Suspension Links

8-23-06

   
 
 

This is a primer on suspension links.  It will contain a little theory and real world data covering the pros and cons of the various materials out there, what works and what doesn't.

Suspension links are a critical part in a link suspension.  Their job seems simple enough: locate the axles.  This means they must keep the axle from moving side to side and keep the pinion from rotating up or down.  A typical 4 link uses 2 or more of the links positioned so they form a triangle. This triangulation is what constrains the axle from moving side to side and typically the more triangulation the better. 3 link and radius arm suspensions require a  link that runs from the frame to the axle, perpendicular to the frame to position the axle side to side. For the most part the links are always in tension or compression, meaning the forces acting on them are inline with the link and either trying to stretch the material or squish it.  This is the ideal loading scenario for a round suspension link.

Due to the nature of our sport the lower links come into contact with trail obstacles which introduces forces pushing on the link from the sides.  The worst case scenario is a force acting on the center of the link, pushing perpendicular to the link. In this case if the force acing on the link is greater than the link materials yield strength, the link will bend and maintain that shape. The length of the link also factors into the equation, a shorter link is less likely to bend than a longer one.   

Broken Johnny Joint style misalignment bushing.

I have yet to see a suspension link completely fail on the trail.  Typically, complete failure occurs in the joint used to take up misalignment (Johnny joint, rod end, etc) or the mounting tabs for the misalignment joint (rod end). If the rod end used for misalignment and the mounting tabs on the chassis and axle are beefy enough that they are no longer the weak area on the suspension link you usually see the suspension link deform and not return to it's original shape. 

When designing your links you really need to treat the link, rod ends and mounting points as a complete system.  It makes no sense fabricating an unbreakable link but leaving the rod end as a glaring weak point. For this reason I like to use the 4 Bar Link Calculator (4BLC) created by Dan Barcroft and Greg Blanchette to model my suspension and then analyze my link options.  I like how their program uses your suspension geometry, rod end strength and link material to determine a safety factor for these four parameters: link yield, link buckling, link bending and rod end breaking.  You will need to take some measurements off of your suspension and have a good idea of the weight of your rig along with the weights of your axles but gathering this data is well worth it in the end since you will have a better idea of how all the components will work together which will let you better optimize your suspension components. 

So once you plug your suspension numbers and vehicle weights into the program you can start looking at link materials.  Our goal here is to have the factors of safety roughly equal among the yield, buckling and rod end. Again there is no sense in having a 30X safety factor of your link buckling if your rod end only has a 2X safety factor for breakage.   The last thing to look at is the factor of safety vs. bending which is much tougher to get up there.  


DOM

This is the most used material for suspension links.  It is readily available in most areas, fairly strong and affordably priced. To attach a rod end the tubing can either be threaded, threaded inserts can be welded to the ends of the tubing or a Johhny Joint style rod end can be welded directly to the tubing. 

  Steels are graded by their carbon content (more carbon makes the steel harder but also more brittle), you are looking for 1020 or 1026 grade. In the 4BLC the strength numbers for this kind of tubing fall between the Steel 1018 selection and the 4130N selection.  If you want exact numbers for the yield strength you can ask for material certificates when you buy the tubing.  You could also error on the side of caution and choose the weaker material (1018) when analyzing your suspension.  One thing to keep in mind is the program assumes heavy walled material, if you plan to get the wall thickness by sleeving the tube it will not be as strong as the single heavy walled tube. 

While playing around with the numbers you will notice the outer diameter of the tubing has the greatest affect on the strength of the link followed by the wall thickness and to a lesser degree the link length.  If it weren't for gravity or cost I think most of us would run 2.00 dia x .50 wall links and never look back.  Unfortunately you have to accelerate, stop and carry that extra mass up every vertical, straightaway and curve in the trail so in the grand scheme of things that extra weight does affect your crawlers performance. 

Broken rod end & cross member.  Link material was 2" solid 7075.

When looking at the factors of safety (FS) keep in mind there is always the potential to exceed the factor of safety in your design since there is no way for you to anticipate all possible conditions the link will be put thru. Realistically if you can get the FS above 15 for the yield, 6 for buckling, 2 for bending and 6 for the rod end you should have a stout setup that should survive most hard hits.  I base this personal rule of thumb on the past few years of usage on the two suspension I designed. The worst carnage I experienced was a few bowed links and some rod ends which became loose. Based on the hits and rolls the suspension took I am pretty happy with those results.  By using bigger components I could have avoided some of this but then again I may have experienced worse breakage by moving the weak link to the link mount cross member on the chassis. Like everything you have to make some trade offs, for me I'd rather have wear items that need to be replaced periodically vs. a hard to repair break.

Technical Data:

Note - all numbers taken from the Machinery's Handbook and may vary slightly from what is displayed in the 4BLC program.

1018/1020/1026 Steel (Cold Drawn) - Yield Strength 50,000psi, Ultimate Strength 60,000 psi


Aluminum

Like steel there are many grades of aluminum out there.  The most suitable and readily available for usage on suspension links is 7075-T6.  6061-T6 is even more common but has a yield strength far below most DOM tubing and should be avoided.

 7075-T6 is used extensively in the aircraft industry for it's light weight and high strength. Compared to DOM it is much lighter and has a higher yield strength.  The downside is its elastic modulus is about 1/3 of any steel which means it is more flexible and would be more prone to buckling than equivalently sized DOM. For this reason links made from aluminum are always made from solid material so the link will be more resistant to flexing.  To attach a rod end you will have to tap the ends to the desired thread size. Since most people won't have access to the equipment required to make a large diameter aluminum link you may have to purchase yours like I did.  I went thru Summit Machine for my 1.75" and 2" diameter links.  Shipping was very reasonable and their price was comparable to buying the DOM tubing and tubing inserts to make an equivalent DOM link.

When looking at the differences between aluminum and DOM links in the 4BLC you will have to set the wall thickness for 1/2 the outer diameter of the aluminum link to account for it being solid. A good starting point is 1.50" diameter solid for upper links,  1.75" diameter solid for lower front links and 2.00" diameter solid for the rear lower links. You will see that the FS in regards to bending and yield will be better with the aluminum links compared to DOM links so if you find you can't fit a large enough DOM link in your design to get the FS up where you want it you may try switching to a solid aluminum link.

As you can see in the pictures above I have no room to go to a larger diameter steel link in latest buggy. I bent one of the lower steels links by wedging a rock between the link and tire (a rather bizarre occurrence).  This particular link was made from sleeving 1.50" x .250 wall DOM with some 1.75 x .120 wall DOM.  The switch from sleeved 1.75" dia x. 375 wall tubing to solid 1.75" diameter 7075 bar netted me a higher FS in bending and Yield while dropping the weight of the lower link nearly in half (17 to 9 lbs).   

Another benefit to the aluminum is it will flex farther than an equivalent steel link and go back to it's original shape more readily.  This would seem to be an ideal characteristic for a piece you know will come into contact with trail obstacles. I am hoping this extra compliance will take some of the shock load off of my rod ends, possibly extending their life.  Time will tell if this theory pans out. 

Technical Data:

6061-T6 Ultimate Strength 45,000 psi, Yield Strength 40,000psi

7075-T6 Ultimate Strength 83,000 psi, Yield Strength 73,000psi


Chromoly

Chromoly steel tubing has the best strength to weight ratio of any material used in suspension links.  With this material you can get away with using a much thinner wall on the link and still have the equivalent strength of a heavier walled piece of DOM tubing.  Sounds great but there is a catch (there always is), to get the full strength of the material it has to be heat treated after all end machining and welding.  In addition, if you choose to weld threaded inserts into the tubing it must be tig welded for maximum strength otherwise you might as well use DOM.  So while this tubing offers the ultimate strength it is also a material most of us do not have the tools to work with and you would need to buy the links cut to length, tapped for rod ends and then finally heat treated.  If you are interested in this material you can contact www.polyperformance.com.

Technical Data:

4130 Normalized - Ultimate Strength 97,000 psi, Yield Strength 63,000psi

4340 Normalized - Ultimate Strength 185,000 psi, Yield Strength 125,000psi

4340 Quenched and Tempered (1000F) - Ultimate Strength 170,000 psi, Yield Strength 156,000psi


Link Tabs

For link tab construction, 1/4" thick steel holds up well.  I have used 3/16" thick material but over time the bolt holes tends to wallow out. For the lower link mounts on the axle I have used 5/16" thick material since they will see contact with the rocks.

Bolts are another potential weak link.  I prefer a minimum bolt diameter of 5/8" for link mounts (grade 8).  I have seen a lot of half inch bolts break on the trails.


Bent Links

I say avoid putting bends in your links at all costs, especially bent panhard bars.  Once you put a bend in a link it loses most of the strength it had and will be very easy to bend further.  To add strength back in you have to heavily gusset the link which takes up space and adds a lot of weight.

The biggest argument for bent links is for extra ground clearance.  I see a couple holes in this argument.  First, if your lower links angle out and attach at the ends of the axle tubes near the tires, they will move up and down with the tire making clearance a non-issue if your are putting your tires in the right spot. Second, straight lower links provide some protection for your rear drive shaft if they are slightly lower.  I would rather have the link be the first point of contact rather than my drive shaft. Lastly a straight lower link acts as a nice ramp to help lift the axle up and over an obstacle if you do end up contacting the link. I would think a bent link would allow an even larger obstacle to get closer to the rear axle which may make it impossible to drag the axle over the obstacle. 


Bracing a Link

Sleeving a link with another piece of material is the most common form of bracing a suspension link.  While not as strong as a single piece of tubing with an equivalent wall thickness it does help.  I have also had success with placing a rib of 3/8" or 1/2" x 1.00" material along the top edge of my lower suspension links. This extra material helps prevent bowing due to hitting obstacles with the bottom of the link but it doesn't not provide any appreciable strength against side hits.  


 

Cross Member Material

I have always used .120 wall round tubing for my link mount cross members.  I honestly would have used .250 wall material if I had some on hand at the time and this is one spot where overkill isn't a bad idea.

I would avoid using square material for the cross member as the stresses on the link mount will be concentrated on a single face.  The link mount shown above was welded along two faces of a 2 x 1 x 3/16 wall piece of rectangular tubing.  The material failed after a few months right next to the weld.

Since I am using relatively thin material on my cross members I have come up with ways to brace the cross members so the loads are better distributed. Shown above is a plate designed to brace the lower link mount cross member.  The plate extends nearly the entire length of the cross member and extends over the top of the link mounts. 


Rod Ends

I am using 3/4 x 5/8 rod ends (40klb rating) on all of my links.  For a buggy under 3500lbs these work good and I end up replacing one due to slop every year or so. I thought about stepping up to 7/8 rod ends but their load ratings are the same as the high end 3/4" rod ends,  If you want to go bigger I'd suggest going to at least 1" rod ends.  Since I built up my rig the prices on these larger rod ends has become very competitive so they are much more affordable than they used to be.

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.

When I designed my links I made them so the rod end would be fully threaded into the link.  If you have any threads showing on the smaller rod ends that is where the weak point will be and you will notice them bend after hard use.   

I haven't used the cartridge style joints (Johnny joint, Rubicon Express, etc).  I have seen the tube they are mounted in fail as shown above and there are no published strength ratings for these style of joints.  With that said I do know a lot of people running them that have had no issues so they are something to consider especially since they run $30-40