Coilover Spring Selection & Setup

Coilover Spring Selection Setup

CHOOSING THE RIGHT COILOVERS IS SOMETIMES CONSIDERED A DARK ART. FIND OUT HOW THE EXPERTS DO IT!!

For the lucky few that do decide that standard OE suspension is never going to cut it and bite the bullet to fit coilover suspension, then here are a few basics that can save you some time in working out what spring rates you may need.

The biggest advantage of a coilover style shock absorber is as follows

The machined component design makes them easily adjustable, replaceable, and tunable. The ability to fit a wide variety of readily available coil spring rates and sizes make them ideal for both supporting the vehicle's weight and absorbing the inconsistencies in the road. Also, you can finely tune your coils and shock tuning to give you the utmost performance out of your suspension. Coils can be supplied in a huge range of spring rates and are universal fitment to suit all 2, 2.5 or 3-inch-diameter shock bodies.

Superior Engineering carry a huge range of springs to suit most 2.0in and 2.5in body coilovers, from 10in - 18in free lengths and spring rates from 80lb/in to 800lb/in.

For off-road work, dual-rate coilovers tend to be the most common setups. This configuration gives you the advantage of being able to set the initial rate which is often smooth and comfortable and is a combination of both the top and bottom springs working together. Then when further compressed over bigger bumps off-road, the soft 'tender' (top) spring reaches a point that the heavy 'main' (bottom) spring takes over increasing the final rate for the remainder of the shock travel.

It's a little confusing but here goes! Two differently rated coils make up one single rate when stacked, as they 'share' the load. Sadly it's not just as simple as adding their rates together. Even two 150-pound rate coils stacked on top of each other do not equal a 150-pound spring rate.

Here is a simple equation to help you figure out your rate:

Initial rate = (upper coil x lower coil) / (upper coil + lower coil)

EG: (using 150lb/in top and 250lb/in bottom) = (150 x 250) / (150 + 250) = 93.75lb/in initial rate.

This initial spring rate will be your spring rate until your coilover compresses enough for the coil slider (spring spacer) to stop at the dual-rate adjuster and the lower coil to become the only actively moving coil.

When it comes to choosing coils

it's a good rule of thumb to aim for around ~100 pounds of difference in spring rates of your two coils (example: a 150 top over 250 bottom). This difference in coil rate is referred to as the step-up rate (SUR) and gives you a change in the spring rates from the softer combined rate to a firm single heavy rate as the shock gets to the last 20-40% of compression.

There is no rule on this but generally slower vehicles can use 100-200% step-up rate and higher speed rigs 200- 300%. The point in which the SUR comes into play is adjusted using the threaded dual-rate nuts which are on the outer body of the shock. These 'nuts' are usually two round rings that can be wound up and down to sets the point at which they make contact with the spring slider that separates the top and bottom springs.

Coil lengths

should preferably be at least the same as the travel of the shock ie 14-inch travel shocks get two 14-inch long coils. Ideally if the shock mounting hardware permits, a slightly longer bottom spring can help position the dual rate slider better on the shock and stop any chance of it dropping away from the shock body at full extension.

So how do I pick the right spring rate to get the height I need?

First thing is to know the "sprung weight" at each end of your rig, once this and a few other variables are known it's normally easy to get fairly close.

For example: if you have 1000kg of sprung weight in the front end and you want 6in shaft showing of a 14in travel shock and 1in of pre-load, mounted at 80degrees off the diff, then:

Initial Rate = [ ((Sprung Weight x 2.2)/2) / ((Shock Travel - Bump travel)+ preload) ] / sine Shock-angle = [ ((1000kg x 2.2)/2) / ((14 - 6) +1) ] / sine 80 = 124lb/in (combined rate).

A combined initial rate of 124lb/in could be achieved by using - 200lb/in top coil and 350lb/in main coils.

The length and spring rate of the coils

play a huge part in the vehicles final ride height when selecting coils. Other factors such as mounting ratio, position or angle of the shock will need to be factored in to get everything spot on.

Pitching is the front-to-rear up and down motion (bobbing), apart from making for a terrible ride it will drastically reduce traction over bumps, leading to loss of vehicle control. For medium to high speed off-road use, suspension frequency is best somewhere around 10-20% higher in the rear than the front. Variations in spring rate from front to rear play a big part in Pitch.

Suspension frequency

Can be calculated if we know the wheel rate and the corner sprung weight, but we will go deeper into that another time!

When installing and calculating spring rates, try and start with a preload of the coils somewhere between 1/2in and 4in to keep the springs seated and to ensure proper droop/extension, wheel rate and bottoming control. Low preload is better for handling and reducing spring sag when adding weight but is less effective at pushing heavy diffs & wheels back on the ground as they pound through the bumps.

When choosing springs, there is never a clear right or wrong - balance and compromise are essential, and often the best solution for off-roading is something like "as soft as practical but still as firm as possible".

VEHICLE USE	SUSPENSION HEIGHT (% DROOP)		SUSPENSION FREQUENCY (Hz)
	FRONT	REAR	FRONT	REAR
SLOW, FLEXY CRAWLER	80	70	0.85	0.95
ALL PURPOSE OFF-ROAD	50	40-50	1.0-1.1	1.15-1.25
STREET, H’WAY,HIGH SPEED OFF-ROAD	30-50	25-60	1.3-1.45	1.4-1.55

Contact Superior Engineering for more information or help selecting from our extensive range of 2.0in and 2.5in Remote Reservoir Coilovers, as well as the huge range of premium Coil Springs and mounting accessories to suit most applications.