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Rear traction bar discussion.

shackle up for sure. More forgiving on setup and ultimately easier on parts. I'd type out the whole explanation of why but I'd rather do it on a white board.
 
OK, let's try this.

The bar that is attached to the rear axle, assuming those are not pivot points and the upper and lower bar are fixed in relation to each other. That bar wants to rotate around the axle centerline which causes the shackle end to go up in an arc.

Now, the shackle will roll fore or aft depending upon the alignment at the time the force is applied. It wants to rotate until it finds the tangent to the arc. That's the sweet spot where the bar lifts on the chassis and plants the tires with negligible front to rear pull on the axle.

So, going back to what I had said earlier, the alignment is almost never perfect so with the shackle pointed down, it would potentially want to rotate the shackle all the way around until its in the upward orientation and hits that tangent. It can't do it because of other limiting factors but it puts a lot of stress on poly joints, bolts, heims whatever. The springs and bar/shackle are fighting the antiwrap bar for lateral control of the axle.

Now, if you have the shackle up to begin with, let's say 90* relative to the bar at rest and again, acknowledging that its never in the perfect spot when you're using the truck and the suspension is in motion. Now apply the force on the same arc, the shackle will be closer to the sweet spot all of the time which in turn diminishes the amount of bind or fight between the springs and bar/shackle.

Easier on parts and more forgiving to set up.
 
So does the angle of the two fixed points in relation to the top bar have any effect?

What about the main bar being level at ride height?

Seems like moving the shackle lower or higher compared to the axle effects squat immensely.
 
I'm sure there is an optimal angle for the 2 bars (likely as horizontal as possible) but that's probably difficult to even get close to on a lifted truck.

Something that I think gets overlooked is the 2 mounting points in relation to the horizontal center line of the axle tube. I've seen some trucks set up with the lower mount at or above the bottom of the axle tube (ground clearance?), this bar can't be helping rotational control - might as well only have the upper bar.
 
nvrenuf said:
I'm sure there is an optimal angle for the 2 bars (likely as horizontal as possible) but that's probably difficult to even get close to on a lifted truck.

Something that I think gets overlooked is the 2 mounting points in relation to the horizontal center line of the axle tube. I've seen some trucks set up with the lower mount at or above the bottom of the axle tube (ground clearance?), this bar can't be helping rotational control - might as well only have the upper bar.
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I believe vertical separation itself is not as important as total separation from the axle centerline in any orientation. It's all about leverage. I have a "laid back" style axle bracket set that is one of the higher clearance versions like you mentioned. It puts the lower bushing a little below axle centerline vertically and say 3" forward of axle center front to back. The rear bushing is about 4-5 inches above axle centerline vertically and say 3" behind axle centerline front to back. What you absolutely don't want is brackets that are too close to each other in relation to axle centerline like one at mid height on the tube in front of the axle and then one directly on the top of the axle tube. It's a point A to point B in a single plane perpendicular to the axle's axis type issue as i understand it.

Toyota FJ80 radius arms are a perfect example of what i am talking about. Check them out if you haven't seen them before.

Now, i think it may put more stress on the system with the laid back style but i am certainly no engineer. If it does I've built it plenty strong and it's a compromise i can live with
 
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I'm running DIYs brackets which are swept a bit. My plan was to attempt to use the lower bar a hair lower than the driveshaft to protect it as much as possible. It's not super thick. .090 or so. So I'm doubting it will absorb 7000lbs on it. So I was trying to engineer the traction bar to absorb the hits.
 
You definitely need some triangulation on the axle ends of the bar, to close together isn't good.

The lower bar should be below the axle tube just a smidge at least IMHO. I try to keep at least 6" vertical separation between the ends.

This is the part of the discussion where you get more specific depending on the vehicle and its intended use. I think a bar assembly that is more horizontal and longer is better however, in the off road world that's not gona work very well for obvious reasons.

On my current build I've designed a shorter bar just to play around with it. We'll see what happens someday.
 
Basically the idea is main bar as level as possible, shackle as low as you can get it, as long as possible and the fixed axle mounts as far apart as possible while retaining ground clearance all while the shackle is 90* to the main bar and ride height. Seems simple enough. Suburban has plenty of room for a long bar under it. My height is a bit of a problem.
 
If you have one of the mounts in front of the axle instead of behind it for clearance reasons, you can alleviate most of the issue by triangulating the upper and lower bars together with a rigid connection close to the axle.

Think of it this way, the axle wants to rotate about the center of the ring gear. So with one link above and one link below, upon acceleration the top will be under tension and the bottom compression. The further apart the mounts the less the force for the same torque.

Now if you move just the lower link in front of the axle, it will leave the top link under tension, but the bottom link will want to bend up with the rotation instead of forward. So you can brace it to the top link to create a rigid triangle to help prevent that and you may need thicker tubing.

If you have the links under compression and tension only with no bending moment, that triangulation is likely not necessary and you can likely use thinner tubing.
 
I'm thinking with the DIY laid back mounts, with the bottom mount slightly below the pinion centerline around the bottom of the axle tube and the top mount somewhere near the top of the tube will be ok on that end. Figure then a ?in shackle tied onto the rear doubler mount about the same height as the output shaft so the DS angle and the TB angle are the same or very similar.

Sound good? I know it's over thinking but massive antisquat just looks awful IMO.
 
Ok, you’re building a traction bar and your material choices are 1.75 x .120 round DOM or 2” x .250 square tube - you have both, which do you use?
 
nvrenuf said:
Ok, you’re building a traction bar and your material choices are 1.75 x .120 round DOM or 2” x .250 square tube - you have both, which do you use?
Click to expand...
.250 wall will hold up well. .

120” would look nicer and probably do fine in that diameter if you get some triangulation
 
I brain farted... I have .250 wall in both. I was really just wondering if the 1.75 would be considered too “small”.

Is one shape stronger than the other?
 
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