The Project Road guide to using adjustable dampers for maximized performance

The purpose of shock absorbers in the suspension is not actually to absorb shock (that is one the spring’s jobs) but rather to dampen vibrations and oscillations and to control weight transfer.  Damper rates determine how quickly the springs - and thus the wheel - can move up (compression or bump) and down (rebound).  The resistance to initial spring travel affects weight transfer off of and onto individual tires, and so is directly responsible for the vehicle’s transitional balance.  

The dampers only affect vehicle dynamics when the suspension is moving.  While a car with beefier aftermarket pistons will feel stiffer due to the higher resistance to wheel travel, once all weight has been transferred, the shock has no effect on handling.  Dampers are intended to damp oscillations, not support load.

The key to understanding the best way to adjust shocks for your particular application is knowing the damper’s roll in the suspension.  Here is a quick run-down.

Dampers will:

• Control oscillations of the wheel
• Determine how quickly and to where weight is transferred
• Allow for changes in transitional balance of the vehicle
• Make changes in responses to driver input
• Affect ride quality and comfort

Dampers will not:

• Change the vehicle’s steady-state balance
• Limit the full amount of wheel travel or total body roll (1)

There are unique settings within the shock that affect how they control matters.  Valving can be altered to control piston speeds between a range of around 3-20 in/sec. The faster dampening determines how the wheels will react to road irregularities and also determines ride quality.  High speed damper rates need to be matched with spring rates to keep the contact patch in check.  With valving that is too stiff the tire will skip over rough roads, provide a harsh ride, and cause premature shock wear.  Conversely, go too soft and the result will be oscillations over undulating roads – a floaty or bouncy feel – and may also lead to the springs wearing excessively.  Correct high speed damper valving/rates are best determined by a suspension professional for each individual application depending on vehicle weight, spring and wheel rates, and how the car will be used.

Low to medium speed valving determines how quickly weight is transferred to and from each tire during slower wheel movement.  These speeds are the result of load shifting due to driver input – in other words, how the car responds to your commands.  Since low speed dampening is what is adjustable on typical aftermarket shocks, this article will focus on changing these settings, either with adjustable shocks or by having them revalved by a professional, to change the balance of the car and how it reacts to steering, brake, and throttle applications.

Since the amount of downward force (weight) determines the coefficient of friction, or grip, the tire can offer, how much - and how quickly - weight is shifted to each corner is the deciding factor in the car’s transitional balance.  Vehicle mass is constant, so any weight removed from one tire will always be added to another.  This means that even if you only make adjustments to one corner or end, the result will be intrinsically tied to all of the contact patches and can often have more than one effect.

Compression (Bump) 

Compression damping controls how quickly the wheel can move up during suspension travel.  The stiffer the valving, the more resistance to movement the wheel will have, causing weight to be transferred onto that tire more quickly.  All else being equal, stiffening compression will result in quicker reactions to steering inputs, moving weight to the outside tire and off of the inside tire.   The same is true of fore/aft weight transfer as power and brake applications are made.  Stiffer bump settings in the rear will make weight transfer onto the rear tires more quickly under acceleration or when releasing the brake.  Similarly, firmer front compression will make weight transfer onto the front tires (and off of the rear) with throttle lift or brake application.

Rebound

Rebound settings determine how quickly weight transfers off of a wheel when it moves downward as the spring extends.  A stiff rebound setting will allow the wheel to move slower, thus transferring weight off that tire quicker.  Conversely, softer rebound allows the wheel move down more freely, keeping firmer contact with the ground for better traction.  Rebound stiffness determines how quickly an inside wheel unloads while turning or how fast the load moves from the rear due to throttle lift/braking or front under acceleration.

(1) In some cases, adding very stiff shocks to relatively soft springs – as sometimes occurs in stock class autocross – can have a similar effect as higher spring rates.  While this is an unusual occurrence (and ride quality will suffer) it is a way to increase transitional wheel rates (but not reduce total body roll during extended cornering) when changing springs is not an option, but is not the ideal way to stiffen the suspension.