Off Road Suspension: Getting the perfect set-up

In Motorcycle Maintenance by Julian0 Comments

As motorcycles get more and more complex, it’s easy to think that the science behind the suspension on your bike is almost too much to understand. Yet knowing how your suspension works and what effect the various adjustments have will help you get the best out of your riding.  A bad suspension set up can turn your bike into an uncompromising and unpleasant machine, whereas getting it right can improve your riding dramatically and progress your skills to the next level.

Just a quick ‘cover-all’ here – always check the manual for your bike before making any adjustments OK?

Crner action

THE BASICS 

OK so suspension can be broken down into just two essential parts – namely the spring and the damper. Both have very different functions but are relatively useless without each other. The spring holds the weight of the machine and serves to absorb impacts from the ground through riding. However, just absorbing impact alone would not be any use, as the bike would continue to bob up and down like Mr Bobblehead. Consequently, damping is used to slow both the compression of the spring as it squeezes together – unsurprisingly called the compression damping, and how fast it returns to the original dimensions – the rebound damping.  Most modern bikes will have adjustment available for the spring and both compression and rebound damping, and it’s getting the balance between these factors right that will make your bike handle well.

KTM Rear shock

At the rear of the machine, most off-road bikes will run an external spring on a single centrally mounted shock absorber, commonly actuated through a linkage system. Damping is inside the structure of the unit, and again commonly there is an external reservoir to the damping system.

At the front, the springs are held internally and up to recently, within both legs. However, the development of SFF systems – Separate Fork Function – for motocross bikes has allowed manufacturers to split the springing and damping function between the two side, one leg holding the spring, the other handling the damping. For enduro and trail machines, this is less common and most will have both springs and damping systems held internally within each fork leg.

For both front and rear suspension, manufacturers have been developing suspension systems that use air rather than conventional springs, backed up with oil damping. While the rear systems are still very much restricted to the top end race teams, air forks are very much in evidence in the motocross market, although customer response has varied between enthusiastic and sceptical.

Getting your suspension set up is essential to making your bike handle well, but it’s certainly not a simple one hit process. The main factors are how heavy you are and how aggressive a rider you are, but after that there are all manner of other factors to take into account from terrain to weather conditions, and in the case of adventure bikes, how much luggage you are packing.

KTM Forks
All about the suspension

Most bikes are set up for an average rider who weighs around 12 to 13 stone, so if you are vastly outside this midline, then set-up is going to be harder. Don’t assume that the setting is correct for you even if you are that weight – you still need to check it and adjust accordingly whether your bike is brand new or second-hand.

The best way to get things sorted is to aim to get the bike to a setting that is roughly right for most of the conditions you ride and your style of riding, then look to tweak things slightly to cope with the conditions and terrain on the day.

ALL ABOUT THE SAG 

First stop on setting the suspension up for your bike is to set the sag. If you don’t know what this is, sag is the amount the suspension goes down under the weight of the bike alone and under the weight of you sat on the machine. This forms a baseline from which you can adjust everything else.

Sag image 1

To start, put your bike onto a paddock stand or centre stand so that both wheels are off the ground. Measure from a point on the swing arm somewhere close to the spindle to a point on the rear mudguard or the top of the number boards. Mark this point you are using with a bit of Gaffer tape so that you come back to the same point.

Sag image 2

Then put on all your riding kit including your helmet and boots.  Take the bike off the stand and sit on it and bounce up and down a few times to overcome any what’s called ‘stiction’, then get a friend, partner, intelligent child or passer by to measure between the same two points.  Subtracting the second measurement from the first will give you the sag – simple. For most dirt bikes, a rule of thumb is around 100mm, but you should check your manual for the recommended figure.

Moving the pre-load

If the sag isn’t at the required level, you are going to need to adjust the preload of the shock spring.  Release the pre-load collar lock  bolt, and if the sag was more than recommended you will need to tighten the pre-load adjuster – hence compressing the spring – ideally using either a specialist tool in the OEM toolkit, or more commonly a bar of drift and a hammer.  If the sag is less than the recommended level, you will need to slacken off the spring pre-load. In either case, be gentle as you are moving the adjuster and look to do it slowly rather than with jerks on the OEM tool or massive clouts with a drift. Keep sitting on the bike between adjustments to take more measurements until you reach the require sag.

Free sag

The final stage of the process is not to measure the free sag, which will tell you if the spring in your bike is correct for your weight. With the bike off the centre stand and no rider measure the difference between the previous two points, ideally with someone else holding it upright. Compare this to the static sag, which will tell you how much the machine sags under it’s own weight.  If it’s between 20mm and 30mm then you can be confident that the spring is right for your weight, and you can retighten the pre-load collar.

Rear springs

If however it’s less than this it’s likely that the spring is too soft, as to achieve the required sag has required too much pre-loading of the spring.  If it’s more than the 20-300mm ideal, then it’s because the spring is too hard. Both scenarios will mean that you need to get another spring for your shock, but that said if you are 20 stone and the average weight is 12 stone, you might have guessed that!  Companies like Hyperpro or K-Tech provide a wide range of springs to cope with both lighter and heavier riders. Fitting a new spring is not difficult if you have both the tools and the knowledge of what is required. If you don’t have both, leave it to a professional.

Front forks

When it comes to the front of the bike, the process is nothing like as easy or indeed as achievable for the average rider.  Most conventional forks don’t have any way to adjust the preload on the springs, so the same level of tuning is not possible. That said, enduro bikes will sometimes have pre-load adjustable via a nut on the top of the fork and the modern breed of air forks have almost infinite adjustments, all of which are possible with nothing more complex than a pump. You do however need to make the adjustments in the correct order, or you risk reducing the travel and totally messing up the suspension! If in real difficulties, you could always read the manual …

As a rule of thumb, your sag and free sag should be roughly the same as the figures you used for the rear, otherwise the bike is going to be unbalanced straight away.

If this is not the case, setting up your forks is best done by a suspension specialist who will be able to identify whether you need pre-load spacers or totally new springs to get the bike to behave correctly. Pay up and look big.

THE DAMPER THE BETTER? 

OK so now we’ve sorted the spring, we need to look at the damping. Compression damping, particularly in the case of rear suspension, is commonly spilt into low speed and high-speed damping. But the terms relate to how fast the shaft of the shock is moving, rather than the actual speed of the bike. Stutter bumps or  rocky trails will mean the shock is moving up and down quickly, whereas whooped-out sand tracks will mean the shock is moving more slowly.

Low speed compression damping adjustment is usually by turning the adjuster at the top of the shock absorber reservoir with a flat head screwdriver by the required amount of clicks. High speed compression damping is adjusted using a spanner on the outer nut at the top of the shock reservoir

Either way you want enough compression damping to ensure that you don’t smash though the shock’s movement when you hit bumps, and enough rebound damping that the bike isn’t bouncing like an excited puppy, but not so slow that the shock hasn’t returned for the next bump.

The rebound damping adjuster will be located at the bottom of the shock absorber and can be adjusted with a flat head screwdriver again.

Compression damping adjustment

At the front of the bike, the compression an rebound damping adjusters are in different positions, with rebound damping adjusted using a flat head screwdriver  or an easy-turn adjuster at the top of the fork unit, and compression damping made in the same way using the adjuster at the bottom of the fork ( after removing the rubber cap).

With all fork adjustments, make sure you let out any trapped air using the air-bleed screws before any changes are made – you can see it on the image below, along with the rebound adjuster and the pre-load adjuster.

Front fork image

So what happens if you slacken off the rebound? Well, the ride will feel particularly plush and smooth as the wheel will move quickly back, but if you take it too far and the handling will become vague and loose. As you increase the rebound damping, the ride moves from plush and into harsh.

Rebound damping adjustment

But if less rebound makes the ride nice and plush, why would you want to increase it? The answer is because having too little rebound reduces traction – the wheel pushes back down in a relatively uncontrolled way that can lead to skipping and hence loss of drive.

To make things a little easier here’s a quick guide.

COMPRESSION DAMPING 

TOO MUCH 

The action of the shock is harsh with far too much resistance to movement and the full suspension travel can’t be used

TOO LITTLE 

The bike feels unstable and uncontrollable with the suspension travelling through the full stoke, breaking traction as it pogos across the ground

REBOUND DAMPING 

TOO MUCH 

The bike sits low and returns to the correct height very slowly resulting in poor traction and loss of drive and a harsh ride

TOO LITTLE 

The bike feels nervous and unsettled as the suspension jumps around with every bump

Riding image

As you may be gathering now, it’s a delicate balance to get it right and the answer is going to be trial end error if you are going to do it yourself rather than pass this over to a bone-fide suspension specialist. Start at the manufacturer’s OEM settings from your manual and make adjustments in really small increments, marking down what you are changing. Bear in mind that if you do change the springs or increase the preload, then this will feel like the compression takes longer as the suspension will be generally stiffer, so in this case you may need to back off the suspension damping.

Add in the fact that changes to the rear suspension will affect the way the front suspension works and vice versa, then you can see that this is complicated stuff – perhaps why so many riders never touch the stock settings!

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FAULT FINDER 

So if all the information above has made you feel like you want to tweak your suspension and improve your ride, then here’s our handy fault finder. Work out where your bike is not performing well and the chart will tell you how to solve it.

GENERAL HANDLING 

PROBLEM:

Suspension feels too hard, ride is harsh and travel not being used

SOLUTION:

Rearreduce compression damping, reduce spring preload or change spring /spring rate

Front – reduce compression damping and change springs / spring rate

PROBLEM: Suspension feels too soft, bike is unstable and bike bottoms out

SOLUTION:  

Rear – increase the compression damping, increase spring preload or change spring / spring rate

Front – Increase compression damping or change springs /spring rate

CORNERING 

PROBLEM: Bike loses traction and breaks away

SOLUTION:  

Rear – back off the rebound, reduce spring rate

Front – back off rebound, drop forks through the clamps to reduce steering angle

PROBLEM: Rear of bike squats under acceleration

SOLUTION: Increase rear compression damping. Increase spring pre-load or spring rate

PROBLEM: Bike dives in corners

SOLUTION:  Increase fork rebound, fork oil level or compression damping

PROBLEM: Bike turns too swiftly

SOLUTION: Increase both rebound or compression, drop forks through clamps to increase steering angle

Cornering

STRAIGHTS

PROBLEM: Bike bottoms out on larger bumps

SOLUTION:

Rear – Increase compression, increase preload or spring rate

Front – Increase compression, increase spring rate or oil level

PROBLEM: Rear of bike kicks around and refuses to stay in line

SOLUTION:  Reduce rear spring rate / change spring, increase rebound damping

PROBLEM: Handlebar / head shake

SOLUTION: Increase compression damping,  change springs / lower spring rate.

PROBLEM: Bike sits low in suspension travel

SOLUTION:

Rear – Increase spring rate, reduce rebound compression

Front – Increase spring rate, reduce rebound compression

DROP -OFFS & JUMPS

PROBLEM: Bike bottoms out on landing

SOLUTION:

Rear – increase compression damping, increase spring rate

Front – Increase compression damping or oil level

PROBLEM: Suspension rebounds too quickly, bouncing the bike back up

SOLUTION:

Rear – Increase rebound damping

Front – Increase rebound damping, reduce compression damping, reduce spring rate.

Mr Jump

So hopefully this simple guide has given you enough information to head out to the garage and get busy with the tools. Start with the sag and then take the bike to your favourite track or trail and start experimenting – at worst it will make your bike easier to ride, at best it might even make you a better rider. What’s not to like?

Let us know how you get on using the Ride Expeditions forum pages.

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