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Race Sag and Spring
Rates
Now that the swing arm and linkage are all
greased and free with no damaged parts we can set race sag and get proper
spring rates. You guys with linkless rear suspension systems – KTM PDS,
Husabergs, Canadales, ATK’s – did you do your service work as well?
These bikes need service also.
To set race sag you will need a cycle stand, a
ruler that can measure at least 22 inches, a hammer & a blunt punch (for
turning the preload collar-- NO screwdrivers or chisels), a marking pen,
approximately l5 pounds of weight, and a friend who can read a ruler.
Start by putting your cycle up on the cycle stand with both wheels off the
ground. Next, on the side opposite the exhaust, measure from a fixed
point (I use the top of the swing arm) to a point on the rear fender that
comes to a full inch measurement – i.e. 24”, 25”, etc. I am using
inches because it is hard to find a long ruler with metric measurements.
Place a mark with the marking pen on the swing arm and the fender where you
took your measurement, show your friend these marks and write this
measurement down – it will be your “extended length” reference. This is very
important, so measure 2 or 3 times to make sure you are correct. All
future calculations are based on this “extended length” measurement.
Now take the cycle off the stand and roll it
over near a post, door, fence, or an old car. With the cycle near one
of these items you will be able to sit on the cycle with both feet on the
pegs and use one hand to balance by touching the support you have chosen.
Now mount the cycle with the 15 pounds added. Bounce up and down
several times, standing with shoulders over the handlebars. Next sit
down while touching the support, with your hips over the foot pegs.
Have your friend measure between the two marked points that you measured
earlier. This is the “compressed length”. Subtract this from
your “extended length”. This number is your race sag. Race sag
should be between 3 3/4” and 4 1/4” inches (95 to 105 mm). If you set
the spring at 1/2 inch preload as I suggested in the first column then you
should use no more than 3 complete turns of the preload collar, in either
direction, to adjust race sag. On linkless systems you should use no
more than 2 turns in either direction to adjust race sag. If you have
to turn the preload collars more than these amounts, you need a different
spring: i.e. if you had to loosen up, you need a softer spring, if you had
to tighten up, a stiffer spring. Check your service manual for the
proper race sag for your cycle. On WP shocks with the allen bolt in
the preload collar, do not tighten more than 6 foot pounds. THEY DO
BREAK.
Spring rates are very important. The
same spring can’t work for a 130 pound rider and a 260 pound rider.
Most 125cc bikes are set up from the factory for about a 150 pound
rider, and most 250cc bikes are set up for about a 180 pound rider. A
spring has about a 20 pound rider weight range. Each 10 pounds over or
under the standard rider weight requires an increase or decrease of one step
(.2 kg) in spring rate: i.e. 4.8 kg to 5.0 kg or 4.8kg to 4.6 kg. You
cannot substitute increasing or decreasing preload for the proper spring
rate. In linkless systems, the spring is good for a rider weight range
of about 10 pounds. A soft spring will require too much preload making
the suspension system harsh on small bumps. It WILL NOT increase
bottoming resistance. Only a stiffer spring will increase bottoming
resistance. Also, a stiffer spring will make the shock less harsh on
the small bumps. If a spring is too stiff, the shock will not use full
travel and you will have an excess of free sag. Free sag is the sag of
just the cycle with no rider (from 1” to 1 1/4” for linked and 1 3/8” to 1
5/8” for linkless systems). Improper spring rates will ruin an
otherwise properly valved rear suspension. Get the right spring
– it will be money very wisely spent.
I will finish up with a short discussion of
linkless systems. Because a linkless system uses springs that are 75%
to 100% stiffer than a link type system, preloads, free sag, and spring
ranges become more critical than on the linked systems. Because the
springs are so stiff – i.e. an 180 pound rider on a KTM 250 (linkless) would
use about an 8.8 kg spring and a Kawasaki KX250 (linked system) rider would
use a 5.0 kg spring – you need to run a lot less preload: 1/8” to 3/8”
compared to l/4” to 3/4” for a linked system. If you would run too
soft a spring and run more preload, the rear end would be very harsh in the
first l/3 of travel. Also because of the stiffer springs used on the
linkless systems, free sag will be more than on a linked system – 1 3/8” to
1 5/8” on linkless compared to 1” to 1 1/4” for the linked system.
Your spring range does not cover as much of a rider weight range,
approximately 10 pounds, before the spring needs to be changed. |
