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Unread 12-10-2012, 10:31 AM   #16
mcannon111
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the 3 link calc for a front really aint that accurate, so its not really worth your time to be honest unless you really want to do it. I've never seen a 3 link with panhard not work.

now the 4 link calc is a completely different story.

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Unread 12-10-2012, 10:45 AM   #17
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Quote:
Originally Posted by Climbit View Post
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.
Even with hardly any separation at all, the housing still wouldn't rotate.....the 8" number is generally a good ballpark number for that particular application. The only way you'll see significant rotation is if the links have slip sections like drive shafts.... Breaking it down to the fundamentals like that will help people understand why links are used in the first place over leafs.
Quote:
Originally Posted by mcannon111 View Post
the 3 link calc for a front really aint that accurate, so its not really worth your time to be honest unless you really want to do it. I've never seen a 3 link with panhard not work.

now the 4 link calc is a completely different story.
It's actually not that bad....it uses the same calculations as the rear. You've got to think about the front suspension just as you do the rear only opposite in regards to weight transfer. My front is actually almost a mirror image of my rear, only without an upper link of course. There are definitely ways to screw up a front suspension just as there are with the rear. Viewing it similarly as you view the rear helps to not screw things up.
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Unread 12-10-2012, 04:08 PM   #18
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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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Unread 12-11-2012, 11:39 AM   #19
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So you're talking about pinion tracking during the suspension cycle?

That's something that you need to be conscience about regardless of where you mount the arms.
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Unread 12-11-2012, 05:34 PM   #20
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I am watching this it can be interesting
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Unread 12-12-2012, 09:14 AM   #21
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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.
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Unread 12-12-2012, 10:13 AM   #22
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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!!
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Unread 12-12-2012, 10:23 AM   #23
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Ftgiles.... Sorry! I didn't see your response until after I posted mine. I think we are on the same page but your response is much more educated than mine. I say Ditto to your response...
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Unread 12-12-2012, 12:38 PM   #24
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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.
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Unread 12-12-2012, 12:54 PM   #25
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Agreed..
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Unread 12-12-2012, 01:48 PM   #26
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Quote:
Originally Posted by Climbit View Post
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.
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Unread 12-12-2012, 02:12 PM   #27
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Quote:
Originally Posted by ftgiles View Post
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.
Enter a fine lesson on applied physics.

Most people already know this stuff. They just don't know it.
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Unread 12-12-2012, 02:31 PM   #28
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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

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Unread 12-12-2012, 04:55 PM   #29
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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.
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Unread 12-12-2012, 05:35 PM   #30
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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.
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.
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