Axle Sway/Swing/Steer??

[oberon crawler]

I've been searching and reading tips etc but I haven't been able to find much info about the dreaded Axle sway (or whatever it is)

I'm really in the dark here and I'm not even sure what it means, but as far as i can tell it's something to avoid.

My questions are mainly regarding axle sway in STICK CHASSIS

So

What is it?
Why is it bad for the performance of your rig?
What is some common ways to reduce it?

[JeremyH]

Are you referring to anti-sway bars? I dont think I understand what you mean by "axle sway/swing"...have you got an image or thread that you saw this on?

[bob1961]

i think he's talking bout axle steer...............bob

....

[RCMFMaxxMan]

I think what you are refering to is when the axle moves away from the rig when articulating or when the axle steers when it articulates (sticks one end of the axle further out in front).
First issue: the axle steers when articulating. This is a problem because what if you don't want to turn the way the axle is turning when your rig articulates? If your stick is mounted with a ball joint and two lower links then you'll have to experiment with different mounting positions for the lower links until you have something good. I am sure someone else can elaborate on this better. I am not an expert on suspension geometry. You can get rid of this fuss though if you ditched the lower links and instead used a second ball joint on each end to keep the axle straight and from flopping around. This solves the axle steer problem because now the axle will always be perfectly pointed forward no matter how it articulates.

For the next issue: how the axle moves out from under the rig and off to the side. Think about looking at the truck head on and imagine the axle articulating on its pivot point (where the stick is mounted). You should notice how the axle is kind of like a pendulum. it doesn't stay directly under the truck. Notice that if the stick was mounted higher above the axle, the axle would then swing out further. This obviously is going to make the truck less stable because under articulation the weight of the truck is no longer right on top of its base (the axle/tires). After these thoughts you should come to the conclusion that the closer the stick is mounted to the line that runs between the center of each wheel, the less the axles will swing out and the more the rig will be centered above the axle/tires during articulation.

I hope this gives you some understanding. I don't know how clear that all was so you may have to read it a couple times. And about the axle steer, I am sure someone else can give a little more insight. good luck!

[oberon crawler]

Originally Posted by RCMFMaxxMan

I think what you are refering to is when the axle moves away from the rig when articulating or when the axle steers when it articulates (sticks one end of the axle further out in front).
First issue: the axle steers when articulating. This is a problem because what if you don't want to turn the way the axle is turning when your rig articulates? If your stick is mounted with a ball joint and two lower links then you'll have to experiment with different mounting positions for the lower links until you have something good. I am sure someone else can elaborate on this better. I am not an expert on suspension geometry. You can get rid of this fuss though if you ditched the lower links and instead used a second ball joint on each end to keep the axle straight and from flopping around. This solves the axle steer problem because now the axle will always be perfectly pointed forward no matter how it articulates.

For the next issue: how the axle moves out from under the rig and off to the side. Think about looking at the truck head on and imagine the axle articulating on its pivot point (where the stick is mounted). You should notice how the axle is kind of like a pendulum. it doesn't stay directly under the truck. Notice that if the stick was mounted higher above the axle, the axle would then swing out further. This obviously is going to make the truck less stable because under articulation the weight of the truck is no longer right on top of its base (the axle/tires). After these thoughts you should come to the conclusion that the closer the stick is mounted to the line that runs between the center of each wheel, the less the axles will swing out and the more the rig will be centered above the axle/tires during articulation.

I hope this gives you some understanding. I don't know how clear that all was so you may have to read it a couple times. And about the axle steer, I am sure someone else can give a little more insight. good luck!

Thanks man, I never thought of having double ball joints and ditching the lower links!!

My question comes about from reading posts about ppl giving up on stick chassis because of the bad axle sway, so i'm just trying to understand what it means.

As for your second issue it seems that it's a question of compromise between ground clearance and axle swing! I've gone for enough breakover clearance to clear a 90 deg edge like a step, and i'll learn to drive with the sway (i'll have rear steer anyway). I see others have made the back bone as low as possible to reduce this sway but then you're cutting into your big ground clearance which is why i'm building a stick in the first place.

[RCMFMaxxMan]

If i were you I wouldn't worry about the ground clearance. Just get the backbone mounted as low as you can and you can always bend a hump into the middle of it to get your clearance back. Someone did this once but I can't remember who. 4runnerrick maybe?

[oberon crawler]

Ok i'm thinking, I can just cut some of the stem off my heim joint and lower the BB by 1/2" it will leave less thread for the mounting screw to hold with but it should be ok.
I'll still have 4.25" of GC, Is that enough?

[RCMFMaxxMan]

Originally Posted by oberon crawler

Ok i'm thinking, I can just cut some of the stem off my heim joint and lower the BB by 1/2" it will leave less thread for the mounting screw to hold with but it should be ok.
I'll still have 4.25" of GC, Is that enough?

That sounds like a good amount of clearance. it really depends on your wheelbase though. the longer the wheelbase the more clearance you need. imagine a limo crawling. the belly will hang up on everything and breakover will be like a see-saw. this is also why our 12.5" wheelbases on our 2.2's are content with 2.5" gc.

[oberon crawler]

2.5" I must be doing something wrong, my axial has 3" and it's always hung up on the rocks!!

The stick i'm building with 4.25" GC is also a 2.2, it's my new Berg stick. Maybe i need to go lower still?

[Mnster]

This is how I feel on some of the variables.

Axle steer-axle steer has a large effect on how the rig crawls. The biggest issue I have found with the axle steer on stick crawlers is on approach to rocks. When pressure is applied for example driving into a 90 degree rock the suspension gets all in fuss. Rather then the tire being pressed straight into the terrain firmly to crawl over it. The suspension or axle just buckles back (axle steer) and the tire that was pressed into the rock remains pressed into the rock while the other tires push around it, the tire opposite the tire pressed into rock flexes into the air. You can see this same effect if you just grab two tires diagonal from each other and push them together as if you are trying to shorten your wheelbase. If you have axle steer you will see and feel the truck wanting to twist out of your squeeze.

Double ball joints?
I once came up with a similar idea though I never finished it. It seemed like a good way to eliminate the axle steer. I just liked links for durability and simplistic reasons.
Mean Green Crawling Machine


Clearance- Low is better, you want to keep the flex rotation nearest the axle as possible. If you go to high you get a lot of the axle walking under the truck which causes the weight to be thrown off either side of the truck when flexing making it unstable. I feel the biggest benifit of the stick is that you can run it low. I mean you have a round chassis plate less then a 1/2 wide. Many times you can find a single groove in a rock to aim for if not it skids easy.

[oberon crawler]

Originally Posted by Mnster

This is how I feel on some of the variables.

Axle steer-axle steer has a large effect on how the rig crawls. The biggest issue I have found with the axle steer on stick crawlers is on approach to rocks. When pressure is applied for example driving into a 90 degree rock the suspension gets all in fuss. Rather then the tire being pressed straight into the terrain firmly to crawl over it. The suspension or axle just buckles back (axle steer) and the tire that was pressed into the rock remains pressed into the rock while the other tires push around it, the tire opposite the tire pressed into rock flexes into the air. You can see this same effect if you just grab two tires diagonal from each other and push them together as if you are trying to shorten your wheelbase. If you have axle steer you will see and feel the truck wanting to twist out of your squeeze.

Double ball joints?
I once came up with a similar idea though I never finished it. It seemed like a good way to eliminate the axle steer. I just liked links for durability and simplistic reasons.
Mean Green Crawling Machine


Clearance- Low is better, you want to keep the flex rotation nearest the axle as possible. If you go to high you get a lot of the axle walking under the truck which causes the weight to be thrown off either side of the truck when flexing making it unstable. I feel the biggest benifit of the stick is that you can run it low. I mean you have a round chassis plate less then a 1/2 wide. Many times you can find a single groove in a rock to aim for if not it skids easy.

I had a look at your super build thread and it looks really good, I've chopped my ball joints and i'm now just a bit under 4" GC and if i want to go lower i have to do major mods.

So when you say the "axle buckles back" under the rig, is this because the backbone is being pressured into rotating against the shock springs thus allowing the lower link on the pressured side of the BB to move up and around the BB? Is that how the axle manages to move back?

I just tried it on my axial and when i push the right front and right rear tires toward each other the lower links on that side push up in the middle at the chassis plate and the whole chassis twists allowing the tyres to get pushed together, meanwhile the other two tyres (left front and left rear do the opposite and move apart) Is that what axle steer is?