gt1guy
Apparently a racist
That makes sense on both counts.
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Linked Suspensions Bible
- Thread starter Treefrog
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- 3 link 4 link suspension tech
That makes sense on both counts.
93blackxj
Well-known member
That's not all engine torque. Right front raised as well. If it was all engine torque right side would compress and left would raise. Since both front shocks extend there is obviously a pitching torque acting on the chassis transmitted through the links. But I do agree a good bit is from the driveline.
93blackxj
Well-known member
What surprised you about the results?
Can you explain the "link torque about chassis"? If 0 torque is no squatting, wouldn't suspension 1 be creating less clockwise torque about CG than 2? Link vertical force at chassis makes sense. Seems like to me torque would be less on suspension 1 since it has less vertical force and squatting more under accel since it is a much lower AS.
Edit: Is the torque value the "reaction" torque from the ground pushing on the tire? I guess this is what you meant in this statement.
And since suspension 1 is squatting more, since some of that load transfer is going through the springs? Is this basically saying that lower AS transfers more force to the ground?
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Treefrog
Book Wheeler
That S2 with AS over 100, based on the torque, still wants to squat.
"Link torque about chassis" is the amount of torque about the sprung CG caused by the forces from the links.
From anti squat values, S1 will squat more. So I would expect more torque on the sprung mass. Which is what the numbers show.
Vertical force is only part of it. There is also the horizontal forces. The math I'm using is perpendicular distance from the link's extended line.
93blackxj
Well-known member
I understand this but I think I am misunderstanding your sign convention. To me, the upper tension force is the negative moment (Clockwise) and lower compression force is positive (CCW). Making the total moment on the CG counter clockwise. I did the numbers myself, and either way you come up with the same value. It just confused me for a minute.
What about the IC location? Suspension 1 IC is located way out front of the CG with 305LB vertical force. Suspension 2 IC is behind the CG with vertical force of 725lbs. That is about the only explanation that I see for suspension 2 to lift
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Treefrog
Book Wheeler
I uploaded the spreadsheet so people can experiment for themselves. I can upload so pictures of high anti with an IC out front later.
93blackxj
Well-known member
Ive played around with it. Something definitely isn't adding up to what I would expect
Treefrog
Book Wheeler
Edit now that I'm not outside hoping to see a tornado: I was not expecting for the torque to support squating at higher of AS values.
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Treefrog
Book Wheeler
TLDR: We can't find the 3d IC the same way we found the 2d one. So we have to look backtrack on the path to getting the 2d IC for a method of finding the 3d IC. Jump to after the line for some discussion on why it is relevant and what it means.
Hopefully, anyone who has made it this far in the bible understands what the IC is. To expand the IC, there is two ways to view it, geometrically and on a body in motion. The first one looks at how the system moves while the second looks at how it is moving. And per the physics definition of IC, it only exists as a point if the motion is planar. If the motion is not planar, it is an instantaneous rotational axis. A body in motion is rotating and sliding along this axis at the point in time. The equations for the instantaneous rotational axis are based around a body in motion. A 2d IC is fairly easy to find. The roll axis not so much. And we want a geometric point about which the axle (or chassis) is orbiting not a line of rotation.
So how do we find this point? Do to so, I'll describe how we find find the 2d IC, a value that is defined by motion, but is often solved statically. To do so, lets take a trip back to Calculus 1. In particular, how we the find the solve the slope of a curve. The slope of a curve can be defined by the rise over run of two points on this curve. As the X distance between these points decreases, the slope gets more accurate. It should be most accurate when the difference in X is 0. But that means we have to divide the rise by a run of zero. So, what we do is use an infinitely small, but not zero, x difference.
Now, lets go back to the static 2d IC. We can apply this same concept to finding an IC. Using the side view of our suspension as an example, we move it up slightly and down slightly and take note of a the location of a point on the axle after movement. These two points and the location of the point at the static location can be used to draw a circle. The center of the circle, is the IC. As the movements get smaller, the IC becomes more accurate. To do this without moving the suspension (run = 0 in the curve example), we take two points that move in a known way. In our case, a circle. With an infinitely small movement on a circle we move in a straight line, the tangent. We also have a line, from our point to the circle center. In an instantaneous image, the tangent applies to any circle with it's center on that line. With 2 lines, the intersection is a point that we can draw 2 circles that meet the known tangents. This intersection is the IC.
Now lets apply this to our 3d system. We move the axle around a little bit and get the center of orbit. In the previous example, we were able to draw a circle. But in 3d space we move about a sphere. Much like a 2d circle can be defined by 3 points, a sphere can be defined by 4. So we can move the axle a bit. One wheel up, then down, and the other wheel up and down. This works with any arbitrary point. The smaller our movements, the better the result.
But what if, like 2d, we take it at points of known movement. We should be able to find it without moving anything. And it just so happens that a linked suspension needs 4 links to be considered constrained. And those 4 links each have a point that moves with the axle. But this does not work. Because the 4 lines never intersect, we cannot find common point of movement for the known points.
So we go back to the slight movements and finding a sphere. This is not too hard. From our 4 points, we make 2 different groups of 3 points and make 2 circles with them. We then take make a line normal to the circles through their centers. Where the two lines intersect is the center of our sphere.
Just like we can see how the suspension moves around the chassis, we can see how the chassis moves around the axle. I wonder if we can treat this like the side view IC and draw lines from the contact patches to figure out a 3d equivalent to anti values. I think that looking at the how the chassis orbits around the axle is likely more important than looking at the inverse since our axles are fixed to the ground. Much like the 2d IC, it is probably a good idea to keep it from switching which end of the vehicle it is on. It seems like keeping the chassis' center of movement as small as possible will help keep the vehicle predictable.
Hopefully, anyone who has made it this far in the bible understands what the IC is. To expand the IC, there is two ways to view it, geometrically and on a body in motion. The first one looks at how the system moves while the second looks at how it is moving. And per the physics definition of IC, it only exists as a point if the motion is planar. If the motion is not planar, it is an instantaneous rotational axis. A body in motion is rotating and sliding along this axis at the point in time. The equations for the instantaneous rotational axis are based around a body in motion. A 2d IC is fairly easy to find. The roll axis not so much. And we want a geometric point about which the axle (or chassis) is orbiting not a line of rotation.
So how do we find this point? Do to so, I'll describe how we find find the 2d IC, a value that is defined by motion, but is often solved statically. To do so, lets take a trip back to Calculus 1. In particular, how we the find the solve the slope of a curve. The slope of a curve can be defined by the rise over run of two points on this curve. As the X distance between these points decreases, the slope gets more accurate. It should be most accurate when the difference in X is 0. But that means we have to divide the rise by a run of zero. So, what we do is use an infinitely small, but not zero, x difference.
Now, lets go back to the static 2d IC. We can apply this same concept to finding an IC. Using the side view of our suspension as an example, we move it up slightly and down slightly and take note of a the location of a point on the axle after movement. These two points and the location of the point at the static location can be used to draw a circle. The center of the circle, is the IC. As the movements get smaller, the IC becomes more accurate. To do this without moving the suspension (run = 0 in the curve example), we take two points that move in a known way. In our case, a circle. With an infinitely small movement on a circle we move in a straight line, the tangent. We also have a line, from our point to the circle center. In an instantaneous image, the tangent applies to any circle with it's center on that line. With 2 lines, the intersection is a point that we can draw 2 circles that meet the known tangents. This intersection is the IC.
Now lets apply this to our 3d system. We move the axle around a little bit and get the center of orbit. In the previous example, we were able to draw a circle. But in 3d space we move about a sphere. Much like a 2d circle can be defined by 3 points, a sphere can be defined by 4. So we can move the axle a bit. One wheel up, then down, and the other wheel up and down. This works with any arbitrary point. The smaller our movements, the better the result.
But what if, like 2d, we take it at points of known movement. We should be able to find it without moving anything. And it just so happens that a linked suspension needs 4 links to be considered constrained. And those 4 links each have a point that moves with the axle. But this does not work. Because the 4 lines never intersect, we cannot find common point of movement for the known points.
So we go back to the slight movements and finding a sphere. This is not too hard. From our 4 points, we make 2 different groups of 3 points and make 2 circles with them. We then take make a line normal to the circles through their centers. Where the two lines intersect is the center of our sphere.
Just like we can see how the suspension moves around the chassis, we can see how the chassis moves around the axle. I wonder if we can treat this like the side view IC and draw lines from the contact patches to figure out a 3d equivalent to anti values. I think that looking at the how the chassis orbits around the axle is likely more important than looking at the inverse since our axles are fixed to the ground. Much like the 2d IC, it is probably a good idea to keep it from switching which end of the vehicle it is on. It seems like keeping the chassis' center of movement as small as possible will help keep the vehicle predictable.
Treefrog
Book Wheeler
As I play around with the code some more, I am finding that which small movements you pick matter a lot. The results for moving the wheels up and down while holding the opposite one steady greatly varies from moving both wheels up and down and moving the wheels in opposite directions. Also, the 2d method of finding the IC should apply to even suspension movement on a symmetric suspension, but it does not seem to line up.
Treefrog
Book Wheeler
So, I should probably share some pictures of what the movement centers look like. So here is a standard front and rear triangulated 4 link. I am pretty sure that the geometry is the default in the 4 link calc. I think the takeaway here is the variety of shapes of surfaces that the axle orbits about. In case the text is too small, yellow is the front.
And here is what the chassis's movement looks like to the axle:
I still do not really understand why the 2D IC and movement centers did not agree. Maybe it has to do the axle not twisting about the 2d IC.
And here is what the chassis's movement looks like to the axle:
I still do not really understand why the 2D IC and movement centers did not agree. Maybe it has to do the axle not twisting about the 2d IC.
Treefrog
Book Wheeler
While rereading this thread wondering if there was anything I had forgotten, learned more about, or had learned enough to figure out, I came across the discussion a few of us had a bit over three years ago regarding portals and antisquat. At the time we had reached the conclusion that portals changed antisquat values, and that the offsetting of the axle tube has an affect. And that was about it.
Even though I barely believe in antisquat anymore, it is still an easy reference value. So I figured this would be a good one to revisit. I wanted to find the antisquat drawing for something with portals. In particular solid axles, though an independent suspension drawing should be easy to find as a offshoot. If we have the drawing, we can get antisquat.
It has occurred to me that the offset does not play any special roll. That is, the links being higher up does not change how we draw things.
Portals change where we start the line that goes to the IC.
It struck me that the key was in looking at why we start the anti line at the wheel hub for independent acceleration and at the contact point for independent braking and solid axle. But we are not going to the wheel hub or contact point, we are going to where the force is applied. But why is the force at the contact patch when reacting to torque? It is because that torque the housing or knuckle reacts to is the acceleration force times a distance from the wheel center. In the case of a ring and pinion gear or brakes, that distance is the tire radius. For an independent suspension the force still exists, but there is no torque. So the distance must be 0.
So we should be able to apply this force and distance approach to portals. The force is fixed in magnitude and direction. Therefore we can only control the distance.
When the gearboxes input and output shafts rotate opposite directions, the housing torque is the sum of the magnitudes in the direction of the output shaft's torque. If they rotate the same direction, the reaction torque is the output - input.
The reaction torque around the ring gear is opposite in direction and equal in magnitude to the axle shaft torque. Combine this with the gearbox torque from above, and deal with some sign stuff and we get equations for equivalent distances.
Solid axles:
Even gear portals: d = 1+2/GR
Odd gear portals: d = 1-2/GR
Positive is above the tire.
Independent:
Even gear portals: d = 1+1/GR
Odd gear portals: d = 1-1/GR
Positive is above the tire.
Even though I barely believe in antisquat anymore, it is still an easy reference value. So I figured this would be a good one to revisit. I wanted to find the antisquat drawing for something with portals. In particular solid axles, though an independent suspension drawing should be easy to find as a offshoot. If we have the drawing, we can get antisquat.
It has occurred to me that the offset does not play any special roll. That is, the links being higher up does not change how we draw things.
Portals change where we start the line that goes to the IC.
It struck me that the key was in looking at why we start the anti line at the wheel hub for independent acceleration and at the contact point for independent braking and solid axle. But we are not going to the wheel hub or contact point, we are going to where the force is applied. But why is the force at the contact patch when reacting to torque? It is because that torque the housing or knuckle reacts to is the acceleration force times a distance from the wheel center. In the case of a ring and pinion gear or brakes, that distance is the tire radius. For an independent suspension the force still exists, but there is no torque. So the distance must be 0.
So we should be able to apply this force and distance approach to portals. The force is fixed in magnitude and direction. Therefore we can only control the distance.
When the gearboxes input and output shafts rotate opposite directions, the housing torque is the sum of the magnitudes in the direction of the output shaft's torque. If they rotate the same direction, the reaction torque is the output - input.
The reaction torque around the ring gear is opposite in direction and equal in magnitude to the axle shaft torque. Combine this with the gearbox torque from above, and deal with some sign stuff and we get equations for equivalent distances.
Solid axles:
Even gear portals: d = 1+2/GR
Odd gear portals: d = 1-2/GR
Positive is above the tire.
Independent:
Even gear portals: d = 1+1/GR
Odd gear portals: d = 1-1/GR
Positive is above the tire.
Treefrog
Book Wheeler
In post 125 I looked at to suspensions that give the same 2d results. And the results were rather inconclusive as to the differences. It was a few percent at most. So I ran the suspensions again to see if the orbit centers are different.
Up first is what the rear axle orbits around. The full up travel points are labeled. Normal is both uppers and lower at full width. The other is changed to half width.
And how the chassis orbits around the rear axle. The full up travel points are labeled. Normal is both uppers and lower at full width. The other is changed to half width.
Up first is what the rear axle orbits around. The full up travel points are labeled. Normal is both uppers and lower at full width. The other is changed to half width.
And how the chassis orbits around the rear axle. The full up travel points are labeled. Normal is both uppers and lower at full width. The other is changed to half width.
Treefrog
Book Wheeler
We often say that high antisquat rides rough. There are some obvious influences like how vertical its movement is and how far away the IC is. But why does suspension geometry that resists squat cause a rougher ride?
For simplicity, we have accelerated long enough that everything has settled.
With a neutral suspension, 0% AS, the links do not resist. This means that the springs take the load. And how far the vehicle squats is dependent on the spring rate. The lower the spring rate, the more it squats.
With AS geometry, the suspension resists the squat. Put another way, we have less distance for the same squatting force. Meaning we have a higher overall suspension spring rate. Higher spring rate, stiffer ride.
But why does this affect the ride at a constant speed? The suspension is not reacting to acceleration. It is reacting to drive force. The force for constant speed is less than when accelerating, but it is not zero.
For simplicity, we have accelerated long enough that everything has settled.
With a neutral suspension, 0% AS, the links do not resist. This means that the springs take the load. And how far the vehicle squats is dependent on the spring rate. The lower the spring rate, the more it squats.
With AS geometry, the suspension resists the squat. Put another way, we have less distance for the same squatting force. Meaning we have a higher overall suspension spring rate. Higher spring rate, stiffer ride.
But why does this affect the ride at a constant speed? The suspension is not reacting to acceleration. It is reacting to drive force. The force for constant speed is less than when accelerating, but it is not zero.
Treefrog
Book Wheeler
I want to look a mixing the new portal formulas into some of the forced base suspension discussion earlier this year. This is to look at what designing with the end in mind might result in. The big change from when we last looked at resultant forces is that upper link is either doing very little or is in compression. We should be able to do things like have no torque at the chassis while having vertical force. Or countering the torque while still having downward force.
gt1guy
Apparently a racist
My understanding is that even with 100%AS and under acceleration, the suspension is still free to move.
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gt1guy
Apparently a racist
Seems I hit reply
The lateral load transfer carrying ability that AS has, is done through the upper and lower chassis mounts and compression/tension on the links. The axle and shocks are still free to react to bumps in the road. I don't believe that would affect the spring rates or felt ride harshness.
The lateral load transfer carrying ability that AS has, is done through the upper and lower chassis mounts and compression/tension on the links. The axle and shocks are still free to react to bumps in the road. I don't believe that would affect the spring rates or felt ride harshness.
gt1guy
Apparently a racist
I used to think that AS was putting the suspension in some kind of bind to hold the load transfer. Which would most definitely give the feeling of increasing spring rates.
But that isn't the case.
The links, acting through the chassis mounts, are keeping the springs from ever seeing the load transfer. So the end result is basically a upward push on the chassis at the point where the links mount.
Since the forces on the links are only compression (lowers) and tension (uppers) the joints still have freedom to move.
The shocks have no idea there was/is any load transfer trying to take place. They are free to do their shock thing.
But that isn't the case.
The links, acting through the chassis mounts, are keeping the springs from ever seeing the load transfer. So the end result is basically a upward push on the chassis at the point where the links mount.
Since the forces on the links are only compression (lowers) and tension (uppers) the joints still have freedom to move.
The shocks have no idea there was/is any load transfer trying to take place. They are free to do their shock thing.
Treefrog
Book Wheeler
Seems I hit reply
The lateral load transfer carrying ability that AS has, is done through the upper and lower chassis mounts and compression/tension on the links. The axle and shocks are still free to react to bumps in the road. I don't believe that would affect the spring rates or felt ride harshness.
The links and axle are free to move around. It is the movement of the axle relative to the chassis that stiffens. I am viewing not as the springs do not see the load transfer, but as the springs have assistance in resisting the load transfer.
Agreed that the shocks play no part.
Treefrog
Book Wheeler
A quick follow up to the orbit gifs in Post 164. In 164, I looked at the axle's movement observed from the chassis and the chassis's movement observed from the axle. I wanted to take this a step farther and look at the axle's movement observed from the axle's point of view, and similar for the chassis.
Up first is the axle's orbit point viewed from the axle:
And the chassis orbit point viewed from the chassis:
Up first is the axle's orbit point viewed from the axle:
And the chassis orbit point viewed from the chassis:
Weasel
Red Skull Member
anyone watching these "tech" videos on the 74 weld portals?
About 10mins in and there are several items I don't fully agree with and functions of suspension geometry as I have understood them in prior conversations. Not sure I can get through 55 mins of this.
About 10mins in and there are several items I don't fully agree with and functions of suspension geometry as I have understood them in prior conversations. Not sure I can get through 55 mins of this.
AgitatedPancake
Frobot
I'm listening to it now, honestly not too bad. I do have a bone to pick with one statement about how anti squat/anti dive countering weight transfer, instead of specifying that the weight transfer is the same, but the transfer seen by the springs is reduced. But that very well could have just been an oversimplified statement for the sake of discussion. Like ~22 mins in at the moment
Weasel
Red Skull Member
I might pick it up more later than. Yeah I didn't like that either and the mention of unloading of IC as they are both more involved.
I think their portal boxes are amazing works of art but don't agree with the implication of Factory suspension is the "end all be all of design" or that their is only one right way.
For sure there are some junk systems out there but that's not true of all of them. It's a convenient line that helps sell a product.
I think their portal boxes are amazing works of art but don't agree with the implication of Factory suspension is the "end all be all of design" or that their is only one right way.
For sure there are some junk systems out there but that's not true of all of them. It's a convenient line that helps sell a product.
AgitatedPancake
Frobot
A lot of the discussion I've heard so far has been a valid "everything is a compromise" rationale, for most of the discussion they're just comparing everything out there.
Alright just taking notes as I listen - I have a bit of a bone to pick with their hydraulic assist rant that starts at like 37 minutes. They're talking about loss of streetability with assist, and that's just due to their hydraulic configuration imo
Alright just taking notes as I listen - I have a bit of a bone to pick with their hydraulic assist rant that starts at like 37 minutes. They're talking about loss of streetability with assist, and that's just due to their hydraulic configuration imo
Weasel
Red Skull Member
Wierd my tow rig has hydro assist and it's the best steering rig I have. So yeah that seems a bit odd. Bummer with YouTube is feedback and discussion.
AgitatedPancake
Frobot
Agreed, I have one rig with single ended assist and one with double ended, and they're both excellent. My guess is the steering pump fitting orofice is too large, so they're getting too much high RPM flow. Granted I'm talking out of my ass about a setup I know nothing about, but once my eyes opened to what that fitting diameter does...the symptoms of it being oversized are everywhere
Will Scarlet
The universe is vast...
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That is the longest sales ad I've seen. Interesting, but they went over everything and in the end pushed their product as the superior compromise.
Treefrog
Book Wheeler
Just listened to it. I was quite impressed. If it were 2010 to 2015 on PBB, that video would have been the first thing I would have linked to for "welcome to suspension 101". And those two very much talked like they were from that era, as does most of the established people in the industry.
And in my opinion, the general understanding of solid axle linked suspension reached the level needed for the average wheeler to create a good enough suspension in that time frame.
That said, there was a number of things that were just wrong enough to bug me. But the majority of those were more rabbit hole things or things that were worded for the average guy who just wants to lift his JK.
As it stands, I only have a few real complaints. Half of which are centered around not discussing the cons of portals.
Weasel
Red Skull Member
Probably right,
I started with a bit of bias for a couple reasons:
1 - Quinin had a IG post on math is math and math is right and you can't ague with math. Which isn't true because math doesn't ever get you to one solution. It provides you with s method to design multiple solution sets with a variety of parameters.
2 - if your going to proclaim yourselves as experts then the little items above should be discussed correctly even if they are incidental (not sure that the word I'm looking for).
I started with a bit of bias for a couple reasons:
1 - Quinin had a IG post on math is math and math is right and you can't ague with math. Which isn't true because math doesn't ever get you to one solution. It provides you with s method to design multiple solution sets with a variety of parameters.
2 - if your going to proclaim yourselves as experts then the little items above should be discussed correctly even if they are incidental (not sure that the word I'm looking for).