Suspension Tuning with Terry Hay

This entry is part 11 of 320 in the series Adventure Rider Issue #1

Suspension demystified

With Shock Treatment’s Terry Hay

So, you’ve bought a new bike.While there are many modifications you may have in mind, or perhaps a few personal touches to make that bike truly yours, there are few changes that will bring the rewards of improved handling.

The standard suspension package on most current motorcycles has seen the benefit of an evolutionary process that has made the modern bike a far more agreeable, friendly beast. Longer suspension travel, improved ground clearance,easier steering and greater impact absorption have widened the scope for both application and type of terrain that motorcycles can now negotiate. Part of this evolution has been the provision of greater levels of adjustability. We now have the ability to tune the bike to suit our intended tasks or personal preferences. Unfortunately for some, all this adjustment adds an unwanted level of confusion, but it’s really not so hard to understand. So, where do we start?

Basic first

These days most bikes’ stock suspension set-ups seem to favour a rider who weighs between 75kg and 85kg, and are sprung and damped accordingly. If you fall outside this zone, there’s no need to worry. There are aftermarket springs available for most bikes and rider weights. Damping can also be tailored to suit individual needs.

If you’re lucky enough to fit the weight category that the bike was set for, or you have procured the appropriate springs, the next step is to optimise the set-up to suit you.

First step: springs

The fork springs should initially be installed with five millimetres to 10mm of preload. The shock spring should have between 10mm and 14mm. Now we set the ‘sag’. Starting with the rear of the bike, we use three measurements to determine the correct sag and see just how suitable the spring is for our application.

Consult the specifications page in your manual to determine the amount of rear-wheel travel your motorcycle has.

Step 1: free sag

• Place the bike on a suitable stand so the rear wheel is unweighted and can fully extend.
• Measure from the rear axle to a point on the back of the bike that is as close to vertical as possible (a piece of tape on the rear guard works well).
• Note the measurement.That measurement is your free sag.

Step 2: rider sag

• Set the bike on the ground and have the rider get on and assume their normal riding position. This should also be done in the complete riding outfit with all additional gear – bumbag, CamelBak, fuel load, camping gear, etc. Fuel levels are best set to halfway as they’re variable.
• Measure between the same two points as used for free sag. This measurement is your rider sag.

The difference is…

We’re generally looking for the bike to settle between 30 to 35 per cent of the rear-wheel travel when fully laden. For example, if your bike has 300mm of travel we are looking for 90mm to 105mm of rider sag.

This allows for a tuning range to suit your application or hard/soft preference. With less sag the bike will steer quicker, but with more sag the bike will generally be more stable. Most people will start with 100mm and adjust from there.

The sag will be adjusted by moving the preload collars on the rear shock up or down and remeasuring until the desired figure is achieved.

Step 3: static sag

This measurement is taken using the same two points as we used for our first two measurements, only this time the rider is off the bike and the bike is held upright, allowing the rear to settle under the bike’s own weight. Ideally the static sag should be between 10 per cent and 15 per cent.

So on a bike with 300mm of travel we’re looking for between 30mm and 45mm of static sag. If your measurements fall outside this range then your spring is not correct for your needs and should be replaced.

Important note: static sag is an indicator only and not a tuning point. If your spring is not quite right, but you can’t replace it immediately, don’t compromise your rider sag to try and achieve a more appropriate static sag measurement.

Front sag

I personally consider front sag to be fraught with inaccuracy due to the friction and binding associated with the telescopic fork, and rather than a measurement, I prefer a dynamic tuning option. By that I mean, “ride the bike”.

Your spring preload will determine the final ride height of the bike and will be relative to braking performance as well as initial feel.

Too little spring preload and the bike will dive excessively under brakes. Too much and the front will ride high, producing slow steering and being more prone to wheel deflections from square-edged bumps. The most important thing here is to achieve a balanced feel and have the front work well with the rear of the bike. Some bikes, like KTMs and Husabergs, have external preload adjusters. Others can be adjusted internally using spacers or preload shims.

Damping

Most bikes have adjustable compression and rebound damping these days, allowing changes to the level of comfort, control and traction. The greatest mistakes people make when adjusting their damping is with the rebound circuit. If we view this logically we will find that compression is the action of the suspension when an impact is experienced and the rebound is the reaction or the recovery. Sadly, we tend to notice the reaction more than the initial action, creating confusion in the diagnosis of the problem incurred.

So let’s simplify this. If a fork didn’t compress, it wouldn’t rebound.

Above left: Fork Compression Adjustment.Above right: Shock Compression and Spring Preload Adjustment.

As a fork or shock compresses, energy is stored in the springs. This energy needs to be released and this provides the mechanism for the suspension to reset itself, ready to handle the next bump or just to get the wheel back on the ground to restore or maintain grip.

All too often people slow the rebound in an effort to stabilise wheel movements that are more likely occurring due to the compression phase. If the compression is too hard we’ll experience wheel deflection due to the lack ofcompliance. If the compression is too soft we’ll see the suspension move too far through its stroke, storing excess energy in the springs as a result.

This energy will have to be released on the rebound stroke and excessive movement will occur.

Knowing this, it makes perfect sense for us to address the compression first before touching the rebound adjusters. Ride the bike and adjust the compression until the desired feel is achieved. Once you’re happy with this we can look at the rebound. Be aware: you’ll find a bike with the rebound a little quick will function much better than one with the rebound too slow. Ride the bike and speed the rebound up. As the wheel reactions get quicker, your grip will improve. Go too far and the bike may start to feel a little loose or tend to wallow.

Come back in on the adjuster until the best blend of grip and control is achieved. Now you’re set. Note the position of the adjusters for future reference.All your testing should be done on a familiar track at a moderate pace. As your bike gets dialled in and you feel more comfortable you can increase your speed (within reason). Remember: nobody ever won a trophy during testing.