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post #31 of 39 Old 12-18-2012, 03:57 PM
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Originally Posted by ftgiles View Post
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.

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post #32 of 39 Old 12-18-2012, 04:25 PM
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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.
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post #33 of 39 Old 12-18-2012, 05:52 PM
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Originally Posted by Necromancer_tat View Post
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.
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.
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post #34 of 39 Old 12-18-2012, 06:18 PM
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Originally Posted by ftgiles View Post
You're information is correct. The forces against the control arms are resultant forces that originate at the tire's contact patch.

Draw a line vertical from the tire's contact patch through the lower control arm pivot point and then through 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.

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.
Thanks for breaking that down a bit for me, it helps to be able to visualize it in my head better! I'm assuming that since the bushing material would be the same when using the same size links, that a larger separation distance will allow said bushings better control over the axle, and using the same logic in reverse I would assume once again that reducing the link separation on the axle side would require larger arms/joints/bushings (Read: Stronger) to retain the same control that larger link separation achieves.
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post #35 of 39 Old 12-18-2012, 07:01 PM
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Originally Posted by Necromancer_tat View Post
Thanks for breaking that down a bit for me, it helps to be able to visualize it in my head better! I'm assuming that since the bushing material would be the same when using the same size links, that a larger separation distance will allow said bushings better control over the axle, and using the same logic in reverse I would assume once again that reducing the link separation on the axle side would require larger arms/joints/bushings (Read: Stronger) to retain the same control that larger link separation achieves.
You've got it. Here's a quick picture to plug some numbers into. Put stock tire and control arm distances in and then compare your proposed. Very revealing.


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post #36 of 39 Old 12-19-2012, 04:27 PM
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You've got it. Here's a quick picture to plug some numbers into. Put stock tire and control arm distances in and then compare your proposed. Very revealing.


Thanks for the diagram and equation! After looking at it and thinking a bit, I now have several other questions and ideas. I'm gonna do a little Googling and see what I can figure out, but I might do some drawings in MS Paint and make another thread instead of steering this one off course.
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post #37 of 39 Old 01-05-2016, 03:08 PM
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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.
Absolutely agree based on my personal experience. I have short arms run with no lift than the same arms run with 3,5 inch lift. No axle mouts modifications no frame mounts modification. Point is after SYE a nd CV instal than pinion angle set up the LCA mounts are pretty higher and UCA mounts a bit backwards. I DOES change the forces applied from tire grip to them in relation ship to the rest of the suspension and body. I can experienced premature fail of UCA joints(stock mounts allow to use 2inch joints) and to solve this need to strenght this part of suspension. So if you want to relocate stock mounts at the axle the separation is only one thing to consider. To use the same size of joints like LCA for the UCA is a good point
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post #38 of 39 Old 01-05-2016, 03:34 PM
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Originally Posted by VladoTJ View Post
Absolutely agree based on my personal experience. I have short arms run with no lift than the same arms run with 3,5 inch lift. No axle mouts modifications no frame mounts modification. Point is after SYE a nd CV instal than pinion angle set up the LCA mounts are pretty higher and UCA mounts a bit backwards. I DOES change the forces applied from tire grip to them in relation ship to the rest of the suspension and body. I can experienced premature fail of UCA joints(stock mounts allow to use 2inch joints) and to solve this need to strenght this part of suspension. So if you want to relocate stock mounts at the axle the separation is only one thing to consider. To use the same size of joints like LCA for the UCA is a good point
It is a triangle so it depens on where the force is coming from and where the force is applied respectively how is transfered. To establish this is not that easy as suspension geometry calculator work. You need to take into account tire size and weight, engine torgue or gears in other words if you put a bigger tires and change the gears to get a stock tire/stock gears performance with stock size of UCA joints and 3,5 ich short arm lift you will end up finding them as a weak point. If you put axle mounts even higher of the axle tube center line the force transfer to the UCA will be even more dramatic no matter you keep the separation. That is I deal with now I was strongly considering high clearence CA mounting with better geometry. I ended up with using the same size of Duroflex joints for lowers and uppers concurently with axle mounts relocation and I am still not confident about symetrical or nearly symetrical rotation force transfer between uppers and lowers BUT time will prove.
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post #39 of 39 Old 01-05-2016, 03:58 PM
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Originally Posted by VladoTJ View Post
Absolutely agree based on my personal experience. I have short arms run with no lift than the same arms run with 3,5 inch lift. No axle mouts modifications no frame mounts modification. Point is after SYE a nd CV instal than pinion angle set up the LCA mounts are pretty higher and UCA mounts a bit backwards. I DOES change the forces applied from tire grip to them in relationship to the rest of the suspension and body. I can experienced premature fail of UCA joints(stock mounts allow to use 2inch joints) and to solve this need to strenght this part of suspension. So if you want to relocate stock mounts at the axle the separation is only one thing to consider. You can increase the separation but who wants? To use the same size of joints like LCA for the UCA is a good point
It depens on where the force is coming from and where the force is applied respectively how is transfered. To establish this is not that easy as suspension geometry calculator work. You need to take into account tire size and weight, engine torgue or gears in other words if you put a bigger tires and change the gears to get a stock tire/stock gears performance with stock size of UCA joints and 3,5 ich short arm lift you will end up finding them as a weak point. If you put axle mounts even higher of the axle tube center line the force transfer to the UCA will be even more dramatic no matter you keep the separation. You need either to increase the separation or use larger UCA joints to deal with it. That is I deal with now I was strongly considering high clearence CA mounting with better geometry. I ended up with using the same size of Duroflex joints for lowers and uppers concurently with axle mounts relocation and I am still not confident that rotation force transfer between uppers and lowers will wear out the joints symetrically BUT time will prove.
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