Since I've opened this can of worms, might as well try to finish it. Sag settings. What should they be, how to measure, and what we can tell about the appropriateness of our spring rate from them. This is also pretty generic for sportbikes in general so it's a good thing to understand.
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, 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.)
When acceptable ranges can't be accomplished, it requires changing the spring. It will become clear as we go. :
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 ductape 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
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.
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!
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.
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:
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.
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).
Got that? Make sure.
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,
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?
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 ficticious 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.
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.
At this point we should have a good grasp on the basic concepts and terminology as it relates to the springs. What we should
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.
This is continued in another post titled: FINAL EXAMPLE, CONCEPTS APPLIED
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