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
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
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,
one vendor online that does this and you may find a local shop that can do this
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
Update 7-5-07 - I did a more extensive discussion
of suspension links in this
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.
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.