Jeep What exactly is Radius Arm Unloading?

[YotaDaniel]

I have read a ton of threads online about radius arm vs 3 link vs 4 link for XJs, and a lot of people say that a radius arm is more likely to unload on a steep climb. So what do people mean by unloading? What is physically happening?

Daniel

 

[STAG]

Geometry. I'll share what I know/think I understand.

In a radius arm you have one point on the drivers side and one on the passenger side, where each link arm ties the axle into the frame (er... Unibody ) and the link arm travels in a fixed radius (hence the name) in relation to the body. It's basically zero resistance and no binding (within the limits of the coil spring travel). Which can be both a good and a bad thing.

When your tire is spinning and trying to grab traction it is trying to rotate the axle housing in the opposite direction. (Every action has an equal and opposite reaction, physics yo). Your radius arm does its job at keeping the housing from rotating and power is applied in a forward motion. However not all the power goes towards rotating the tires, because of the zero resistance and binding of travel in a radius arm, some of the power easily transfers as lifting the front of the jeeps body. (Think when you are at a red light and gun it, the weight transfers towards the back, and the rear sags and the front lifts.) in a radius arm, this is extremely noticeable because the geometry does not apply any anti-squat, or "the good part or link binding)

With a proper 4 link or the like, (with vertical separation, proper instant-centers and link lengths and all that jazz) you create some "good binding" as your axle housing tries to rotate again it is pushing on your upper link and actually pulling a bit on your lower link. With the right geometry this makes it so no power is applied in lifting your jeeps body.

It all makes sense to me I hope it may to you.

Same principles (but reversed) apply to anti-dive or the anti-squat of braking. Like when you see a newer ford (with a factory radius arm front suspension) slam on the brakes his front end dives down. This can be avoided with a multi-link setup.

Also applies to roll-axis when you bring in link triangulation. You can fight body roll and unloading with proper geometry.


Im out of time for a bit but there is a link calc in my signature.

 

[Greg]

Here's a pretty in-depth discussion about radius arms and how they can unload. - http://www.rme4x4.com/showthread.php?75649-Radius-arm-front-suspensions-Hopping

 

[I Lean]

Chocflip201 is almost right, but 180 degrees off. Radius arms have a lot of anti-dive built in, meaning they resist front end compression under braking, and they "want" to suck the front end down under power. If you have a short radius arm (or a lot of anti-dive in a muli-link setup) that can lead to hopping during climbs.

Unloading is a separate issue, not necessarily related to suspension geometry. Any suspension that is free to move is also free to unload as the weight transfers rearward. How much that happens is due to shock and spring rates, weight distribution, and center of gravity. More weight, more weight up high, more weight out back, all make the front unload more when you get steep.

If everything else works well in the suspension system, the only thing needed is something to limit the unloading--either a limiting strap, or a winch rope on the axle. (essentially an adjustable limiting strap)

 

[Kevin B.]

Carl, can you paint a better picture of the unloading itself? I'm picturing a violent release of tension, but I'm having a hard time seeing it. I've heard reference to double-shackle setups unloading, and I can't get my head around the physics of that either.

 

[STAG]

Ha I'm terrible.

 

[I Lean]

Carl, can you paint a better picture of the unloading itself? I'm picturing a violent release of tension, but I'm having a hard time seeing it. I've heard reference to double-shackle setups unloading, and I can't get my head around the physics of that either.

No violence (usually), just the front suspension extending when the nose of the rig is pointing skyward. You'll naturally transfer weight toward the rear axle when climbing something, and if the front extends you'll transfer that much MORE weight rearward, leaving less traction from the front end. Simply keeping the droop controlled keeps as much weight as possible on the front end, so climbs end up easier. (you can similarly help yourself by not having extra weight hanging off the top or rear end of the rig)

Here's a great shot of a horribly unloading vehicle. Air shox are probably the worst offenders for this, which is why you see such a drastic change from even a mild climb as shown.

 

[B2-Bomber]

OK, so what is the most desirable way to set up a front end?

i hate to sound pedestrian but i love my leaf springs up front

 

[I Lean]

The most desirable is whatever gets you and your vehicle out on the roads and trails.

 

[B2-Bomber]

^^YES I know, i mean in superbole. what on paper is the most desirable?

 

[gertsch]

My grand Cherokee is running radius arms on from and they work super. It's been able to climb anything without any real issues. I am a fan of four links and this is my first lift using a radius arm but I haven't felt unstable or disappointed in its performance yet. Just my .02 for what it's might be worth.

 

[B2-Bomber]

I am a fan of four links and this is my first lift using a radius arm

4 links still have radius arms, with 2 upper locating arms,

do you mean this is your first "lift using a 3 link"...?

that is unless my vernacular is mistaken, a radius arm is a lower forward arm, and are called "trailing arms" when on the rear

upper arms are called "locating arms"

traction (or "anti wrap" bars) maintain pinion angle under load when the pinion tries to "climb" the ring gear, or when it tries do dive

a 3 link almost always (except on a few land rovers etc) uses a trac bar/panhard bar to control side-to-side movement, and radius arms to control front-to-back movement.

triangulated 4 links do not need a panhard/trac bar

non-triangulated 4 links do need a panhard/track bar to control side-to-side movment, then called "5 link" or "4 link with panhard"

 

[gertsch]

By 4 link I'm meaning the upper arms don't bolt it the lower arms. They have there own mounts on the frame. Like what Teraflex uses on ther tj suspension verses say rubicon express. For long arms.

 

[I Lean]

I think for the purposes of this discussion, a "radius arm" is assumed to mean all axle-side attachment points merge into one control arm. (or to use your terminology, the locating arms attach to the radius arms) As opposed to any other design using separate upper and lower links, whether with or without a panhard.

So a "radius arm" suspension = two "traction bars", plus the necessary panhard.

 

[gertsch]

I think for the purposes of this discussion, a "radius arm" is assumed to mean all axle-side attachment points merge into one control arm. (or to use your terminology, the locating arms attach to the radius arms) As opposed to any other design using separate upper and lower links, whether with or without a panhard.

So a "radius arm" suspension = two "traction bars", plus the necessary panhard.

This is what I was trying to say! Thank you!

 

[B2-Bomber]

So a "radius arm" suspension = two "traction bars", plus the necessary panhard.

i should have typed out that there are 2 different type of rad arms:

the first is anchored at the axle end, and does not have a bushing or pivot to allow the axle to twist, and is ideal when the frame end of the arm is able to be mounted inline (longitudally, and vertically) with the u-joint on the t-case end. this way proper pinion angle is maintained throught the entire travel of the suspension

the second has a pivot on the axle end, and allows the axle to twist, this style is better when the frame end of the arm can not logistically be placed in-line longitudally, and vertically with the u joint on the t-case end. the use of other links (either 2 triangulated locating arms, or a traction bar) is then used to maintain proper pinion angle throughout the travel of the suspension.

unless i am way off base here..?
i have been wrong before, i used to believe that it was called a panhard bar when on the rear, and a track bar when on the front, but a bunch of jeepers here made sure to set me straight that it is an "either-or" thing and both names bork for both, based on what mood a person is in when they are talking about it.

 

[Herzog]

Solid at the axle and a pivot at the frame (single point) is a wish-bone and is a different animal. You can't have a functioning arm system with independent joints at the frame and a solid mount at the axle, won't work.

Radius = bushing at the axle (high & low) that converge somewhere on the arm, either solid or jointed. Looks like a y with bushings at the top of the "y" (axle side) and a joint at the bottom of the "y" (frame side) and one arm on each side (driver and passenger). It's a radius arm because the arc of travel is is a radius and the housing stays static.

 

[B2-Bomber]

Solid at the axle and a pivot at the frame (single point) is a wish-bone and is a different animal.

but solid at axle, and pivot at frame is exactly how a ford ttb radius arm is......and that is undoubtedly considered a rad arm

It's a radius arm because the arc of travel is is a radius and the housing stays static.

i thought a rad arm got its name because it determines the radius of the travel of the axle?

Radius = bushing at the axle (high & low) that converge somewhere on the arm, either solid or jointed. Looks like a y with bushings at the top of the "y" (axle side) and a joint at the bottom of the "y" (frame side) and one arm on each side (driver and passenger). It's a radius arm because the arc of travel is is a radius and the housing stays static.

i have seen those done several times, but again the ford rad arms have no bushing on the axle side, nor, are they specifically shaped like a y
they are just solidly mounted to the axle at 1 point with 2 bolts

and the housing on a ttb can't "remain static" in relation to the rad arm because of the 2 extremely large "beams" that tie to the frame transversely not allowing any twisting of the housing. but they still have rad arms

 

[I Lean]

Ford radius arms DO have a bushing in between the axle and the radius arm--it's commonly called the "C bushing".

Unless you're talking about TTB front ends, which are pretty much different animals, and don't behave very similarly to the solid-axle version at all.

 

[B2-Bomber]

Unless you're talking about TTB front ends, which are pretty much different animals, and don't behave very similarly to the solid-axle version at all.

i am, and even said "TTB"

they have rad arms, and no bushings, the bushings are not somehow the defining characteristic of a radius arm