Setting Suspension Preload

From Forum Member "Dad". See this link.

There are two sag numbers. One is called "Free Sag". That is the amount that the suspension will compress without the rider on the bike. Just the bike, standing upright, all by itself, on level ground.

The other is called "Static Sag". That is the amount that the suspension will compress with the rider on the bike, standing upright, on level ground.

The sag is adjustable by changing the "pre-load" on the spring. That is accomplished by turning that spanner nut on top of the spring for the rear (loosen the top lock nut first and then turn clockwise to increase preload), or changing the spring spacer lengths in the forks at the front. OR, most newer bikes have adjuster screws on the top of the front fork tubes that allow some pre-load adjustment without requiring modification to the internal spacer length. (Unfortunately, the EX isn't one of those.) Sad When acceptable ranges can't be accomplished, it requires changing the spring. It will become clear as we go. Roll Eyes Grin

GETTING THE MEASUREMENTS
For this segment we'll focus on the rear but the basic principles/theory apply to the front equally. There will be details that vary but the core principles remain the same.

You'll need a tape measure, a friend or two to help with holding the bike and getting the readings, and I recommend some duct tape to attach the measure to the bike. Extend the measure and lock it in the extended position. Tape the free end to the body directly above the rear axle, letting the locked tape measure body hang down, measure tape in line with the axle. (Place a strip of duct tape on the body to serve as a paint protector, then tape the measure to this.) It is nice to align a whole number with the reference point you chose on the axle. This would be with the suspension topped out, extended fully against the top stop. Now proceed to getting your measurements.

At the front, measure the fork movement directly. Place a wire tie around one of the fork tubes to use as a sliding reference point. As you compress the suspension it will slide the tie up the leg, holding the travel reference point to be measured.

FREE SAG, NO RIDER
At the rear. The desired "Free sag" is a pretty well fixed number and should be between 1/4" up to as little as just zero. Any tighter and the bike's suspension can't extend sufficiently after hitting a bump to maintain tire contact with the ground. We'll look at this more, later. It is measured by first lifting up on the bike to be sure the suspension is fully extended, topped out. That can be done by pulling it over on the sidestand or just lifting up on the grab rail. In this position, check that the axle reference you are measuring to aligns with a whole number on the tape measure. It makes it easier to get your measurements if it is aligned with a whole number. Then, gently let just the weight of the bike settle down on the suspension. Make sure the bike's vertical and you're not lifting on it or pushing down on it, just steadying it so it won't fall. Record the change in measurement at the axle, the amount that the suspension compressed from topped out.

Now, to cancel out the stiction of the suspension components from your measurements, bike still vertical, push down on the bike, compressing the suspension slightly, then gently release it. Let the bike come up slowly and when settled, record that dimension. The difference between the two is the stiction of the suspension components and needs to be cancelled out. Take 1/2 of that difference between the two measurements and either add it to the first measurement (compression, the one from letting the bike settle from the fully extended position), or subtract it from the measurement taken by compressing it and letting it rise (rebound). That number is the "free sag", corrected for component stiction.

If you got NO movement from topped out to the weight settled on the suspension, then the spring may be too tight, too much pre-load, and we have no way of knowing how much that is. Of course, it's possible that the suspension is just topped out, not pressing hard against the top stop but just settled against it. To determine if that's the case, with the bike standing upright, apply the lightest of downward force on the bike, just the force of a lightly placed fingertip. If it starts to move right away, that could be considered zero and may be left alone at this time, BUT... only the lightest of downward force, just a fingertip's effort. Otherwise, you have to back the spring pre-load off and measure until you get at least some sag, or at least a number that we can comfortably call zero as we just defined it. (More on this later.)

As long as you have some "free sag" and have recorded that number, corrected for stiction, then proceed to measuring the "static sag". We'll adjust after we get both of these numbers and digest what they tell us.

At the front. All of the basics remain the same but the methods of getting the measurements will be slightly different. Place a wire tie around one fork leg. Slide it down to engage the fork leg. Now, it will slide with the fork movements, pushed to a position by the travel. As you lift the bike the tie will remain at its highest travel point so the travel can be measured directly from the tie to the top of the fork leg by topping the suspension out and measuring the exposed fork leg directly. For rebound dimensions you'll be bouncing the front down and releasing slowly, therefore will push the wire tie past the point that you're trying to measure. For those dimensions, once the load has settled, hold that position while the person recording the measurements slides the wire tie down to engage the fork. Then unload the bike, top it out, and record the measurement. All of the principles remain the same. Only the method of measuring has changed.

STATIC SAG, RIDER ABOARD
"Static sag" is the measurement of the suspension travel with the rider aboard, feet on the pegs, in riding position. To get this measurement it will take a person or two to hold the bike upright and one to read the tape measure. The rider should assume riding position and try to remain still to assure the accuracy of the measurements. To measure the rear it may help to steady the bike from the front to have the least possible effect on the rear readings, and from the rear for measuring the front.

Proceed in the exact same fashion you did getting the "free sag" numbers, front and rear, except rider aboard this time. The measurement is that from the suspension topped out at full up travel to compressed with the rider aboard, bike held vertical. (This why it's handy to set a whole number on your scale to the topped position at the beginning. No need to recheck it. It doesn't change.) Lift the bike slightly and let it settle gently, recording the number. Then compress it and let it rise gently, recording that number. Take 1/2 of the difference between those two readings and add it to the compressing number or subtract it from the rebounding number, the same as you did getting the "free sag", cancelling out the stiction. Now we've got the "static sag" number, corrected for stiction.

Again, measuring the front repeats the steps described for "free sag", but with the rider aboard. OK?

Checking the measurements a couple of times is good practice to be sure you haven't accidentally pushed on the bike in a fashion that will alter the readings.

NOW, we got 'em, what do we do wit' 'em. Wink

SO... JUST HOW DOES THE SPRING WORK?

Well, first, let's think through what the spring is doing and get rid of an often held but wrong idea. The idea that winding down on the spring makes it stiffer. It DOESN'T! Shocked Bear with me on this, and don't proceed until you understand it. It is critical in the whole picture and once understood, you'll never refer to "winding down" on the spring as "making it stiffer" ever again... because it doesn't and you'll be part of the select few who understand why. Wink Shocked Grin

EXAMPLE:

We'll pick some nice round numbers out of the air to demonstrate the principle, keeping the math simple, focusing on the principle as it applies to the spring ONLY. The example will focus on the rear but the same principles apply at the front. (BTW,this principle is generic to springs but we're using a suspension example.)

As the need for a specific number arises, we'll use: Here goes. Smiley Spring, all by itself, not in a bike. It's a 100"# linear spring. That means the spring compresses 1" for each 100# of load applied to it. If we apply a 200# load, it compresses 2"... and so on... (until it's coil bound, of course). Wink Got that? Make sure.
 * 1) a 100"# linear spring.
 * 2) a 100# force each end from a 200# rider sitting on the bike.
 * 3) a 200# bike, balanced with 100# at each end.
 * 4) 5" total suspension travel
 * 5) For simplicity, we'll place the spring in straight alignment/compression with the axle so we don't have to adjust for the leverage/ratios of the swing arm or rocker linkages.

OK. Now, we have this spring in the bike, pre-loaded with the adjustment nut to a number, let's use 1/2". As such, we have the nut wound down 1/2" so have pre-loaded it with 50# of force. Got it so far? Make sure. (Linear spring, 100"#, wound down 1/2" = 50# of force.)

OK. Now, if we were measuring the "free sag", what would we do? We'd lift on the bike to be sure it's topped out. Right? The spring will extend until it comes up against the adjuster nut. The nut that we wound down 1/2", pre-loading the spring with 50# of force. Right? That's our starting point. Now, when we stand the bike up to measure the sag, applying its 100# of force to the spring, it will move down, compress. How far? An additional 1/2", yes? The first 10, 20, right through to 50# never caused any movement. Why? Because we already had a 50# pre-load in the spring. At 51# and beyond we started to get movement. Got it so far? Make sure. We would have a reading of 1/2" "free sag". (With a 50# pre-load on a 100"# spring, placing 100# load on the spring results in 1/2" additional movement.)

OK, if you're still with me, Wink let's look at the rider aboard. We already determined that our example has 1/2" "free sag". Now we're going to place an additional 100# force on the spring by putting the rider on the bike. We were already compressed 1/2" (free sag) due to the settings we had so when the rider gets aboard, we add another 100# so the spring compresses another 1". We now have a "static sag" of 1 1/2". Right? We had a 50# pre-load, applied a 100# force to the spring from the weight of the bike alone, compressing it 1/2" additional, then added 100# more, compressing it another 1", to the present measurement of 1 1/2". Got it? Cool, huh? Smiley

Now, let's play with our adjusting nut, see if we can make this example stiffer.... or do we just move it around, wind it up OR wind it down. Let's turn the nut down another 1/2", adding another 1/2" pre-load. That has us at 1" pre-load on our 100 pounds per inch spring. So what happens to our sag readings? Our pre-load is at 100# and our bike only weight is 100#, so the "free sag" goes to just zero. Right? Got it? Therefore it follows that our "static sag" goes to 1", right? Our "free sag" is now zero so when we put our rider aboard, his 100# force moves the bike down 1". Hmmm.

Winding the nut down hasn't done anything but move the starting height that all of this occurs. Putting 100# weight on it moved it 1", same as before, even though we wound down 1/2" on the spring. We didn't change the amount the suspension moved. The same load moved it the same amount, just from a different starting point. Got it? Did we "stiffen" anything? No.

Now, just to complete the circle, we'll back the nut off to zero spring pre-load. We stand the bike up with its 100# load and measure our "free sag". It will be 1", right? And then we put our rider aboard with his additional 100# and our "static sag" will go to 2", right? If you're seeing it and agreeing, you've got it... this far.

OK, so all of the screwing we do on that nut has not directly changed the spring's stiffness, the loads, the spring's ability to handle loads, NOTHING... except the ride height, moved it up or down. BTW, we don't want to adjust ride height there. While it effects ride height, we don't want to adjust ride height there. More on that later.

Now, and this is important. All we want to use the adjuster nut for is to move the spring's operating range to optimize the available suspension travel, getting it in the most useable, practical operating range. The best range for both up and down movements, to allow the wheel to follow the road the best it can.

Let's continue to use our frictionless suspension above, one more exercise at the extreme to underscore the point. We wouldn't want to adjust it to have 4" of "free sag" in a suspension that has a max travel of 5", would we? As soon as our rider got aboard, his 100# would move it down another 1" to 5" "static sag", the full operating range of our suspension. The first bump we hit, the suspension could absorb nothing. It's bottomed. Not that it is a likely scenario to see anything this extreme, it's still demonstrative of the principles involved and adding spring tension at this point... or taking it away, we accomplish nothing. The suspension is completely out of its operating range, unable to do a thing for us.

And to complete this circle of extreme examples, what if we pre-loaded the spring 2". It will be pre-loaded with 200# of force. Stand the bike up on its own, applying its 100# force, and the suspension never moves, right? Now put the additional 100# force of the rider aboard. The suspension still doesn't move, right? Make sure you're following this far. Now, the first bump you hit, the suspension moves a bit because it exceeds the 200# load and pre-load, the bike goes up a bit, and then as you clear the bump, IMMEDIATELY the wheel leaves the ground!... because we've got it set topped out hard against the adjuster nut, unable to extend. Shocked Not good. Wink But this is EXACTLY the nature of the problem you create as you try to wind down on too soft of a spring to get an appropriate "static sag", having ignored the "free sag". More on this later. Smiley

CONCEPT SUMMARY
At this point we should have a good grasp on the basic concepts and terminology as it relates to the springs. What we should understand is:
 * 1) The definition of "free sag" and static sag".
 * 2) How to measure them.
 * 3) A given spring has given capacities, and will move a predictable amount for a known load. (A 100"# spring moves 1" for each 100# applied. 200# will move it 2", etc.)
 * 4) The adjuster nut changes the height, the position in the available travel range that the spring moves as its capacity to support the applied load dictates. It does not stiffen the spring. It does move the range in which the spring operates.
 * 5) The travel range is fixed by the shock travel length at the rear, and the fork travel length at the front.

CONCEPTS APPLIED
One more example to see how all of this theory applies to a riding scenario. We'll use our same theoretical bike.

What if the spring installed was much weaker, say 25"# for example. And the rider load was still 100#. Let's set the "free sag" at zero, the highest it should ever be set, ideally. We will have to wind 4" of pre-load into it to accomplish that but so what? We got it at zero like we wanted, right? The rider climbs aboard and the "static sag" goes to 4". Right? So we only have 1", of a total 5", of suspension travel left to absorb load variations. Hmmm. What else might be happening?

Well, let's think about this a minute, consider what we DO know at this point. What do you think happens as we hit mild variations in the road or load the suspension just from the centrifugal force pushing down on the suspension from the mass of the bike and rider leaned into a bend? Without knowing what's perfect, what would a 100"# spring do differently than the 25"# spring, all else being the same? Zero "free sag" for both, the same 100# rider load, but those two springs, a 25"# and a 100"#.

Let's assign some numbers to the load in this scenario so we can see it. We'll say that the imperfections in the road, as well as centrifugal cornering force, caused a 25# additional load compressing, then went over a rise causing a 100# unloading of the applied weight.

The 100# spring bike starts with a free sag of zero. The rider gets aboard, applying his 100# force so the suspension compresses 1" (1" static sag). We start down the road, through a corner, applying an additional 25# force so the total sag, total downward travel, went to 1 1/4". OK? Now the bike advances to the 100# unloading imperfection. The resultant suspension travel is what? The bike was at 1" compressed, "static sag", the load is reduced by 100# from the road imperfection, we have a 100"# spring, so the suspension travels upward 1", positioning the suspension at just 0" sag. Follow that so far? Cool

So, here's the summary of our suspension action for the 100# spring as it travelled our theoretical road: Now the 25# spring bike, same scenario. Start with the same zero free sag. The rider climbs aboard, applying his 100# load, so the suspension goes to 4" compressed (4" static sag). We start down the same road, applying the additional 25# force, so the total sag, total downward travel, goes to 5" (just bottomed). OK? Now the bike advances to the 100# unloading imperfection. The resultant suspension travel is what? The bike is at 4" compressed, "static sag", the load is reduced by 100# from the road imperfection, we have a 25"# spring, so the suspension travels upward 4", positioning the suspension at just 0" sag. See it so far? Cool
 * Max sag: 1 1/4".
 * Min sag: 0"
 * Total that the suspension travelled: 1 1/4"

So, here's the summary of our suspension action for the 25# spring as it travelled the same theoretical road: WOW, WHAT A DIFFERENCE! The exact same road, the exact same rider, the exact same conditions, one bike used the whole 5" of suspension travel while the other, again in the exact same conditions, used a total of 1 1/4". NOW, what do you think our "exact same rider" felt riding those two bikes through the exact same conditions. Wink Grin One of those bikes would be inclined to feel planted, inspire rider confidence, while the other is wallowing down the road, maxing out the suspension's total travel.
 * Max sag: 5".
 * Min sag: 0"
 * Total that the suspension travelled: 5"

HOW FREE AND STATIC SAG EFFECT THINGS
Now that we know how to properly measure static and free sag, what can we tell about our suspension from those two numbers?

The difference between free and static sag will tell us if the springs installed are appropriate for the bike and rider without knowing anything about the specific spring rate, bike weight, or rider weight. This does NOT mean those aren't good bits of information to have. In fact, that's the data that the tuners use to predict and recommend the proper components for your bike, sight unseen. All it means is, we are striving to optimize the suspension in its travel in an application, and that the tell-tale measurements, the final proof of our work, are the free and static sag. They can be calculated, accurately predicted, and bench adjusted if sufficient data is available, but at the end of the day, the proper components will fall into the range and the improper won't. If our bike measures in that range, we don't need to know the details of why, just that they are, and let that final fact be our documentation that the components are correct and the same we would have arrived at with all of the data. Understanding all of the original theory should make this evident. If it's not, re-read the theory portion until you can satisfactorily understand the claims just made.

OK, so what should those numbers be? The difference between free and static sag for typical sportbikes should be approximately 1" for street use. That can be reduced, stiffer, to about 7/8" (maybe 3/4" at the rear but...), and softer to as much as 1 1/8". Any stiffer than 7/8" is really asking for trouble with the suspension being able to skip on corner bumps. As you get softer, beyond the 1 1/8" range, the flaws aren't as absolute as being too stiff, but the rider comfort and positive feeling of control diminish. At 1 1/4" it's already showing on hard corners and wallowing may appear, especially with a soft shock. By 1 3/8" expect wallowing at the rear and a generally unplanted feel as you push the bike harder.

For the track, all of the above numbers can be reduced by 1/8". That would mean that the minimum differential between the two numbers would be 3/4" (maybe 5/8" at the rear, but...) at the stiffer end and softer to about 1". Notice that the two, street and track, overlap. Soft on the track might be considered stiff on the street. Consider that to be in the realm of rider preference, not right or wrong.

RECOMMENDED SETTINGS
For the street or track, free sag should be set between 1/4" (6mm) and up to just zero. Static sag for the street should fall between 1" (25mm) and 1 3/8" (35mm). On the track that can be reduced to as little as 3/4" (20mm) and as much as 1 1/4" (32mm).

So where should I set mine? The ranges are fairly loose, aren't they? The end to be most concerned about is the top out possibility from a spring that's too stiff and a static or free sag not sufficient. A spring that's on the stiff side of the range then set at the minimal end of the static sag, will wind up with a minimal free sag as well, and may not be a good choice. The potential to top out on a harsh bump, not allowing the wheel to reach the ground, becomes greater as those two numbers are reduced. A spring that falls on the stiffer side of acceptable might be better set with the free sag at the greater end of its range allowing the static sag to fall in an acceptable portion of its range.

On the street, front sag should be conservative, that's to say, "When in doubt, add a little". You don't want the front too restricted. You might get away with a rear a little on the tight side but not the front.

Example: If we have a sag differential of 7/8" and have the free sag set at zero, then the static sag will be only 7/8". This may be trouble. If we set the free sag at 3/16" it will add directly to the static sag and will end up at 1 1/16". I don't think I'd want any less for this scenario, maybe even another 1/16" (to 1/4") free sag, 1 1/8" static sag. On the other hand, if we had a spring that was compressing 1 1/8", we could run the free sag up at 1/8" and still have a comfortable static sag of 1 1/4".

The shock settings, which we have not addressed because we don't have any, Wink will also come into play here. I'll mention it because it's worth being aware of, just not much of an option for these. Shock settings can have an almost overlapping effect as we get on the edge. A spring that's softer on its scale can be compensated for with a slightly stiffer shock setting and the opposite also holds true. BUT... a shock NEVER supports weight dynamically and can therefore NEVER fix a true spring problem. It can and does only assist by controlling the springs' movements in the short term.

Now you're in the range! Slight tweeks from this point may show some improvement in handling but you have it in the range and can say so with some confidence. The biggest thing to consider if you've reduced sag is to be aware that it may tend to skip on bumps. If it does, consider adding some sag. If we had shock adjustments we might look there, too.

Ride height should not be changed with the spring adjustments. That should be evident by now. Get the springs right, sags right, then change ride height externally, like with the links at the rear or sliding the tubes in the trees at the front. Some shocks can be shimmed at the attachment points and even others have adjustability built in. It's a different setting. Even though setting the sag effects it, it's NOT where we adjust it, right? Smiley OK, just a little. Wink Keep it in mind when setting sags, but keep the point in mind too. It's NOT your ride height adjustment. You don't get much there and it can create more trouble elsewhere by wasting useable suspension travel.

On the street, front sag should be conservative, that's to say, "When in doubt, add a little". You don't want the front too stiff. The above numbers should work fine but weigh the whole picture carefully if you choose to get on the stiff side of the spring range, or minimal sag settings within their range. Good roads, maybe. Bad roads, trouble. You might get away with a rear a little on the stiff side but not the front. It's unnerving when the front's not planted.

Fork oil level has an effect on the spring rate in the front. As such, it's a critical number. The air that is trapped in a fork tube has a piston effect on the fork leg and works as a progressive air spring, adding force over and above that of the spring compressing. Raising or lowering the oil level effects the volume of air, therefore the amount of the spring effect that compressing it affords. A higher level (up to the extreme point that it would hydraulically lock the front) will have the effect of making a front spring that's linear in its characteristics, act progressive when both the air and spring's forces are combined. This is critical for the extreme front loads that can occur under hard braking, placing ALL of the weight of the whole package on just the front wheel. As such, it's important.

Guidelines for oil height will come with a set of springs and will be very close if the springs are properly sized for the weight. To prove the appropriateness of the level, a wire tie left on the fork leg will indicate the max travel under hard braking. It should not bottom but at the same time, should not leave a great amount of unused travel. If we aren't using it, why have it? Right? Real hard braking, smooth surface, should leave maybe 1/2" travel. That's the reserve for hard braking and a bump. If the spring's right it will handle the load nicely and the fork oil assist will be sufficient to handle the extremes of braking. BTW, an appropriate fork oil level with the appropriate springs will typically fall in at about 5" to 5 1/4". Not because it's etched in stone but the factors that are in play tend to fall in that close, one application to another. Still check the recommended numbers for your starting point. Chances are they'll be perfect. Those guys have done this before. Wink