Excellent question — you asked not what preload is or how to set it, but what it does. That’s excellent because it gets to the matter of why preload matters.

So here you are, the weight of you and the bike connected to the wheels by springs, front and back. (No, they’re not necessarily literal springs, as modern suspension uses compressed gasses or fluids to accomplish the same thing, often in combination with actual springs, but that’s not significant in this context.)

Those springs need to move up and down.

• When you hit a bump, the suspension need to compress.
• When you hit a pothole, the suspension needs to extend.

Don’t forget that: Suspension needs to travel down, but it also needs to travel up. It needs “room” -- travel — to do both, and typically you need about 30% travel available both up and down.

Now, you take the weight of the bike — motor, chassis, transmission, and all the bits and bobs — and set that on the suspension. What happens? You compress it a little bit. That’s static sag: weight of the bike presses down and lowers the suspension a few millimeters. This is adjusted for at the factory, because the weight of the bike is obviously known. (Advanced riders may fiddle with the static as well, but let’s stick to basics).

Next, you go and sit on the bike. Depending on the bike and you, that could add anywhere from 20% (a 70 kg rider on a 350 kg Harley) or 100% (a 100 kg powerlifter on a 100 kg dirt bike). Either way, a significant change.

So what happens? The suspension compresses more, and there is less travel available. Add luggage, and you’re loading up the suspension even more. Add a passenger, and now the two humans on the bike can outweigh the bike itself.

Back to the basics: Your suspension needs to be able to travel up and down, and your fat, average, or skinny ass(es) is (are) now compressing it and removing some of that travel. You’re also changing the geometry of the bike by lowering it — that’s rider sag. Static + rider sag = how much your bike “sits down”. This affects handling, not just suspension travel.

Your bike and suspension is designed for a certain height and suspension travel.

What to do?

Well, you adjust the tension — how much weight it takes to compress the spring, or gas canister, or whatever magic your suspension uses. That’s your preload.

Preload dials in how much weight it takes to compress the spring X millimeters. The reason you have to set it, not the factory, is the factory doesn’t know how much you weigh and whether you’ll ride cross country with your lead ingot collection in the side cases and Shrek riding pillion.

Too much preload for your weight and you won’t be using enough suspension travel. It’ll be stiff and harsh. When your suspension can’t react properly to bumps and dips the tires lose contact with the ground momentarily. Too little, and you’ll bottom out. Picture, for example, braking hard. The front dives into the forks, bottoms out, and — oops — hits a bump; there’s no more travel available to soak that up, so your front tire bounces momentarily right when you need the traction. Bad times.

So, preload is the tension in your suspension. It affects handling as well as comfort, and it needs to be set for you on your bike because you don’t weigh the same as other riders, who don’t weigh the same as other riders, and so on.

Unless you have electronic suspension, in which case the bike sorts it out and you didn’t need to read this at all, it’s the first thing you should get sorted, the baseline before you fiddle with compression and rebound damping. If you don’t know how to do the measurements and adjust your settings yourself, find a local shop.