Sag and Preload

author: Graham Byrnes PhD

Roughly speaking, there are two kinds of road irregularities: ones that stick up (bumps), and ones that drop down (also known as, wait for it, holes). To cope with the first, your suspension needs to compress, for the second it needs to extend.

It follows that when you are riding along on your bike, the suspension should be neither completely bottomed out, nor fully extended (topped out). Where exactly in the travel it sits, relative to fully extended, is called the sag. A general rule of thumb is that the front sag should be about 30-35% of travel, while the back should be at about 25%. That works out to be 30-40mm at the front and 25-35mm at the back, for most bikes.

You adjust the sag by adjusting the preload.

It’s worth discussing preload for a bit, just what it does and doesn’t do. Preload is simply the amount the springs are compressed while the suspension is fully extended. A typical pair of sports bike fork springs are about 8.5-9.5N/mm. For them to exert enough force to hold up the front of said sports bike and rider, they need to be compressed 50-55mm, but we only want, say, 35mm of sag. Come what may, the spring is going to collapse its 50mm (for example), so we have to give it a headstart of 15mm. That way, after 35mm of sag the spring will have compressed a total of 50mm and everything is sweet.

Adding more preload just means the fork won’t need to compress as far for the spring to get its 50mm, so the bike will sit a little higher at the front. The story is the same at the rear, but complicated by leverage ratios we’ll get to in a later article.

Note that, with some exceptions, that’s all it does. Preload makes the bike sit higher, or lower. It does not make the spring stiffer. So if someone tells you that you should reduce your preload to make the bike feel less harsh, they probably don’t have a clue.

In many cases, less preload will make the bike somewhat more harsh. That’s because the suspension will sit lower in the stroke, closer to the rubber bump stop on the shock or the hydraulic lock stop on the forks (we’ll get to that later… suffice to say it serves to stop people who do bad wheelies from hurting their bike). Also, trapped air in the fork makes the spring rate increase at the bottom of the stroke, while many rear linkage systems do the same at the rear.

The exception: rebound springs

Several bikes now use long springs that act to push the fork down. These are called top-out, or rebound, or negative springs. Usually they only operate over the first 1/3 of fork travel. Now it’s not what you’d immediately guess, but if you trap something between two springs pushing in opposite pairs, their rates add. You’ll have to trust me on this, or look up stacked pairs in the an engineering book. A hint: as one spring extends, it helps less to extend the spring, while the other resists more.

Now this means that the spring rate in the top part of the travel (above the ideal sag point) is stiffer, typically by 30-40%. So instead of needing to compress 50mm to hold up the bike, only 30-35mm are needed, from the point where the springs balance each other out. Which means that the fork has no apparent preload (the actual springs are preloaded against each other, but you can’t tell from outside).

If you are accelerating hard on a 2004 GSXR1000R (which has only very short top-out springs and about 15mm preload) and the wheel is just brushing the surface, the front suspension is essentially rigid: it needs at least 15mm worth of spring force to start the wheel moving at all because of the preload. That means any small bumps are likely to bounce the wheel in the air, or cause a tank-slapper.

If you repeat this on a CBR1000RR, which has long top-out springs and no externally apparent preload, even a small bump will cause the suspension to compress a bit. Along with a forward weight bias and a few other things, this is the reason the Honda is more stable under acceleration, not some hocus-pocus about the rear shock not being connected to the frame.

The catch is, if you try reducing the sag by increasing preload, you’ll find yourself riding along in the zone where the top-out spring is still operating. Which means the spring rate is 30-40% stiffer than it should be, and it will feel harsh!

Bike sag, or static sag

You can also measure the sag without rider. Which is more important? Personally I don’t much care how my bike handles when I’m not on it… However you’ll often be presented with target values for bike sag, or rather the difference between bike and total sag. The argument is that the difference between the two sags tells you if the spring rates are correct. It does, but only vaguely and in some cases.

Let’s think about it. You measure the bike sag, which is how much spring compression is required, less preload, to hold up each end of the bike. Then you climb on and get someone to measure the total sag (instructions are elsewhere on this site). What have you done? You seen how much extra the springs have compressed when the extra force due to your weight is added. So the difference in sag is proportional to the spring rate.

Problem 1:

The recommended difference for the forks is about 20mm. You’ll be doing very well to measure it within 2mm, due to fork stiction, which means an error of 10%. However when riding, you’ll feel quite a difference with a 5% change in spring rate.

Problem 2:

The difference in sag is proportional only to the rider’s weight, not the bike’s. So if you use the sag-difference method to choose springs, you’ll end up with the same rate on a 300kg cruiser or a 170kg superbike as on a 72kg 125GP bike, if the rider is the same.

Problem 3:

For the same reason as above, the relationship between front and rear rates will depend only on where the rider sits on the bike, not how much weight the bike puts on each wheel.

Using this method does have some value as a rough guide, but only on bikes of a certain type.

Summary

Setting the sag allows the bike to have the correct compromise for dealing with both bumps and holes. Preload adjusts sag (and so ride-height), not stiffness. However it can change stiffness in unexpected ways due to rebound springs and progressive linkage systems
Too much sag/too little preload will make the bike feel: harsh on bumps, twitchy (front) or slow turning (rear).
Too little sag/too much preload will make the bike feel: skittish on bumps when accelerating (front) or braking (rear), twitchy (rear) or slow-turning (rear).

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