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.