4 Link Primer



I am no expert on suspension design but I figured I could at least share some of the knowledge I have acquired on the subject over the last few years.  This will be in no particular order and I will add stuff as I come up with it. 

Radius arm, 3 link, 4 link, panhard bars???

There are pro's and cons to all the different types of link setups but the real determining factor for what you use usually boils down to what you can fit.  The main thing to keep in mind is that the linkages must constrain the axle housing to keep it from rotating and so it cannot move side to side.  The length of your links will determine the wheelbase.  To control rotation a link must be placed above and below the axle centerline.  The greater the distance between these link mounting points the less stress the mounting tabs will be subjected to.  To locate the axle side to side you either need to triangulate the links by angling them inward or outward from the frame to the axle housing, or use a panhard bar which runs from the frame to the axle housing parallel to the axle housing.  If you use a panhard bar the other links should be straight with no triangulation to avoid binding between the panhard bar and links. These are the basics to a link suspension.  Here's an interesting link I found which covers a lot of the various suspension types.

Supporting the Load

Air bags, coil springs, coilover shocks, and air shocks are just a few of the ways used to support the weight of a vehicle with a linked suspension.   In order to figure out the proper spring rates for any of these devices, you will need to know the unsprung weight of your vehicle and if possible the unsprung weight at each corner.  Unsprung means everything supported by the suspension which is usually everything above the axles plus half the suspension weight.  Most of us guess at this, I'd suggest taking your rig to a truck stop so you can get a front/back weight, then search the internet for the weights of your axles.  This will get you close to what you need and will help the vendor who sells you your spring/coilover/airshocks make good recommendations.

Air Bags - have to be mounted further up on the links to get a lot of travel out of them which means your links need to be very strong and cannot rotate.

Coil Springs - Simple but little ride height adjustment (unless you build an adjustable mount).  The springs need to be captured top and bottom.  You'll also need room for a shock to dampen the suspension. Cons are limited spring rates.

Coilover shocks - Basically a large shock with a coil or coils that are attached to the body of the shock.  Pluses are lots of adjustment, easy to tune for specific vehicles due to hundreds of spring rates (single, dual and triple springs are possible) more compact than coil springs alone.  Travel is only limited by how you mount the coil over.  The springs determine the spring rate so generally speaking you can easily get a linear or very steep spring rate change as the springs compress.  The biggest con of a coilover is the price. For most applications you are looking at the cost of the shock ($250-500), dual rate kit ($50-75) and a pair of springs ($35-100) per coilover. You will also need a way to charge the shock with nitrogen (really only needed once unless you have a leak)

Air shocks - Very affordable ($200-300ea), small and light weight (approx 6-8lbs).  The spring rate is determined by the level of oil and nitrogen in the shocks so you don't need to buy multiple springs to dial in the ride height of the vehicle. The biggest con I have seen is the spring rate curve is very steep as you compress the shock. Also, tuning the shock can be difficult since you have to fully extend it to charge it or change the oil level (think 4 feet of wheel travel and then trying to get the chassis high enough so the air shock is fully extended) .  Another possible problem, most air shocks can only support approx 1000lbs of unsprung weight per corner which seems like a lot but this means you have to run them at high nitrogen pressures for a 4000lb chassis which can result in a harsh ride.  Lastly the entire vehicle's weight is basically supported by one shaft seal, any small leak instantly renders the shock useless and you will be riding your bump stops for the rest of the day (better plan on bump stops). Access to a nitrogen tank and regulator would be handy to have since every time you want to make an adjustment you will need to chare the shocks. The nitrogen tank will also be handy if you live in a warm climate and travel to a cold climate since the air shocks loose some height when exposed to a colder environment.

Coilover vs. Air Shock

Rockcrawling competitions kind of introduced the air shock to the recreational rockcrawling crowd. Comp rigs use them for their light weight and small form factor, two things that also benefit a recreational crawler.  However what you don't see comp rigs doing is driving long distances.  Many only have 1-2 gallons of fuel on board and are limited to 10-30mph due to hundreds of pounds of steel shot or water in their tires.  This means for the most part the people who discovered the air shocks don't ride as nice as coilovers do on the bumpy roads to and from the trails are the recreational wheelers. 

Reason for air shocks riding harsh involves the spring rate curve.  With an air shock the spring rate curve is very steep and shoots up to near infinity near the end of stroke. The more up-stroke you have on your buggy the worse this will be since it takes more nitrogen to get the shaft of the air shock sticking out further which makes the spring rate curve start even higher. Since most rubber bump stops compress 2-3 inches you really need 5-7" of shaft showing at ride height to get the best ride possible out of an air shock.  If you keep nitrogen pressure low (thus having little shaft showing) you end up riding the bump stops which can make for a jarring ride. There will be a few exceptions to what I said above, running air bumps would help keep the buggy off of the stiff rubber bump stops and a really lightweight buggy (Under 2500lbs total) wouldn't require as much nitrogen as a heavier buggy so the spring rate curve would start out much lower. I may sound down on the air shocks but that is not true, they are pretty amazing devices with some shortcomings.  You will need to deiced for yourself if these shortcoming matter to you or are outweighed by the advantages,  Here's a great write-up from Pirate4x4.com which goes into great detail on the air shocks.  I believe there is also a spreadsheet you can use to see the possible spring rates with different nitrogen and oil levels.

Coilovers on the other hand can have very shallow spring rate curves, especially when you factor in dual and triple spring rate kits. This makes it easy to get your rig to the desired ride height yet still have a cushy suspension.  Making minor ride height adjustments is also easier with the coilovers via the threaded collars.  I can say the only downside I have seen is you have to buy parts (springs) to adjust the spring rate, so you may end up with extra sets of springs after further tweaking.  One option to avoid this is to  buy from someone who will swap them out until you find the spring rate combo you like, Polyperformance is one vendor online that does this and you may find a local shop that can do this as well.


So if you decide on coilovers I can make some recommendations.  For lighter rigs (3000lbs of unsprung weight) I would stick to 2" body, 14" travel emulsion shocks with a dual rate kit. This will allow you to use 2 commonly available 14" long springs (QA1 coils from Summit Racing are the cheapest)  and you shouldn't need a helper spring kit to keep tension on the coils when the shock is at full extension.  The problem with the 16" travel shocks is there are very few 16" springs on the market and those that are out there start at a 200lb/in spring rate.  Project BMP uses a pair of 14" coils, one 150 lb/in and the other 175lb/in.  Since they are only 14" long there is not enough spring tension at full droop and the lower coil bucket can (and has) fall off. I solved this with a helper spring kit and some custom tabs that maintains some pressure (not enough in some cases) on the coils as the shock extends. The helper spring kit adds another $60 per shock.


Update 7-5-07 - I did a more extensive discussion of suspension links in this write-up.

Links are very important because if one breaks and you don't notice it right away (or even if you do) things can snowball fast.  One of the guys in our club bent a link on Anaconda one time and the subsequent movement of the axle broke the shock mounting eye off of one of the coilovers and then broke the driveshaft which next took out the fuel pump. The lesson we learned is don't skimp on the links. That being said you have your choice of aluminum or steel links, I am not totally sold on aluminum links.  I like the weight savings over steel but I don't like the failure mode of aluminum.  Basically it will bend more than steel and spring back, but if you go past the elastic limit, aluminum fails completely.  When you go past the elastic limit on steel it just retains the bent shape. I don't see a problem with using aluminum for upper links since they generally have less stress placed on them compared to the lower links and should not come in contact with trail obstacles.

What joint you use to connect the links to the suspension tabs on the chassis and axle housing are just as important as the link materials.  A failure here could also cause an expensive repair bill. There are numerous types of joints out there the most common types being a simple spring bushing, rod end or Johhny Joint.  I am not a fan of spring bushings since they have to bind, then deflect when the axle articulates causing premature wear, however this type of joint can keep a link from rotating if used on one end.  The Johhny Joint style swivel joints are fairly inexpensive and pretty common.  There are a few different styles mainly differing in the ball seat material and ball retaining mechanism.  The one reason why I didn't go this route is no one test their joints and publishes a max load rating.  The one I have seen fail actually tore the DOM tubing which houses the joint right where it was welded to the links, this was on a Samurai with Toyota axles and 37" tires.  This leaves my preference, a spherical rod ends (also referred to as heim joints).

 A 3 piece heim joint is the strongest for our application and you need to position the joints so they see minimal side loads since they are not tested for loads in that direction, only radially. The smallest heim joint I have seen used successfully is a 3/4 x 5/8, the 3/4 is the size of the shank and the 5/8 is the bolt size.  A 3/4 x 5/8 heim is actually stronger than a 3/4 x 3/4 heim since the bolt hole thru the center ball is smaller leaving more material.   Look for at least a static radial load rating of 20,000, the higher the better. Rod End Supply sells the XM10-12 (RH thread) XML10-12 (LH thread) for about $26, the joints are rated for 40,572lbs and are located on page 7 of their on-line catalog.

7/8 rod end next to a 3/4 rod end

Edit 7-5-07 - Expert Offroad sells the XMR14 for $27 with a jam nut.  These 7/8 rod ends use a 3/4" bolt and are rated for 55k lbs.  Ballistic Fabrication makes reducers and misalignment spacers to step the bolt hole down to 5/8 if desired.

If you do steel links I'd recommend using at least 1.50 x .250 DOM sleeved with 1.75 x .120 wall for the lowers, uppers can be 1.50 x .250 wall or if you can find it 1.25 x .120 wall. Notice all the above combos have a 1" ID. You can then get threaded tube inserts (3/4-16 to match the rod ends) from Coleman Racing.  #15590 is LH, #15589 is Rh and they cost about $5.


If you are shorter on time than money you can buy Shock tabs from A&A Mfg on the web, they sell .25 thick tabs with .50 holes in various sizes for under $2 ea (#AA-026-H). I bought the tall ones and cut them down to length (#AA-181-A). I also picked up my lower link mount tabs for the axles from Bluetorch Fab. They sell a kit with beefy 5/16 tabs for about $20 (four tabs). No way I could make those for that if I counted my labor.

On the subject of tabs; I get asked a lot how to mount the tabs for a shock.  For the most part if you mount the tabs on the axle parallel to the axle tube and then the chassis tabs close to perpendicular to the axle tube your shock should be bind free.  You'll still want to cycle the suspension and double check but this setup allows the lower joint on the shock to handle most of the angle the axle will see. I realize most cars and trucks on the road have the upper and lower shock mounting bolts parallel to the axle tubes but they were not designed for a lot of wheel travel and can get away with that configuration.