General 4 Link Suspension Design Information

Yay!Gurrr

Yay!Gurrr

Better Faster Stronger
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Joined
Mar 17, 2005
Location
N. N. Raleigh, NC
First, Let me be clear. This is NOT my information, I'm simply parroting this from information I've read elsewhere. I've only condensed and simplified parts and wording so I could understand it better. That said... Hope this helps others as much as it did me...
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So to say it again (cut and paste job here)-
- try to make your links parallel when veiwed from the side
- make them as long as you reasonably can
- make them as flat as you reasonably can
- make the roll axis as high as you reasonably can
- vertical separation should be 6" or more. More especially for more horsepower and/or bigger tires.
- make the triangulated portion 40 degrees or more for decent lateral control.
If you do this then you will end up with as little anti squat as possible and with the least body roll as possible and as the suspension cycles things will remain as stable as possible. Now IMHO this is the best place to start when building a link suspension - something that gets the power to the ground in the most stable way possible ..... meaning that it has the least possible chance of starting to hop on throttling climbs or walking a wheel undernieth on slow articulating climbs and it has the least chance of rolling you over on the off camber climbs.
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If you do this you will have a linked setup that will be the most nicely behaved setup that you can produce - you will break less rear axles and diffs and you will roll over and flip over backwards the least.
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Now if you find that your rear suspension doesent really hook up and bite into the rocks on the throttling climbs and it doesent hop at all then you can start to experiment with more anti squat. Easest way to do this is to lower the chassis mount of the upper links. If you are smart you will build this option into your links when you first build it so that you can play with it later. But IMO you are best to start with the least amount of antisquat as possible and then work your way towards more antisquat. Most people build links because their rigs hop and buck and generally behave really badly when the power comes on so IMO you are best to start with the other extreme.
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In the majority of cases IMO you will still end up with more antisquat than a totally 100% neutrally handling rig. Just by running a lifted rig with big tyres will generally give more then enough anti squat even if you try to totally minimise it.
As a side note I just got a copy of the super crawl video from last year. Now if you watch it carefully you can look at the rigs that hop badly on the climbs, the rigs that hop a little bit and the rigs that dont hop at all. Now it is easy to see that the rigs that dont hop at all run the least amount of antisquat (by looking at the rear link angles) ... the ones that hop a little bit have a bit more antisquat and the ones that hop a lot run a lot of antisquat. Its that simple.
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OK I take a stab at link for dummies. Most will be like ErikB posted which are good. I'll expound a bit more.
-I would recommend a Tri-4link or 3link wishbone, for the rear. If planning front I would also recommend unless physically impossible due to driveline configuration.
-Place the converging Links on top of the axle with upper tri or wishbone, OR at the lower crossmember for lower links. This will keep your roll center fairly high. You can do both for a double Tri-links.
-Keep the tri links angled as much as the chassis you are building will allow or axle. It hard to go with a hard fast number here because widths of chassis and axles vary so much. Example: my 48" on center widthed chassis isn't going to allow for as much as a regular sized frame.
-Keep the remaining links wide on the axle attachment point. This will allow for more stability. Here again you have to account for the tire throughout their travel and if they steer. Just like leaf spring, you don't want the tires rubbing a lot on the links, so would be OK, but I like none.
-Vertical separation is important to take the torque rotation from the axle under power, and even more important with portal axles. Place the lower link attachment points at the axle so the brackets are not lower than the axle tube. You can go lower but will take away clearance and hang up on stuff. Then place the upper axle mounting position fairly high, again you chassis setup will dictate this a lot (especially in front under the engine). See how much compression you want, what the links will hit and make slightly lower. Once under the rig it will be clear how much, and too much isn't necessarily a good thing.
-After all of the above, you should have the axle mounting placed, or a good idea where they will go, then focus on the chassis. The lowers should be placed in the chassis to be tucked away so you have good clearance and don't get hung up. This would be either frame mounted or cross member mounted. If you have to build lower because of chassis clearance issues build a ramped mount.
-The last set of links to place are the chassis upper, all others you should have a good idea where they go from the above text. Here is where I would build in some adjustment to the suspension. All other points can be solid mount with no adjustment because they take into account other thing for best placement. Make a attachment point with several (3 min) holes that are on the same arc as the link at ride height. You will also see that the body or chassis will dictate the location of this bracket and will probably allow for some positions that have less link separation that at the axle and maybe one parallel. These adjustment holes will allow for some tuning of your anti-squat and IC. They will allow for smaller adjustments than if the lower chassis mount was adjustable.
-Lastly build strong brackets, especially at the chassis and convergence of the tri link or wishbone.
That will give you a good start. This is by no means the be all end oll of building a link suspension I probably left out a ton of stuff. Look at a lot of different suspensions and you will get a good idea. You really have to see what will fit with your chassis and axle, and go from there. Unless your are building a full on tube chassis from the ground up, your configuration will dictate a lot of how the links will end up. This is even the case when building custom chassis, its just easier to move stuff when building from scratch.
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The reason why the uppers should be shorter than the lowers is so that as the rear axle mover away from the frame the links the anti squat will become less.
So that if you start to climb with a certain amount of anti squat which will cause the rear axle to move away from the frame this movement will then reduce the anti squat value which lessons the tendency for the axle to keep moving away. This results in a more stable setup which will hop less on the steep climbs. It actually lets you run more anti squat at ride height so that you can get a good horizontal launch while still remaining stable on the steep climbs because the anti squat value will lesson all by itself.
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If the links are all the same length as the axle moves away from the chassis the anti squat will increase which will cause the axle to move away more and the only way it can stabalise is by the extension on the rear springs (meaning the rear springs will extend more than if you ran short upper links).
I wouldnt be too concerned with the pinion angle changing - thats what uni joints are for. Probably should run a loose centre limiting strap to stop the rear from totally dropping out anyway.
 
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The roll centre is the point that the body wants to rotate about on the axle or the point that the axle supports the body laterally (or supports the body in a sideways direction). The simplest example is a triangulated 3 link with the upper links triangulated to a single point on top of the axle. This point is the roll centre. If you are talking about roll centre what most people are concerned about is the height of the roll centre. If the roll centre is high it will reduce the amount of body roll that you get when on side slopes. This is a good thing because it keeps everything stable in the off camber stuff and is why a lot of people try to get their roll centre up high - to reduce body roll. The roll centre can be thought of as the point where the body is supported laterally (or sideways). So that if the body is supported laterally up high it wont have as much tendency to induce body roll than if it was to be supported down low. If a rig uses a panhard rod then the roll centre is the mid point of the panhard rod.
When you start to look at triangulated 4 links things start to get a bit more complex and you should be really talking about the roll axis as apposed to a single point. But in simple terms if only the upper links are triangulated then the roll centre is high (means less body roll) and if only the lowers are triangulated then you get a low roll centre (means more body roll). If both the uppers and lowers are triangulated then the height of the roll centre will still be high and the angle of the roll axis will be more horizontal. Now without going into it too much a horizontal roll axis produces little roll induced rear steer (which may or may not be usefull on a rock crawler). Also if you can get the roll axis to be sloping down a bit (going from the rear to the front) then it will behave better on the road (again this may not worry you on you trail rig)
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The above statement ABSOLUTELY has merit to it and should not be ignored. By a combination of link placement and length, you can engineer the suspension to transfer more or less load through AS in different positions of travel. With the higher speed trends, one of the things I'm designing into the suspensions is EXACTLY what is being talked about by Strange Rover. By designing the placement of the uppers on the chassis in such a position that as the travel droops I get consistent or less AS it is doing 2 things I'm looking for... The axle will be consistent in AS more through the droop, meaning as the axle nears full droop, it will not be trying to walk under the rig. Instead it will be consistent when I mash the throttle whether I'm flexed out or setting level... AND, this can work both ways if designed where AS increases as bump travel nears the last few inches of compression... the suspension will load the links in such a way as to resist full compression on it's own. IF, you are looking to go fast in the rough.. I started looking at these effects on the suspension as a way to let the links help the shocks, and not fight them... As the suspension droops out across whoops, I want the valving in the shock to be doing the work and not find resistance from absorbing the bump from high AS as the tire touches the face of the whoop... conversely, I want the links to help the shocks resist bottoming out with highest AS seen right at the last few inches of bump travel. THAT will make a stable vehicle in both climbing and GETIIN IT!!
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It works very similar to a double-triangulated 4-link. The main difference is that the roll axis is always parallel to the upper links and runs through the imaginary intersection of the lowers.
*** Do this if possible ***
On a dual-tri (or wishbone upper):
1) You can tune the height and slope of your roll axis by changing the upper axle mounts
2) you can change your IC by changing the upper frame mounts. Changing one doesn't have much affect on the other.
That's not as easy w/ the DSI/AMJ setup since changing one directly affects the other. You can do it by changing the lower mounts instead, but that's not easy either since we usually want max ground clearance down there and therefore don't have as much room to play.
A big part of the debate over the DSI/AMJ setup is that in order to have a flat roll axis, you end up with a relatively high AS value. This may or may not be a bad thing. The guys running it say it works great for them.
(anyone have any info on the DSI/AMJ setup ? I searched a while ago and wasn't able to find anything. Some of the pbb info goes back several years and the threads are goto hell..
 
And I found this in a good thread also, I can't remember who posted it... anyway... This is very good info..

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I can't say that there is a generic set of numbers to work with... When I lay out the design work on a new rig, there are tons of stuff I'm keeping in mind.

The quick and dirty version..

1.) I lay out the suspension geometry first and map out the range of travel up and down...and draw in the tire size. This lets you see what kind of clearance you need.

2.) Look at the length of your shocks at your intended ride height. take a compass and locate where the shock will be located on the axle and draw an arc for the range that the top of the shock will be at ride height for the front and rear shocks..

This will give you some constraints to try to keep your tube work inside of... You know that tubing has to hit the arcs to mount the shocks and you know that the base of the chassis has to hit the control arm locations as well as clear the links..

3.) carefully measure your engine combo and draw it in like a block... this will let you see where the top of the trans will be, the top of the engine and oil pan... how far foreword the accessories set, etc. Make a template to move around on the drawing if you have too. determine where the bellhousing will be in the chassis.

4.) get the actual seat you plan on using and set down in it. get someone to measure how tall you are from the ground for head height, how far your arms reach out when your comfortable, where your feet are when they are comfortable where your elbow is when your holding a steering wheel... plot these measurements in the drawing and it will tell you where to put the seat, how high the beltline of the chassis needs to be to be comfortable and how high the roof needs to be to be safe...

5.) Start measuring each component and place where it will go in the chassis... Think ahead of EVERYTHING, then go look for parts that fit the space or design the space to fit the parts. I'd strongly encourage looking at the generic parts that are available and design the chassis to fit them... this will save lots of money in your build, custom parts get expensive quick!!

All these plotted points will start to let you know where the main structure of the chassis needs to be to hit critical suspension points and fit components while saving enough room to be comfortable. From there you have to play with component sizes and the lines in the chassis to make it look good...

Even knowing all the "critical dimensions" doesn't ensure the rig will have style. Sometimes changing the angle of even a single tube just a few degrees can have a HUGE impact on the attitude and character of a chassis... The main thing is to spend a lot of time drafting out your ideas and don't even think about trying to build until your happy with it on paper and can see that things will fit where you intended.
 
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