Suspension parts, what they do.

**Blknight.aus** · 22nd December 2009, 07:00 AM

its been asked What do the various bits of your suspension do and what effect does changing them have.

So in the same vein as the grey box of evil heres suspension 101..

I'll break it down into the 3 basic configurations that are on landrovers, leaf, coils(and I'll touch on airbags) and independant suspension but to start with I'll cover the componentry thats common to all

Now we all know that the suspension is there to make the ride nicer but thats just a handy side effect the real purpose of the suspension is to keep all the wheels on the ground so that you can put your power, steering and stopping efforts to good use and thats never the same twice due to a lot of variables that interact with each other. For instance what works well on an F1 car which is designed to go really fast round a really smooth track would be totally useless in a baja comp truck which has to go relatively fast over no road at all.

In a nutshell theres only a few classes of components in any suspension system sometimes one part will do more that one job but if you look closely all the classes are almost always there.

The first thing you have is control components. These are the bits that hold the axle (be it a full beam or a stub) and allow it to move. They are probabley the most form varied component

the next thing is some kind of a spring, this applies force to the axle to support the weight of the vehicle and theres several basic types and about a million variations of each you get coil and leaf springs, torsion bars and lastomers, finally hydraulics and air.

that gets the wheel to move up and down and the following components serve to restrict its movement.

Shock absorbers serve to slow the wheels up and down movement and stop bouncing and finally

Bump stops and Check straps. These limit the total movement of the axle to prevent damage to other components on the vehicle.

**Blknight.aus** · 22nd December 2009, 07:41 AM

The components that are common in all systems are Bump stops, Shock absorbors and bushes

The Bump Stop

The bump stop has a simple job to do it stops the axle from moving up so far in the suspension that it hits the chassis or over compresses the spring and shock absorber.

They take advantage of a rubber (or rubber like substance) that while nearly non-compressible is malleable. As the it gets force applied to it it begins to distend and the more it distends the more force it takes to do so. As an example to "compress" a 10cm long block might take 1KG of force for the first cm an additional 2 for the second 3 more for the 3rd and so on up to full "compression". Each cm of movement take exponentially more effort.

1cm=1kg
2cm=3kg
3cm=6kg
4cm=10kg
5cm=15kg

you get the idea.

without bump stops if you get suspension components going metal to metal they can get damaged.

if you get axle-chassis contact you can bend/break either item
If you get over compression of the shocky you will either bend the shocky which destroys it or punch it off the mount by breaking either the mount or mounting point.
if you get spring binding (in the case of a coil) or over extention (in the case of a leaf) you will either break the mount or snap the spring.

the wrong composition of the stop risks bending the axle or arm that its attached to if its too stiff and all the above problems of metal to metal contact if its missing or too soft.

positioned to far into the travel and you rob yourself of travel and risk bending the axle or arm and too far back in the travel risks the problem of metal on metal at the spring or the shocky..

Related to the bump stop is the check strap, usually only seen on long travel or leaf spring setups it prevents hyper extention issues of bits traveling so far that they fall out of alignment.

The shock absorber
The shock absorber or Dampner slows the motion of the suspension and prevents bounce. a simple demo is with a rubber ball, a hard surface and wet sponge. bounce the ball on a hard surface and it comes up fairly high, gravity stops it from coming back to full height and given enough time it will gradually come to a rest.

Place a damp sponge on the floor and drop the ball onto that. The sponge acts as a shock absorber and the ball should just about stop immediately.

in a nutshell thats what your shock absorber does stops the bounce. They can also preform seconday functions. Depending on consturction they can also act as a check strap, a bump stop and a locating device for the springs as well as being part of the load bearing system (the spring)

Bushes
Bushes are a simple version of a bearing, they allow the suspension components to move and flex at the pivot and connection points.

**Slunnie** · 22nd December 2009, 08:44 AM

Out of interest, some manufacturers also design the bumpstops as a part of the springing system. They are delierately made large and positioned just off the axle so that the progressiveness of the spring is significantly ramped up before the actualy bumpstop action begins. Its all rubber still and a cheap and unrefined way of doing what air bumpstops do in offroad racing vehicles. These bumpstops are a lot longer than normal bumpstops.

**Blknight.aus** · 22nd December 2009, 12:27 PM

This is the post about the bits that actually take the weight of the vehicle.

theres half a dozen different ways of holding up the weight of your vehicle.

First and easiest is the leaf spring, basically a set of metal bars heat treated in a certain way and bolted up to the chassis with the axle bolted to it, Its been around since the horse and cart.

The nice thing about the spring leaf is that it does (or can do) the lot. IT locates the axle, it controls the reactive torque, it can provide the dampening (this is achieved by internal friction between the leaves) and if setup the right way eliminates the need for bump stops. But its usually the least flexiable of the lot. Its biggest advantage is huge load carrying and its simplicity.

next in line is the eleastomers. Basically big bump stops great loadability but thats about it as a primary suspension component when used in place of a traditional coil type spring, pretty rare to see these days.

then we're into coil springs, basically a big coil of thick wire tempered up to be a spring, its biggest advantage is the massive number of combinations of material, thickness, tempering, shape and rates that can be combined to come up with all the varying ways to carry the load. depending on the configuration of the rest of the suspension its easily the best bang for buck when it comes to flexability and load bearing. The only things that cant be readily designed into the spring is torque reaction control and locating the axle.

The next sort of load bearer is the torsion bar. Basically a lever on a bar thats anchored at one end. The simplest way of demonstrating it is with a wooden ruler, Grab it one end in each hand and twist. when you let go of one side it springs back flat, thats the concept of a torsion bar. they come in a few flavors using either metal or a lastomer to provide the spring action. The biggest draw back to the torsion bar setup is the range of motion is limited and exceeding it will usually damage the bar and the range of weight and progression is limited, however its light and simple to construct.

Airbag suspension, other than the material used for the springing, is generally the same as coil springs but instead of a metal spring you have a rubber bag full of air the biggest disadvantage to airbags is if you run out of air, hole or unseat the bag your running on the bumpstops and any support the shocky is designed to offer. The biggest advantage is the ease of which the load bearing and height of the suspension can be changed. Air in equals more height or load and take it out to lower it down.

Hydraulic suspension is again similar to Airbags and coil springs in that a hydraulic actuator replaces the spring but it has more in common with an airbag than a coil spring becuase somewhere in the system will be a gas charged accumulator that provides the space for the oil to comrpess to when the suspension is travelling. Its biggest drawback is its weight and complexity but its advantages include bi directional axle postion control and the ability to lock the axle in a given position Its also got gargantuan load bearing ability and if setup to do so can use variable porting to negate the need for a shock absorber.

**dmdigital** · 22nd December 2009, 05:21 PM

You mean the suspension isn't there to keep you in suspense

Sorry couldn't resist that

But seriously, I hope this will help a few peoples understanding.

**samuelclarke** · 22nd December 2009, 09:28 PM

Just wanted to say thanks Dave for taking the time to put this down. I already understand most of how it all works, but for those still figuring it out this is a great writeup (and I learnt some new stuff too).

**Blknight.aus** · 15th May 2010, 06:21 PM

Knew I forgot one...

The locating bits.

These are the bits of the suspension that keep the wheels where they are ment to be, more or less under the vehicle and control most of the forces that act on the wheel.

as previously mentioned some suspension setups use other parts of the suspension to provides some of the control measures so lets first look at what forces are in play that we need to control.

First longitudinal location. This is the forwards and backwards motion of the axle. Without this the first time you nailed the loud pedal in a 2 wheel drive vehicle the front axle would quickly become friends with the rear axle which would have driven itself forwards to say hi. the reverse holds true when braking, with no longitudinal control the rear axle would quickly find itself catching up with the front axle.

Longitudinal location in most rovers is taken care of by the leaf spring, the hockey sticks in the front of a coiler or the radius rods/arms down the back. If you have independent suspension then its the shape of the control arms that provide the longitudinal location. (actually its the bolts that hold the arms to the diff but thats just getting picky).

next up is Lateral location this is the left and right location of the wheel. If the suspension didnt control this youd very quickly scrub out the sides of your tyres on the bodywork and the chassis. Again in a leafer its the spring leaf that provides the location, in a coiler its the panhard rod up the front, the Aframe down the back (the 5 point watts link in a D2) and the control arms in an independent suspension setup.

Then theres Torque control. This is the resistance to the twisting action applied to the wheel by the brakes and the half shafts. Without torque control the first time you hit the brakes or gave it the bikkies the wheel would remain doing what it was doing and the axle would merrily spin in the opposite direction to what you wanted it to. Thank you Sir Issac Newton. A great example of poor torque control is numpties on high powered bikes. One Twist of the wrist sees them standing on the ground while the bike turns itself upside down in front of them. In a landy again its the leafs, the front (and rear in a D2) hockey sticks and the Aframe that provide torque control with the control arms in an independent suspension vehicle taking the brunt again.

The last thing to be controlled is the vertical motion of the suspension, Its fairly Rare to have this controlled by anything other than the springs bumps stops and check straps but it does happen occasionally usually in torsion bar setups.

And thats it for your suspension componentry.....

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