4-Link Tech

AssBurns

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#1
lets talk about 4-Link

To download the Triaged 3-Link and 4-Link calculators, click the link and scroll to the bottom right of the page for the excel spreadsheet based calculators.

https://patooyee.com/calculators/calculators.htm

I'm going to try to explain some things to look for when building your own 3-link or 4-link, and using the calculators. Feel free to correct anything I may have said that may be incorrect.

First, if you want to design your own suspension, you should understand the basics of suspension geometry, and the key words that you need to know when trying to come up with the best overall design for your needs. Remember, there will be a lot of compromising on suspension design. Not everything will be perfect due to packaging constraints.

Some things you should know before tackling this on your own. (Play with the calculators to visually see how changes effect each of these factors).

  • Roll Center - The point at which the sprung mass rolls in relation to the unsprung mass when lateral forces are applied. Measured in height in relation to the ground (You will see/hear this term a lot)
  • Instant Center - The convergence point at which suspension components hypothetically meet. Measured in height from ground, and distance from axle centerline. (i.e Draw a line along the upper and lower links until the lines converge. That point is the Instant Center)
  • Roll Axis Angle/Inclination - The axis at which the sprung mass rotates in relation to the unsprung mass when lateral forces are applied. Measured in degrees (There are multiple forms of Roll Axis Angles to consider when designing suspension.)
    • Vehicle Roll Axis Angle - The hypothetical line between the front roll center and the rear roll center.
    • Front/Rear Specific - The hypothetical line between the instant center and the roll center (This is important when eliminating roll steer)
  • Anti-Squat (Rear suspension) - The effect on how the suspension behaves when inertia or torque is induced during acceleration. The value is described as a percentage.
  • Anti-Dive (Front suspension) - The effect on how the suspension behaves when braking forces are applied. The value is described as a percentage.
  • Roll Steer - When one side of the axle is further forward or back than the other side when there is body roll. This causes a steering effect. (Highly undesirable, especially at higher speeds)

Some characteristics and numbers to shoot for when designing your own suspension:

Roll Center Height
Higher roll center means less leverage the center of gravity has on your suspension, which means less body roll. Too high is not always a good thing though. The higher the roll center is, the more likely you are to have jacking. Jacking is similar to how anti-squat effects your suspension, but on a lateral force instead of acceleration. If you have too much jacking, your vehicle will want to rise as it corners and transfer more weight to the outside tire which is not a good thing. For rock crawling, a higher roll center that is almost as high as your COG (Center of Gravity), is typically desired to help eliminate body roll when side hilling. A higher roll center in the rear can also help reduce your roll axis angle. For desert racing a lower roll center (a few inches above the axle centerline) can be desirable to help reduce jacking, and a sway bar can be used to help combat body roll.

Instant Center
I don't know the answer to this. This is a lot of discussion on this when it comes to drag cars, but I am not really sure the goals on this besides try to keep it in a somewhat consistent location throughout travel.

Roll Axis Angle/Inclination
High numbers are no bueno! The sweet spot seems to be somewhere between -2* to +2* with more emphasis on the -2*.
Too high and you'll get roll oversteer, which becomes super sketchy at high speeds and cornering. As you get body roll, it will want to oversteer further into the corner causing you to have to counter steer constantly. Also with a high positive Roll Axis Angle, it will want to lift the front corner of the vehicle (3 wheeling) as its cornering or crawling, especially with the rear end locked.
If you have too low of a negative Roll Axis Angle, it will have roll understeer which is less sketchy than roll oversteer, but it wall cause you to have to steer further into corners as the body rolls. Understeer in corners will cause the vehicle to want to plow through corners. A little understeer is much better than oversteer when considering roll axis angle. It will make the vehicle much more stable at higher speeds, and will want to stay planted through corners or crawling.
You want the rear Roll Center to be slightly higher than the front Roll Center to get a slight negative vehicle Roll Axis Angle.

Anti-Squat
This is more of a preference thing than a hard-set rule. You'll see different types of vehicles running different percentages, and every driver will have their own preference on what they feel is best. This is also something you'll have to take with a grain of salt because you cannot know your exact Anti-Squat percentage without knowing your EXACT location of your Center of Gravity. The general estimate on COG height is the top bolt on your transmission bellhousing. If you have a top heavy SUV or lot's of weight up top, you may want to add an inch or so to compensate for this.

Rock Crawling % - I've seen everything from 50% to 150% with good results. These days, we are seeing more people stick closer to the 100% range as a good happy medium. I personally prefer less than 100% even in crawling since it helps avoid wheel hop. I feel 80-100% Anti-Squat is a good range for most crawler based vehicles.

Desert Racing % - We see the desert trucks running much lower Anti-Squat % so that there is less force being transferred through the links and more being transferred through the shocks/springs which are easily tuned. I wanna say that these trucks are running somewhere between 25% - 60% Anti-Squat.

Dual Sport % - I personally think a vehicle that does everything from rock crawling to desert running to daily driving drives best with an Anti-Squat somewhere between 70% - 80%. This gives a more plush ride when under acceleration, and still has enough to keep the vehicle planted when crawling up ledges or hills. Too low and you'll be squatting a lot which is annoying when daily driving, and doesn't seem to perform too well when crawling.


Anti-Dive -
There are a couple different ways that Anti-Dive is measured.

Inertia Induced Anti-Dive - This is caused by the vehicle decelerating and the inertia from the vehicles unsprung mass putting force on the front suspension components. This is measured the same way as you would measure Anti-Squat, but reversed. With a low Anti-Dive % (Lesss than 100%), the vehicle will dive under deceleration, and rise under hard acceleration. With a high Anti-Dive (Greater than 100%) the front will rise under deceleration and dive under hard acceleration. With a 100% Anti-Dive, the front will not rise or dive under acceleration or deceleration.
100% Anti-Dive seems to be a general starting point.

Brake Induced Anti-Dive - This is a tricky one to explain. When you apply the brakes, the rotational forces that are then applied on the spindle/knuckle will cause the suspension to behave certain ways depending on how the links/arms are laid out. If you have links/arms that are parallel with the ground at ride height, you will have no brake induced Anti-Dive because the rotational forces being transferred through the suspension into the chassis would be equal in both directions. Now lets say the lower link is parallel to the ground and the upper slopes down to the rear as it meets the chassis. When you apply the brakes, the rotational forces will want to pull the top of the spindle forward and down. The steeper the angle between the uppers and lowers (the closer the instant center to the axle centerline), the more you'll get this brake induced Anti-Squat. You have have a very low inertia induced anti-dive, but have a lot of brake induced anti-dive (and vice-versa). This effect is also controlled by front/rear brake bias. The more front brake bias, the more this effect will be noticed. Also, the taller the spindle/knuckle, the higher rotational load transfer you will get in this brake induced anti-dive. This is a bigger deal in A-arm front suspension than linked suspension.
Lot's of good info in this subject here - https://www.pirate4x4.com/forum/gen...basic-ifs-irs-anti-squat-dive-tech-twist.html



TO BE CONTINUED
 
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#2
So I haven't used the link calculator. But so far the guidance I am going by will be about 10" link separation at the axle because I'll have 40" tires. The other piece of advice that I got from a trusted source is to try and keep the lower link below 5 degress relative to the ground.

I don't have any real world experience for what kind of anti squat/dive I should aim for for the type of wheeling I will do. Plus I don't know anyone with links.

If anyone has any info they would like to share maybe we can use this as a gathering place.
 

AssBurns

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#3
@AssBurns

I like that rear bumper. Came out nice.

Question on your link setup. I am hoping to start mine next month. Did you use the calculator to get you starting numbers and then adjust for room on the truck? I am having trouble finding info on anti squat/dive. I understand it but am not sure real world numbers to have. I am going to be going slower and mostly rock crawling. My uneducated guess is to look for 80%?

You are more go fast? Do you mind sharing what you ended up with? How have you liked it?
Yes, I used the calculator all the way through the process. What I did is take some rough measurements to get an idea of what would fit in the space I have to work with, then figure out mounting brackets, crossmembers, and axle truss. Once you determine what parts you’ll be using, get specific measurements based on those brackets, etc. Then crunch those numbers into the calculator to see how it works out, and adjust as necessary. You’ll always run into compromises, so figure out what you want most and try your best to work around that. For me it was eliminating roll steer by keeping the roll axis angle low (about -4° for me. -2° is about ideal) .

Anti-Squat is for the rear suspension and Anti-Dive is front suspension. Depending on what you are looking to build your truck for, is how you should determine how much anti-squat to build in. If you are building an all around wheeler, I’d say somewhere between 70%-80%. If you are building more rock crawler based and less daily driving, then go a little more than 80%. If you are going for a higher speed build, then stay under 70%. That’s kind of a decent starting point to work around. You can always adjust your frame side mounts down the line to dial in your anti-squat if you don’t like the way it’s setup.
 
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Yes, I used the calculator all the way through the process. What I did is take some rough measurements to get an idea of what would fit in the space I have to work with, then figure out mounting brackets, crossmembers, and axle truss. Once you determine what parts you’ll be using, get specific measurements based on those brackets, etc. Then crunch those numbers into the calculator to see how it works out, and adjust as necessary. You’ll always run into compromises, so figure out what you want most and try your best to work around that. For me it was eliminating roll steer by keeping the roll axis angle low (about -4° for me. -2° is about ideal) .

Anti-Squat is for the rear suspension and Anti-Dive is front suspension. Depending on what you are looking to build your truck for, is how you should determine how much anti-squat to build in. If you are building an all around wheeler, I’d say somewhere between 70%-80%. If you are building more rock crawler based and less daily driving, then go a little more than 80%. If you are going for a higher speed build, then stay under 70%. That’s kind of a decent starting point to work around. You can always adjust your frame side mounts down the line to dial in your anti-squat if you don’t like the way it’s setup.
Super awesome. Thanks. I'll post up info and pics as once I get started on the project.
 

AssBurns

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#5
Super awesome. Thanks. I'll post up info and pics as once I get started on the project.
Are you going to set it up as a dedicated crawler or an all arounder vehicle that’s focused on crawling?

The 25% vertical separation rule on the axle side is just a rule of thumb. With your setup you probably won’t have any issues, but unless you are running super high HP, you can easily get away with a little less than 25% if needed. I agree about keeping the lower links below 5°, you want them to be as flat as possible without reducing your clearance/breakover angle too much. It could be a tough thing to balance.
 
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#6
Are you going to set it up as a dedicated crawler or an all arounder vehicle that’s focused on crawling?

The 25% vertical separation rule on the axle side is just a rule of thumb. With your setup you probably won’t have any issues, but unless you are running super high HP, you can easily get away with a little less than 25% if needed. I agree about keeping the lower links below 5°, you want them to be as flat as possible without reducing your clearance/breakover angle too much. It could be a tough thing to balance.
I think realistically it will be mostly crawler. But do want the capability to drive it to the trail. Most trails are within an hour drive so I think it would be ok to lean more in that direction.
 

AssBurns

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#7
I think realistically it will be mostly crawler. But do want the capability to drive it to the trail. Most trails are within an hour drive so I think it would be ok to lean more in that direction.
I’d say focus on a lower roll axis then (-2° being ideal), and maybe somewhere between 80%-100% A/S.

That should give you a pretty solid crawling setup that’s good for everything. From there figure out packaging into the space you have available, while keeping ground clearance.
 
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#8
I’d say focus on a lower roll axis then (-2° being ideal), and maybe somewhere between 80%-100% A/S.

That should give you a pretty solid crawling setup that’s good for everything. From there figure out packaging into the space you have available, while keeping ground clearance.
Awesome thanks for sharing. :beerscheers:
 

AssBurns

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#13
I ran 1.25 ruffstuff heims on my 4500. Gonna run all summit machine joints on my new build though.
Those Summit Machine joints are really similar to Ballistic Joints, right? I remember looking into them when I built my old links with stock suspension, but their website sucks balls.

What's your reasoning for Summit Machine's products over the other hundreds of options we have these days?
 

AssBurns

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#16
Yes sir, apparently they are the tits. I know a few guys who race on them and haven't even needed a rebuild yet after a few years, and they're fully rebuildable and greaseable. Course they also cost like twice as much as heims.....
Geeze, if they cost that much, these guys should at least have a fucking website that works. Gotta call them and place an order like its 1995 still?
 

KevinK

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#19
Here to snoop on how else I might burn future monies. A buddy is about to have this Tundra rear linked at LSK. I'm kinda interested in building a crawler at some point in the distant future. Want to learn all the cool stuffs.
 
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