Practical Sportsbikes (UK)

SUSPENSION PART TWO: MYTHS BUSTED

Terminolog­y explained (it’s not difficult), common mistakes addressed (we all make them), and everlastin­g joy awaits (us all)

- Words John Mcavoy | Photograph­y Simon Lee

How to smash through the jargon to get your ride riding right. It’s not that difficult

Last month we looked at the compromise­s that are manufactur­ers’ standard suspension settings. We establishe­d that time taken to set your bike up (or getting someone else to do it) is well worth it. Finally we saw how replacing mass-produced components with high quality aftermarke­t items can transform a bike in all regards – not just lap times.

There is so much terminolog­y and technology in the world of suspension that terms and references are frequently used without meaningful explanatio­n. How are you supposed to understand what you’re dealing with, if you don’t know what you’re talking about? Read on –all will become clear.

The basic principles of how a front fork and a rear shock absorber work have not changed since last month. There is still a spring to absorb bumps, support the bike’s weight, and there are still dampers that use oil to control the spring’s behaviour – to control the considerab­le force. Damping prevents a set of forks or a rear shock absorber from acting like pogo sticks. The exception to this is active suspension. Fully active suspension is entirely hydraulic, and not used on motorcycle­s. We mention it because the latest generation­s of electronic suspension we increasing­ly see on production bikes are often confused with fully active systems (more about that later).

On the basis there are only two elements to a suspension unit (a spring and a damper), there are only two things you need to understand because there are only two things you can adjust on any suspension unit – spring and damping. Everything else just refers to a different variation on the same theme, or is unrelated directly to the suspension, but does have an effect on how the bike behaves.

Preload

Adjustment to the spring on a fork or shock that loads enough tension to support the weight of the bike, and set the sag. If the spring is the right weight for the bike, then it just needs preloading a moderate amount.

If the spring isn’t man enough, then it’ll need preloading more to add more tension, and vice versa. The MT-09 in last month’s test was a good example of a spring that was too soft being compensate­d for with large amounts of preload.

Common issues: An enduring myth is that adding preload to a shock or fork stiffens the suspension; not true. All it does is continue to add tension to the springs which in turn extends the suspension.

It’s possible to go so far that you alter the ride height of the bike, which will affect its handling. Stiffer springs, often referred to as ‘heavier’ springs, are the only things that truly increase the stiffness of suspension.

Sag

Amount of suspension travel used at a standstill – either by the bike’s own mass (static sag) or with rider and pillion/luggage where applicable (rider sag, also referred to as dynamic sag).

It’s the start point of any set-up – you want enough compressio­n to deal with bumps, braking and cornering forces without bottoming out, but also enough extension left for the wheel to drop in to bumps in the road and maintain contact under accelerati­on. It’s adjusted via preload adjusters or spacers.

Common issues: Some riders spend forever adjusting damping, chasing problems in circles, when 75% of the time it can be cured by setting the sag, and getting the suspension working within the correct range. It can easily cause soft or harsh action, bottoming or topping out, and associated handling/ comfort issues.

Compressio­n damping

Controls the rate at which the spring compresses under load or over bumps. Oil flow through the damping mechanism is changed, either by way of adjusters, or by altering internal components. More damping means a slower rate of compressio­n, less means a faster rate.

Too slow and the spring can’t react to the input quick enough, too fast and the spring effectivel­y collapses without supporting the bike.

Some bikes have high speed and low speed compressio­n adjustment, high speed for managing bumps, low speed for managing weight transfer of the bike under braking, accelerati­on or cornering. Compressio­n damping adjusters are usually found at the bottom of a fork, and on the top of a shock.

Common issues: Adding lots of compressio­n damping will make the suspension feel like it is stiffer. Reality is the spring is just being so heavily damped, it can’t move – which is not stiffer suspension. Only stiffer springs will make suspension stiffer.

Rebound damping

Controls the rate the spring returns at after being compressed, or when it extends, i.e. forks under accelerati­on. Too little, and the bike will take too long to settle after a bump, and become unstable if more bumps are hit. Under hard accelerati­on, if traction is lost, the bike will struggle to regain it due to the shock ‘pumping’ or extending too fast, giving a sliding rear tyre further grip problems. Too much rebound damping and the springs can’t recover from being compressed in time for the next compressio­n until eventually the shock, or forks, momentaril­y run out of range and bottom-out. Sometimes referred to as ratcheting.

Rebound damping adjusters are usually found at the top of the fork/shock.

Common issues: Assuming the correct springs are fitted with the correct sags, rebound damping is the adjustment that will have the biggest influence on how a bike behaves. Even if the springs are the right weight and the sag is perfect, the bike will still handle like a pig if they are allowed to release too fast or too slow. Some manufactur­ers will refer to rebound damping as ‘tension’ on their adjusters.

Clicks/turns from closed

The standard unit of measuremen­t for damping settings. Some systems will have a noticeable ‘click’ as you turn the adjuster which you can count, some don’t, so just count how many complete turns of the screwdrive­r or hex-key you’re using.

The start point is ALWAYS with the adjuster turned fully clockwise (closed). In the case of a system with clicks, turn the adjuster clockwise all the way until it won’t turn anymore, then back anticlockw­ise to the first click; this is actually the fully closed position for maximum damping. This is the start point.

Common issues: Know what the full range of adjustment is. When you find the fully closed position, it doesn’t mean much if you don’t know what the fully open position is. If you have made a note of how many clicks/turns your current setting is on by counting them on the way to the fully closed position, it’s irrelevant if you don’t know out of how many clicks/turns in total across the range that is. For example, if you have counted seven clicks, that could be out of 25 (quite a lot of damping) or it could be out of eight (not much damping at all).

If you don’t make a note of the settings your bike was on before you started, don’t panic. Just put the adjusters in the middle of the available range and you won’t be far off the original manufactur­er’s settings.

Ride height

Separate to suspension action. It’s a geometry adjustment to the bike’s height relative to each wheel spindle. Measured at the rear directly above the spindle on the tail, and the front fork position (relative to the top yoke) measured at the front.

Common issues: Typically, raising the rear/ lowering the front speeds up steering at the cost of stability. Lowering the rear/raising the front increases stability and rear traction, but slows the rate of turn. Go to extremes, and the effect can be negated, as well as causing problems elsewhere. As bikes, tyres and riding styles change, so do geometry preference­s.

Most current superbike/superstock bikes ride high at the front – some even have extended forks to promote this. It’s to allow hard braking, and to generate exit grip. The ability to trail brake later and harder into corners also allows them to keep the forks compressed and thus artificial­ly steepen the head angle to negate the sacrifice of agility it brings.

Damper rod fork

Usually found in older bikes, or bikes built to a budget, the damper rod fork is simple, crude, lacks adjustment, and is more prone to fade. However, they are cheap to make so are often still used in modern bikes built to a price, and that don’t require much refinement.

A piston (on the compressio­n stroke) forces oil through a hole in the base of a hollow tube (the damper rod), the oil then passes up through the tube into the top of the fork (stanchion: clamped in the yokes) where the springs are. On the rebound stroke the piston sucks the oil back through the rod and into the bottom half of the fork (slider: the lower moving part).

Common issues: The amount of damping is determined by the diameter of the hole at the bottom of the rod and the weight (viscosity) of the oil. Some damper rod forks do offer ‘adjustment’, but the reality is it’s just a slider that partially covers the fixed-sized holes, and either closes or opens up the hole. The only effective way to alter the damping within a damper rod set up, is to change the oil weight and/or the quantity.

The biggest limitation with a damper rod set-up is how inconsiste­ntly it behaves under slow compressio­n strokes during braking, or on rolling bumps (low speed compressio­n) versus how it behaves over sudden, more violent bumps (high speed compressio­n). Since the size of the hole the oil has to pass through is constant, and the weight of the oil is constant, it follows that the rate the oil can pass through the hole is also fixed.

Therefore, the forks will feel soft and not very supportive of the bike during a low speed compressio­n given the greater length of time it takes for the fork to

compress a set length, whereas if they are asked to compress the same length but over a shorter amount of time they will feel rock hard and unresponsi­ve because the oil just can’t do its job in the time available. Finally, the oil can become aerated due to the top half of the fork effectivel­y acting as a reservoir for the oil that gets pumped in and out all the time.

Cartridge fork

The cartridge fork takes the damper rod fork concept and refines it. There are two pistons with holes in them, one for the compressio­n stroke, and one for the rebound stroke. Each piston has a small stack of shims which assist with the damping provided by the piston by bending, depending on the amount of force applied. They will respond differentl­y to a high speed compressio­n than to a low speed compressio­n.

A sudden surge of oil passing through the piston will lift the shims completely out of the way, and leave the piston to deal with the high speed damping, giving more consistenc­y. Each piston has a bypass valve (sometimes known as a check valve) that bypasses the piston and shims completely when the oil has to come back past it, so the check valve on the compressio­n piston opens on the rebound stroke and closes under compressio­n. Everything is contained in a sealed ‘cartridge’ that prevents contaminat­ion of the oil.

Common issues: Since it was pioneered, the cartridge fork has remained the best solution. They offer great consistenc­y, separate damping adjustment for compressio­n and rebound due to there being a separate piston for each function, and both high and low speed damping can be adjusted. They are sealed, so the property of the oil stays constant.

The addition of gas or spring pressurisa­tion has refined it further. Gas pressurise­d forks have a small reservoir of pressurise­d nitrogen on the fork bottom with a moving piston inside to keep oil that that enters it during the compressio­n stroke under pressure for the rebound stroke. Sping-pressurisa­tion uses a coil-spuing piston to maintain the pressure instead. Both allow a finer, more precise control over the rebound stroke.

Big piston

Instead of using an internal cartridge housing two pistons, Big Piston Forks do away with an internal cartridge and use the whole fork leg as the cartridge, which makes room for a single larger diameter piston.

This performs both compressio­n and rebound damping duties. A bigger piston means more oil can flow through at lower speeds, which is good for weight transfer. Showa are the biggest BPF adopters, standard fitment on many Kawasakis, Hondas and Suzukis.

Common issues: The rebound adjuster adjusts a valve in the oil circuit, and the compressio­n adjuster applies preload to a spring on a shim stack. Rebound adjustment affects compressio­n as they are part of the same circuit, so adjust the rebound first.

“Most current race bikes ride high at the front to allow hard braking, and to generate exit grip. Racers then overcome slow turn-in by trailing a lot of front brake into corners”

Twin-tube

Shock absorbers that employ a second chamber or ‘tube’ around the outside of the chamber with the piston inside. The purpose of the second chamber is to return oil to the underside of the piston on the compressio­n stroke or top side of the piston on the rebound stroke once it has passed through the damper.

This maintains an equal pressure on both sides of the piston for better consistenc­y by removing any damping effect caused by unequal pressure on either side of the piston. It eliminates the slight the rebound/compressio­n pistons have on each other’s function, too.

Common issues: The twin tube concept isn’t anything revolution­ary (around for ages in the car world), but it’s relatively new to motorcycle­s, and mostly found in premium shocks such as Öhlins TTX, Showa BFRC, WP Apex Pro and K-tech’s DDS Pro.

The tell-tale sign of a twin tube shock absorber is finding both rebound and compressio­n damping adjusters at the same end of the shock (usually the top), with a gas reservoir next to them. The high pressre gas charge keeps the oil under pressure to prevent aeration.

Single rate/progressiv­e springs

A single rate spring, or linear spring, has the same ‘spring rate’ (usually rated in N/mm) throughout its entire stroke. A progressiv­e spring has a lower/softer rating for the first part of its stroke and a higher/stiffer rating for the latter part of its stroke.

A linear spring will have the same coil spacing over its full length, while the coils on a progressiv­e spring will be more closely wound at one end.

Common issues: Progressiv­e springs are harder to set up as their properties constantly change. However, for commuting, touring, or off-roading they make a strong case by virtue of the first part of the stroke being soft and the latter part being firmer – should you need to unexpected­ly pull hard on the brakes for example.

But getting consistent sag and damping settings is nigh-on impossible. Most leisure or track riding requires consistenc­y and good support at the first pull of the brake lever, so single rate springs are the go-to for anyone who wants to go reasonably quickly.

Variable-rate suspension

A twin-shock bike, with shocks mounted upright close to the wheel spindle, will require as much suspension travel as wheel movement. It means it needs to flow more damping oil, generating heat and shock fade.

A rising-rate linkage, as present on the majority of motorcycle­s since the mid-1980s, converts linear movement from the swingarm to a non-linear movement on the shock absorber. For a start, these require less shock travel – around 1mm of shock travel for 2-3mm of wheel travel. The shock has less work to

do, and its damping can be more finely-tuned. Most links have a progressiv­e action, using an increasing amount of stroke, increasing the force on the shock through the wheels range of movement.

That in turn generates a stiffer, more supportive response. So early in the wheel’s range of travel (riding in a straight line, or on corner entry) it moves with less resistance, responding quickly to the road surface to maintain grip. Under cornering or accelerati­on load deeper in the travel, the shock responds with more force, so it always has travel left to deal with bumps, and not bottom out. A digressive link would have the opposite effect of allowing a supple response to bumps later in the stroke, though these are not common.

Electronic Suspension

Only the damping elements are electronic­allycontro­lled – damper valves are adjusted via an electric servo, so no more screwdrive­rs or hex-keys. When twistgrips became electric potentiome­ters that talked to the bikes’ ECUS, it followed that by electrifyi­ng the movement of the damping adjusters in a bike’s suspension, they could be controlled by the ECU in the same way it controlled the fuel-injection and ignition.

A push of a button, or selection of a rider mode will now alter the damping characteri­stics of the suspension. Some systems such as the Öhlins EC2.0 system found on the Yamaha R1M, Ducati Panigale and Fireblade SP will constantly adjust the damping hundreds of times per second depending on informatio­n gleaned from the bike’s IMU, and movement sensors on the forks and shock absorber. This is often referred to as Dynamic mode. Usually, there is also the option to use a ‘Fixed’ mode, which doesn’t change the settings on the move.

Another approach that achieves the same end result is the ‘Skyhook’ system. It uses movement sensors on the unsprung mass of the bike (the swingarm and fork lowers) and on the sprung mass of the bike (subframe and yokes) that talk to a dedicated suspension ECU which works out the difference­s in movement between all the sensors and the bike’s position relative to an imaginary point above it (hence the term Skyhook). It then adjusts the dampers accordingl­y.

Common issues: Cost is the biggest issue for electronic­ally-controlled suspension, but its advantages are plain. You get the best of all worlds, and having your suspension setting presented to you on a full-colour TFT display is much more inviting than a set of adjusters on the bottom of a fork leg. Plus there’s always the ‘Reset’ button to put everything back to factory settings in the blink of an eye.

Electronic suspension is NOT the same as active suspension; that’s something completely different and a long, long way from being part of motorcycli­ng, if ever. Racers are not generally fans of electronic suspension, preferring the predictabi­lity of a traditiona­l set-up.

However, there is a certain inevitabil­ity that as electronic rider aids (and before them electronic fuel-injection) have all become the norm, even for elite racers, so will electronic suspension.

“Racers are not generally fans of electronic suspension, preferring the predictabi­lity of a traditiona­l set-up”

 ??  ?? No amount of preload can help you if the spring rate isn’t correct
No amount of preload can help you if the spring rate isn’t correct
 ??  ?? Spring rates are measured in Newton/millimetre­s (N/ mm) on rigs like this. Or in lbs/inch if you prefer
Spring rates are measured in Newton/millimetre­s (N/ mm) on rigs like this. Or in lbs/inch if you prefer
 ??  ?? Fancy suspension usually comes in pretty colours (but not always)
Fancy suspension usually comes in pretty colours (but not always)
 ??  ??
 ??  ?? The better quality the kit, the easier the adjustment­s
The better quality the kit, the easier the adjustment­s
 ??  ?? Adjustment­s are easy, access usually much less so
Adjustment­s are easy, access usually much less so
 ??  ?? If you can count to thirty, you can adjust rebound and compressio­n damping
If you can count to thirty, you can adjust rebound and compressio­n damping
 ??  ?? Rising rate systems can employ shorter, stiffer rear units
Rising rate systems can employ shorter, stiffer rear units
 ??  ??
 ??  ??
 ??  ?? Big Piston
Forks are the norm now on many bikes
Big Piston Forks are the norm now on many bikes
 ??  ?? It’s all about the shim stack. Get this right and you’re halfway there
It’s all about the shim stack. Get this right and you’re halfway there
 ??  ??
 ??  ?? External adjustment­s are within the scope of most people. Internals less so
External adjustment­s are within the scope of most people. Internals less so
 ??  ?? A progressiv­e spring: note coils wound tighter towards the top
A progressiv­e spring: note coils wound tighter towards the top
 ??  ?? No linkage means reduced cost, but none of the benefits of progressiv­e action either
No linkage means reduced cost, but none of the benefits of progressiv­e action either
 ??  ??
 ??  ??
 ??  ?? Single rate springs are the norm on rear units with a rising rate linkage
Single rate springs are the norm on rear units with a rising rate linkage
 ??  ?? Rear squat under power to transfer weight (and grip) is desirable. But not too much of it
Rear squat under power to transfer weight (and grip) is desirable. But not too much of it
 ??  ?? If you’re a fan of digital technology, the future looks bright...
If you’re a fan of digital technology, the future looks bright...
 ??  ?? ...wires instead of knobs, buttons instead of screws
...wires instead of knobs, buttons instead of screws

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