Explaining Adjustable suspension (coilovers)

[Kenneth]

I have been asked questions about suspension quite a bit lately and seen a few on the forum. I thought it might be useful to explain the following bits and pieces.

I have attached a crude picture that should help visually explain some of the affects of different adjustments.

The purpose of suspension is to keep all tyres in constant and even contact with the road. This isn't always possible of course, but the more effectively this is done, the greater your grip will be.

Adjustable shock body (Length):
This allows you to set the ride height. Shortening means that the wheels will not come down as far when you raise the car. (Actually makes it easier to take wheels off and put them back on :) )

Obviously this allows you to lower the car which in turn lowers your centre of gravity. The following adjustments allow you to also stop you tearing a new one in the bottom of your car.

Spring rate:
If your spring rate is 10Kg/mm (Linear spring) then you require 10Kg of weight to compress the spring 1mm.
If you put 100Kg onto a spring rated at 10Kg/mm then the spring would move 10mm.

Increasing spring rate means there will be less change in height of any wheel given the same force applied. If you hit a bump in the road, a force is applied to the wheel to make it deflect. Higher spring rate will cause slow variations in the road surface to be transmitted to the vehicle body (going over bumps at low speed) as the spring will cause the wheel to return to its original position before the irregularity in the road is past. As a result you get a bouncy ride.

At high speed a higher spring rate allows the wheel to return to its usual position faster, stopping you bottoming out and absorbing irregularities more comfortably as you pass them quicker. If your car is very low, a higher spring rate will allow you to absorb more energy from the road without bottoming out. As a general rule, the lower you are the higher your spring rate needs to be to hold your car off the ground.
Using only preload will only keep you off the ground until the next bump in the road...

Adjustable preload:
Preloading the springs puts a certain amount of force on the spring without adding weight. So If you were to preload the spring by 50Kg, then putting 100Kg of weight would only move a spring rated at 10Kg/mm only 5mm instead of 10.

Preload differs vastly from spring rate, and should not be confused. If you place preload on the spring in your car, you will not change the ride quality of the vehicle. what WILL change is how far your vehicle sags on the suspension before resting at the ride height.
With no preload and a soft spring, your shock has to be long so that it can take up the weight of the car and leave enough ground clearance to be usable. This is why jacking up most standard cars creates such a large gap between tyre and wheel guard.
Because of this, preload does have an effect on ride height. The higher the preload the less sag you will get and therefore the higher your car will sit off the ground (spring rate and shock length being the same)

Coilovers that do not have separate length adjustment use preload to set ride height. This works, but is not always ideal.

Adjustable Dampers:
Dampers allow you to change the valve orifice size. The point in dampers is to create resistance when you try and move the suspension quickly.
The valve orifice creates resistance to the fluid trying to move past (or through) the piston. The smaller the orifice, the harder it is to move the piston quickly. ('quickly' is the key word here)
With car suspension, a large orifice means a more comfortable ride at low speed as the wheels can conform to larger road level changes without transmitting them to the body of the car. Small deviations in the road surface cause small changes in the piston position and due to low resistance in the valve, the damper piston is able to move freely through the liquid. The down side is that the damper is unable to absorb large forces well.

making the damper hard (reducing the orifice in the piston) means the piston doesn't want to move quickly even at low speed. This causes deviations in the road surface to be transmitted to the vehicle body more.
The benefit is at high speed when there is a lot of force on the damper, it is able to resist the huge amount of energy thrown at it when going over even small bumps.

There are 2 directions that the piston moves and it is important to consider the dampening force in each direction. Compression dampening pushes against the force pushing the wheel up into the guard, or hitting a bump in the road. Rebound dampening slows down how fast the spring can return the wheel to the height it wants to be at.

In most damper adjustable suspension for street use either both the compression and rebound are changed at the same or the compression only.

Questions or corrections welcomed.

[Munta]

SUPER WAGON DAMPER ( AS ABOVE + REBOUND AND EDFC COMPTABLE )

GT WAGON ( AS ABOVE BUT WITH ADJUSABLE PILLOW BALL UPPER MOUNTS )

whats the difference between the coilovers with pillow ball upper mounts and the 1s without? how 'much' better are they? are they worth spend the xtra $$ on?

[Kenneth]

SUPER WAGON DAMPER ( AS ABOVE + REBOUND AND EDFC COMPTABLE )

GT WAGON ( AS ABOVE BUT WITH ADJUSABLE PILLOW BALL UPPER MOUNTS )

whats the difference between the coilovers with pillow ball upper mounts and the 1s without? how 'much' better are they? are they worth spend the xtra $$ on?

They are to keep alignment and increase steering response with suspension travel and to transmit force directly to the chassis.

I would severely doubt they give any benefit to the Galant/Legnum as we have multi-link suspension and don't use the suspension strut for alignment, camber or any of that crap. Our suspension basically sits there and goes up and down.

Also, pillow ball mounts transmit more vibration to the chassis, which on a road car will just be noisy.

Multi-link suspension is basically a more complicated version of double wishbone suspension

[Kenneth]

Ok, to be more correct with pillow ball mounts... They provide a more direct response to bumps in the road.

Basically the rubber mount at the top will absorb some shock but not necessarily in a consistent manner.

You could say it is similar to replacing your suspension bushings with solid alloy... response will be great, but damn it is going to be uncomfortable.

Unless you change your suspension bushings and want to go full out track performance, I wouldn't bother with them. (unless you want them for **** factor)

Now if you had McPherson strut suspension I think it might be another story...

[Beastlee]

Excellent piece of information. Hopefully this will help those with adjustable suspension to get it right.
Rep on it's way.

[bernmc]

I still Don't quite get the preload bit...?

Have some rep anyway ;)

[psbarham]

I still Don't quite get the preload bit...?
its for bikes but it might explain it a bit


C ommonly asked questions like, “What’s the difference between spring rate and preload?” and “Can’t I make the spring stiffer by cranking more preload?” require delving a little deeper into the mechanics of spring forces.
Lets back up and define what spring rate and preload really are. “Spring rate” reflects the stiffness of the spring and is measured in kilograms per millimeter or pounds per inch. One of the ways to test spring rate is to first measure the spring’s “free length” -- the uninstalled length -- and then put weights onto it, measuring the resulting compression with the addition of each weight. “Straight-rate” springs maintain a constant rate of compression throughout their travel. If you are testing a straight-rate spring and you plot these points on a graph, you will end up with something that looks like Figure 1.

A stiffer spring would also start at zero force, but its compression rate would increase at a steeper angle, as in Figure 2. Notice that at 20 mm travel, the softer spring requires only 10kg force, while the stiffer spring requires 20kg. The stiffer spring at 1.0kg/mm is both twice as stiff as the softer spring, which is 0.5kg/mm.

Now let’s take the original spring and install it in the fork. As it’s installed, it gets compressed, or preloaded, a small amount. “Preload” (or “preload length”) is the distance the spring is compressed from its free length as it’s installed with the suspension fully extended. Just a note here on the difference between preload and preload adjuster: All bikes that I am aware of have preload. Some do not have external preload adjusters, but they do have preload. All forks can have preload adjusted internally by changing spring spacer length, though sometimes it takes special spacers. Forks that have external preload adjustment have preload even when set at the minimum adjustment.

The “preload force” is the initial force the spring exerts on the of the fork tube with the fork fully extended. Referring now to Figure 3, you can see that when preload is added to the spring, it effectively shifts the curve (line) to the left proportional to the amount of the preload force. In this case, the preload length is 35mm and the resultant preload force is 17.5kg at zero travel. In other words, with this 0.5kg/mm spring and a setting at 35mm preload, you would have to put more than 17.5kg force on the end of the fork tube to create any movement at all.

For a straight-rate spring, the relationship between force, spring rate and travel is described by the equation: F=K x L (or force [F} equals spring rate [K] times length [L]).

When you tighten the adjusting collar on a shock or increase the preload length by tightening the adjuster on the fork, you are indeed increasing the initial force exerted by the springs. This decreases sag, making the bike ride higher. It does not, however, increase the spring rate.

For example, you can achieve a targeted sag on the fork even with a spring that is too light (or soft) if you use a lot of preload. You can also achieve that same sage with a spring that is way too heavy by using very little or no preload. Let’s look at just one fork leg on paper. Refer to Figure 4 and notice that the softer spring has 35mm preload and therefore has 17.5kg force at zero travel (fully extended). Let us assume the front end has 30mm of static sag. At 30mm of sag, the total spring force is 32.5kg. This means that each fork spring must push up 32.5kg to create a 30mm sag figure. Any combination of spring rate and preload that gives 32.5kg force at 30mm travel will create the same sag. Notice the stiffer 1.0kg/mm spring has 2.5mm preload and, at 30 mm travel, also creates 32.5kg force. This means that they will both have the same sag; however, they will perform totally differently.

[Munta]

thanx heaps guys. i think im just going to go for some coilovers with damper adjustment.... sorry to hijack...

but who now has ability to get good prices on new tein coilovers?

[Kenneth]

I still Don't quite get the preload bit...?

Have some rep anyway ;)

Think of it this way,

Imagine you have a set of scales and when you stand on them, for each kilo you weigh the needle goes up one kilo.
ie:
actual weight - weight shown
1Kg - 1Kg
2Kg - 2Kg
3Kg - 3Kg
...
50Kg - 50Kg

Same scales but lets say you weigh so much your scales go off the scale (excuse the pun... lol).
Adding preload to the scales would add an offset at which the scales started reading. If you did a 50Kg preload, then the scales wouldn't start registering until you put 50Kg of weight on them.

However (and this is the important bit) after that they would STILL go up 1 kilo for each kilo of weight.
ie:
Actual weight - Weight shown
1Kg - 0Kg
2Kg - 0Kg
3Kg - 0Kg
...
50Kg - 0Kg
51Kg - 1Kg
52Kg - 2Kg
53Kg - 3Kg

If you changed the spring rate instead, say to 2x what it was, instead of having an offset, you would read 1Kg on the scale for every 2Kg of weight.
ie
1Kg - 0.5Kg
2Kg - 1Kg
4Kg - 2Kg
...
50Kg - 25Kg
52Kg - 26Kg

So, with preload the amount the spring moves per Kg of weight does not change, however the point at which it starts moving does. this means you can use a soft spring (for comfort), but put the preload up so that you get good ground clearance.
On a damper this is important because it means that you can have a softer spring without sacrificing suspension travel.