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Discussion Starter · #1 · (Edited)
Been sorting out some handling problems, and looking with a magnifying glass at the rear suspension. Front is good, I just went through that. Everything is visibly good, corner-balanced, nothing binding, spherical rod ends, all is OK. Shocks set even, limit straps loose, tires Ok, I have looked at everything. Running an Intermediate spring package, 500 front, 350 rear. Due to back issues, I cannot go firmer. On the track, I do set the shocks quite firm, lower level settings on the street for a comfortable ride.

I will be adding more aero over the winter to help with high speed stability on bigger race tracks, but that has no affect on bumps.

Two symptoms:
1 - Very unstable at really high speeds (120+)
2 - Odd circular gyration when rebounding off a bump

I have found that the front alignment was badly toed out, so a quick plate alignment brought it back to toe in, and mostly neutralized the instability, but it needs to be checked again to make final adjustments.

I will put the car up on stands, disconnect the shocks, and check the rear for free movement. Maybe that will show something?

I run the QA-1 DA shocks. Do I still need the limit straps? Could I have a bad shock? They do respond to adjustments in Compression and rebound.

Anything else I should look at or consider?

Many thanks for your ideas and experiences.

Dan
 

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Discussion Starter · #2 ·
Did some quick research, and saw a few noting the 3-Link bracket could be cracked/broken? Mine is a MKII, and may have an older, weaker design? I have tried rocking the car sideways, no noise or big lateral movements.

What part of the 3-link bracket fails?

Also seeing some notes about the Panhard bar adjustment. Mine is level at ride height, but will re-check
 

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Discussion Starter · #3 ·
Another thought, recounting a great conversation with Gordon:

Could I have the shocks way to soft, so there is enough rebound to swing the panhard bar through it's arc, causing the "wag"?
 

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I don't 'think' the panhard moves enough in it's arc to matter. Here is a pic of Wade's banana
Gas Auto part Bicycle part Metal Nickel

bracket. I 'think' he had partially welded it to the diff casting. I didn't feel comfortable welding there so
this is how I reinforced mine. The red arrows show that there is also a square tube (hard to see) running from the left side of the banana to the web on the diff where the OE 4 link bar was mounted.
Automotive tire Motor vehicle Bumper Bicycle part Rim

You may want to check that the rear axle is square to the frame. I used a laser to do that or you can use a
straight piece of wood trim. Hold it against the outside of the rear tire 1/3 to 1/2 tire diameter off the floor and set the other end on the floor near the front tire. Then measure in to the 4 inch frame tube just behind the front tire. It would also be good to figure a way to measure rear toe. I have heard of axle housings getting tweeked so worth checking. I made up this after I installed the irs. Since there was a brake line running down the outer side of the left tube, I lay this down, push it far enough that it is past the tube, rotate it to vertical and pull out until it contacts the tube. You don't need the scale of course. just another 90 and short piece of pvc to give you something to measure to that is easier to access.
Tire Wheel Automotive tire Tread Wood

What bushings are in your LCAs? If they are the plastic ones FFR used to supply, I'd take them all apart to check. There were 3 pieces of poly where the outers are relatively soft and the center is quite hard. FFR now has a spherical kit to replace them and they also have the real nice aluminum arms.
 

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Here is a link to an older thread on the other forum. std 4 link vs opt 3 link rear suspension.
Post 34 may be of interest.
Softer springs in the rear will result in the outside rear suspension compressing more when loading up in a corner. If your lower control arms are not initially setup to be parallel with the ground at normal ride height then, there will be a change in wheel base (compared side to side) when one side is compressed more than the other. Using the lower traction bracket control arm mounting holes causes the control arms to have a significant downward angle to the rear. This will result in the rear of the car steering towards the outside of the corner under suspension compression and back when the suspension unloads. A fast, hard left turn will result in the rear steering to the right as the suspension loads and then back to the left as it unloads. Would be really noticeable when driving aggressively and the feeling is amplified because we sit very close to the rear wheels.

HTH
Norm
 

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Discussion Starter · #6 ·
Norm,

Interesting, I had not heard about the lower arms. Mine is a very late MKII, and I do not recall seeing extra holes for the lower arms on the brackets. I will check.

I also already have spherical rod ended Lower arms. I had their alignment checked. When I installed them, I set up a jig, so the eyelets were identical to the FFR tubular Rear arms.
 

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Discussion Starter · #7 ·
Brought this over, as it is definitely great info. Thank you for showing it to me.


On level ground, my LCA are at 5deg. That 3-link support in the photo is level.

[IMG alt="Click image for larger version.
Name: LCA-angle.jpg Views: 70 Size: 124.4 KB ID: 161936"]https://thefactoryfiveforum.com/attachment.php?attachmentid=161936&d=1644185748&thumb=1[/IMG]


Now, luckily (?) there are holes above the LCA that I can use that look to be almost perfect for level LCA. No idea now why I chose the lower not the upper holes originally. But my larger/taller tires have made the problem worse I suspect. Kinda dumb to ask the question, but should I move the LCA into the top hole and make them level?

Here's a shot from other side where you can see the upper hole.

[IMG alt="Click image for larger version.
Name: LCA-angle2.jpg Views: 52 Size: 152.9 KB ID: 161937"]https://thefactoryfiveforum.com/attachment.php?attachmentid=161937&d=1644186114&thumb=1[/IMG]


The short answer is “probably” but for every adjustment you make on a suspension, there is more than one single effect. Not enough space here to cover a lot of detail so I’ll try to keep it simple. And for others following, this is specific to a solid rear axle car (or solid front axle for you Jeep, Ford, and Ram fans) with a 3-link or 4-link suspension, or trailing arms.

The LCAs control the wheelbase. They swing in an arc so as the suspension travels through bump and rebound, the wheelbase changes. No matter what angle the LCAs are on, if they both travel the same direction and the same amount, the wheelbase on each side stays constant, i.e., they both get longer or shorter by the same amount. The wheelbase is at the longest length when the LCAs are level with the ground. This is the neutral position as if the axle moves up or down from this position, the wheelbase gets shorter. The shortening effect is exponential, that’s why lifted Wranglers have way more roll & bump steer and why I say the farther from parallel to the ground the LCAs are, the more roll steer.

The trouble starts when the LCAs are at in incline and the two side do not travel the same direction or the same amount. I didn’t have a lot of time tonight but I did find one video that sort of explains this about halfway through. You can also demonstrate this for yourself with a broom handle. Here’s how: Hold a broom handle in your hands out in front of you like doing curls with your forearms parallel to the ground and the broom handle perfectly horizontal (neutral position). With your forearms parallel to the ground, lift the broom handle (that’s your solid rear axle) up and down by bending your elbows while keeping the broom handle horizontal. The distance from your body to the tips of the broom handle changes but the distance is the same for each tip. Now starting with your forearms in the neutral position, lift one side and lower the other side the same amount. Again, the distance from your body to the tips are equal. This simulates body roll. Now start with your forearms at a 45-eg to the ground simulating your LCAs at an extreme angle. Now once again, lift one side and lower the other the same amount. Notice that the broom handle is skewed – one side gets further away from your body and one gets closer. This is what happens when your LCAs are not parallel to the ground and you experience body roll or run over a pot hole with only one rear tire. The axle skews and acts like a forklift, the rear tires execute an uncommanded turn.

Now, I haven’t covered how this affect instant center and anti-squat so I’ll leave you to research that for yourself.
 

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Discussion Starter · #8 ·
Norm,

I am not active on the other forum, but do appreciate the info. Please send a PM to NAZ, and thank him for me
 

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Norm,

I am not active on the other forum, but do appreciate the info. Please send a PM to NAZ, and thank him for me
Will do!
Switching to the upper traction bracket holes and coil overs versus the Mustang springs greatly improved the handling characteristics of my car. I can't launch as hard as I used to without smoking the tires. I believe this is a combined result of the tires getting old and the weight transfer not as great due to the control arm position. Gladly accepted the loss of drag racing ability to get rid of unsettling rear end movements when driving on rough roads.

Norm
 

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Discussion Starter · #10 · (Edited)
One other thing that seemed counter-intuitive to be was swapping the rear LCA's to the Spherical Rod ends. My FFR issued arms had no binding or problems, and due to their construction would swing only in the one axis. Going to the rod ends would allow movement in other directions. So, just seemed a step backwards to me.

Yes, the Panhard bar is supposed to cancel that. And I have to check the frame mount to see if anything in the Frame mount is broken.
 

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Discussion Starter · #11 ·
Just checked, I do have the second upper hole set on the axle brackets. Next, will check the angle of the LCA
 
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