Suspension travel upgrades on an 80

[SUMOTOY]

There is no problem with my drawing - ask any of the other people on the thread. Ask an engineer.

I can pick any point on the radius arm, or the axle, or even the tire, and as I track that point as the suspension moves, it's going to draw a circle centered on the frame mount - just as I've drawn. Can't we get past this point.

Do a close up of the axle mount. Your drawing shows it intersects the axle centerline. I say it never will unless you can draw a straight line from the frame pivot centerline, thru axle mount 1 centerline, thru axle centerline, thru axle mount 2 centerline.

Drawing is not correct. And given the dimension of x, I suspect it doesn't require a real closeup drawing. Try measuring your truck and solve for x.

SJ

 

[Walking Eagle]

Drawing is not correct. And given the dimension of x, I suspect it doesn't require a real closeup drawing. Try measuring your truck and solve for x.

SJ

Still not getting through to Sumo -

but hey, I'm starting to see a trend....

Man... this has been quite ammusing but the sad truth is that the ONLY 2 points in space that matter are the center of the pivot and the reference point in space I.E. the center of the axle.

SUMO, the arcs have the same dimensions (an arc is defined by its radius and angle, no?) but they are in different locations.

The paths are different.

Puuuhhleeezz recognize this fundamental fact of nature.

It doesn't matter what shape or where the arms attach to the axle the tires will follow an arc that's radius is equal to the distance of the rear pivot point to the axles center.

Sumo - How can anything rotate around a point at a radius of anything other than the distance from it (the axle) to said point of rotation (frame mount)? So, it rotates, in a perfect circle, but the distance from it to the center of the circle is something other than the radius of the circle? See how that doesn't make sense? Or is it rotating in something other than a perfect circle? With a single point of mounting / rotation, it really can't do anything but rotate in a circle! A four bar linkage, two of which are used on a four-link or 5 link rear suspension (sounds wierd I know), will give you a motion that isn't a circle (http://www.mines.edu/fs_home/dgolson/mce302-1.html or http://www.mines.edu/fs_home/dgolson/mce302-1.html). But not the front suspension on an 80 series.

If the axle is rotating (it is), it has to rotate around a point (the frame mount), and if it's rotating around that point, it has to stay equidistant from that point through it's whole rotation. So, the radius the axle rotates in, has to be the distance from it, to the axle mount, which is consistant through the whole rotation. I don't know anymore ways of explaining the same thing.

 

[landtank]

If the axle is rotating (it is), it has to rotate around a point (the frame mount), and if it's rotating around that point, it has to stay equidistant from that point through it's whole rotation. So, the radius the axle rotates in, has to be the distance from it, to the axle mount, which is consistant through the whole rotation. I don't know anymore ways of explaining the same thing.

While this is true in relation to the frame mount it doesn't mean that the axle will remain perpendicular to the frame.

Scenario 1:

The leading arms are horizontal when in a static position. If one was to flex the axle so that one side raised an equal amount to the other side being dropped then the distance on both sides of the axle horizonatally to the frame mount would be less and equal.

Scenario 2:

The leading arms are angled down in a static position. If one was to flex the axle so that one side raised an equal amount to the other side being dropped then the axle would be at a angle to the frame. This is do to the fact that when the side being raised moves through it's arc the axle's position horizontally to the frame mount is lengthened while the side that is dropping is moving horizontally closer to the frame mount. This would cause the axle to have a angle to it in relation to the frame.

This is called bump steer where the angle produced by going over a bump will cause the vehicle to turn. I'm not sure if this is what Sumotoy is getting at but it is of little or no concern to a lifted 80.

 

[T Y L E R]

If one was to flex the axle so that one side raised an equal amount to the other side being dropped then the axle would be at a angle to the frame. This is do to the fact that when the side being raised moves through it's arc the axle's position horizontally to the frame mount is lengthened while the side that is dropping is moving horizontally closer to the frame mount. This would cause the axle to have a angle to it in relation to the frame.

Rick, can you flesh this out a bit I'm trying to visualize/understand this lengthening.


Thanx,



TY

 

[Hayes]

Rick, can you flesh this out a bit I'm trying to visualize/understand this lengthening.

TY

Think about how the axle doesn't just move up and down, it also moves a little bit fore (comressed up) and aft (drooped down) as it travels along its arcs.

Now think about what would happen if one side of your axle was moved forward while one side was moved back.

Hayes

 

[landtank]

Ty, imagine that the axle is in a position where it is horizontally aligned (level) to the mounting bracket (pivot point). The horizontal distance would be 33" (there abouts). Now imagine that the axle is in a 45* position to the mounting point, the horizontal distance would then be half the length of the arm or 16.5". Now from that 45* position if you move the axle up it would lengthen the horizontal distance and if you were to further drop the arm it would shorten it.

This only happens when the arms hold the axle in a static position where the axle is not level with the pivot point. When the axle is held level to the pivot point it is already at it's max horizontal distance and all moves whether up or down will shorten the horizontal distance keeping the axle straight or perpendicular to the frame.

There are some factors that govern the effect of this on the driver.

1. the amount of bump steer induced
2. wheel base, the longer the wheel base the less the effect has on course change
3. the speed your driving.

With all these things considered you would typically be going rather slow to flex the axle enough to create enough bump steer to alter your path and it would be a natural course correction for you to deal with.

By chance if you did flex that axle enough at a speed that you would notice the course change you are probably already out of control.

 

[spressomon]

Hey, if this goes on a couple more weeks we will have a complete 80 suspension drawn up

Or two forums...80 & 100...melted down

 

[T Y L E R]

Thanks Hayes, Rick ..

I haev another query now though :

The horizontal distance would be 33" (there abouts). Now imagine that the axle is in a 45* position to the mounting point, the horizontal distance would then be half the length of the arm or 16.5". .

Rick it's been a long time since I took geometry so bear with me. Are you saying when the arm swings down to 45° that the horizontal distance from hub to mounting pivot point would now be 'half' of the original horizontal component? That doesn't make sense to me if I have that much correct. It seems to me that the horizontal measure @ 45° would not be halved, but would only lose a fraction of that ... I was thinking it would have to drop down to 60° or something to be halved.


What am I missing?


TY

 

[Walking Eagle]

You can look at it this way. As the arm goes down it also goes back. If the opposite side doesn't move at all, the axle will twist. The side of the axle that goes down also comes back to some degree, and the axle is no longer perpendicular to the frame - and it starts to steer the truck a little. The flex in the bushings is what allows this slight misalignment. My dwgs probably over exagurate the effect. The alxe is 24" from the pivot point and the dip is 6.25" - so it's exagurated.

This is about where Sumo will (or should) pipe in and say that if the passenger side moves up from the horizontal state (though he and I will argue if horizontal is defined by the axle to pivot relation, or some imagnary point to the pivot), it too will move backward. and if the bump on the passenger side, makes that tire go up the same amount the pot hole makes the driver's side go down, the the axle will straighten out - though back a bit from static - as you look at it from above. (like Land Tank was saying). From the front of course it will be rotated clockwise.

Now - if I add a lift to the rig, and don't drop the pivot point, the arms are going to point down. So when they move up they're going to move forward not backward. And when they move down they will move backward more severly. The bump steer effect will be more. This is also what Land Tank said. I'm just adding on, and trying to help everyone understand - and I like dwg. pretty pictures. More is kinda a subjective term. It may be 1/10 of a inch or 2", I don't know - haven't gone to the point of calculating it all out, and even if I did, I'm not sure how much effect the front axle momentaryily not being square to the frame has on the steering. You might see 1/2" or more and never notice it when driving. Or a 1/4"' could make it undriveable - I'm not going to comment one way or the other, cause I don't know.

You can account for this though. You can lower your frame mounts, get the arms more level and minimize the front to back motion that way. Or, you can add longer arms that don't have as much forward and back motion through the same amount of travel as shorter arms. Or you can do nothing if that's what you find acceptable (just no matter what mods you make or don't make, don't have an accident with my wife and kid).

 

[Hayes]

Thanks Hayes, Rick ..

I haev another query now though :

Rick it's been a long time since I took geometry so bear with me. Are you saying when the arm swings down to 45° that the horizontal distance from hub to mounting pivot point would now be 'half' of the original horizontal component? That doesn't make sense to me if I have that much correct. It seems to me that the horizontal measure @ 45° would not be halved, but would only lose a fraction of that ... I was thinking it would have to drop down to 60° or something to be halved.

What am I missing?

TY

You're geometry has not failed you.
60 degrees would halve the horizontal distance, not 45.

Cosine of 60 degrees.

But you get the idea as far as the bump steer goes.

I think if your leading arm was 60 degrees down from the frame, you'd have some serious problems...

Hayes

 

[Walking Eagle]

Rick it's been a long time since I took geometry so bear with me. Are you saying when the arm swings down to 45° that the horizontal distance from hub to mounting pivot point would now be 'half' of the original horizontal component? That doesn't make sense to me if I have that much correct. It seems to me that the horizontal measure @ 45° would not be halved, but would only lose a fraction of that ... I was thinking it would have to drop down to 60° or something to be halved.


What am I missing?


TY

nothing. I couldn't remember either, so I checked. What he's saying is good, just off by a few degrees

 

[landtank]

sorry about the discrepency guys, I was trying to keep it as simple as possible to help in the understanding as to what I was getting at and I F'd up.

 

[Walking Eagle]

sorry about the discrepency guys, I was trying to keep it as simple as possible to help in the understanding as to what I was getting at and I F'd up.

Not a big deal - at least you're not making stuff up, or convoluting the issue.


Hey, how many posts do we have to have to out do the cheerleader thread?

 

[Hayes]

sorry about the discrepency guys, I was trying to keep it as simple as possible to help in the understanding as to what I was getting at and I F'd up.

Yeah. What the hell were you thinking?!! You could have warped Tyler's sense of trigonometry permenently!!
Some people...







;p



Sorry, just thought this thread could use a little more humor.

Hayes

 

[landtank]

Well the true of the matter is, I'm 48 years old and the last geometry class I had was in 11th grade in high school when I was 16. Senility is setting in I guess.

 

[Hayes]

...and I'm a 31 year old high school science teacher--I get exposed to plenty of the stuff most of us forgot years ago. I also get exposed to plenty of sarcasm and juvenile humor. It's rubbing off I guess.

I hope you detected the sarcasm.

Hayes

 

[landtank]

sure did Hayes, No problem.

 

[landtank]

While here lets move on a little and talk about how this axle angularity can actually improves the handling of our lifted LC over drop brackets or even the stock setup.

Because we've lifted the front and the rear so that the pivot point is higher than the axle, both axles will be at angles during a cornering manuver because of body lean. The arms on the axle side that is on the outside of the turn position the axle closer to the ends of the vehicle while the side on the isde of the turn are moved toward the center of the truck.

So the axles look like this in a right hand turn direction -> \------/. While this is very slight it gives a 4 wheel steering effect and aids in control during normal driving. A benefit over having the axles positioned horizontal to the pivot point where no steering effect would happen.

Now in LxTreme's case (not bashing just an example) where he lowered the LC so the axle is sitting higher than the pivot point his axles respond in the opposite way in a right hand turn direction -> /--------\. This actually creates a condition where the more you turn and increase body roll the more the truck wants to go straight.

As a side note: this is why the shackles on a leaf srung truck are at the outer ends of the springs so this steering effect will help control the vehicle at speeds.

 

[mickey_rubicon]

To improve travel I am running without front and rear swaybars withe an OME lift, assuming that it improves travel. I am not sure if I noticed anything. It has been like this years now, and I have not noticed any changes in the street performance.

 

[landtank]

The steering effect is small and increases with lift height. Like bump steer, it's there an calculable but not necessarily felt