Thread: Adjusting pinion angle w/4 bar rear end

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Mt deuce has a triangulated 4 bar rear end setup....If I wanted to tilt the pinion downward, which of the 4 bars should I adjust? Should the top bars be shortened, and not touch the bottom, or should the bottom bars be lengthened and not touch the top bars,... Or do all 4 need to be adjusted together? Not sure I even need to do this, but in just "eyeballing" it tonite, I think my pinion may be aimed a little higher uphill than my transmission is aimed downward, which could be where this vibration that I still have is coming from. I'm gonna borrow a protractor from a buddy and check it angles to be positive, but am just wondering what the approch to adjusting the pinion is with this type of 4 bar setup.

Go to Sears or similar and buy yourself a $ 10 angle finder. Place it on a flat machined surface of the rear end where the u joint enters it, and take a reading. Do the same to the transmission. You want about 3 degrees down on tranny and up same amount on the pinion. It can vary a little but that is pretty ideal.

As for advice on the four bars, someone else will have to help there, I have never owned them. Here is a picture of what you want to buy.

Don

Torsional loads make the pinion rise. If you load this thing it's going to make the 4 bar set change instant centers as the bars torque and pull, thus changing the "original" centerline of your instant center for the car. I always recommend 2 degrees of down ward (and this only a recommendation). I can't not be hands on to see it so I only recommend 2 degrees. Adjust bars equally so as to not disturb the instant center of the bar angles. Does your tranny have a downward angle? Vibrations will ocur when the two intersect points (Trans and pinion intersect points) are varied as such with pinion downward, tranny downward. Be on the safe side and get the tranny to point directly at the pinion. A couple maybe slightly in a degree angle up above the pinion intersect will get you a smoother operation and allow for the pinion the work the bars under load.

Nitrowarrior is right on with his 2 degrees down comment.

I run that setting on the parallel 4 bars on my Deuce highboy roadster.
45,000 miles, runs smooth, never had a U-joint problem.

Only thing I would add is, 4 bar adjustment will move the rear axle fore or aft changing the tire/fenderwell relationship.
Most times it's not much of a change so no big deal, but if you crank a lot of the thread in or out you'll notice the change.

If you like where the tire sits in the fenderwell now, split the difference between the upper and lower 4 bars.
One goes in the other goes out, tire/fenderwell relationship remains the same, pinion angle changes.

Some of this advice runs contrary to what I have always done, and what is generally recommended for street rods.

Nitro,

If you load this thing it's going to make the 4 bar set change instant centers as the bars torque and pull

Since in most triangulated 4-bar suspensions, the upper and lower bars are parallel, I can't see any reason for a change in instant center. Am I wrong here?

Be on the safe side and get the tranny to point directly at the pinion.

I've always heard that this is a good idea on a race car, but undesirable on a street car.

Are you guys recommending that both the trans and the pinion point down? Conventional wisdom has the pinion pointing up and the trans pointing down in equal angles. That makes the centerline of the pinion and the centerline of the trans output shaft parallel.

http://www.iedls.com/ptsetup.html

If you're running the bars parallel just bebopping down the road, Parallel is fine for suspension travel. If this thing is going to launched and you want the tires to plant well, you need to adjust the uppers in relation to lowers to create an instant center more condusive to the center of gravity of the car. Like I said thogh, if it's just for cruising around, run them where you have them. Go back on this thread and look at the diagram posted earlier. It gives you a better understanding of where you should set them as far as angles go for your suspension set up. If it were mine I would stick with 2-3 tranny down and 2-3 pinion down.

When running 4 bars, the pinion should tilt up the same number of degrees that the transmission points down. Most v8 engines are set up so that when the carburetor base is level the tranny tailstock will angle down 3 to 3 1/2 degrees. To keep your wheel centered in the wheelwell,for every full turn that you shorten the bottom 4 bars, then lengthen the top 4 bars the same amount, or vice versa. To have the tranny pointed down and the pinion pointed up is absolutely wrong---that setup is only for drag racing, when wrap up in the springs, 4 bars, and yes, even the frame, will bring the pinion up to level. This is absolutely the only time a pinion should point down---full on drag racing.

Originally Posted by brianrupnow

When running 4 bars, the pinion should tilt up the same number of degrees that the transmission points down.

This is what I've always heard too... I've never heard of aiming both the pinion and the trans downward, unless maybe you're building a drag car thats gonna be launching at 9k or something. Right now it appears my trans points down, and my pinion points up, but I believe the pinion angle is greater than the trans angle, necessitating a slight tilt downward to the pinion. That "angle locator" posted above is actually a protractor similar to the one I am hopefully borrowing this week to determine my exact angles, but as i said, visually right now, I'm gonna say my pinion needs tilted down a bit. I wasn't sure which bars i needed to adjust, but it sounds like if I want my wheels to stay where they are, i need to adjust top AND bottom bars.

Keeping going Joe. You're gonna this thing just right yet. Brian, we never asked him how much travel this rear end is going to have. I worry about pinion traveling up a crossing over the nuetral centerline thus putting it into a u joint bind. This is why I recommend 2 deg down for good measure. I hate not being hands on. The mind's eye can only do so much in visualizing it. Thank's for your input guys. I like a good effort to help all gals & guys out.

With equal length 4 bars, the angle of the pinion does not change, regardless of the suspension travel. It is a true parallelogram, If the leading pivot points of the 4 bars are vertically aligned, and the trailing pivot points are vertically aligned, at a "neutral position" (which in a perfect world, will be with full weight of body, engine, and chassis, resting on the springs, and the 4 bars running at 90 degrees to the vertical) the pinion angle will not vary either above or below the "nuetral centerline". There are other factors that are introduced when the top and bottom 4-bars are not of equal length, but as far as I know, we are not dealing with that here.