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Hillbillie Mod
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Discussion Starter #1 (Edited by Moderator)
From Sport Rider

(Let me know if yall get x's on the pics)

Technicalities: Suspension and Springs

Beyond nuts and bolts

By Paul Thede

Figure 1
What's all this ruckus about suspension these days? It seems everyone is clued in that suspension setup can be a key to riding fast and safely, but how do you do it? No matter what shock or fork you have, they all require proper adjustment to work to their maximum potential. Suspension tuning isn't rocket science, and if you follow step-by-step procedures you can make remarkable improvements in your bike's handling characteristics.

Figure 2
The first step to setting up any bike is to set the spring sag and determine if you have the correct-rate springs. Spring sag is the amount the springs compress between fully topped out and fully loaded with the rider on board in riding position. It is also referred to as static ride height or static sag. My company, Race Tech, (909/594-7755) has an advanced method of checking spring sag that I'll describe.

If you've ever measured sag before, you may have noticed that if you check it three or four times, you can get three or four different numbers without changing anything. We'll tell you why this occurs and how to handle it.

Rear end

Step 1: Extend the suspension completely by getting the wheel off the ground. It helps to have a few friends around. On bikes with sidestands the bike can usually be carefully rocked up on the stand to unload the suspension. Most race stands will not work because the suspension will still be loaded by resting on the swingarm rather than the wheel. Measure the distance from the axle vertically to some point on the chassis (metric figures are easiest and more precise; Figure 1). Mark this reference point because you'll need to refer to it again. This measurement is L1. If the measurement is not exactly vertical the sag numbers will be inaccurate (too low).

Step 2: Take the bike off the stand and put the rider on board in riding position. Have a third person balance the bike from the front. If accuracy is important to you, you must take friction of the linkage into account. This is where our procedure is different: We take two additional measurements. First, push down on the rear end about 25mm (1") and let it extend very slowly. Where it stops, measure the distance between the axle and the mark on the chassis again. If there were no drag in the linkage the bike would come up a little further. It's important that you do not bounce! This measurement is L2. Step 3: Have your assistant lift up on the rear of the bike about 25mm and let it down very slowly. Where it stops, measure it. If there were no drag it would drop a little further. Remember, don't bounce! This measurement is L3.

Step 4: The spring sag is in the middle of these two measurements. In fact, if there were no drag in the linkage, L2 and L3 would be the same. To get the actual sag figure you find the midpoint by averaging the two numbers and subtracting them from the fully extended measurement L1: static spring sag = L1 - [(L2 + L3) / 2]. Step 5: Adjust the preload with whatever method applies to your bike. Spring collars are common, and some benefit from the use of special tools. In a pinch you can use a blunt chisel to unlock the collars and turn the main adjusting collar. If you have too much sag you need more preload; if you have too little sag you need less preload. For roadrace bikes, rear sag is typically 25 to 30mm. Street riders usually use 30 to 35mm. Bikes set up for the track are a compromise when ridden on the street. The firmer settings commonly used on the track are generally not recommended (or desirable) for road work.

You might notice the Sag Master measuring tool (available from Race Tech) in the pictures. It's a special tool made to assist you in measuring sag by allowing you to read sag directly without subtracting. It can also be used as a standard tape measure. Measuring front-end sag is very similar to the rear. However, it's much more critical to take seal drag into account on the front end because it is more pronounced.

Front end

Step 1: Extend the fork completely and measure from the wiper (the dust seal atop the slider) to the bottom of the triple clamp (or lower fork casting on inverted forks; Figure 2). This measurement is L1.

Step 2: Take the bike off the sidestand, and put the rider on board in riding position. Get an assistant to balance the bike from the rear, then push down on the front end and let it extend very slowly. Where it stops, measure the distance between the wiper and the bottom of the triple clamp again. Do not bounce. This measurement is L2.

Step 3: Lift up on the front end and let it drop very slowly. Where it stops, measure again. Don't bounce. This measurement is L3. Once again, L2 and L3 are different due to stiction or drag in the seals and bushings, which is particularly high for telescopic front ends.

Step 4: Just as with the front, halfway between L2 and L3 is where the sag would be with no drag or stiction. Therefore L2 and L3 must be averaged and subtracted from L1 to calculate true spring sag: static spring sag = L1 - [(L2 + L3) / 2]. Step 5: To adjust sag use the preload adjusters, if available, or vary the length of the preload spacers inside the fork.

Street bikes run between 25 and 33 percent of their total travel, which equates to 30 to 35mm. Roadrace bikes usually run between 25 and 30mm. This method of checking sag and taking stiction into account also allows you to check the drag of the linkage and seals. It follows that the greater the difference between the measurements (pushing down and pulling up), the worse the stiction. A good linkage (rear sag) has less than 3mm (0.12") difference, and a bad one has more than 10mm (0.39"). Good forks have less than 15mm difference, and we've seen forks with more than 50mm. (Gee, I wonder why they're harsh?) It's important to stress that there is no magic number. If you like the feel of the bike with less or more sag than these guidelines, great. Your personal sag and front-to-rear sag bias will depend on chassis geometry, track or road conditions, tire selection and rider weight and riding preference.

Using different sag front and rear will have a huge effect on steering characteristics. More sag on the front or less sag on the rear will make the bike turn more quickly. Less sag on the front or more sag on the rear will make the bike turn more slowly. Increasing sag will also decrease bottoming resistance, though spring rate has a bigger effect than sag. Racers often use less sag to keep the bike higher off the ground for more ground clearance, and since roadracers work with braking and steering forces greater than we see on the street, they require a stiffer setup. Of course, setting spring sag is only the first step of dialing in your suspension, so stay tuned for future articles on spring rates and damping.

Hillbillie Mod
20,155 Posts
Discussion Starter #2 (Edited by Moderator)
more from SR

Commonly asked questions like, "What's the difference between spring rate and preload?" and "Can't I make the spring stiffer by cranking in more preload?" require delving a little deeper into the mechanics of spring forces.
A stiffer spring would also start at zero force, but its compression rate would increase at a steeper angle, as in Figure 2. Notice that at 20mm travel, the softer spring requires only 10kg force, while the stiffer spring requires 20kg. The stiffer spring at 1.0kg/mm is both twice as steep and twice as stiff as the softer spring, which is 0.5kg/mm.


Let's back up and define what spring rate and preload really are. "Spring rate" reflects the stiffness of the spring and is measured in kilograms per millimeter or pounds per inch. One of the ways to test spring rate is to first measure the spring's "free length"-the uninstalled length-and then put weights onto it, measuring the resulting compression with the addition of each weight. "Straight-rate" springs maintain a constant rate of compression throughout their travel. If you are testing a straight-rate spring and you plot these points on a graph, you will end up with something that looks like Figure 1.

Now let's take the original spring and install it in the fork. As it's installed, it gets compressed, or preloaded, a small amount. "Preload" (or "preload length") is the distance the spring is compressed from its free length as it's installed with the suspension fully extended.


Just a note here on the difference between preload and preload adjusters: All bikes that I am aware of have preload. Some bikes do not have external preload adjusters, but they do have preload. All forks can have preload adjusted internally by changing spring spacer length, though sometimes it takes special spacers. Forks that have external preload adjustment have preload even when set at the minimum adjustment.

The "preload force" is the initial force the spring exerts on the end of the fork tube with the fork fully extended. Referring now to Figure 3, you can see that when preload is added to the spring, it effectively shifts the curve (line) to the left proportional to the amount of the preload force. In this case, the preload length is 35mm and the resultant preload force is 17.5kg at zero travel. In other words, with this 0.5kg/mm spring and a setting at 35mm preload, you would have to put more than 17.5kg force on the end of the fork tube to create any movement at all.


For a straight-rate spring, the relationship between force, spring rate and travel is described by the equation: F = K x L (or force [F] equals spring rate [K] times length [L]).

When you tighten the adjusting collar on a shock or increase the preload length by tightening the adjuster on the fork, you are indeed increasing the initial force exerted by the springs. This decreases sag, making the bike ride higher. It does not, however, increase the spring rate.

For example, you can achieve a targeted sag on the fork even with a spring that is too light (or soft) if you use a lot of preload. You can also achieve that same sag with a spring that is way too heavy by using very little or no preload. Let's look at just one fork leg on paper. Refer to Figure 4 and notice that the softer spring has 35mm preload and therefore has 17.5kg force at zero travel (fully extended). Let us assume the front end has 30mm of static sag. At 30mm of sag, the total spring force is 32.5kg. This means that each fork spring must push up 32.5kg to create a 30mm sag figure. Any combination of spring rate and preload that gives 32.5kg force at 30mm travel will create the same sag. Notice the stiffer, 1.0kg/mm spring has 2.5mm preload and, at 30mm travel, also creates 32.5kg force. This means they will both have the same sag; however, they will perform totally differently.


The quality of the ride will suffer with a spring that is either too soft or too stiff. The spring with a rate that's too soft will dive and bottom easily because the spring doesn't provide enough additional force as it gets deeper into the travel. The spring that has a very stiff rate will feel harsh, like it's hitting a barrier or very stiff spot.

A few more measurements will show if your spring rates are in the ballpark. Set the sag to standard settings (see Technicalities, August '95) and then measure the "free sag." "Free sag" is the amount the bike settles under its own weight. Use exactly the same procedure as when checking static sag, but without the rider on board. Street and roadrace bikes require 0 to 5mm of free sag on the rear, but should not "top out" hard. "Topping out" occurs when the suspension extends to its limit. It should barely have enough force to top out without the rider on board. If it takes a lot of force to compress it at all, you can bet it needs a different spring. On the front, expect to see 5 to 10mm of free sag.

When the static sag is correct and the free sag is less than the minimum recommended (e.g., it tops out hard), you need a heavier spring rate with less preload. A lighter spring is recommended when the free sag is more than the maximum recommended.

Most bikes, but not all, are set up with fork springs that are too soft for aggressive riding. Keep in mind that personal preference, conditions and type of riding come into play when setting up suspension. Racers generally use higher spring rates with less preload than street riders. When in doubt, consult a good suspension tuner.

Static sag for a particular bike and rider combination is dependent only on spring rate and preload. Springs are position sensitive and only care where they are in their travel, not how fast they're being compressed. Damping settings, on the other hand, are dynamic forces. In other words, damping forces only occur when there's vertical suspension movement. This means they do not affect static sag, as sag is measured when there is no movement.

Just a couple of final thoughts about air and oil level. The addition of air into the fork tubes on models with air valves will have a huge effect on both sag and harshness. We don't recommend the use of air as a tuning variable because harshness generally increases a lot for the relatively small benefit in bottoming resistance. It is almost like adding spring preload. The use of additional air is quite effective on touring bikes for temporarily changing the load-carrying capacity, like when packing double. Oil level changes will affect the total spring-type force, but only as the fork gets to the bottom of its stroke and, therefore, does not affect sag.

Now, of course, we haven't even talked about damping and how this affects the overall ride and handling characteristics of a sport bike-but that's another story.
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