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.
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
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.
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
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.
6061-T6 Ultimate Strength 45,000 psi, Yield Strength
7075-T6 Ultimate Strength 83,000 psi, Yield Strength
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.
4130 Normalized - Ultimate Strength 97,000 psi, Yield
4340 Normalized - Ultimate Strength 185,000 psi, Yield
4340 Quenched and Tempered (1000F) - Ultimate Strength
170,000 psi, Yield Strength 156,000psi
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.
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.
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.