4 Link Tech or Four Link Tech

[lowbudgetjunk]

So far I have had good luck with learning the basics for air shocks and coilovers. I am getting closer to burning in some tabs and want to make sure I get the basics for the rear 4 link down pat. I am going to start with this and make a second post about the 3 link in the front.

The goal is to start with the average wheelers set up for a stable 4 link and then let some of y'all add questions and comments that are way better at this.

My disclaimer is still f#ck going on Pirate. I don't mind links to it, but would rather cut off one of my balls then participate.

Feel free to add thoughts and keep the bullshit somewhere else. Once I get back home, I will take some measurements and I hope some of you guys help point me in the right direction.

Sent from my XT1049 using Tapatalk

 

[fl-krawler]

Take them with a grain of salt, because every driver, every rig, and every terrain will have a different setup that works best for them. These are my general rules when building susp. for customers and friends.

Roll center height needs to be as high as humanly possible, and should be higher than the frame lower link mounting point

lower links should be atleast 36" long, and minimum of 15 degrees of triangulation.. Also try to get them as flat as possible. I try to not let the frame end of the link mount be more than 2-3" higher than the axle end at ride height.

The taller the upper link mount is on the axle, the more stable the rig will be and the better it will climb.

Upper links separation from the lower links at the frame end should be between 70-85% of the separation at the axle end.

Upper links should be around 85% of the length of the lowers

Instant center (the imaginary point where the upper and lower links converge) should always be in front of the front axle.. never behind it.. Ideally in a perfect world you would want it right on top of the front axle.

Antisquat: I've found numbers as low as 50% work better for going fast, to 100% for rigs that climb a lot of vertical ledges. Personally I like around 80-85% and the above rules get me pretty close to that number usually

 

[Craig_c]

From the mouth of Eric Miller, lower links flat as possible, upper links slightly downhill going torwards the frame, and enough triangulation to support it.

There are no magic link lengths, but everyone has their preference. My U4 Buggys links are 27" before adding the length of the heims and it works great.

Use the calculator as a base, then get stuff where it actually fits.

 

[mac5005]

Here are some design points to shoot for:

As close to zero pinion angle change as possible.

If you have some pinion angle change, ideally have the pinion rotate up slightly at full droop, but this means longer uppers than lowers wen viewed from the side horizontally.

Lower link length close to tire diameter.

You will have to compromise travel antisquat with pinion change.

Uppers at 67-70% of lowers will keep antisquat through travel consistent, but will have more pinion angle change.

Compromise for what's important to you, and design for what style driveline you will run.

You want at minimum, 40 degrees of total triangulation when viewed from the top. Add the upper and lower triangulation amounts, when triangulated in opposing directions.

If you can't get 40 degrees, plan for a panhard (track) bar to locate the axle laterally.

Get as much triangulation as possible when comparing the uppers and lower links, when viewed from the side.

This means uppers as high as possible on the axle, and lowers as low as practical.

Generally separation at axle should be a minimum of 25% of tire diameter. Separation at the frame around 50-75% of the axle separation.

You want antisquat between 50-90%.

Typically make frame side uppers adjustable with points at 60,70, and 80%.

Start at 70% and see what you think.

Too little and you will lifting the front too much under throttle, and not forcing axle into ground.

Too much and you are wasting HP on extending the suspension as weight transfers rearward.

Too much will also cause wheel hop as the suspension loads the tires looking for traction, and then extend the suspension, removing weight on rear tires and then the tire spins. As soon as it spins slightly, the suspension will return to ride height, and process starts over. Wheel hop.

You want roll axis angle as close to zero as possible.

Generally uppers as high, flat, and parallel to the ground at ride height as possible.

Lowers as low as possible without being anchors.

Typical link lengths similar to tire size , and/or driveshaft length.


With all that said, if you can make all those things happen, and all that fit the chassis side mounts, no matter if it's single or double triangulated, straight uppers, or lowers,
You will be happy with the setup.

 

[lowbudgetjunk]

http://www.patooyee.com/calculators/calculators.htm

Got to searching some tonight. First one that popped up and it was on JJ's site.

 

[Elliott]

See where in the side shot how they cross out front in the imaginary lines, that's called your instant center, I always run my uppers flat and my lowers angled so I can get that where I want it and keep the ride height I want. If you start looking there are some that they cross in the rear instead of the front, that will not work well for you, it will act like the suspension is bound up.

 

[Neal3000]

Some good reading here-

http://www.pirate4x4.com/forum/general-4x4-discussion/168577-link-suspensions-dummies.html

These basic rules have worked well for me

- try to make your links parallel when veiwed from the side
- make them as long as you reasonably can, 40" max
- 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.
-uppers 75% of lowers
- make uppers adjustible

 

[Rokcrler]

Re:

Here is the 75% rule explained by Jessie Haines:
75% Rule Explained!

For probably 15 years, I've been hearing about the 75% rule when it comes to suspension geometry: "your upper links should be 75% the length of your lower links". But why? Never once have I heard an intelligent explain of why. I'm guessing that at some point, someone read it in an engineering book somewhere, it got printed in a magazine, and it was then know as the rule! I can't say for certain the theory behind the 75% rule, but I believe I have a pretty solid theory on the basis for it (for a rear suspension).

I'm not going to spend hours explaining everything about anti-squat, but the change in AS throughout suspension travel is why the 75% rule was created. With your uppers shorter than your lowers, as your suspension droops, the AS will decrease. As the suspension compresses, your AS will increase. That means as the suspension droops, the geometry changes will DECREASE the forces pushing your axle down, away from the frame. As the suspension compresses, the change in geometry will INCREASE those same forces. This means the suspension will be less likely to unload as it droops. It also stiffens the suspension as it gets closer to bottoming out.

This sounds great in theory! The reality is, the 75% rule shouldn't be at the top of your priority list when you're building your suspension. Things like packaging, pinion angle change, and caster change may be more important in your situation. One example, on an Ultra4 or race truck, pinion angle change should be a higher priority. When the suspension goes full droop at 70 mph, you really don't want your pinion angle dropping and causing vibrations. In other situations, your driveline angle may bind at full droop, and you could remedy this by changing the length of the upper links.

Because AS forces are reversed on a front suspension, the 75% theory doesn't really make any sense on a front end. I can only assume the "rule" was based on rear suspension only. Designing a 4-link is almost always a compromise. You just need to figure out what's important to for your situation.

 

[ForMud]

There is something about link suspensions I can't seem much information about. That is how to match the front to the back to make them work together?
I linked the back of my rig last year and I followed the rules as space permitted. Next year I plan on doing the front, what should I try aim for? I'm talking about AS numbers and instant center numbers. I don't the two fighting each other.
Also pros and cons between four and three links would be helpful.

 

[wayne86]

There is something about link suspensions I can't seem much information about. That is how to match the front to the back to make them work together?
I linked the back of my rig last year and I followed the rules as space permitted. Next year I plan on doing the front, what should I try aim for? I'm talking about AS numbers and instant center numbers. I don't the two fighting each other.
Also pros and cons between four and three links would be helpful.

Good question. I have been thinking the same thing. I am currently building a jeep with front and rear four link. I have come to the conclusion that I am overthinking everything and I'm just mounting where things fit and look (with common sense) correct. I can then cycle the suspension and check my pinion angle to make sure nothing silly is going. This may be wrong and I may get flamed for it but look at all the different builds on here and other forums and there are all kinds of configurations.

 

[BUG-E J]

Good question. I have been thinking the same thing. I am currently building a jeep with front and rear four link. I have come to the conclusion that I am overthinking everything and I'm just mounting where things fit and look (with common sense) correct. I can then cycle the suspension and check my pinion angle to make sure nothing silly is going. This may be wrong and I may get flamed for it but look at all the different builds on here and other forums and there are all kinds of configurations.

Best way to do it. Shock angle and correct valving have more to do with the way your suspension acts than people realize

 

[TRD]

As someone who deals with and trys to compensate for poor geometry:

Rig build priorities:
1: Low COG
2: Good suspension geometry
3. Good shock/suspension tuning

Suspension geometry priorities:
1: Flat lower links!
2: Seriously, make the lower links flat!
3: Triangulate the upper and lower links (not always possible, lower link triangulation is highest priority)
4: Static anti-squat (as MAC mentioned)
5: A/S and Pinion change trade off

Good shock tuning can't make up for bad geometry.
Good geometry can't make up for a bad COG.

 

[mac5005]

Just wanted to update some.

You will have to compromise your front AS change through travel and balance that with caster change through travel.

Caster change through travel is same as pinion angle change.

This is where you must design the suspension to match your drive line style.

A drive line with single joints at both ends works best when pinion angle remains parallel with the output angle.

In a perfect world, the pinion would rotate down in droop, and up in bump, and the angle to the chassis stay the same through travel.

This would also make the caster angle stay consistent through travel, so that
Steering would be consistent through travel.

However, this style would bind the driveline if using a double Cardan front driveshaft.

Front AS #s becomes antidive (AD) while braking.

+AD(AS) in the calc equates to Anti-lift forces on the front suspension under throttle in 4wd.

Rear AS #s in triaged calc is based on 100% rear torque and traction bias.

Imagine triaged calculator is in 2wd.

You must reduce this AS % number by the torque and traction split front to rear for being in 4wd. The same applies to the front, when using the calculator to look at front #s.

Another note: It's important to check your design through bump and droop to make sure nothing goes crazy. When you are going up a 45 degree climb, the force from Gravity is still downward, but now at 45 degree angle to the chassis.

This means your suspension is no longer at static ride height for flat ground.

There is more sprung weight on links and less on springs, raising the ride height when going uphill.

If vertical, in theory there would be no spring weight on the springs, and it all would be on the links, and would be at full droop.

This is why you do not want AS to increase in droop.

 

[BustedKnucklefilms]

These may help.
ROCK RODS TECH TIP - Anti Squat and Anti Dive
ROCK RODS TECH TIP - 4 Link Suspension Design 101

 

[lowbudgetjunk]

Keep it coming fellas. The videos are nice to get a good visual as well as theory

 

[ForMud]

Just wanted to update some.

You will have to compromise your front AS change through travel and balance that with caster change through travel.

Caster change through travel is same as pinion angle change.

This is where you must design the suspension to match your drive line style.

A drive line with single joints at both ends works best when pinion angle remains parallel with the output angle.

In a perfect world, the pinion would rotate down in droop, and up in bump, and the angle to the chassis stay the same through travel.

This would also make the caster angle stay consistent through travel, so that
Steering would be consistent through travel.

However, this style would bind the driveline if using a double Cardan front driveshaft.

Front AS #s becomes antidive (AD) while braking.

+AD(AS) in the calc equates to Anti-lift forces on the front suspension under throttle in 4wd.

Rear AS #s in triaged calc is based on 100% rear torque and traction bias.

Imagine triaged calculator is in 2wd.

You must reduce this AS % number by the torque and traction split front to rear for being in 4wd. The same applies to the front, when using the calculator to look at front #s.

Another note: It's important to check your design through bump and droop to make sure nothing goes crazy. When you are going up a 45 degree climb, the force from Gravity is still downward, but now at 45 degree angle to the chassis.

This means your suspension is no longer at static ride height for flat ground.

There is more sprung weight on links and less on springs, raising the ride height when going uphill.

If vertical, in theory there would be no spring weight on the springs, and it all would be on the links, and would be at full droop.

This is why you do not want AS to increase in droop.

Am I correct in thinking, because I have full hydro steering I shouldn't worry about caster and the effects on the steering?
What effects the pinion movement? The length of the upper link compared to the lower?

 

[BUG-E J]

If your caster gets funky your turning radius goes to ****

 

[84mallcrawl]

If your caster gets funky your turning radius goes to ****

and caster the opposite direction is just as bad. It feels like you are a circus bear balancing on a ball.

can someone with better knowledge put some measurements in here or at least guidelines to follow?

 

[Beerj]

I was told 6 to 8 degrees is what you want.

 

[BUG-E J]

I was told 6 to 8 degrees is what you want.

Honestly on a solid front axle that will not be driven much on the street the best thing to do is get the high steer arms as flat as possible. Caster is the angle of the upper and lower ball joint. The more caster you have the straighter it drives to a certain point. Helps your return to center also.