Rear suspension??? 4-link, radius arms, or leafs? and anti-squat (1 Viewer)

[88chevota]

Ok so someone passed on to me some free coils from an 80 series cruiser. I also have some stock 4 link stuff from a JK 4 door jeep.

I am reading a little about anti-squat and the figures people are throwing out there are all over the place. I am confused as ever.

my basic understanding is that the higher the number the more the rear end will push up? the lower the number the more it squats? Is that right? somewhere in all that you can find the middle and it stays level.

I could build a 4 link for pretty cheap but I could really mess up the squat or anti-squat whatever. seems really complicated too.

I like my leaf springs because they are simple and stable, but don't flex super well.

How would a long arm radius set up handle like what I have in the front currently?

I have johnny joints at the frame end, stock bushings at the axle. They are 36" long so it keeps a really nice angle. My front will flex way better than my rear end. I keep saying to myself why not do they same thing for the rear?

Am I going to get some weird anti-squat or squat issues with that in the rear? It seems that radius would move in the same manner as leaf springs. Coils may cause it to hop, but if I get the right shocks wouldn't that reduce hop?
I have seen someone running radius arms in the rear of an 80 series (Dusty, I think?)

I know my front radius will flex really well with no binding and I could build this cheaper than a 4 link, and it is simpler.

Chime in and hopefully I will not be more confused than I am now.

 

[mookie]

Link Suspension for Dummies:

some other folks on here may disagree with this but it worked out great for me......IMO if you can follow these basic rules you'll have a good working setup and anti-squat won't be a problem....the problem is that usually there's only about one way to get all this stuff to fit under a stock rig so it's not easy.....unlike a buggy where you can really set all your link numbers in stone before you even start.

1. get your lower links as flat or as close to 5 degrees as possible

2. try to make your lowers at least 40" long, the longer the better

3. make your uppers about 75% of the lowers length

3. shoot for around 8" of link seperation at the axle

4. adjustable link brackets for the top links.....like 4 holes on the frame mounts and at least two on the axle

5. get as much triangulation as possible, you'll prolly have to go with straight lowers and triangulated uppers

6. initially set it up with the upper and lower links parallel when the upper links are in the top frame and axle bracket holes and then adjust as needed.......the RIGHT amount of anti-squat will help you climb better, but you have to just drive it and then adjust to how you like it.

 

[GLTHFJ60]

 

[88chevota]

That makes the four link sound a little bit simpler.

anyone have ideas on a radius arm for the rear?

 

[BellyDoc]

Radius arms start to bind if you set up for a lot of twist travel.

I can demonstrate that with some hand waving, but it's harder to describe using words.

Stick out two fingers on each hand (like the peace sign) and stick your elbows against your sides with your peace signs pointing forward. Now your arms are like radius arms.

Imagine that you are holding a stick pinched between your fingers in both hands. This represents the axle. Radius arms each hold onto the axle in two positions, usually top/bottom, to resist axle wrap.

Now, "twist" travel your imaginary axle.

As you approach a 90 degree articulation, you'll notice that your "peace signs" are almost 90 degrees off of eachother around the imaginary axle.

If you articulated at your shoulders instead of your elbows, simulating longer radius arms, you'd decrease this effect, but not eliminate it. You can also angle your fingers somewhat to reduce, but not eliminate, this issue.

Radius arms require flexible bushing mounts to accommodate this phenomenon, and they're limited by it in a way that 4-links are not.

They can be done well, but the key is to design the right system for the amount of travel that's it's supposed to get.

Now you know everything about radius arms that I do.

As to 4-linking, there's a lot to learn and understand, and I'm no expert. However, I'm fascinated by the topic even after having built a front and rear triangulated 4 link buggy, and I continue to learn things all the time.

The concept of antisquat isn't rocket science, but it helps to think of it as rocket science. In a rocket, there's some point within the mass of metal parts that's the center of gravity. If the rocket engines are attached accurately and squarely, so the force of the thrust points right toward the center of gravity, then the thrust will push the rocket straight. If the rocket engine is off to the side, or angled away from the center of gravity, then firing it will spin the rocket like a pinwheel.

The same is true of wheel thrust on your truck. If the force applied to the vehicle points at a certain spot, then you just launch. If it's pointing above or below, then you'll induce some rotation forces that will move the parts of the suspension relative to eachother.

If the force is aimed UNDER that point, it will tend to "pinwheel" the mass of the truck back as you accelerate. That's called "squatting" because the rear end dips as weight is transferred onto the rear springs. If you aim the force OVER that point, then the rear end may actually climb when you launch.

Depending on the geometry of a link construction, the force can be aimed in a lot of different ways.

Antisquat is expressed as a percent, where 100% represents where the force is aimed at that point.

There's a lot more to it, though.

One thing is that the geometry of the links can induce some steering angle with articulation. The axle can vary from where it starts perpendicular to the long axis of the vehicle. If it moves a lot, you can get some instability in the steering at speed, or when crawling, the rear can crab to the side. There can be either "oversteer" or "understeer" depending on the geometry and it's worth learning about how that happens.

Another thing worth thinking about is the induced changes in pinion angle to the drive shaft. If you build links so that they sit basically parallel as viewed from the side, then the angle of the pinion doesn't change as the axle cycles up and down. It's like one of those parallelogram link desk lamps. You set the lamp direction and then you can move it all over the desk and it keeps the same attitude to the desk surface. If, on the other hand, the links converge sharply to the frame, like a triangle viewed from the side, then the pinion angle will tend to aim toward the point of that triangle throughout it's cycle.

It's worth seeing that with diagrams and really understanding it before you finalize a plan.

In some ways, I'd say getting an initial plan to work in terms of link and drive shaft clearance, pinion angle and steering geometry is key, and getting antisquat is a later consideration. However, the reason I'd say that is because antisquat is just a guess if you don't have accurate knowledge of exactly where the vehicle's center of gravity is. Unless you're a stud engineer with meticulous mathematical models of the whole design, you're only going to know where the center of gravity is AFTER you build it and physically weigh it and do some calculations. You can start with a guess, (top center bellhousing bolt) but I have no idea how often that works out to be accurate in the long run.

So, the adjustable link comment is key. You have to be ready to tune the antisquat based on performance and considerations after the build is complete. The easy way to do that is to have angle adjustments available for the upper links.

Here's a good write up I recently bumbled across about 4 link basics. It's as good as I've seen, and very readable.

Suspension Encyclopedia/4Wheel Underground

 

[Tapage]

BDoc .. outstanding explanation .. !

 

[GLTHFJ60]

Wow, very cool. I've never read such a complete explanation of anti squat before, thank you!

 

[88chevota]

Definitely a great explanation.

I built these radius arms for the front. with the johnny joints it allows it to twist and not bind up. it gives great flex. how would these handle in the rear? What kind of squat issues will I run into? I also am running 14" travel shocks in the front and they completely stretch out.

 

[BellyDoc]

I don't know anything about the antisquat/antidive involved in radius arms. I've heard that it isn't great, but I haven't seen anything that helps me understand how to calculate it.

I'll hunt and if I find, I'll post back.

 

[lenross1]

Great link! thanks doc!

 

[Dusty]

My radius arms out back flex great and bounce very little. (manifestations of antisquat minimized) I would do them again. But the radius joints need servicing wheras my 3 link front needs little attention. I have had to change 3 bushings in as many years on the radius arms. I use the large rubicon express bushings.

Only advice I would give if you are going to do radius arms out back is to use only rubber bushings at all 4 joints in each arm so that all 4 joints can share the misalignment. You will change joints less often if u do this. I used only rubber bushings at the axle and ballistic joints (jj's) at the "y" and evolution .750's at the axle.

Also I would make all 4 joints screw in type rubber joints. I recommend the new ballistic 1.250 threaded replaceable rubber joint. We used these to link my dads jeep . They have the same dimensions as a standard johny joint. The reason for the screw in joint is so you can easily replace the rubber in the joint if you trash the rubber. You don't want to have to take the entire arm off to replace the rubber in 1 bushing which you will have to do if you cant easily unscrew the whole bushing assembly. These bushings (same as a rubicon express bushing) will take a vice (not a press) to replace the rubber so you can't do the fix on the truck. I keep a rebuilt bushings in the truck so if I ever Need to replace a busted bushings I can unscrew the bad one relatively quickly and put a fresh bushings on. That said I have never changed the bushings on the trail because I never let them get that bad