4-Link rear air ride question

[NPloysa]

On Kelderman's site they have two versions of the rear air ride, one under the Dodge category and if you go to the price page and click on the link for the Dodge Short Box and Long box pic.



On the latter one, it seems like both arms are the same length some what like this:



0---------0



0---------0





Got me so far? That is with the arms level, say bags 50% full.



Now, when the bags increase or decrease pressure, the arms change angle, shifting the axle back and forth, correct? They look like this:



0

. . \

... \

... . 0



0

. . \

... \

... . 0



Do not pay attention to the periods in the last illustration, nor the extreme exagerated angle of the arms. I used them to position the backspaces since it moved everthing over. Just pay attention the the 0's which are the ends of the arms and the lines, --- or \ which are the arms, parallel to ground and angled to the ground, respectively.



How does that not change the axle position?



Thanks, Nick

 

[bmoeller]

Haven't used/seen the setup in person, but if it acts the same as a parallel linkage used on some farm equipment (which it should), it will move it forward or backwards a little bit in the up/downward travel. It will, if the change in height is enough, it will also slightly change the pinion angle. Should be very little of either in this case though. Most of the movement is in the most extreme up/downwards postions. Won't have that in this situation.

 

[SRadke]

The travel simply is not great enough to allow that much axle travel forth and aft. You're right though, in an extreme situation it would move and the amout it moves per height change is dependant on the length of the arms.



-Scott

 

[Turbo Thom]

Scott is right is that the amount the the axle would want to move is is congruent to the distance the 4 links would travel in a radius. Just remember the pinion angle will change with a greater height addition. I had a fellow here want to install a 4 link system into a 1500 for drag racing. He was wanting to shift/transfer weight to the rear at the line. Air up the front and dump the rear. That will work, and the force will be transfered to the bump stops in the bags. But remember the pinion angle will change. That is the beauty of the 4 link system. The pinion angle can be change at will. What he will need to do is at home, set up the truck up with out rear air and set the angle. Then set the angle with road ride air heigth. Drive to the track and then set the known pinion angle without rear air. Drive around all day like that. When you get ready to hit the road, adjust it back and head home.



I don't see any other way he could do what he wants to do. Oh yeah, and he's installing a 01 5. 9 Cummins at 550 HP in there also. Must be nice to be 25 and roll in lots of $$$$$$$$.





And another project we're thinking about is cutting a long bed 2500 down for a short bed to fit. A short bed reg cab 2500. I think that would be kinda neat.



. later.



. . Preston. .

 

[NPloysa]

Thanks guys. One other question. On the other type of 4-link, the one listed under the Dodge application page, uses the same axle mount system, but the bottom arm is much longer, maybe by 18" or so, would that keep the axle centered?



Nick

 

[CIverson]

The more parallel the arms are to each other the less caster and pininon angle change throughout the travel.



Also the longer the arms and the more parallel they are to the frame the less change in forward and backward movement of the axle when the suspension extends and compresses.

 

[NVR FNSH]

Equal length & parallel arms do not change the pinion angle. Any deviation from equal length & parallel will result in a changing pinion angle as the suspension is cycled.



Brian

 

[NPloysa]

Originally posted by NVR FNSH

Equal length & parallel arms do not change the pinion angle. Any deviation from equal length & parallel will result in a changing pinion angle as the suspension is cycled.



Brian

So a longer arm on the bottom would have a longer radius therefore keeping the axle centered more than parallel arms moving the same amount up and down... ?



Nick

 

[SRadke]

No, that's not what he said. If you desire no change in pinion angle the arms must be the same lenth. If one is longer it will swing a larger radius and cause the axle to twist (change pinion angle).



If all you desire is to keep the axle as centered as possible through a longer suspension travel you have to make the arms longer.



It's a bit extreme compared to what we're talking about here but notice how long the arms are on these trucks:



#ad




They use longer arms because you will get the best ride when the arms are level but because making backets to allow for that in this case is rediculouse the next best thing is to move the pivots ahead and lengthen the arms to make them closer to level. Clear as mud?



-Scott

 

[SRadke]

Ok so I looked a little closer at the picture and they aren't using a four link system in the rear but what I said is still true.



-Scott

 

[Turbo Thom]

Scott:

A very good explaination.



. . Preston. .

 

[BigBadDodge]

http://www.rpmnet.com/techart/4link.shtml



BBD

 

[NPloysa]

Thanks BBD, I PMed you also.