Deviousfred

This will take a little out of the box thinking. Sorry if this has been posted before but I'm looking to get some feedback on this. On a thread over at VMF we had a little discussion on adjustable strut rods. There was some mention as to the best mounting option when it came to mounting the new strut rod to the strut support brace. Now before we go any further I’d like to point out some of the terms that will be used in my post.

Original Strut Rod - No explenation needed

After Market Strut Rod - example pictures below


Strut Support Brace - pictured below


Original Strut Rod Mount on Strut Support Brace - pictured below


LCA (Lower Control Arm) - no explenation needed

LCA Mounting Location - where the LCA mounts to the subframe

Strut Pivot - where the strut rod pivots. Not sure how to explain this much more



First the strut pivot on the original design is created at the midpoint or meeting of the two strut rod bushings. This location is at the original strut rod mount on the strut support brace. Someone mentioned that the original strut rod mount on the on the strut support brace was in-line with the LCA mounting location. What this would mean to me is that a line drawn from the strut rod mount on the strut support brace to the LCA mounting location would make a right angle (90 degree) with the LCA. See figure 1.



If this is so, then since both the strut rod pivot and LCA pivot lie in the same line we can treat this as an axis of rotation. So, when the wheel moves up and down in the suspension travel it will travel in a path around this axis. What this means is that the wheel will travel directly up and down which is what the busing end on the LCA is designed to do.

Now, let’s take a look at the majority of aftermarket adjustable strut rods. Most of these use either a clevis and heim-joint or a ball/socket design (TCP) where the original strut rod was originally mounted with bushings. What does this mean? Well this would mean that the new strut pivot is now at the heim-joint or ball/socket depending on the design. This new pivot has now been moved slightly (<1” usually) further back from the original strut rod mount on the strut support brace. This is where it gets interesting so pay attention. Now we have to draw a line from the new strut pivot to the LCA mounting location. This line is now NOT in-line with the LCA. This new line is not a right angle (90 degrees) with the LCA. Instead, this new line is at a smaller angle from the LCA (<90 degrees.) See figure 2.



This new line formed between the new strut pivot and the LCA mounting location is our new axis of rotation. Try to picture this in your head in 3-D. When the wheel travels up and down it will also experience some forward and back movement. The original LCA is not designed to handle this kind of movement. Most aftermarket LCAs have spherical bushings and can handle this. However, this also means that negative caster will be changing constantly as wheel travels up. Since the original LCA is not designed to accommodate this type of movement it will cause bind. Probably not a huge amount, but there will be some bind.

So what does all this mean? I believe the strut pivot should be kept in-line with the LCA mounting location. I will be building my own set of adjustable strut rods with heim-joints in the near future but will be cutting out the original strut rod mount on the strut support brace, drilling a hole across the strut support brace, and placing the pivot at the same exact location it was with the original set-up. It will probably require some strengthening to the strut support brace with a plate and some gussets.

I must say that I am no expert in suspension, but based on what I do know and engineering material I have and continue to study, this is what I concluded. Also, this whole theory is based on the assumption that the line between the original strut rod mount on the strut support brace and the LCA mounting location makes a right angle (90 degrees) with the LCA. I’m just waiting for someone to actually take a measurement and send this whole post and my theory down the drain.

Thoughts?

Scott H.

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JamesW

I like to think that the 1" back heim joint on the end of the strut rod will rotate in an eliptical fashion and not force the LCA bushing to bind.

Least that's what I'm hoping for when I do this same homemade strut rod setup.

Deviousfred

well if the heim has the ability to allow movement around an axisparallel to the one the LCA rotates around it would work without bind or creating any greater pull on the arm. I think it could work.

67mustang302

Even if the strut rod is in line with the LCA pivot point, you have to account for the fact that the strut rod isn't in the same horizontal plane as the mointing points parallel to the ground. The strut rod is angled downward, so there's always going to be some forward/backward movement of the LCA during suspension travel, but yeah, it's so minor that you'll never notice it. Theheim joint setups are nice, but they're primarily to allow for more positive location of the LCA and allow for free movement. The entire job of the strut rod is simply to locate the LCA front/back relative to the chassis, and the LCA pivot pointand the strut rods are bushed to allow for movement. With soft bushings that can be a problem, since they allow so much movement that LCA deflection occurs because the strut rod is not able to positively locate the LCA, with harder bushings, such as poly you get much more positive location of the LCA, but there's less ability to deflect, which can lead to a stiffer ride. The advantage of the heim joints is that they have no play like a bushing so you get positive control arm locating, yet they havea very wide range of motion, so full suspension articulation can occur.

Basically, don't wast you're time making a whole new bracket yadda yadda. If you could measure it on an alignment rack, I doubt the difference in caster change from LCA movement rearward on suspension compression would be more than a couple tenths of a degree between mouting the strut rod at the factory point, or where the aftermarket kits mount. Besides, that's why you adjust caster anyway, so that on corning with suspension compression you have the proper amount of camber in the tire(if you're running positive caster, which you should if you want good steering control and cornering).

Remember, that strut rod is also angled downwards as well, and as the suspension travels through it's range the change in angle of the strut rod relative to the ground/chassis will have a minor effect on caster, moving the LCA forwards or backwards, but not enough for you to feel. Making up a whole new bracket isn't going to change that, and the effect of a slightly different mounting location will be definately not worth the time involved.

Norm Peterson

Disclaimer: My work computer does not display all of the pictures. That I keep getting a number of "Security Alerts" in order to reply or edit might have something to do with it.

Anyway, with any design of strut rod that is fixed to the lateral link (which I believe you have identified as the LCA) with two bolts you essentially have a single (lower) control arm with a pair of chassis side pivots. It looks differently than, say, the "L" shaped front control arm on the S197, the LCA that'sunder a Fox/SN95, or even what's under mythumbnail car, but it functions in precisely the same way.

If the strut rodis rod-ended at both ends, it would be possible for the lateral link to rotate torsionally with respect to the plane containing the assembly, but the lower ball joint path would remain essentially the same (it would be somewhat dependent on the location of the rod end between the strut rod and the lateral link).

If it's numbers that you want for evaluating the differences that a 1" relocation of the forward pivot will cause, you'll want a 3-D suspension program. Either that or wrestle through some 3-D geometry and equations for planes on your own. A 2-D approach isn't enough.

But qualitatively, your caster gain will increase (caster varies with suspension travel except under specific geometry). IOW and with numbers pulled out of my hat for illustration only, suppose that the stock pivot points gave you an additional 0.25° of caster per inch of bump travel. The relocated strut rod pivot might shift that up to something like 0.3°/inch. Or it could change a very slight negative caster gain into an equally slight positive number. It won't be a huge change, as the small angular shift in the pivot axis suggests. In terms of effects, there will be slight changes in things like stability under braking, roll center migration, bumpsteer, and turn-in response, but that's where needing the real numbers comes in (followed by some judgement). Not everybody would notice these differences, and fewer still would be able to identify the reason. Braking loads applied at the chassispoints will increase slightly due to the slightly shorter fore/aft distance separating them.

Since the chassis pivot axis is changed, whatever bushing exists at the chassis side of the lateral link will need to accommodate this, via adequate compliance and/or angular range of motion.

Maybe somebody has a chassis dimensions picture from the shop manual - it might have some of the dimensions that you're looking for. I thought I had one somewhere, but I guess it's not on this computer.


Norm

[IMG]local://upfiles/62186/1F64E1823E6C4B49A70B19655A7B53FE.jpg[/IMG]

Deviousfred

well, if you think about it since the strut rod mounting point and the LCA mounting point are in-line and both mounting locations lie on the same plane, as the ball joint travels around the axis of rotation the strut rod will not try to change lengths. take an acrylic 30/60/90trianlge. Pretend the 90 degree corner is the LCA mounting location, the 30 degree corner is the strut rod mount location, and the 60 degree corner is the strut rod/LCA meeting point. The side oppostie to the 60 degree angle is our axis of rotation. if you rotate the triangle around this axis the strut rod stays in constant contact with the LCA. You have to remember because of the strut rod mounting design it not only has the ability to move up and down but also every other direction although movement is somewhat limited.

JamesW

I think this horse can be soundly considered beat to death.

Deviousfred

just trying to arrive at an accurate answer. I believe things are improved by asking questions and figuring out how and why stuff works the way it does.

Scott H.

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