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)
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.
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.
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.
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.
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.
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.
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.
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”
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.
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.
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”