Suspension Tactics

By: Jason P. Laskowski

Technical Director 

Introduction 

I’ve created the following document to give everyone a better understanding on how to tune suspension settings whether it’s your ATV or your street rod. 

Dampening (compression & rebound) 

Compression is when a shock is compressed (duh) and rebound is when it extends itself after compression.  By introducing dampening we have the ability to adjust how fast or slow a shock can do either. 

In most cases the spring (or spring rate) directly effects the compression (sometimes called pre-load). 

Almost all movement on earth is damped in some way. Damping can be viewed as some sort of friction that wants to stop or slow a given movement. 

Suspension systems usually use oil, which flows through a wide variety of valving systems (holes).  The damping force that is created increases with the speed the oil that is forced through the holes. 

By introducing complex valving systems we can tune this adjustment to suit specific suspension requirements. 

A typical damping device consists of an oil filled cylinder and a shaft with a piston on it, both of which travel through the oil. On the piston there are valves (holes) of some sort where oil can flow through. When the suspension moves in some direction, the piston is forced through the oil and oil is forced through the valves. 

Air damping basically works just as oil damping, only that air flows through the valves instead of oil. This means that the holes in the valves must be much smaller of course.  Air damping is generally not as good as oil damping, since air is compressible, which makes it more difficult to control / adjust the amount of damping. 

As I mentioned earlier the spring (or spring rate) primarily effects compression and / or sag.  Sag refers to the amount the rear of your vehicle will squat with rider onboard.  In my opinion sag is the foundation to suspension tuning.  In other words sag should be adjusted prior to any other suspension component. 

For specifics regarding how / where to adjust your shocks please refer to your owner’s manual or mechanics guide. 

Adjusting Sag (pre-load) 

Sag can be measured from any solid point on the frame (e.g. the grab bar on ATV’s).  I’ll be using an ATV in this example.  With the vehicle on a flat somewhat level surface measure the height of the grab bar from the ground.  Now sit on the quad in a normal riding position and have someone take another measurement ensuring to measure from the same point(s).  The difference between these two measurements should not exceed 35% of your suspensions full travel.  This percentage may vary depending on your ATV’s application (i.e. MX, Flat Track, etc.)  For example, if you have 12” of rear travel the ATV should not squat more than 4.2” when you’re seated on it (Max Travel x 0.35) = max sag. 

Usually your rear shock will have two adjusting rings which you can tighten or loosen to adjust the shock’s compression / pre-load.  This will also change the sag respectively.  

Adjusting Rebound

Rebound is the speed at which the wheel returns to the ground.  Rebound is not only responsible for straight line handling but also is the energy that holds your wheels to the ground in a corner and prevents volatile bouncing while tackling whoops.

Step 1:

Out on the track, find a good corner, preferably a short sweeper. The front should compress to set up for a turn.  The speed at which the shocks rebound is the energy that pushes your front end into the ground. If the shocks are allowed to rebound too quickly, the energy will be used up too early and the front wheels will slide to the outside. If the shocks rebound too slowly, the front end will tuck under and turn too soon to the inside. Adjust accordingly.

Step 2:

With the shocks handling well in the corners, go on to rougher sections of the track.  The shock action should be smooth and the wheels should return to the ground quickly.  They should not bounce off jumps or deflect off berms. 

Alignment 

Wheel positioning determines a number of performance aspects, including tire wear, braking control, vehicle stability in both straight lines and in turns, steering ease and overall vehicle 'handling.'  For all of these reasons, it's imperative to gain an understanding of wheel angles, their purpose, and their usefulness in obtaining proper chassis geometry. 

While a shop may possess an advanced computerized alignment system that requires no calculations on the part of the technician, it is nonetheless important to understand the basics.  

A basic alignment consists of the following: toe, camber, and castor. 

Toe 

The toe angle, as viewed from overhead, refers to the difference between the front tread centerline to the rear tread centerline of the two tires on the same axle.

The toe reading is generally in tenths of an inch (or millimeters).  As viewed from overhead, toe determines whether or not the tires roll straight down the road.  We can refer to toe as the 'angle of attack' for the tires as the vehicle is driven forward.  If the front of the tires point towards the centerline of the vehicle, with both tires aiming inboard, this is called 'toe-in'.  If the front of the tires are aiming in an outboard direction, away from the centerline of the vehicle, this is referred to as 'toe-out.'

Generally speaking, rear-drive vehicles usually require that front toe is set at a slight toe-in, usually 1/16 inch to 1/8 inch. This is done to anticipate the likely toe-out movement as the vehicle travels forward, due to imperfect tolerances in the front suspension bushings / bearings.  Many front-drive vehicles often specify a toe-out setting on the front wheels, in anticipation of forward acceleration wherein the front tires may 'crawl' inboard. 

In either case, the toe setting that will be achieved during the alignment is designed to compensate for the expected action during the vehicle's forward movement.  In order to travel down the road in a theoretically perfect 'zero toe' condition, you must first compensate by adjusting the front wheels with a slight toe-in or toe-out, knowing that the front wheels will try to splay in or out to a zero toe when in motion.  The object is always to achieve a zero toe angle as the vehicle travels forward in a straight line.  In the case of ATV’s more than likely you’ll need a very slight toe-in (0.5mm +/- 1.0mm) 

In some cases, the rear wheels are adjustable for toe angle as well. Common examples involve independent rear suspensions like Corvette, Jaguar, Datsun Z and those with trailing axles as found in front-wheel drive (FWD) cars.

Toe adjustment at the front is performed easily by adjusting the threaded adjuster sleeves on multi-link steering systems or the tie rod ends on rack systems.  Rear toe on some adjustable designs require rotation of cam bolts, the use of special shims or in a few cases, adjustment to threaded adjuster rods. 

Camber 

The camber angle refers to the inward or outward tilt of the tire when viewed from the front or rear of the vehicle. Negative camber exists when the top of the tire leans inward.  If the top of the tire leans outward, positive camber exists.  If the tire is positioned at a true vertical, it's set at zero-camber.  Camber angles are measured in degrees.  The camber angle is adjusted to maximize tire wear, handling and directional stability.

The majority of passenger vehicles are designed to use a slight positive camber angle at the front wheels. This is done to reduce steering effort, to increase highway-speed directional control (less road wander at speed), and to compensate for the added weight of driver and passengers. 

It's common for ATV’s and road-course competition drivers to request a negative camber at the front wheels, in order to maximize tire tread contact in severe turns. 

Though the front wheels may be set in a severe negative camber and tire tread contact may be minimal when the vehicle travels straight…chances are, the inner tread will only actually contact the road.  When the vehicle enters a hard right turn, the tire tries to flex and 'roll-over.'  The result is full-tread contact of the left front tire during the turn.  With full-tread contact in a hard turn, the driver maximizes his tire 'bite,' and is able to better grip the road surface.  Keep in mind that this is primarily true on dry road conditions.  In severe wet-surface conditions, there may not be sufficient tire grip to generate enough rollover to obtain a full-width tread contact patch.  In that case, a camber setting closer to zero would likely perform better.  However, because it's impractical to readjust the alignment every time the weather changes, performance drivers must choose between an optimum dry or optimum wet weather setting, or make a slight compromise between the two optimum settings. 

Adjustment of the front axle camber angle varies with suspension design.  On double-A-arm (upper and lower control arms) designs, the upper control arm's frame may require that flat shims be added or removed to adjust camber. 

If the upper control arm's pivot shaft is located outboard of the frame (as most ATV’s are), adding shims will move the wheel in a positive direction, while removing shims will move the wheel in a negative direction.  If the pivot shaft is located inboard of the frame, shims are added to create negative camber and removed to create positive camber.

On strut-type front suspensions, camber adjustment may take place at either the top strut tower (moving the top of the strut inboard for negative camber or outboard for positive) or at the bottom of the strut bracket where it bolts to the spindle.  Far too often, vehicle manufacturers offer little or no camber adjustment provision.  In this case, the use of aftermarket camber plates at the top, eccentric bolts, bushings at the bottom, or the entire a-arm assembly is required.  

Usually, if the adjustment is to be made at the lower strut bracket, the upper mounting hole will be the stationary point, while the lower hole will provide angle movement. 

On vehicles with rear camber adjustment (rear axles on some FWD cars, and independent rear suspensions), camber may be adjusted with the use of shims at the hubs or by adjusting eccentric pivot bushings. 

Camber / toe relationship 

It's important to realize that any change to camber will affect the toe setting.  If the steering arm is located in front of the front axle centerline, the toe will move out as negative camber is induced; the toe will move inboard if positive camber is induced.  When the steering arms are located behind the front axle centerline (like most ATV’s) and you change to a more negative camber angle, toe will move inboard.  If a more positive camber angle is achieved, the toe will move outboard.  The point is to always check and readjust toe whenever a camber change is made.  You cannot alter camber in any way without affecting the toe setting. 

Caster 

Caster refers to the tilt angle of the front spindles, as viewed from the side of the vehicle.  It involves the relationship between the top of the spindle and the bottom of the spindle at its attachment and pivot points.  Imagine a front suspension with upper and lower control arms.  As viewed from the side of the vehicle, the caster angle refers to the location of the upper ball joint to that of the lower ball joint.  If you draw an imaginary line between the two, that angle represents caster.  In the case of a strut-equipped front suspension, you can imagine a line drawn from the lower ball joint to the top of the strut tower.

When the top of the spindle is set back (placing the upper ball joint closer to the rear of the vehicle, with the lower ball joint closer to the front of the vehicle), this is called a positive caster angle.  All vehicles use a positive caster angle. 

The reason we have an offset angle between upper and lower spindle points is to allow optimum steering and handling for a given vehicle.  A positive caster angle is used to allow predictable steering of the vehicle.  If the caster was zero with the upper ball joint exactly above the lower ball joint, steering would be erratic, the vehicle would be very hard to control and the front wheels would wander all over the road.  By creating a backward 'lean' (a positive caster angle), the vertical travel of the front wheels is 'dampened.'  The more positive the caster angle, the more controllable the vehicle is at higher speeds.  As caster is lessened (closer to zero), the vehicle's front wheels will react faster to steering input, but highway stability will be decreased. 

Caster settings on many vehicles are fixed and can't be easily adjusted independently.  Replacement of damaged parts is commonly required in some cases.  Front double-A-arm suspensions allow easy caster adjustment at the upper control arm with the use of shims or supplied adjusting rods.  Strut suspensions may be adjusted through use of camber/caster top tower plates. 

Camber/caster relationship 

Since camber and caster are both affected by changes at the same mounting points, a change in caster can affect camber, and vice-versa.   In the case of an upper control arm adjustment, adding or removing exactly the same amount at both front and rear of the shaft will only affect camber.  By adjusting different amounts at front and rear of the shaft, both camber and caster are affected. 

Steering axis inclination (SAI) 

The SAI is a fixed angle, determined by the spindle or the strut.  As viewed from the front of the vehicle, this refers to the relationship of two lines: From the spindle attachments (the line drawn from the upper to the lower ball joints, or from the lower ball joint to the strut tower) to a reference point.  Depending on the type of alignment equipment being used, this reference point will either be a true vertical that passes through the center of the wheel hub, or the true centerline of the tire, from top center of the tread to the bottom center of the tread.  It sounds confusing, but all we're doing is checking these two lines to make sure the spindle or control arms haven't been bent.  A change to the specified SAI simply indicates that damage has occurred and parts need to be replaced. 

Scrub radius 

This is simply an extension of reading the SAI.  A change in scrub radius simply means that the camber is incorrect or damaged parts are in place.  Viewed from the front of the vehicle, this is basically the measurement of distance between the camber angle of the wheel and the line drawn through the spindle's upper and lower ball joints, measured directly at the point where the tire actually contacts the road surface. 

Pre-alignment Checklist

Test-drive. 

Check vehicle ride height and correct. 

Always check and correct tire inflation. 

Inspect tire condition and replace/rotate as needed.  If tires or wheels are unidirectional, follow proper rotation procedure. 

Check tire sizes.  Brand and model must be same at all four wheels (auto).  Sizes must be the same on each axle. 

Inspect steering components for looseness, wear, damage and improper installation. 

Inspect suspension parts for damage and wear. 

Inspect the brake system for potential causes of vehicle pull in addition to an overall system check.  This is a good time to perform a basic safety check, too.  Check calipers, hoses, hard lines, master reservoir, etc. 

Check drive line items such as driveshafts, U-joints and CV joints. A vibration complaint may be caused by a worn joint or by a driveshaft that has lost a balancing weight (auto). 

If the vehicle features front-wheel drive (FWD), check for evidence of recent engine or transmission work.  The engine-mounting cradle may have been reinstalled incorrectly.  Remember engine cradle placement can affect front suspension geometry by creating incorrect/uneven steering axis inclination or included angle, which makes it impossible to align correctly. 

Quick Reference

 References:

How to Make Your Car Handle
by Fred Puhn (Paperback - September 1983) 

Competiton Car Suspension : Design, Construction, Tuning
by Allan Staniforth (Hardcover - December 1999) 

Street Rodder's Chassis & Suspension Handbook
by Street Rodder Magazine (Editor) (Paperback - November 2000) 

Automotive Suspension and Steering Systems
by Thomas W. Birch (Paperback - May 1992) 

Auto Suspension and Steering Technology
by Chris Johanson (Paperback - June 2000) 

A Short Course on Wheel Alignment by Charles Ofria

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