The Truth About Sag: Ride Height, Traction and Handling Balance

Morning everyone. Setting sag is one of the most misunderstood — and often completely forgotten — parts of getting a motorcycle to not only go around corners properly, but also travel in a straight line.

For the most part, riders read a specification in a magazine — yes, I’m old — find one online, or copy whatever their mate has put into their bike. Generally, whatever number they use will be wrong. Not necessarily because the number itself is bad, but because it was developed for a different rider, different springs, different terrain, different speed and often a completely different use.

First: measure it consistently

I’m not particularly religious about exactly how you measure sag. Measure it however you want — but make it repeatable, otherwise the measurement is useless.

For motocross and off-road bikes, that generally means measuring with the rider standing in their normal riding position. On a road bike, it will generally mean sitting in the normal riding position. Most importantly, measure as closely as possible in line with the wheel’s arc of travel.

• Same rider position.
• Same gear.
• Same fuel condition.
• Same measurement points.

Pretty simple so far. Oi.

Why do we need sag at all?

Suspension needs to move in both directions. Everyone understands that suspension has to compress when the wheel hits a bump. What gets forgotten is that the suspension also has to extend when the wheel encounters a dip, hole or depression in the ground.

If the wheel cannot extend into that depression, the tyre loses contact with the ground. Once tyre contact disappears, so does traction. That means wheel spin, reduced braking grip, reduced cornering grip, instability and ultimately loss of control.

Slow rebound damping can make this considerably worse, but that is another Technical Series article.

Sag exists because we deliberately position the suspension part-way through its available travel so that there is usable movement available in both compression and extension.

The one-third starting point

A useful general starting point for rider sag is approximately one-third of available wheel travel.

That is a starting point — not a commandment carved into stone. From there we make allowances for intended use, chassis geometry, front-to-rear balance, suspension design, frame flex, rider position, riding style, spring rates and the handling character we are trying to create.

The important part is understanding why the number exists.

Stop thinking about preload as turns

This is probably the biggest change I would like riders to make in the way they think about suspension setup.

Instead of thinking “I’ve added two turns of preload,” think “I’ve changed the ride height.”

Adding preload increases the installed force in the spring and reduces sag. The motorcycle therefore sits higher relative to the ground. Removing preload increases sag and lowers the motorcycle.

Once you start thinking of preload as a ride-height adjustment, the effect it has on handling becomes much easier to understand.

Rear ride height changes the motorcycle

Increase rear sag — lowering the rear of the motorcycle — and, assuming the front remains unchanged, the bike generally becomes more stable, slower to change direction, heavier to turn and more inclined to transfer weight rearward and wheelie under acceleration.

Reduce rear sag — raising the rear — and the motorcycle generally becomes more responsive, easier to turn, less stable and potentially more prone to rear tyre spin under acceleration.

Neither is automatically right or wrong. We are looking for a balance: enough extension travel to maintain traction, enough compression travel to absorb bumps, enough stability to give the rider confidence and enough agility to allow the motorcycle to turn.

A motorcycle is a complete system

This is why I often say motorcycles behave more like fighter aircraft than cars. Everything interacts. Change one parameter and you influence several others at the same time.

If the fork spring is too soft, or the compression damping is insufficient, the fork may dive excessively under braking. That changes rake, trail, wheelbase and weight distribution. The bike may suddenly become nervous or unstable entering braking bumps.

The rider may then try to correct that instability by changing rear sag. Now we are using the rear suspension to compensate for a problem originating at the front. That is how suspension setups get chased around in circles.

Yes — the forks have sag too

I have deliberately introduced fork sag here because it is something almost every rider — and surprisingly many workshops — pays very little attention to.

But exactly the same principles apply. The fork needs extension travel. Its ride height changes geometry. Its sag affects chassis attitude. And its relationship with rear sag has a major influence on how the motorcycle behaves.

The motorcycle does not know which end you remembered to measure. It only knows where both wheels are relative to the chassis.

So what are the correct numbers?

Here is the part people usually do not like. There is no single magic number.

On my KX250, for example, the sweet spot is approximately 52 mm fork sag and 112 mm rear rider sag. That works for my bike, my setup, my weight, my riding and what I want the motorcycle to do. It does not automatically mean those numbers will work for you.

You are unlikely to find your perfect setup printed in a magazine. You find it by riding.

Measure the bike. Ride it. Make one controlled change. Ride it again. Take notes. Then repeat.

Mountain Race Shop Takeaway

• Sag is not simply a number — it establishes where the suspension operates within its travel.
• Suspension needs extension travel as well as compression travel.
• Preload is easier to understand when treated as a ride-height adjustment.
• More rear sag generally increases stability but reduces agility.
• Less rear sag generally increases agility but can reduce stability and rear traction.
• Front sag matters as well as rear sag.
• The correct sag setting is the one that creates the best front-to-rear chassis balance for the rider and the job.

That is exactly what the Saturday morning setup sessions at Mountain Race Shop are about. At the workshop we can measure, change, ride, come back, measure again and actually learn what the motorcycle is telling us — rather than guessing between races.

Enjoy.