Anti Squat and Suspension Tech

[Mnster]

Ok I have been reading up on this off and on since the thread has started. However, some of the pictures in this thread do not really match with your trucks. This is understandable as a drag car and a rock crawler have very different link geometry and suspension needs. Obviously since you people rather learned as you went, some of the statements you people made in the beginning are no longer are relevant. All of this rather sent me in a spiral of which is the correct way to do things. Anyways, after dropping a coil setup that was ok for a droop setup that sucked I went for a 50/50 droop and extension setup. I now notice my bad link geometry more then ever before as the suspension is no longer contained or limited by the shocks. The truck seems to push the rear axle under the truck and as a side effect unsettles or makes the front light on the up hills. The steepest slope I can climb is right at 55 degrees. I feel my center of gravity is very low with the running gear low and air soft pellets. I have 2 ¾ worth of belly clearance so the truck is not riding very high. The torque twist is bad now which may be related as once the front is light the chassis can rotate without a force pushing back. What would you recommend I do? I am thinking the right thing to do would be to raise the rear upper links higher off the axle to reduce the amount of anti squat. I would then run a flip-flop link setup like Cole to eliminate some of the twist. Or on the other hand, am I backwards in my theories? It’s a lot of work to design a new link mount around my dig setup I want to be sure I know what I am doing before I commit on the remake.

[Cole82]

Ben either move the links closer together on the axle or further away on the chassis. That will make your rear links parallel. Think of it as if you had no intersection it will do more pushing going up the hill than lifting the truck rotating the axle under.

If you make the the links further apart on the axle it will RAISE the chassis up. Increasing anti squat

[Mnster]

So this is how it works then.

[EeePee]

The tires are spinning the wrong direction.

[JohnRobHolmes]

And whether it is squatting or rising under power depends on where the links intersect (your instant center) in relation to the line of force (draw a line from the rear contact patch to the front tire at the height of your chassis CG).

And read this http://www.raceglides.com.au/TechInfo.htm

[JohnRobHolmes]

The pink line is your Line of Force. As you can see your links intersect above the line. When you give the vehicle acceleration the rear will rise. To remedy the situation some you can either lower the ride height, raise the chassis CG (I guessed here) or lower the rear upper link on the axle a bit. To make the susp totally neutral you will need to flatten the rear lower link more. Your CG is low enough that it isnt easy to set your ride to 100% AS (neutral) without changing the lower link. Another option would be to raise the rear link both at chassis and axle. This could hit the 100%AS spot.

Your torque twist is caused by the lack of limiting springs or straps on the decompression side of the suspension I imagine. You have to have something to push back and recenter the chassis once the initial acceleration has stopped.

[JohnRobHolmes]

I played around a bit more. It looks like raising the mount point of the rear upper link at the chassis a smidge would solve your large AS issues. You still need to control the decompression side of the shocks though.

[EeePee]

Originally Posted by johnrobholmes

The pink line is your Line of Force.

Without knowing where the CG point is, your Line of Force can't be correct.

Another option would be to raise the rear link both at chassis and axle.

That's what I would do.

Here's mine. I found the CG (pink dot) using the old string trick, hanging the truck from many points and visualizing the CG point. May not be dead on, but it's close enough. I'm in the anti squat side of things. With equal oil weight, and equal spring rate all around, set up mid stroke at ride height, I get only a little bit of torque twist action.

[JohnRobHolmes]

If I understand correctly, isnt it the CG of just the Chassis since the rest of the weight is unsprung on a crawler?

I did make an assumption on his CG though.

[EeePee]

I believe that the CG is based on the entire truck. Going up hill or sidehilling, all the weight on the axles will have an effect on the entire truck.

So, the weight transfer still happens from the CG point.

Maybe?

[JohnRobHolmes]

I know that at least the front axles would have an effect on the CG when climbing, so those should probably be included in the CG. I have confused myself at this point.

Should I keep my mouth shut or pose the idea that the CG changes as the suspension cycles, as well as the IC of the links.

[EeePee]

Originally Posted by johnrobholmes

Should I keep my mouth shut or pose the idea that the CG changes as the suspension cycles, as well as the IC of the links.

It does. And I remember our Tech writer Jay (Roktoy) posed that question someplace on this board. I believe it went unanswered, as it becomes quite complicated at that point. I think he asked what happens to the IC and what effect the links have on the chassis when the axles articulate.

The Line of Force changes drastically.

I feel the best approach is to tune your truck to work it's best in the middle of it's suspension range. Like the mid stroke set up. And of course don't go too extreme with your link geometry.

[carmatic]

Originally Posted by EeePee

I believe that the CG is based on the entire truck. Going up hill or sidehilling, all the weight on the axles will have an effect on the entire truck.

So, the weight transfer still happens from the CG point.

Maybe?

the weight transfer is purely from the sprung part of the suspension, assuming that the axles' own cg are in the center of their rotation throughout the articulation

this is the way i see it... the links only push in the direction they are pointing, and the truck moves as if it is being held at the instant center... torque from the wheels causes the lower and upper links to not push as hard as each other, and it will be as if a non-existing link was pushing from where the tyre touches the ground towards the instantaneous center
to put it another way, the gearbox wants to rotate in the opposite direction of the wheels, and if you arrange the links in such a way as to make the gearbox move up or down when it rotates like this, you get squat or anti squat

[EeePee]

Well, I don't like to leave anything hanging, so I got in contact with a buddy of mine who works for Honda's suspension R&D department.

"CG of a vehicle is of the whole vehicle. but you should try to lock the suspension in the normal ride height when measuring. if your talking squat/anti-squat then technically it should be of the chassis. the anti stuff just comes from determining where forces can be applied to a mass to prevent it from moving (going through the cg). that's why they are always in %'s. so for that the chassis is the mass, so no suspension. but in cars, the cg of the vehicle is good enough. for a RC crawler where the sus is probably close to half the mass of the chassis it might be more important to seperate. I could look through some books next week if I get time if you really want to know"

Then I mentioned how much heavier our axles and wheel/tires are compared to the chassis and he said, "i was just guessing. but yes, if they are heavier, probably even more like what we discussed."

So, armed with that information, my CG point will move to a much more central location and higher up, my Line of Force angle will increase putting my IC darn near that Line of Force. Putting me closer to a nuetral set up.

It does behave more like a nuetral set up would than an anti squatter.

You guys following this?

[Cole82]

Originally Posted by EeePee

Well, I don't like to leave anything hanging, so I got in contact with a buddy of mine who works for Honda's suspension R&D department.

"CG of a vehicle is of the whole vehicle. but you should try to lock the suspension in the normal ride height when measuring. if your talking squat/anti-squat then technically it should be of the chassis. the anti stuff just comes from determining where forces can be applied to a mass to prevent it from moving (going through the cg). that's why they are always in %'s. so for that the chassis is the mass, so no suspension. but in cars, the cg of the vehicle is good enough. for a RC crawler where the sus is probably close to half the mass of the chassis it might be more important to seperate. I could look through some books next week if I get time if you really want to know"

Then I mentioned how much heavier our axles and wheel/tires are compared to the chassis and he said, "i was just guessing. but yes, if they are heavier, probably even more like what we discussed."

So, armed with that information, my CG point will move to a much more central location and higher up, my Line of Force angle will increase putting my IC darn near that Line of Force. Putting me closer to a nuetral set up.

It does behave more like a nuetral set up would than an anti squatter.

You guys following this?

Now that's some good to know info. I have to go refigure mine now.

[carmatic]

yes, if your axles are heavy, what will happen is that instead of the vehicle pivoting at the front wheels when you are accelerating (that is why you draw the line from the bottom of the rear tyre till it reaches the front axle) , the chassis will simply rotate on its own axis more, the front springs will extend or compress while the rear springs do the opposite, without so much as a change in the weight distribution, since most of the weight is in the axles

[sfrankland]

EeePee could you redraw your setup with the new COG based on what your buddy advised you of. I need to see it to get my head around it. I am a hands on visual person.

[EeePee]

Alright.

[EeePee]

See how much closer my IC is to the Line of Force?

Pure guess as to where the CG is.

[carmatic]

im realizing more and more, that crawlers are shockingly complex machines....

i have an idea...you know one of those clotheshanger things that you use to hang socks and underwear from? take the wheels off, and tie the axles with equal length strings to the hanger , try to make the strings as vertical as possible... change where the strings attach on the hanger until the truck hangs evenly , the cg should be straght down from the center of the hanger... or has anyone else come up with this idea already and i havent searched the forum hard enough....

to find the vertical location of the center of gravity, put the wheels on, and tie the string to the tyre so that it hangs by the middle of the wheel , and stagger the strings sideways where they tie onto the clotheshanger so that they point inwards , forming a 'trapezoid' at the front and back of your truck... rock the hanger slightly and feel how difficult it is to rotate it, and keep on staggering it until its the easiest to rock... the CG would be tricky to find but mathematically you need to somehow find the angles that the strings are at, and imagine that there are lines at right angles to the string where it is tied to the tyre... the point at which these lines intersect is your cg...like, its some high school trigonometry so i dont feel like typing it here....