the idea behind pointing the front of the rods in / \ is to lessen the amount of "compliance" needed about the roll center. It would be the opposite if you went \ / as the distance is more.
I love the smart guy answer! This is the correct response to the original and restated question. It's a strength thing, like Halfdozen said. The upper bars are asked to do double duty in locating the axle in two planes, for and aft AND side for side.
I believe the roll center is also lower if the links point in towards the front versus out towards the front - all other things being equal
Both directions work. If you want to really confuse things, look up a Satchell Link rear. The convergent point of the non parallel bars is the center of the roll axis. Much like the middle of a panhard rod is the center or the center pivot of a Watts Link.
The GNX Buick did away with the uppers and added a panhard bar and torque arm. This helped with the GNX's propensity in Buick language to "exceed the static coefficient of friction". Hoop (Ford at birth, Buick by Choice)
Every fox based mustang had the setup shown below. Ford sold a couple million cars with this setup. It's not perfect, but it does work. Lot's Mustangs run deep into the 11's on this design. They also handle pretty well.
if your talking about a straight bump thats completely uniform so the axle move up and down equally then yeah, your system wont bind, but a properly designed and set up triangulated 4 link shouldn't either.
When the links are triangulated inward to the rear the Roll-Centre height [or instant centre ] lowers as the suspension is extended When the links are triangulated inward to the front the Roll-Centre height [or instant centre ] lowers as the suspension is compressed The best method of locating a constant Roll-Centre height is a Watt-Linkage with the Bell Crank mounted on the centre of the axle
Most of what we build on our street rods is compromise - for appearance, or clearance, or budget. But we know by experience that some systems work, don't break, and can be built for a reasonable price. Elpolako probably spends more time than most working these things out in CAD, and understands the constraints of the vehicle he's working on. If you were around in the 60's, we saw some really wild front suspensions on top fuelers and altereds from big name chassis builders like Woody Gilmore which turned out to be not the hot tip. Most of them went back to the torsion bar / locating arm setup quickly. It worked for that application, as do either the parallel 4 link or the triangulated 4 link on street rods- personally I like the triangulated setup because it (correctly designed) eliminates the panhard bar.
I like Panhard rod because it is easily adjustable to fine tune rear roll stiffness. A lot cheaper than 10 different rear sway bars. No matter what it is I am setting up I have a marked off 100 foot radius in a parking lot. This allows a way to evaluate mid corner balance. I realize we aren't building autocrossers normally but you still want a relatively balanced or mildly understeering car as a safe easy to drive car. You never know when you will overshoot a decreasing radius freeway exit and that's bad time to find out you have terminal oversteer. A panhard bar gives you a lot of adjustability. jm2c Hoop
Some of these guys are very knowledgeable about suspensions and have a ton of experience. They're giving you some great info. I agree, it looks like your set up will bind under body roll and when the suspension on one side sees more travel than the other. And yes your set up will stress the upper mounting brackets.
Putting them at angle is fine it keeps from having to have a panhard bar. They need to be straight on high horse power cars but will need some sort of side to side locator for the housing.
I'm not getting how the shorter lower control arms can move in the same arc as the longer torque arms that is rigidly attached to the rear end?
in the 4x world, we would mount the arm to a shackle on the frame end. that way the 2 links locate the axle front to back and the arm just controls axle wrap
I emailed a company that builds everything from 4-links to custom made control arms, and asked them the same question I asked here, and this was his response. "The way guys build triangulated 4 links 90% of the time is incorrect. Excuse my crude examples but this is how most setups look. Now imagine how these bars are going to travel up and down. They are going to want to close in on each other, due to the geometry of the pivot points. As the rear end travels up and down from theoretical ride height with the pivot points set up the way they are here, the triangle will close up. Putting the bushings in a bind." "The way pivot points should be setup for ZERO binding, is as follows. All the pivot points in the 4 link should be parallel with each other. An even better way of saying it would be that the points need to all run on the same axis in order for nothing to bind. " (this is the way mine is setup) I also mentioned that OEM suspensions are done that way, and here was his response to that, "Yeah but cars from the 60's, 70's and 80's used huge rubber bushings. There is so much give in those bushings that the geometry made up for it. Ever notice when you tear down and original Impala that the rubber bushings are completely fucked? Ever wonder why they're so fucked up? This is also true in an 80's G-Body. Yes the triangulated links aren't like I said they should be, but look at the size of those rubber bushings! We all use really tight spec'd Urethane bushings in our cars and trucks these days, and for the most part guys are using Johnny Joints or Heims on at least one end of the trailing arms, or on one end of a wishbone. Go have a look under a late model car. Doesn't matter the make or model, I guarantee you every pivot point lies along the same axis. (give or take a slight front end alignment angle) "Binding" can't just be shrugged off. It's a pretty important thing to think about. You start taking things for granted and before long you'll figure out why you should have payed more attention to it in the first place. Looking at the pictures you sent me of your setup, I'd say you did it the "correct" way. Like I said, 90% of people will tell you it's wrong, but trust me, you got it right. I hope this helps."
I'd be more worried about the frame rail that the rear appears to be sitting on. I saw bags, if that is your geometry with the air out then your ride height geometry is going to be terrible. I'm not going to get into 'why' as it'll start another whootiedo debate. Kinda wonderin what the 'u' bolt is doing, must be your jig for setting everything up right?
all of this is why i build 3-link with panhard bar setups instead of a 4-link. i think why most here dont think too much about it is because in a hotrod application is because there isnt that much articulation or travel overall. i always overthink stuff like this because it make a huge difference when you build something with 15" + of wheel travel. i know a lot of guys on here also dont pay any attention to roll centers or anti squat. a lot can be learned from triaged and his calculators http://www.pirate4x4.com/forum/gene...-version-my-4-link-analyzer-request-help.html
i used the u bolts to keep everything square, that is roughly my ride hight. i set my pinion angle up at ride hight. but the lower links are coming out to get adjustable ends.
I think there is confusion as to 'angle' and 'parallel'. Elevation wise, i like to keep street car link bars parallel to the ground and each other. The object is to let them rise and fall at the same 'rate', that keeps the rear from rolling around. Looking down from the top you can 'angle' the bars to keep the rear situated, i've never done it but you could probably angle both upper and lower. Generally it is simpler to angle the uppers like the Fox body Mustang another mentioned. Get a couple beers, some matchsticks, a bottle cap and clear a spot on your coffee table to make one side of a miniature 4 link. Tape the matchsticks or toothpicks to the top and bottom of the round part of the bottle cap, bisect the cap with a line for the pinion and use your fingers to pin the other end of the matchsticks and push the cap thru its' motions and watch the effect on the line you drew as you try different 'attaching' points. Try to keep the pinion line level.
To one degree or another ALL the 4-link rear suspensions we are talking about here have inherent "binding". That includes parallel links. As a result, without rubber bushings there are conditions where something has to flex, bend, or have play to allow free suspension movement. Solid Monoball / Heim / spherical pivots aren't a great choice for the street. Even with boots and frequent lube they quickly develop play. Beyond that they transmit a lot of noise to the chassis, get noisy themselves, and maximize the peak loads ther suspension and chassis are subjected to. It doesn't take a lot of compliance to alleviate the binding situation. In most cases the big marshmallow bushings most production cars have are a lot more than what's needed to prevent binding. In most layouts, relatively thin and hard bushings are adequate.
Made sense to me -- the ends of the rods parallel to the axle.... Whether it is better, or not, is another question.
I think that with the small amount of movement we see in our cars its really a non-issue either way. When I imagine either setup in an extremely articulated position, I honestly don't consider EITHER as perfect. Any single link, with 90* ends or otherwise, is gonna see a lot of bushing flex due to the tube joining them being unable to twist torsionally. It's just the way it is! The original P & J parallel 4 bars had rubber bushings for compliance...eventually going to stiffer urethane bushings to add some roll resistance by forcing the 4 link tubes to try to twist and thus act as a sort of anti-roll bar. The result? The locknuts were known to loosen and allow the tube to rotate on the threads! In any tubular 4 link that binding and twist resistance is built in due to the tube stiffness. The physical bushing position being one way or the other might settle your mind, but it really does little to actually eliminate binding!
That's all well and good if you aren't interested in learning about rear axle ride geometry. Personally, I have always had the question that the OP has.... which is why I used a parallel 4 bar and a panhard bar on my current ride. If you have compliant bushings and +/- 2 inch rear axle travel it probably doesn't make much difference....
Post 51 nails it. Note "As the rear end travels up and down from theoretical ride height with the pivot points set up the way they are here, the triangle will close up. Putting the bushings in a bind" Simple geometry.
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