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Suspension Basics: Part 1
By
Sean Coker
Suspension, in a perfect world, would not be needed. Anyway you
slice it, suspension robs power, alters geometry and draws attention
away from riding. But the fact remains that we need suspension to
deal with the irregularities all around us: potholes, ripples in
pavement, overlapping sections of asphalt, etc. So what do all
those knobs on our bike do and how can they help make me a better
rider?
A properly set up suspension allows a motorcycle to better deal
with the inconsistencies of real world riding. With the help of a
couple of friends, a few minutes and some note taking skills, you
too can properly set up a bike for free. Modern bikes are frequently
equipped with adjustable preload, dampening, and compression
settings. These three settings are often found on the front and rear
of the bike and help determine the nature of the bike’s handling.
Getting started, Setting Sag
There are two types of sag: static and free. Static sag involves
how much the suspension compresses under the weight of the rider and
free sag involves how much compression occurs under the weight of
the bike alone. Measuring static sag is easier with the help of a
few extra hands. Start by measuring the total amount of suspension
travel by lifting the front end completely off the ground. For
traditional forks, measure from the top of the dust cover to the
bottom of the triple tree and for upside down/inverted forks measure
from the dust cover to the top of the axel mount. Record this
measurement as L1.
Mount the bike and have someone depress the forks, letting them
rise until they stop, this measurement is known as L2. Get off the
bike and measure how much the suspension sags from its own weight
(free sag), this is known as L3. Static sag is= L1- (L2+L3)/2.
Street riders should aim for anywhere from 1.25-1.5 inches of static
sag for the front end. Now the rear wheel.
The procedure for measuring the rear suspension is nearly the
same. Pick a spot directly above the rear axel and use this point
for measuring. Using a center stand or some buddies to lift the sub
frame until the rear wheel is off the ground (race stands will not
work because they are still loading the suspension) and measure from
the axel to our point of reference. Record this measurement as L1.
Mount the bike and have a friend stabilize it from the front. Have a
second friend depress the rear about an inch (do not bounce!) and
let the suspension ride to a resting place. Record this measurement
as L2. Now have your friend lift up the rear of the bike an inch and
let the suspension drop to its resting place. Record this
measurement as L3. Spring sag is actually in between L2 and L3, the
difference is the frictional drag of the linkage. Again use the
formula static sag= L1- (L2+L3)/2. If there is too little or too
much sag then an adjustment is needed, which is done via preload.
Preload is the initial compression of a spring and is
accomplished generally by a slotted screw that depresses a spacer on
the front forks and a ramped collar (or collars) on the rear. By
screwing in the preload adjuster, or tightening the rear spring
collar, we are adding more preload. If there is too much sag add
preload, and conversely too little sag is remedied by removing
preload. Remember, preload is a minor adjustment and if no amount of
preload will set proper static sag, different springs are needed.
Note: some bikes are equipped with a dampening adjustment to the
inside of the preload adjuster. Adjusting preload will not change
dampening settings.
If your bike does not come equipped with a preload adjustment on
the front, fear not. Raise the front of the bike and remove the fork
caps. Identify any stock spacer located inside the fork and measure
its length. Find the biggest piece of PVC pipe that fits inside of
your fork, and, using a miter box to ensure proper perpendicular
cuts, cut sections of PVC pipe in varying º inch lengths. Make sure
to deburr the edges before placing inside your forks. Write the
length of the spacers on the spacer in a back marker and reinstall
the fork caps. Go about measuring static sag again and adjust spacer
length based on need.
Next month, we dive deeper into suspension settings.
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