Teach Me Suspension (Part 3): Everything Preload

In this part we are specifically going to be talking about preload, what it does, and why we would adjust it.

After we have covered all of that, in part 4 we will be going more into using preload to set rider sag.

But first, a quick recap…

A Quick Recap on Springs

If you remember in part 1 we spoke about springs and the role they play.

A spring’s job (both front and rear) is to support the bike’s weight and allow the wheels to move over bumps while the bike remains in the same position relative to the road.

In order to change the characteristics of the spring so that it better deals with bumps with a rider on board you have two options.

The first is change it for a different spring with a different rate.

A spring’s rate is the 1kg/mm figure we spoke about before, and how if you apply 1kg of force to the top of the spring it will compress 1mm. Likewise if you apply 10kg of force, it will compress by 10mm.

The second option you have to change the spring’s characteristics is to alter the amount of preload that is placed on the spring.

What is Preload?

It is a measurement of how much the spring is mechanically compressed in both the forks and shock when they are fully extended.

As an example, let’s assume you have a front fork out of the motorcycle, and inside that fork you have a space for a spring that measures 320mm.

The spring that you want to put inside the fork actually measures 330mm. That means that you would need to compress the spring by 10mm for it to fit inside your fork tube.

With a rate of 1kg/mm and the spring now compressed by 10mm, you would need to exert more than 10kg of force down on the fork in order to begin compressing it.

For example, if you applied 11kg of force on top of the fork it would only compress by 1mm (not 11mm) because the first 10kg of force you applied was merely overcoming what was being exerted by the spring inside the fork (as a result of it being compressed to fit inside).

By turning the preload adjuster on the top of the fork and adding another 5mm of mechanical compression, the spring would then be exerting 15kg of force back out, meaning you would need to then apply more than 15kg of force in order to begin to compress it.

While the rate remains the same, by adding preload to the spring in your suspension you are changing how the spring reacts to weight that’s applied to it. Whether it be through a rider’s movements, braking, cornering efforts, or the application of power.

By adding preload on the front, for example, in the braking zone there is going to be less of a diving effect as the weight moves forward because it takes more of yours the bike’s weight to overcome the force of the spring pushing back up.

This is why it is often referred to as making the suspension ‘stiffer’, because that’s the impression it gives.

Why Do We Change Preload?

Both hard braking and acceleration have the potential to bottom or top out the suspension respectively.

Bottoming or topping out the suspension is very bad for you because it means the suspension then cannot effectively do its job of keeping the tyre in good contact with the track.

Basically, traction is going to the reduced.

For that reason, while we want to use as much of the suspension’s available range as possible, we don’t want to use it all to the point where you reach the end of suspension travel.

Preload is a way to ensure that that doesn’t happen because it changes how much of the range you use and where in the range the bike sits.

Given that the individual needs of every rider are different, it’s nigh on impossible to find the perfect spring rate for any rider.

Preload lets you fine tune how your bike ‘sits’ with you on board and ensures you don’t use too much travel.

The Springs Don’t Compress With Preload

Ok that’s not strictly true. An unladen, fully extended fork (or shock) spring WILL compress when preload is added.

However, under yours and the bike’s weight, when preload is added the spring in the forks and shock do not compress. The spring remains the same length, but the forks and shock themselves actually extend.

Sounds confusing I know, but let me explain.

Imagine someone holding onto the rear of your bike, letting it rest upright under its own weight.

At that point in time, the springs will be compressed by a given amount based on the rate of the spring and how heavy the bike is.

When you wind on preload, the adjuster starts to push down on the spring and it tries to compress it, but in doing so you are changing the amount of force that the spring pushes straight back up (remember our earlier examples).

That then means that the spring needs MORE weight to be applied to it in order to compress it, thereby cancelling out a portion of the bike’s weight that was originally causing it to compress.

The spring’s length under the bike’s load doesn’t change, but now the fork (or shock) has actually extended.

Here’s a visual example that may help you get your head around it.

The first fork tube is one with zero weight on it. As such it is fully extended.

Our second tube in the picture has had 50kg of weight applied to it. Being that the spring rate is 1kg/mm, the fork compresses by 50mm.

The third tube has had its preload adjuster (red) wound on by 25mm. As you can see, this raises the upper portion of the suspension tube (blue) back up. That upper portion is where the triple clamps would be situated, and so the ride height would be raised by 25mm.

A couple of important things to note are:

1) the spring length inside the tube remains the same between tubes two and three, and…

2) the top of the preload adjuster actually remains the same distance from the bottom of the fork. Only the upper portion moves back up.

Preload Affects Geometry

As I said, winding on preload actually raises the suspension. Conversely, removing preload lowers it. For that reason it’s important to realise that your preload changes directly affect geometry.

Winding on 10mm of preload to our bike’s 1kg/mm spring at the front, for example, would raise the front end by 10mm.

This would have the exact same effect on geometry as raising the triple clamps up the forks by 10mm.

Once you’ve set your sag to the correct range (which we’ll cover next), if you wish you can begin experimenting on how those preload changes affect the geometry, and in turn, steering characteristics of your bike.

You could change your preload adjustment by 4mm to change the ride height by the same amount and then go out and see what affect that has.

Remember, a 4mm change in ride height at either end equates to about a 1mm change in trail.

As always if you want to learn how all this stuff works, you need to go out there and test it, making sure you’re keeping notes of everything as you go.

Ok, that about covers the theory on preload. I hope it wasn’t too confusing for you.

In the next part we are going to be talking about how to change preload and how we use it to set rider sag.