What are the advantages of J-arms?

[bart]

Over a arm of thae same length mounted in the traditional manner ? I had intended to make lowers straight and slightly longer than my triangulated uppers on my new rear suspension. I would mount them higher on the axle than factory and as flat to the frame as possible. I know the J-arms give more clearance immediately in front of the rear tire and act as long arms. Anytthing else? Traction? Thanks for any help.

-Bart

[Allen]

More droop.....

Maybe other benefits as well. I've never looked at them mounted on a Jeep.....

Allen

[mrblaine]

Originally posted by bart
Over a arm of thae same length mounted in the traditional manner ? I had intended to make lowers straight and slightly longer than my triangulated uppers on my new rear suspension. I would mount them higher on the axle than factory and as flat to the frame as possible. I know the J-arms give more clearance immediately in front of the rear tire and act as long arms. Anytthing else? Traction? Thanks for any help.

-Bart

You stated a paradox given a static wheelbase. Unless you intend on mounting the J-arm behind centerline on the front and in front of the centerline on the rear. You also can't make your lowers the same or near the same length as your upper in a normal triangulation.

What you can do is make them slightly longer than the effective length.

The purpose of a J-arm is to mount a longer arm in the same place that you removed a shorter arm from without drastically altering the wheelbase.

Since the J-arm is typically mounted at or very near centerline of the axle tube, the effective radius is now more parallel to the ground and any deviation from parallel at ride height is a compromise. Very similar to a shock's function, any mount angle that deviates from perpendicular to the range of motion in both directions is a compromise and decreases the shock's effectiveness.

I'm not sure how you are getting more clearance in front of the tire. Can you clarify that? Typically the arm goes under the axle and very near the tire and you lose some clearance.

[bart]

Allen- More droop? How?

Blaine- I think we are slightly misunderstanding one another. I plan on making triangulated uppers on my new axle similar to how Currie does. I intend to remake my frame mounts in about the same location as stock, but angle the bracket to take the side loading off of the joint. Option 1 is to make new lower arms that are straight and slightly longer than the triangulated arms. I will make new axle brackets similar to what ChrisO did, but not quite that high. The frame bracket would be a new one mounted forward of the stock one and as close to the frame as would let the joint work properly. Option 2 is Currie J arms used in the normal fashion. Why do you say I cannot make my lowers the same length as my uppers?


You can move your mounts like I intend to and not alter wheelbase. Did you think I was going to use factory mounts and just make the arms longer?

Which makes the J arms act like a longer arm. My longer straight arm, as described, would also make it more parallel to the ground at ride height.

By more clearance in front of the tire, I mean on the frame. Is that not why J arms were made, to have less to hang up on? A typical long arm, like my option1, is moved farther forward on the frame and gives you something else to have to get over a rock before your tire hits. I know the J arms gives you less clearance under the tube and also that they are jointed on both ends and can move out of the way most of the time (do they bang around a lot?).

So I guess my original question was, is it better to have clearance under the tube or at the frame in front of the tire? And are there any advantages to J arms ovr regular "long" arms (about the same effective length) than said clearance? Clear as mud?

-Bart

[mrblaine]

Originally posted by bart
Allen- More droop? How?

Blaine- I think we are slightly misunderstanding one another. I plan on making triangulated uppers on my new axle similar to how Currie does. I intend to remake my frame mounts in about the same location as stock, but angle the bracket to take the side loading off of the joint. Option 1 is to make new lower arms that are straight and slightly longer than the triangulated arms. I will make new axle brackets similar to what ChrisO did, but not quite that high. The frame bracket would be a new one mounted forward of the stock one and as close to the frame as would let the joint work properly. Option 2 is Currie J arms used in the normal fashion. Why do you say I cannot make my lowers the same length as my uppers?

Effective length versus actual is not the same with triangulation. The triangulated arm has to be longer to span the same front to rear distance because it is angled. If an arm went from one side of the frame to the other but only shifted it's rear connection point by a difference of 1", it has an effective length of 1" even though the arm is 36" long or so.

You can move your mounts like I intend to and not alter wheelbase. Did you think I was going to use factory mounts and just make the arms longer?

Which makes the J arms act like a longer arm. My longer straight arm, as described, would also make it more parallel to the ground at ride height.

By more clearance in front of the tire, I mean on the frame. Is that not why J arms were made, to have less to hang up on? A typical long arm, like my option1, is moved farther forward on the frame and gives you something else to have to get over a rock before your tire hits. I know the J arms gives you less clearance under the tube and also that they are jointed on both ends and can move out of the way most of the time (do they bang around a lot?).

So I guess my original question was, is it better to have clearance under the tube or at the frame in front of the tire? And are there any advantages to J arms ovr regular "long" arms (about the same effective length) than said clearance? Clear as mud?

-Bart

We strive for clearance under the tub. The less things you have to hang up on the better off you are. I have never had my arms bang around. I suppose it's possible, but they almost always are loaded and that really limits any freeplay that could occur if you let the joints wear out by not keeping them greased.

The only advantage the J-arm has over the LA is minimalistic install effort. You're not gonna get the same performance out of the J-arm that you will the long arm, but you will get much better performance that any short arm. Sort of in the middle with very easy install effort.

[bart]

Originally posted by mrblaine
Effective length versus actual is not the same with triangulation.

The only advantage the J-arm has over the LA is minimalistic install effort. You're not gonna get the same performance out of the J-arm that you will the long arm, but you will get much better performance that any short arm. Sort of in the middle with very easy install effort.

Good point on triangulation. I need to figure on that. Curriwe uses the stock frame mount location, right? And they effectively lengthen thier lowers. I think that the upper arms are lengthened due to the angle (effectively), but I don't think it is as much as the actual lengthening. I would cite the path of the axle during articulation for this.

Minimal install effort and more frame clearance in front of the tire. How much more minimal is relocating the axle mount than relocating the frame mount for your long arm? Seems the same to me, as most LAs use the factory axle mount. And my Option 1 lowers are not going to be as long as REs or anything, they will be sort of mid-length too.

-Bart