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short arm 3-link?

18K views 41 replies 18 participants last post by  astjp2 
#1 ·
so here is my dilemma:

I need more height. I need to get some more air under the belly. AND I need to get rid of the factory 4link+panhard

I am currently running currie 4.5" coils that give me about 4" of lift over stock. the plan is to sell them and put in a set of synergy suspension (polyperf) 6" coils.

once I have the 6" coils in, I will need to deal with the fact that my CAs will be at an extremely steep angle. right now, the arms are at a pretty good angle, not too steep. I really like my short arms, they travel just fine, I can easily travel a 12" shock with them and they are never hung up on anything. the bigger problem is the inherent bind in the factory suspension.

the answer is a 3-link.

the upper link is easy: cut the factory passenger side mount off the axle, grab a 8" link tower from Ruffstuff or the like, weld on, plate the inside of the unibody and weld on a link bracket. make a link with a pair of 2.5" JJs and be done.

the lowers are another story.

I've been thinking about this a lot recently, and an Idea popped into my brain.
why not cut the LCA mounts off the axle, and move them up flush with the tube and keep the frame side mounts where they are?

that will do a couple things for me:
it will lessen the angle of the CAs with the extra lift height, which is good
since my shock mounts are attached to the lower CA mount it will move them up~2" giving me back the same travel numbers I started with (5"up,6.5"down)
it means I don't have to weld on new lower mounts and make new lower arms, it keeps my arms short so that I don't get hung up on them.
more ground clearance at axle.

some notes:
I will be stepping up to 35"s as well and this will give me the extra fender room I need without loosing any travel.
this must be a streetable rig still, its not a daily driver anymore but it still will see a lot of road miles
this rig will stay full-bodied

so am I retarded or what? thoughts?

here are a few pics of what I currently have for perspective.
LCA mount and shock mount, well below the tube


CA angle at ride height


max droop
 
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#2 ·
Someone a good bit more tech savvy than me will jump on this soon, but here's the problem as I see it: the reason the LCA brackets are below the tube is they provide anti-rotational stability (insert correct term here.) When you have both upper and lower brackets above the axis of rotation (axle shafts) the axle will want to rotate rather than transfer the power to the ground.

Everyone would love to have the extra clearance of having their LCA brackets above the tube, this is the reason no one does it.
 
#3 ·
Someone a good bit more tech savvy than me will jump on this soon, but here's the problem as I see it: the reason the LCA brackets are below the tube is they provide anti-rotational stability (insert correct term here.) When you have both upper and lower brackets above the axis of rotation (axle shafts) the axle will want to rotate rather than transfer the power to the ground.

Everyone would love to have the extra clearance of having their LCA brackets above the tube, this is the reason no one does it.
That's incorrect. The lower control arms will do their job even if they're above the axle centerline. As long as the upper is above the lower, the axle will be constrained. I think you were on the right track in regards to the leverage the control arms have over the rotational forces acting on the housing. The further the mounts are from the centerline, the more leverage the arms have on the housing. The less leverage the arms have, the more stresses are placed on the joints so you've got to use good stuff. Currie joints can handle it just fine.

Climbit, as long as you have more separation at the axle than at the frame, you'll be fine. With the lowers mounted flush with the centerline, the upper will need to be a couple inches higher than they currently are. I run my lowers flush at both ends with about 8" of vertical separation at the axles and 6" at the frame. Obviously you shouldn't just rely on general numbers like that but a 75% ratio is a good ballpark number to shoot for. Build the mounts strong and use good ends and you'll wind up being very happy with it. Feel free to keep the discussion going, it's a good topic.
 
#5 ·
A friend of mine welded my axles up when I did my 3/4 link. I wanted to make the lower arm mounts on the axle even with the axle also. He told me to run them slightly down. His reasoning was more for strength of the mount. They are high enough that they don't really hang up on anything so I am happy. I trust his judgement his builds survive rock and endurance races.
 
#7 ·
i have seen a couple vehicles with the lower CA mounts even with/ above the tube and they rotated enough to cause damage. on level ground it is fine but with power applied with a good amount of suspension compression it allowed too much rotation. now i have no figures to plug into any calc and i wasnt about to run under there and start measuring but it can happen. im sure the placement of the other mounts had a lot to do with it as well. just keep in mind that a lot more is going on under there when wheeling then is easily imagined while building. mine are 45* down and they are still higher then the dana 60 inner c that they are snugged up to so i dont think im loosing any valuable clearance.

if you are going with a larger tire you can afford a longer control arm with respect to issues of hanging up on things. i personally dont understand the paranoia over having completely flat control arms that has been going around lately particularly in the jk world. there are always compromises to be made and i dont think what it takes to run flat arms is worth what it takes to make it happen.
 
#9 ·
i have seen a couple vehicles with the lower CA mounts even with/ above the tube and they rotated enough to cause damage.
The axle housing can't physically rotate unless the arm bends, the joints deflect or the mounts move.
on level ground it is fine but with power applied with a good amount of suspension compression it allowed too much rotation.
See above. The only time I've ever had movement was when my old truss (fairly weak design) bent. The mounts, arms and joints were all fine.
i personally dont understand the paranoia over having completely flat control arms that has been going around lately particularly in the jk world. there are always compromises to be made and i dont think what it takes to run flat arms is worth what it takes to make it happen.
The flatter the arms, generally the more neutral the roll axis angle is. This results in minimal unwanted steering input while articulating. And in theory, the ride quality is optimized but personally, I can't tell a difference between arms sitting at 5 degrees and those sitting at 15 degrees and I consider my senses pretty good at pickup the small differences. All in all, flatter arms perform better than steeper arms. The former is the real advantage to be gained. The latter is what most gullible JK owners worry about. It's really not tough to make it happen--don't jack the rig up to the sky and keep the axle mounts tucked up where they belong.
 
#8 ·
I run my rear lower link mounts level with the axle centerline with absolutely no issues with the axle twisting or breaking mounts. I run 6.5" of separation at the frame and 8" of separation at the axle. As long as you move the upper CA mount up to maintain roughly 8" of separation you shouldn't have any problems with axle rotation. Build the mounts to handle the increased forces and use joints that can handle the load (I run 1.25" Ruffstuff rod ends, 2.5" JJs will work as well).

It's very nice having nothing hang below the axle tube. Know where your pumpkins are and you very rarely get hung up on the rocks.
 
#11 ·
i disagree saw it myself. if i remember right the uppers, that had plenty of separation were set straight up and down with a pinion angle of 0 or before the pinion was set. with the pinion set the upper tower had a very large lean equalling a upper link that was pretty long. couple this with a lower link bracket that was high(honestly dont remember if it was even or above axle tubing) and it can happen. ill state again and clarify the placement of the link mounting other then the axle LCA had a lot to do with it. just having enough "separation" at the axle end is not enough. my point was simply to be weary of all the elements and do thorough travel tests.
 
#15 ·
methinks that something else is going on in those suspensions. if you have 6-8" of vertical separation, it does not matter where your lowers are located, the upper will control all rotational forces.

now if the upper link fails, or there is too much slop in the joints (why I don't like bushings) then you have problems.
 
#12 ·
Well, I've been around plenty of rigs with lower mounts above the axle centerline--most of those are Ultra 4 cars. The housings are held rock solid, as they should be. It makes no logical sense for there to be any movement unless there is that much movement somewhere else in the system. Now in your case, if there wasn't enough triangulation to keep the housing laterally constrained then I can understand what you're describing.

And considering my personal rig has all lower arms at the axle centerline and the housings don't budge....why would I have done that if it wouldn't have prevented the housings from rolling? Take a look at just about any custom suspension--the mounts are normally tucked up. It's a very common practice in order to minimize the angle of the arms and to improve clearance. If there was any decrease in the ability to constrain the housing's rotation, it wouldn't be nearly as common.

Lower mounts above the axle centerline


Think about this--what would happen if you put a jack under the pinion and went to town? It wouldn't rotate anymore than if the lowers were 1" below the axle. It's constrained in that axis exactly the same way.

Lower mounts behind and flush with the axle tube.....
 
#18 ·
no i agree with imped that you can build with the lowers above the axle tubes and never meant to imply otherwise. i just wanted to state a potential hazard ive seen if not paying particular attention to the mounting of the others. where i think we are crossing ideas is im not saying the axle tube somehow magically rotates. just having uppers with enough separation is not the key to having a pinion angle that remains perfect.


this picture is not the vehicle im referring to but the upper link placement looks similar. if you were to move the lowers up above the axle tube and you were to compress the suspension, if allowed to compress enough the pinion angle would exaggerate and you could see some issues. the angle of the uppers is important. the one i saw tore the pinion yoke off. this is simply what i was referring to.
 

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#21 ·
Using a kinematic model is fine as a first step to understanding some of the geometry of vehicle suspensions. A simple link calculator which is geometric or kinematics based like the ones you see referenced all the time in the 4x4 forums is not very revealing when it comes time to actually building a suspension. Taking the springs out and moving the axle around is kinematics. The way a vehicle’s suspension works is based on a “Force” model and not a kinematic model. A force based model does not necessarily follow the principle of instantaneous centers because the magnitude and direction of forces are ever-changing. The static numbers calculated bears little resemblance to the dynamics of a vehicle’s suspension. Geometry is only a part. Stiffness distribution between the front and rear, wheel rate, corner weight, shocks, and anti-sway bars all affect the forces seen by the geometry. None of which are accounted for in a geometric calculator.

Changing the axle control arm mount locations relative to the centerline of the axle does indeed have an effect on the forces seen by the control arms. Moving the lower control arm up on the axle increases the force it sees because of the gain in leverage relative to the tire's contact patch. Simply stated, the joints and mounts on both ends must be strengthened. If some of the separation of the links at the axle is lost, the forces seen by both the upper and lower arms will increase. That means bigger joints and mounts on both ends.

If the separation at the axle is maintained or increased, then the same or smaller upper joints and mounts can be used, but the way the mount interfaces with the axle will have to be stronger. This is the common failure point when extending the upper mounts to just maintain the existing sseparation. Locating the upper arm on the diff side helps in creating a stronger interface.

If raising the axle end of the lower control arm increases the amount of force it sees, and the force is transferred to the frame/body, then the suspension dynamics change. The same thing happens with an off-center three-link. More force is carried by a single arm, instead of two, and deposited asymmetrically. That affects the suspension dynamics and requires stronger joints and mounts, too.

Changing the angle of the arms in relation to each other or changing the pivot points or the length of the arms will all have an effect on pinion control in terms of degrees and the rate of change.

These are just some things to think about and not pointing out anything specifically wrong. Everything you do to your suspension has an effect and it’s usually a compromise with some other characteristics. If you can’t find the compromises then you’re not ready to make the changes. The rock-grabbing lower control arm mounts were not an oversight by some lazy engineers.
 
#31 ·
If the separation at the axle is maintained or increased, then the same or smaller upper joints and mounts can be used, but the way the mount interfaces with the axle will have to be stronger. This is the common failure point when extending the upper mounts to just maintain the existing sseparation. Locating the upper arm on the diff side helps in creating a stronger interface.
Thanks. I always (apparently incorrectly) thought it was to hold the mass at it's heaviest point, the way you explained it makes sense though.
 
#22 ·
As well I am watching. Very interesting. My opinion is it would work either way, but if the lower arms are mounted at or above the center line of the axle they act as a hinge point during axle torque (rotation) thus putting all the rotational force on the upper arms. All the front to back force (under power & braking) would be on the lower arms and now the upper arms act as a hinge point. If the lower arms are located below the center of the axle it spreads ALL the force out more equally. If the upper and lower arms are equally spaced from the center line of axle then you have equal forces on all arms. This is the strongest method, but yes it will compromise ground clearance at the axle. Just my thoughts... Have fun!!
 
#24 ·
I know that by raising the lowers up to the centerline of the axle, all of the rotational force of the axle is then controlled by the upper. thats pretty well known.

thats why the upper needs to be strong as possible. hence me wanting to run 2.5" joints (probably 1.25" heims) and significatly increasing the strength of the mounts. however, by eliminating any locational (is that a word?) forces from it, you reduce its overall load.
 
#26 ·
I know that by raising the lowers up to the centerline of the axle, all of the rotational force of the axle is then controlled by the upper. thats pretty well known.
There is no such thing as a rotational force. Force travels in a straight line. Torque on the other hand is a product of force and lever length causing an object to move about its axis. If there is no axis, there is no torque. Take away the lower arm and now the upper arm becomes an axis. Both arms together remove an axis possibility. Both arms work together to control the force from the tire's contact patch. The direction of force each arm reacts to depends on their orientation with each other. The magnitude of force each arm reacts to depends on the length of the lever being acted through.
 
#28 ·
what I MEANT was that with the lowers even with the centerline of the axle, the upper no longer does any forward or backward locating of the axle, its purpose is to keep the axle from rotating around the lowers.... and to set the pinion angle

sheesh tough room ;)
 
#29 ·
No matter what, the control arms are always locating and constraining the housing. Depending on the situation, the arms are always in a state of tension or compression. If that wasn't the case and I followed your train of thought, I could remove my uppers and my housing wouldn't roll. That's most definitely not the case. And as for pinion angle, I use both sets of arms for that..... in fact, the lowers have a larger impact than the uppers due to their proximity to the axle center line.
 
#30 ·
thats not what I meant.

once you move the lowers up even with the centerline of the axle, you GREATLY reduce the amount of forward/backward force that the upper controls, and you eliminate the amount of rotational force they control. the force is not gone, but the amount of input those links have on that force changes.

as far as pinion location goes, of course both links are used to set the pinion angle, but in practice, the wheelbase is set with the lowers, the pinion with the uppers, then fine adjustments are made to both links to get the axle in the exact place you want with the right angle.
 
#32 ·
I'm also very interested in this topic as I'm doing research for my own build upgrades. I have seen elsewhere it suggested that the link separation at the axle side be a certain percentage of the intended tire diameter in order for them to control the axle effectively. I believe it was close to 25% if I remember right. I'll try and see if I can find the link and post it here. If it's correct a 40" tire should have a link separation of 10" at the axle, but perhaps using bigger joints and stronger mounts and control arms could reduce that amount? If that ratio is accurate then a 35" tire would require 8.75" of separation.
 
#33 ·
You're information is correct. The forces against the control arms are resultant forces that originate from the tire's contact patch.

Draw a straight line vertical from the tire's contact patch through/by the lower control arm pivot point and then through/by the upper arm pivot point. That's the lever that needs to be resolved to understand the forces. And that leverage changes with tire size AND control arm placement.

The force generated at the contact patch is magnified by the lever length between the ground and the lower control arm mount, which is the fulcrum. Then the tire's contact force is magnified in the opposite direction by the length of the lever that exists between the fulcrum and the upper arm's pivot point. The force at the fulcrum is the sum of the force at the contact patch and the force at the upper control arm.

This is a first class lever and follows "lever law" in its theory and calculation. Simple stuff once you define the lever.

If you play with the lever lengths that are affected by moving the control arm mounts and do the force calculations you'll see installing larger tires in combination with moving the control arm mounts further from the ground has a huge impact on the forces seen by the control arms and then transferred into the frame/body. That means a bigger effect on the same anti-squat percentage because that is more force being input, for example. The same anti-squat percentage will behave differently and probably need to be reduced in order to make the vehicle dynamics act in a similar fashion as with smaller tires and lower control arm mounts.
 
#40 ·
its good to be back0:) thanks imped,its been a long time since refreshing my brain felt so good..all you guys,this was a good lesson for anyone to learn about.all inout was a great read,since ive been out of touch for a few yrs.
j9
 
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