Linked Suspension 101 Part 1
Part 1 of 5
Bottom line up front. There are 3 big questions to ask anyone about linked suspension if you're looking for a shop to build a rig for you, or if you're looking at a linked suspension kit. I consider these to cover the absolute minimum knowledge. I'm going to break these questions down and explain them along the way.
Disclaimer: If you're only spending a couple thousand dollars on a solid axle swap, this doesn't apply... Not only that, don't expect a cheap solid axle swap to be built with these things in mind. This is for performance linked suspension. There is a market for people just welding stuff on wherever it fits or building it the "way I've always done it and it works great". Not everyone needs or wants a performance minded rig, and that is totally fine.
- Anti-Dive is how much force or weight transfer is sent into the links.
- It's primarily a concern when braking as it loads the front and compresses the suspension.
- You can control how much of this force is sent into the shock/spring assembly, and how much is eaten up by the links due to their positioning.
- These numbers are represented as percentage values, 0% implies that all the force from the weight transfer is sent to the springs/valving, 100% implies that none of the force from the weight transfer is sent to the springs/valving and instead the links "absorb" it.
Why does it matter?
This first question gives you a good idea of wether or not a shop understands linked suspensions. This is a basic data point that folks who are designing these things can confidently tell you what they are, and why they are.
The biggest red flag you can find is if someone pulls this up for a front 3 link:
Triaged 3 link calculator is based around old drag link rear end calculators. It does not take into account drive bias...
This means that any numbers attained from this calculator will only be accurate if your rear driveshaft breaks and you are driving home in front wheel drive.
Unfortunately the internet is full of advice based on the most simplest of errors. Any numbers you get from folks using this are incorrect to start with (even if we're pretending they scale the truck for COG height). There are several much better free calculators out there, the irate4x4.com one comes to mind as an example of the best free resource out there. There are also some fantastic paid calculators, though the paid versions are almost exclusively for drag racing they do have functionality above that of a free resource.
Asking specifically about the numbers throughout travel weeds out folks using calculators that don't show suspension geometry throughout travel. Suspension are not static fixed objects that don't move. In fact the whole point of a suspension is that it moves. Your numbers at ride height are overrated.
Knowing those numbers in both 2wd and 4wd will let you plot them for both fire-road overlanding and rock crawling (depending on application). For a vehicle that is driven on the street, knowing the anti-dive in 2wd is absolutely crucial, particularly if you're going to valve a solid axle for going fast off-road. It's important to be aware of the suspension geometry throughout the entire range of the suspension doing its job, this lets you dial in specific characteristics or traits for better performance.
2. Wait, you guys are measuring COG height?
- Rules of thumb for Center of Gravity height are not accurate.
- Taking the time to measure out your suspension, then basing all the info you receive from those measurements on a made up number, does not help you plot link geometry.
- Vehicles must be scaled in their final load-out to ensure the links meet your design criteria.
I wouldn't say there is no point in guessing on COG height, as even with a guess you can get some general data from a suspension calculator like driveshaft plunge or caster change. I fail to see the point in going through all the trouble to plot out your suspension just to guess on the data it's all based around. It's kind of like doing a really long math problem and getting the answer wrong because you didn't care to check if the number on the first line was a 2 or a 20.
The easiest and most accurate way for you to calculate COG height is with scales. By scaling the vehicle on flat ground then lifting one end at minimum several feet in the air, you can achieve a fairly accurate "Z" height for where your center of gravity lies. The higher you lift the vehicle the more accurate the data. I was working on a very extensive linked suspension encyclopedia where i had almost 20 different link setups ready to post then realized i was scaling them incorrectly... In my quest to get the most accurate data possible for you guys i was not accounting for the lower end of the vehicle side loading the scales which was altering my numbers. Because of this i started puting the tires on trash bags with a thin film of grease inside so that they could slide freely and load the scales correctly, which put my work behind by almost 6 months.
3. Does Geometry affect shock valving?
You would be hard pressed to find 2 things more closely related on a vehicle. It's important to design something with the end goal in mind.
Imagine you have 2 identical trucks, both with the same exact link geometry and both with 6 inches of up travel. One has 16" coilovers and the other has 10" coilovers.
In that situation both those trucks would need different spring rates, valving, and geometry.
Truck A has 10" of down travel at ride height and we want 2 inches of preload on the springs. That means it must be sprung with a coil that will compress 12" at ride height. If this vehicle has a 900 lb corner weight, that gives it a 75 pound per inch main rate. This might look like a primary spring at 125 and a secondary spring rate of 200.
Truck B is the same truck but with 4" of down travel, give it 2 more for preload and you need a spring rate that is compressed 6 inches at ride height. This comes out to a 150 pound per inch main rate. This might look like a primary of 300 and a secondary at 375.
Truck B has a main spring rate that is double that of its identical twin Truck A. Clearly you would not valve a truck the same with double the spring rate, and therefore your Anti-Dive should be different as well. Heavier spring rates have the advantage of being faster, by that i mean they are quicker to re-accelerate unsprung mass which is great in a solid axle vehicle with the coilover mounted at 1:1 as you're trying to get a ton of weight (get it) moving up and down quickly offroad. With the heavier rate truck if you left the compression valving the same as the soft sprung truck it would be horrible on and offroad, as it wouldn't use hardly any of its suspension travel except at very high speeds. The truck with the heavier springs will generally have some compression valving taken out, and rebound valving added in to control the extra spring force (when compared to its twin). Because the heavier sprung vehicle can re-accelerate faster, it requires less bleed than its twin as well. With the lighter sprung vehicle you may have to add in quite a bit of bleed to fight the initial stiction and aid in the transitions from bump to droop travel.
In this scenario, a lightly sprung vehicle with lots of bleed in its main piston will have significant brake dive at lower anti-dive numbers. The heavier sprung vehicle will have less brake dive so you can get away with a lower anti-dive number overall. Depending on your use case, having low anti's can generally be seen as a good thing for high speed offroad, as well as factoring in your anti-lift to get the front end raised over high velocity sections. It will also reduce wheel hop, as anti's produce a jacking force which also creates a movement of the suspension. Additionally, because your front brakes do a majority of the work in slowing a vehicle down, having weight transfer to the front when braking can improve stopping distance.
The benefits of heavier springs is often offset by how generally compliant and slinky low rate springs are in the rocks for my particular use cases, but i would like to highlight the fact that there is such a thing as "too much droop". Also, no one can seem to agree on what a heavy or light spring is... there are folks literally putting 700 lb springs on the front of toyotas, or 650s with stock valved 6100s. It sort of makes me wonder if in the absence of comparison maybe folks don't know how much performance they're missing out on.
Numbers from calculators are based on perfect traction environments, adding in massive boulders and sandy bits throws a monkey wrench into everything. Building a vehicle with "perfect geometry" is almost never possible due to chassis constraints. It may not even desireable for all-around driveability across many uses, so compromises must be reached based on each build.
It pays dividends to start with the end goal in mind.
Part 2: General overview of suspension terms and how they relate to the real world
Part 3: Using the Link calculator to analyze Bump Steer
Part 4: Determining safety factors for heim and link sizes
Part 5: Valving for going fast with high unsprung weight