms and it didn't want to start spinning the drum, car was bouncing all over. Not sure I'd feel confident with .058 in that scenario.

 

The 0.120 DOM is going to be stronger than the the rest of the components you are bolting into.  If you every get a large lateral hit (curb/pothole) what is the fuse that will blow?

 

On 12/28/2022 at 12:09 PM, JMortensen said:

Slight revisions I'm considering, would appreciate feedback:

1. Using a monoball holder instead of an outer rod end in the rear
2. Welding in 5/8" studs into the strut upright rather than using a long bolt

3. spacing the rod ends away from the strut a little bit. Monoball holder does this automatically, if I did a rod end again would probably use a rod end spacer like so:
 

 

I'm not sure how you are welding studs to the strut but I'd personally weld on brackets that allow me to bolt to the strut using a much smaller (shorter and smaller diameter) bolt that is in the same orientation as the strut tube.  These can the connect to a monoball or the rod end as needed.  Another variation is to weld the monoball cup to the strut tube and use a clevis on the control arm to bolt to the strut.

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I thought about welding a clevis to the rear of the upright and then being able to adjust roll center with spacers. I think you had brought that up years ago. I like the "fix it at the top" idea though, I think that's going to be simpler, won't have to mess with the CVs, etc, so that's the plan for now. Just need to disassemble so that I can get the camber plate shape and then work backwards to make an offset spacer. Was thinking of drilling and tapping the spacer so that I could move the camber plate to the stock position and then 1" back, and running studs in the top to bolt to the chassis and bolting the camber plate to the bottom. Might be a little tricky getting the plate to move far enough without the strut top hitting anything, but I suppose I've reinforced enough in there that I can hack some metal away without affecting things too greatly.

It sounds easy from the office chair...

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2 minutes ago, JMortensen said:

I thought about welding a clevis to the rear of the upright and then being able to adjust roll center with spacers. I think you had brought that up years ago. I like the "fix it at the top" idea though, I think that's going to be simpler, won't have to mess with the CVs, etc, so that's the plan for now. Just need to disassemble so that I can get the camber plate shape and then work backwards to make an offset spacer. Was thinking of drilling and tapping the spacer so that I could move the camber plate to the stock position and then 1" back, and running studs in the top to bolt to the chassis and bolting the camber plate to the bottom. Might be a little tricky getting the plate to move far enough without the strut top hitting anything, but I suppose I've reinforced enough in there that I can hack some metal away without affecting things too greatly.

It sounds easy from the office chair...

 

All in on sounds easy from the office chair :-)  I don't think  I'd make a spacer myself but just move the entire plate over and weld up the old holes.  That way you can still check on things easily and, if needed, doe some basic adjusting.  And since you are rebuilding think about making an end stop that would allow you to easily measure or drop spacers for pre-aligned camber.

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The problem is that there is a transition from the strut top to the area between the wall of the cabin. I'd be putting my camber plate right in that spot. I suppose I could cut out the entire strut top, lay a new piece of .120 sheet in there and put a gigantic slot in it and have 5 degrees of movement or something like that, and then just weld it to the plate on top of strut area and the regular strut area. That'd be weird, but I think doable. Might be a lot of cutting on the underside of the plate that is in there now. I seem to remember a little pocket in that spot that I plated over. 

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