| 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 fully extended
- 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
671 mm (26.4")
Loaded
with rider - 568 mm (22.4")
Race Sag
= 103 mm (4.0")
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.
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.
Unloaded
671 mm (26.4")
Loaded
w/o rider - 651 mm (25.6")
Free Sag
= 20 mm (0.8")
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 XR.
Because of the great absorption quality
of the shock bumper rubber, it may be difficult for you to notice when
your XR'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 XR 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 1½ 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 ½ 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.
Tech:
Main Menu
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The owner of TOO TECH suspension, Rick Johnson, tells us how to set-up
suspension. Rick has graciously given us these tips for you to read.
Here's PART I of Too Tech's Suspension Tuning Tips:
Dialing in your bike's suspension.
The settings in this article are for a '97 YZ 250, GP Desert. Yours may be
different.
STEP 1: Measure suspension "Race Sag": (Most
important adjustment there is) All measurements are made between the rear
fender and the rear axle. The first measurement is made with the bike on a
center stand with the rear wheel in the air.
The second measurement is made standing on the foot pegs with full equipment.
The difference between these two measurements must be 3-3/4" Short track
and Supercross, 3-7/8" Natural terrain moto-cross, 4" Gran Prix and
desert.
STEP 2: TUNE RACE SAG (The other "most important
adjustment")
Adjust the forks to the standard height in the triple clamps before starting
any adjustments.
Increasing the preload on your rear spring will decrease the Race sag. This
will raise the rear of your bike putting more weight on the front wheel and
reduce the front-end rake. This will always make the bike turn sharper. However,
if you tighten the spring too far it will make the bike twitchy and promote
headshake.
Decreasing the preload on the rear spring will increase the Race sag. This
will lower the rear of your bike, putting less weight on the front wheel and
causing it to ride like a "chopper". This will reduce head shake,
making the bike go straighter and be more secure in high speed sections.
However, if you loosen the spring too far the bike will be harder to turn.
To fine tune the spring preload (Race Sag), try tightening the rear spring
adjusting nut 1/2" turn at a time and mentally note how much easier the
front end will dive into a turn and hold the inside line. Continue this spring
tightening until the bike becomes twitchy and unstable, or it feels like you're
always pulling up on the handlebars. Measure and record your Race Sag.
Then try loosening the rear preload 1/2 turn at a time and mentally note how
the rear end "Squats" down and traction increases as you exit each
turn. When you reach the point of excessive front-end lift (wheelies) and loss
of steering, or you begin to have trouble holding a tight turn, the spring is
too loose and you have too much Race Sag. Measure and compare these two
extremes, then reach a compromise between them that balances
"stability" and "tight turning".
Tune the front-end ride height to match the rear end!
If the Race Sag compromise you determined above is close to the typical
measurements listed above, your fork height adjustment in the triple clamps is
probably about right.
Raising the forks in the triple clamps will lower the front end making the
bike turn sharper but will reduce high-speed stability. (Similar to increasing
the rear preload.)
Lowering the forks in the triple clamps will raise the front end making the
bike harder to turn, but will reduce high-speed stability. (Similar to lowering
the rear preload.)
NOTE: Once you have established the best overall ride height front and rear, record
these settings as your baseline. For added stability on a Desert or Gran
Prix track, I always push my forks down about 1/8" to 1/4". For Moto-Cross
I pull them back up to improve turning. To further improve turning on a flat
Supercross style track, tighten the rear spring about 1 turn.
STEP 3: Break in the new suspension valves and oil.
Leave the settings as received for at least 1/2 hour. Put the bike on a
center stand and release the accumulated air from the front forks. Re-measure
the rear shock sag to insure this critical adjustment is still 4 inches. Ride
once again concentrating on any gross problems like suspension bottoming front
or rear, definite harshness.
STEP 4: Adjust compression damping for bottoming.
Rear shock: Increasing your compression damping (the screw on the
shock reservoir), will slow down the compression stroke and decrease rear end
bottoming. Turn you compression adjuster "in" (clockwise) to reduce
bottoming. If you never bottom, try turning your adjuster "out"
(counter clockwise) to soften the compression damping and use more travel.
Slight occasional bottoming is OK, but don't allow the bike to crash down when
bottoming.
Front Forks: Increase the compression damping (the screw at the bottom
of the forks) to slow the compression stroke and decrease front end bottoming.
Turn your compression adjuster "in" (clockwise) to reduce bottoming.
If you never bottom, try turning the adjuster "out" to soften the
compression damping and use more travel.
NOTE: Softer, screw "out", settings provide a plush mushy
feel which works well for cross country racers trying to go straight and
conserve energy. Stiffer, clicker "in", settings hold the suspension
up and out of holes and provide more lift on jump take-offs. Additionally, body
English and throttle changes transfer directly into the dirt instead of getting
lost in a mush suspension.
STEP 5: Adjust compression damping front & rear.
(Note: Bring a small screwdriver with you and make adjustments at your test
track.) It is important to make all damping decisions with the suspension
hot and to immediately test the change.
Making damping decisions in the garage can lead to nasty surprises.
Letting your friends adjust your suspension is also a no - no.
If "bottoming" is noted at either end, the compression damping
should be adjusted "in" (clockwise) to reduce the compression stroke.
The front fork compression adjuster is the slotted screw at the bottom of the
fork. The rear compression adjuster is the screw in the shock reservoir.
STEP 6: Adjust rebound damping front/rear. (Critical adjustment,
change slowly)
If either front or rear tends to kick up, (rebound), more than the other
after landing from a large jump, then more rebound damping is needed at that
end. Adjusting the rebound damper screw "in" or "clockwise"
causes more damping, which causes the suspension to return more slowly to its
original ride height. If the front end bounces up after landing from a jump,
turn the slotted screw at the top of the forks "in" 1 click at a time
to slow their return. If the rear end kicks up after landings, or kicks up side
to side down high-speed straights, turn the slotted screw at the bottom of the
shock "in" 1 click at a time, to slow the rear wheel return.
But remember, too slow a rebound setting causes "packing" because
the suspension does not have time to rebound to its original ride height before
you hit the next bump.
Rule Of Thumb: Run your rebound at both ends "faster"
rather than "slower". When the bike in on the verge of, but not quite,
kicking up after lands, the rebound is just about right.
STEP 7: Balance front end and rear end static ride height.
If the rear end squats under acceleration along with too much front-end lift
and/or the bike doesn't want to turn sharp or easily enough adjust your rear
sage to 3 - 3/4 inches.
If the front end rides low, turns too sharp, and/or tends to Head Shake, try
a combination of lowering the front forks in the triple clamps and adjust your
rear sag to 4 - 1/4 inches.
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