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Spring Preload, Sag & Adjustments

Posted: Thu May 12, 2016 4:39 pm
by 4Strokes
Dirt Bike Spring Preload, Race Sag and Suspension Adjustments
Note: Sample numbers are given for all dimensions on this page. Consult your owner's manual or service manual for your actual dimensions.

Spring Preload & Race Sag Overview
Setting the proper race sag (ride height) is important for competition use. Race sag refers to the amount of rear wheel travel used by your bike at rest, ready to ride, with you on the seat. As a general rule of thumb, the race sag dimension should be about one-third of the maximum travel. Ride height is changed by adjusting the rear suspension spring pre-load.

Spring Preload & Race Sag Adjustment
The following adjustment procedure establishes the correct starting point for any suspension tuning - the proper rear spring preload adjustment for your specific needs. Your bike should be at normal racing weight, including fuel and engine oil. You should wear all your normal protective gear. To calculate the proper adjustment, it's necessary to measure between two fixed points - from the top of the most rearward point of the sub-frame down to the machined edge at the rear of the swing-arm, for three different situations:
  1. Unloaded (without rider): Bike on a stand with rear suspension fully extended
  2. Loaded - with rider: Bike on the ground
  3. Loaded - without rider: Bike on the ground
Calculate the Race Sag Dimension
Support your bike on a stand with the rear wheel off the ground. Measure the "loaded - with rider" and "unloaded" dimensions. Remove the stand, with two helpers available, sit as far forward as possible on the seat, wearing all your protective gear. Ask one helper to steady your bike perfectly upright so you can put both feet on the pegs. Bounce your weight on the seat a couple times to help the suspension overcome any sticking action and settle to a good reference point. Ask the other helper to measure the "loaded - with rider" dimension.

To calculate the race sag dimension, subtract "loaded w/rider" dimension from "unloaded" dimension.
Loaded & Unloaded Dimensions
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671mm (26.4") - Unloaded
568 mm (22.4") - Loaded w/rider
103 mm (4.0") = Race Sag

Adjust spring preload as necessary to obtain the desired handling results:
  • Decreasing the race sag dimension (i.e. 98 mm, 3.9") improves turning ability for tight terrain at the cost of slightly reduced straight line stability.
  • Increasing the race sag dimension (i.e. 108 mm, 4.3") may improve stability on faster terrain with less turns, but will reduce turning performance slightly and may upset the balance between the front and rear suspension, producing a harsher ride. This will happen if the adjustment shifts the effective wheel travel toward the more progressive end of its range.
The ideal race sag (ride height) is 103 mm. This is the center of the recommended 100-105 mm range. Individual preference may produce a race sag from 85-115 mm. It is important to know your ideal race sag measurement before changing spring preload. Different abilities, riding styles, and measuring techniques will vary the ideal race sag among individual riders.
Loaded dimension (without rider)
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671mm (26.4") - Unloaded
651mm (25.6") - Loaded w/o rider
20mm (0.8") = Free Sag

Calculate the Free Sag Dimension
Free sag indicates the distance the rear suspension should sag from the weight of the sprung portion of your bike. To calculate the free sag dimension, subtract the "loaded - without rider" dimension from the "unloaded" dimension. Do this with your bike set at the standard race sag.

With the spring preload set to obtain the proper race sag, the rear suspension should sag 10 to 25 mm (0.39 to 0.98"). If the rear of your bike sags less than 25 mm (0.98") from its own weight, the spring is too stiff for your weight. It's not compressed enough, even though you have the proper race sag adjustment. As a result, the rear suspension will not extend as far as it should.

Spring Rate
If you are lighter or heavier than the average rider and cannot set the proper ride height without altering the correct spring preload, consider an aftermarket spring.

A spring that is too soft for your weight forces you to add excessive spring preload to get the right race sag and, as a result, the rear end is raised. This can cause the rear wheel to unload too much in the air and top out as travel rebounds. The rear end may top out from light braking, or kick sideways over lips and square-edged terrain. It may even top out when you dismount your bike.

Because of the great absorption quality of the shock bumper rubber, it may be difficult for you to notice when your bike's suspension is bottoming out. Some riders may think the damping or perhaps the leverage ratio is too harsh and in reality, the problem is most likely insufficient spring preload or a spring that is too soft. Either situation prevents utilizing the full travel.

Keep in mind that a properly adjusted suspension system may bottom slightly every few minutes at full speed. Adjusting the suspension to avoid this occasional bottoming may cost more in overall suspension performance than it is worth.

A spring that is too firm for your weight will not allow the rear tire to hook up under acceleration and it will pass more bumps on to you.

Suspension Adjustments for Specific Terrain - SOFT SURFACE (Sandy, Loamy)
Typically large rolling bumps will develop which may require an over all stiffer setting.

On soft ground, sand, and especially mud, consider increasing compression damping front and rear. Sand often requires a bit more rebound damping to minimize rear end kick. Although sand bumps are usually larger, there's more distance between them, giving the shock more time to recover. You may want the front suspension a little stiffer for sand tracks to help keep the front end up and improve straight-line stability.

In a muddy event, stiffer aftermarket springs front and rear may help, especially if you are heavier than the average rider. Your bike may be under-sprung because of the added weight of the clinging mud. This additional weight may compress the suspension too much and affect traction.

Front Suspension - SOFT SURFACE
  • Test harder compression damping in one-click increments
  • Test harder rebound damping in one-click increments
  • Increase spring preload in 1-1/2 mm increments
  • Raise the oil level by adding oil in each fork leg in 5 mm increments
Rear Suspension - SOFT SURFACE
  • Decrease race sag (by increasing spring preload) in 5 mm increments. Usually, one complete turn of the spring adjuster nut will produce a 5-6 mm change in race sag.
  • Test harder rebound damping with each increase in spring preload, in one-click increments.
  • Test harder compression damping in one-click increments
Suspension Adjustments for Specific Terrain - HARD SURFACE
On harder ground you may want to decrease compression damping. When an overall softer setting is achieved, the fork may have a tendency to bottom on jump landings. If this occurs, consider raising the fork oil level to make the end of fork travel more progressive so it resists bottoming.

Front Suspension - HARD SURFACE
  • Test softer compression damping in one-click increments
  • Test softer rebound damping in one-click increments
  • Check for dirt in the dust seals
  • Check the fork oil for contamination
  • Check fork alignment, following proper front wheel installation procedure
  • Check for air in the fork legs. Release accumulated air pressure by opening the air bleed screw on each fork cap.
Rear Suspension - HARD SURFACE
  • Set ideal race sag. Avoid making a major change in race sag in this situation.
  • To reduce the impact of small bumps - test softer compression damping in one-click increments.
  • To better absorb the impact of big jump landings - test one-click harder compression damping. If necessary, test two-clicks harder compression damping, followed by a 1/2 turn increase in spring preload. Repeat if necessary.
  • To stabilize the recovery of the rear wheel after jump landings (reduce a spongy feel) - test harder rebound damping in one-click increments
Keywords: How to adjust free sag