Suspension Tuning:

Preload
     
by Dean Baker

Although we are an association of vintage motorcycle enthusiasts, most of us have a “modern” bike or two.  Suppose I were to tell you that you could improve the performance of your motorcycle with no greater investment than a half hour with a couple of friends.  Well, it’s true.  For most of us an increase in the performance of our machine is just a few minutes and zero dollars away, plus it even “personalizes” the bike to its rider, and we all know that’s a good thing.  So, let’s talk suspension.

All of us know that power means nothing unless you can get it to the ground.  Today’s tires are remarkable in their ability to shoulder that responsibility, along with cornering, stopping, and a host of other functions.  Your bike’s suspension is responsible for keeping tires in contact with the ground so that they can do their job.  The suspension must accomplish this task over a wide spectrum of road conditions, while simultaneously giving you a comfortable ride.  On the majority of modern motorcycles this is accomplished at the front by forks with internal springs and damping, and at the rear by a shock with an external spring.  There are varying degrees of adjustability on these components, ranging from preload-only on less expensive parts to fully adjustable compression and rebound damping on the more high-end components.   For our purposes here we are only going to address preload, as it is applicable to most bikes and is the first place to start in dialing in your suspension.

First, let’s begin with a few definitions.   Anyone who has taken a fork apart knows that the springs within the fork tubes are under compression, even when the tube has been removed from the bike.  This is “preload” and simply means the compression the spring is under when the suspension is not subjected to any weight.  Preload is typically adjusted on the forks by a threaded adjuster on the cap, and on the rear shock by lock rings or a ramp-type adjusting collar.  On some bikes, the early Suzuki SV650 for example, there is no external adjuster on the fork and adjusting preload requires changing the length of spacers between the spring and fork cap.

By adjusting the preload, we adjust the “sag” of the bike on its front and rear suspension components.  Sag is the distance the bike settles on its suspension when subjected to load.  The term “free sag” is used to express the compression of the suspension under the weight of the bike alone.  Free sag assists in preventing your suspension from topping out as the fork unloads toward full extension.  Without it, the suspension would have insufficient upward travel with which to work.  “Static sag” is used to express compression from full extension when the rider is on board.  Sometimes this is also referred to as “race sag”.  Do not read too much into the word “race”.   Race sag does not strictly mean the correct sag for racing purposes.  Rather, the terms “race” or “static” sag refer to the suspension travel achieved when the motorcycle is laden with rider(s), riding apparel, and any luggage and/or gear.   Let me mention at this point that, depending on the source, these terms are not absolute and are sometimes interchanged.  The term used is not as important as the concept.

Before we can accurately set our bike’s sag, we need to determine the amount of “stiction” front and rear.   Static friction, or stiction, reduces your suspension’s ability to do its job.  Stiction is the resistance to smooth, friction-free travel of your forks or shock.  Since we don’t live in a friction-free world, some degree of stiction is always present, but in can be compounded.  On the front it can be exacerbated by fork misalignment resulting from an over tightened axle nut, tweaked triple clamps, torqued fenders or incorrect preload.  On the rear shock it is typically a smaller number and, when excessive, is usually caused by linkage problems. 

The amount of stiction should be determined before setting sag.  To determine stiction, have your assistants lift the front of the bike to the full extension of the fork and hold it there.  Measure from the base of the triple clamp to a convenient point on the slider.   See Figure 1.


Figure 1: Measuring fork travel at full extension.

            A tape measure with markings in millimeters makes the measurements and the math much easier.  Also, a centerstand may assist in this particular measurement.   Any point on the slider will do, for example the dust ring or rock guard, just be consistent.  Record this measurement as “A”.  You may also obtain this value from your owner’s manual if you have stock components.

Next, with an assistant steadying the bike, lower the front end so the suspension is supporting the full weight of the bike, no center or side stands.  Lift the front end slightly and release slowly.  This allows the sliders to find a “normal” amount of extension.  Measure again, using the same points, and label this as “B”.  Now, press down on the front of the bike to compress the forks and again gently release.  Measure using the same reference points as before, labeling this measurement as “C”.   The difference between “C” and “B” is the stiction, and is simply caused by friction in the components not allowing the fork to return to the exact same place in its travel.  Some stiction is inevitable because of the fork seals, and on a modern bike anything less than a 10mm value on the forks is pretty good.  More than that and you should investigate the cause. 

Repeat the process for the rear shock, lifting the tail section full extension to get “A”, lifting again slightly and lowering gently to get “B”, and compressing to get “C”.  Again, the points of measurement are not as critical as consistency, but avoid using the bodywork as a reference point as it might flex somewhat during the measurement process, throwing off your numbers.  Consider using the axle nut for the lower point and a solid spot on the subframe directly above the axle for the upper.  The difference between “B” and “C” on the rear should be less than 5mm.  See Figure 2.


Figure 2: Measuring stiction on rear.

Once satisfied that stiction is acceptable, we can set the proper static sag.  This is done with the rider in his gear, sitting feet on the pegs in a normal riding position.  While a helper holds the bike steady, other assistant repeats the same measurements as before, using the same reference points.  See Figure 3.

As in determining stiction, measure for B and C using the extend-release, compress-release method, then add B + C and divide the sum by 2. This is to average out stiction in the suspension travel. This value is subtracted from “A”, the suspension at full extension, to provide static sag. You can calculate the approximate sag you are looking for before you start measuring. Generally, static sag should equal 25%-30% of the full travel of the fork or shock (get the total travel from your owner’s manual), with 25% being better for the track or aggressive sport riding, 30% better for general street use. As an example, say your rear shock has a range of travel of 125mm and you are planning on street use. Multiplying 125mm by 30% gives us 37.5mm as a good baseline for static sag. If you can’t find specifications for your suspension’s range of travel, you can use 30-35mm for street, (25-30mm for the track) as a starting point. Front and rear sag should be about the same.


Figure 3: Measuring static sag on front.

            If your numbers fall within the proper range, you’re finished.  Skip down to checking free sag.   If not, it’s time to dial in the suspension.  Use the preload adjusters on the fork caps to set the front static sag.  Figure 4 shows the preload-only adjusters on the caps of a Honda VFR Interceptor.  Be careful, these adjusters are usually aluminum and don’t react well to the wrong tool.   It’s also a good idea to protect the tank before reaching for the wrenches and screwdrivers.  Make sure both forks are adjusted identically by counting the number of exposed rings on the adjuster.  As preload is increased, fewer rings are visible on the adjuster.  One turn of the adjuster usually equates to about 1mm change in sag.  If you don’t have adjusters on the fork caps, the caps will have to be removed and new spacers installed between the fork caps and springs.  Light aluminum tubing or PVC pipe works well.  Be sure to use quality washers of the correct size between the spacers, caps and springs.


Figure 4:  Preload-only adjuster on VFR 800.  Note rings.

            On a lock-ring type shock, preload is adjusted by tightening or loosening the ring adjuster, shown in Figure 5.


Figure 5: Lock-ring adjuster.

            Note in the photo the two rings.  The top is the lock ring, which must be backed off to allow adjustments to be made by tightening or loosening the lower ring.  I have marked the lower ring with a black marker to tell one full turn of the ring.  Generally, one complete rotation of the ring nut equates to about 1-2 mm change in spring preload.  On ramp-type shocks, shown in Figure 6, preload is adjusted by twisting the ramped collar to a greater or lesser value.  Obviously this type of shock does not allow for the range of adjustments that the ring-type shock does.   Make your adjustments, and then re-measure to see if you have achieved your goal.


Figure 6: Ramp-type preload adjuster.

            Once static sag is properly set, check your free sag.   When static sag is within parameters free sag should be about 5mm-10mm less than full extension.  Use the same procedures that you used to get the values for “B” and “C” in calculating the static sag.  Your original value for “A” (full extension) is still valid, but now you measure for “B” and “C” without the rider onboard.  Record the new “extend and release” and “compress and release” measurements, add them together and divide by two (as before, when calculating static sag,) and subtract the result from “A” to calculate free sag.

Here are a couple of important items.  Write down each measurement and adjustment.  You’ll be working with a lot of numbers and it is easy to get confused.  Plus, by documenting each step you have a baseline to work with as you refine your suspension.  Experimentation is encouraged.  Find a familiar twisty road for your test rides, preferably not too busy and not too smooth.  You will notice changes in the handling when you adjust your suspension, so take it easy.  Make an adjustment, document it, and go for a ride.  Evaluate and document the results.  Repeat as necessary.  There is no hard and fast number for the correct amount of sag.  You may find you like more or less sag than I have recommended.  No problem.  This is all about adjusting the suspension to optimize your riding experience.  Also, take the static sag measurements while approximating the load the bike will be carrying when you ride.  It isn’t absolutely necessary to be in complete riding gear (although you do wear helmet, boots, gloves, jacket, etc when you ride, don’t you?)  Of more importance, are you planning on carrying luggage or bags with you?   Are those bags going to have anything in them?   Do you plan to ride two-up?   When measuring static sag it is imperative that you measure the suspension under the load it will be supporting when you’re riding.

    It’s important to understand what you’re doing while cranking down on that adjuster. Increasing preload does not result in a stiffer spring.  It only changes the amount of load it takes to begin to compress the spring.  Spring stiffness is determined by the manufacturing process, and is usually expressed in lbs/in or kg/cm.   As an example, a 100-lb/in. spring preloaded 2 inches will not compress further with any weight less than 200 pounds, but will compress 1 inch per 100 pounds above that.  Normally, stock springs should work well enough unless you are very light or very heavy, or if you carry gear or ride two-up.  If you find that you are near the limits of your preload adjustments to achieve proper static sag, you may need to consider changing your springs.  Also, if your static sag is correct but free sag is less than recommended  (5-10mm) or if the suspension is topping out, a heavier spring rate is indicated.  Conversely, a lighter rated spring may be in order if free sag is more than desired.

So, what about that performance gain I promised at the beginning of all this?  When your machine’s suspension is properly set up for its rider and conditions, it is more capable of performing the way it was designed.   Remember, riding a motorcycle involves a series of signals and responses between you and the road.  The suspension is the two-way conduit that transmits those signals between you and your tires.  An optimum set up allows optimum control and optimum feedback.  That equates to more confidence.  Confidence allows you to fully utilize the performance capabilities of your machine, whether it is honing your skills at a track day or touring two-up on the back roads of the Hill Country.

I have drawn on several sources to put this article together.  I highly recommend Kevin Cameron’s Sportbike Performance Handbook (MBI Publishing Co.) as an excellent source of information on suspension and any other aspect of motorcycle performance tuning.  I have also used information from “Motorcycle Consumer News” and “Motorcyclist” magazines, as well as tech notes from Race Tech®, manufacturer of quality aftermarket suspension components.  All these sources provided outstanding information.  Thanks also to Bob Hagemann, Dicky Hunter and George Tuttle for assisting with the action shots.  Lastly, remember that motorcycling has enough risks on a properly maintained bike.  While the adjustment procedures I’ve outlined are not complex, if you lack the mechanical skills to go wrenching on something as important as suspension, DON’T!  Get a gearhead friend to help out, or let a pro at your local shop handle it.

A quick review of formulas:

        Stiction = (A – B) – (A – C)
                    Static Sag  = A – (B+C)/2 *
                    Free Sag = A – (B+C)/2 **

        Where:
                            A = Full extension
                            B = Suspension Measurement after Extend and Release
                            C = Suspension Measurement after Compress and Release

*Suspension Measurement with Rider(s) and Gear Onboard
            **Suspension Measurement with Unladen Motorcycle


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Revised: January 29, 2008 .