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Suspension Knowledge: Balance
and Sag
by Quinton Cain of
SUSPENSION DYNAMICS
Let’s clear
up some confusion and dispel some myths. I will explain sag
and how it is measured. I will also explain an often
overlooked aspect of the bike known as balance.
Balance is
what you want to achieve. Sag is what you adjust to get it.
Race sag,
also known as rider sag, is the distance a bike settles down
from its fully extended position when a rider is on board.
Free sag is a
similar measurement, but with no rider on board. Your free
sag should never be zero. Typical ranges are 15 to 35 mm.
Balance is
just what it sounds like. The front and rear of the bike
should go down at approximately the same rate.
Pre-load is
the distance a spring is compressed from its free length.
Increasing the pre-load decreases the sag, and makes the
bike stand up taller.
Measuring
sag: A metric tape measure is the quickest and easiest tool
for measuring sag. You can measure both the front and the
rear, but the rear is most common and is easily adjustable.
You will need two people.
You will take
two measurements, and then subtract them to get the net
distance. Measure from the rear axle to a position on the
fender directly above the axle. Take both measurements at
the same points.
Measure:
(1) with bike on a stand
(so that the rear wheel is fully extended)
(2) with bike off stand
and rider seated on bike
Additional
guidelines:
- Rider should be in full
gear, seated in a neutral position with feet on pegs.
- Bike is on level
ground.
- Bounce the bike up and
down. If the bike does not fall to the same point each time,
average the high and low points, (then consider greasing
your linkage).
A good
starting point for race sag is 100 mm for big bikes, 80 mm
for minis. Adjusting the rear sag without changing the
front will change the bike’s geometry. Balance is more
important than having the perfect sag number. Consider the
following example:
Two riders
share the same bike. One weighs 200 lbs, the other 100
lbs. The bike has been set up with stiffer springs front
and rear for the larger rider. Race sag is set at 100mm,
(free sag measures 20mm). What happens when the lighter
rider gets on board? The race sag will be much less,
probably 80 mm, (free sag remains the same). If you
decrease the pre-load on the shock spring to increase the
sag to 100 mm for the lighter rider, you will have a bike
that looks like a chopper and is not very well balanced.
The front pre-load has not changed, and it will sit up
higher, with or without rider. It would be better for the
lighter rider to ride the bike in a balanced state than to
change the sag so drastically that you alter that balance.
The best
approach on balancing your bike is to get the proper spring
rates for your weight for both the front and rear. Then set
the sag to balance the bike. Then fine tune sag for terrain
(more for sand tracks, less for short, tight tracks). Then
fine tune suspension clickers for performance and handling
(see article in previous issues).
The free sag
to race sag relation can tell you if you have the proper
spring rate for your weight. An increasing distance between
the two indicates the need for a stiffer spring. (Consider
a diet drink.)
If you need
help setting your sag or balancing your bike, call a
suspension tuner.
This technical article
was provided by Quinton Cain of SUSPENSION DYNAMICS located
in Arlington, TX. Phone number is 817-563-6891. SUSPENSION
DYNAMICS offers springs to fit a wide range of riders for
all brands of off-road motorcycles.
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