Triangulated 4- Link

[davee]

I have a 31 Ford that I want to install a triagulated 4-link in. I have 26 " long bars for the lowers and I need to know what the length of the upper trangulated bars should be? (does the length of the bars matter as long as they are parallel upper and lowers?)  

Thanks  
Dave

[slocrow]

I want to install a triagulated 4-link

Hey davee; Welcome. Second, Sorry, I'm kind of dumb about this stuff and can't help in the length (if critical) needed for the upper bars in your 4 bar set up (upper and lower running parallel). Is that what you're doing along with (I'm guessing) a panhard bar, to prevent the rear from moving side to side? I'm under the impression that in a "triangulated" 4-bar set up the top bars are run in from the frame, rearward to the banjo housing at about a 45 degree angle, preventing housing lateral movement.
Is it a regular four bar that your planning (sounds like it) or a triangulated set up? I'm sure someone will hop on and give you some measurements and requirements soon.......Frank

[DrJ]

Depends on the angle of the bar.
I think it's trigonometry?
The only one I remember is if one side is 4 units long and one is 3 units long and the angle between those is 90º, the "long" side will be 5 units long.

[slocrow]

Julius; Isn't that how you correct your corner too 90 degrees when building a structure? As you said, 3' out from the vertex on one leg and 4' out on the other and there should be 5' exactly on the long leg of the triangle if the angle is 90 degrees. If not adjust the loose side until you have 5'/units.
The same thing would apply to the rear T-4bar set up I'd guess though the triangle would be closer to a 30/60/90 when you were through....I think.

[Bob Paulin]

FYI.

In the time that transpired between the first word of the following post and the final period, the forum had another mild "seizure." I was fortunate enough to be able to save what I had written yesterday in order to post it this morning.

What have you heard from your host, F.C.?

=============================================


Here are a few points to ponder.......


The shorter the upper arms are, the sharper they can be angled from the axle to the frame in top view - giving better side-to-side axle location control. That is one of the reasons the uppers are shorter than the lowers in factory applications. They provide the effect of a Panhard bar.

The sharper the upper set angle is pointing down towards the front, the MORE anti-squat you will have - which affects forward traction.

Conversely, too sharp an angle will result in rear-wheel hop on hard braking as the axle tries to pole-vault itself over the upper arms.

Lower arm length and up/down angle in side view will determine roll steer. If they are pointed up from the axle towards the front of the frame, you will usually end up with roll oversteer - or "loose" condition - in which case, as the racers say, the rear end will hit the wall first. If they are pointed down toward the front, it will usually result in roll under steer - or "push" - in which case the front-end will hit the wall first.

Longer arms scribe flatter arcs, thus create less roll steer per degree of roll than shorter arms.

There IS a happy medium.....you will have to find it yourself by jacking the rear axle up/down through roll without the springs.

Roll center is determined by the degree of up/down angle scribed by the upper arm set and the lower arm set in side view.

Stock GM configuration has the lower arms angled inward toward the front in top view - their instant center converging past center nearer the front of the car at about frame height......and the upper arms angled inward towards the back of the car - with their instant center converging slightly behind and above the rear axle itself.

Project a line between these two instant centers, and where it crosses the rear axle is where your rear roll center is located. Changing the angles of the upper and/or lower arms changes the roll center.

For example, pointing the upper arms higher toward the rear (as you would if you were pointing them down towards the front to gain anti-squat) raises their instant center, and raises the projected line where it crosses the axle, thus raising the roll center......

.....but, don't forget that changing arm angles to change roll center also affects anti-squat and roll steer......and vice-versa.

Run a parallel four-link and add a Panhard bar, and you're talking totally different geometry, and roll center configuration.

While all of the above can seriously affect a race car that is being driven at "ten-tenths", there is a markedly greater "fudge-factor" on a street-driven car.......

........but it sure helps to make the car more pleasant to drive if it is somewhere near correct, and not totally out-to-lunch.

If you want a ballpark upper-to-lower arm length ratio and mounting angles, you can measure a coil-spring Mustang or a 78-85 mid-sized GM rear suspension. Both of these cars handle fairly well in stock form, and you won't be far off by copying them.

The "3,4,5 rule" is actually, the sum of the squares of two sides equal the square of the third side.

3x3=9....4x4=16....5x5=25......9+16=25...square root of 25 is 5.  

While using the numbers 3, 4, and 5 is simplest, you don't need to use them.

If you know the length of two sides, you can square them, add the squares together, then find the square root of that number to determine the length of the third side - or hyopotenuse - needs to be in order to have a perfect 90° triangle.

30, 60, 90 really doesn't apply for triangle side measurement.....unless they have already been squared in which case the sides would be......5.4772255, 7.0745966, and 9.4868329.

(How many "little marks" is that on your Stanley 16-foot tape?)

30, 60, 90 could also refer to the angles in a right triangle - which always add up to 180°, but it could also be 45°, 45°, and 90°. Again, if you know one angle - and you know that there is a 90° angle in a right triangle -  you can figure out the third angle.

This little ditty comes in handy when cutting and  fishmouthing tubes in a roll cage.


B.P.

[slocrow]

Thanks Bob.
Boy, we seem to have lost a few posts on this thread along with the pic of a triangled set up.......
Again I think the lengths of my bars in my tri set up are , lowers around 27" and uppers around 17". Hope that helps........Frank

[flt-blk]

I'm running triangulated 4 bars in my A pickup.  I will measure them
when I get home.  I think I may also have a build picture showing their
mounting I can scan.
TZ

[PeterR]

One other consideration with this set up.

Depending on the arm lengths, the angle they make viewed from the side, and the relative position of the gearbox and pinion UJs, there can be considerable variation in driveshaft length over full suspension travel.    

In extreme cases it is enough to pop a slip joint from the gearbox, and the only way to avoid this is to use a two piece drive shaft with the intermediate joint located at the instant center of the rear linkage.  

Before getting too far along the way you can easily determine if it will be a problem later.    After you have decided on arm lengths and pivot points make a drawing about 1:5 scale.

Mark the position of the pinion UJ at normal height, the two extremes and a couple of intermediate points.   (Remember the axle assy is also rotating while moving up and down.)  Join the marks with a smooth curve to show the path the UJ follows.

On the same drawing mark the position of the gearbox UJ.   Measure the distance from the gearbox UJ to the curve to find the shortest and longest shaft length over full suspension travel, then subtract these to determine the variation in haft length.

If this does show reveal a problem, better to know early and decide on a course of action than to be bitten later.

[Dirk35]

Wow Bob, thats good info to keep.

FWIW, if you have access to a junk yard, earily 90s Lincolns with the 8.8 rear-end used a triangulated four link. And the rear ends are fairly easy to narrow for shops that can re-spline. I know there are others, this is just one that I do know about.