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Suspension Knowledge: Balance and Sag

 

by Quinton Cain of SUSPENSION DYNAMICS

 

Letís 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 bikeís 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.

 

Additional Tech Tips

#1 ... Clickers

#2 ... Balance and Sag

#3 ... How Suspension Can Affect Turning

#4 ... Leaking Fork Seals 

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Suspension Dynamics   817-563-6891    qcain@tx.rr.com