4Strokes.com Technical: 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:
Unloaded (without rider): Bike on a stand with rear suspension
Loaded - with rider: Bike on the ground
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 the "loaded
- with rider" dimension from the "unloaded" dimension.
Unloaded - 671mm (26.4")
Loaded w/rider - 568 mm (22.4")
Race Sag = 103 mm (4.0")
Adjust spring preload as necessary to obtain the desired handling
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.
Unloaded - 671mm (26.4")
Loaded w/o rider - 651mm (25.6")
Free Sag = 20mm (0.8")
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
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.
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
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
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.