Suspension and Stuff

A Simple Guide to Suspension..... and Stuff
by Chris Phillips


We shall start with the very basics....

. The Motorcycle
A bike is able to move through this world as it does courtesy of something called the gyroscopic effect, whereby a rotating wheel creates a force ("centrifugal"...which is often mistaken for "centripetal", which relates to a force tending towards the centre ) which travels from it's centre to it's edge, thus keeping it upright.
When the wheel is forced off it's axis, some of that force is then transferred laterally...dragging it inward. On a bikethis transfer is achieved by countersteering and weight-transfer. Countersteering is normally done subconsciously by the rider and occurs when, say in a lefthand corner, the lefthand handlebar is pushed lightly forward, this allows the bike to fall into the corner..the harder you push the further it will fall. The gyro effect will keep the bike at that angle and along with it's overall mass as it's leant into the corner, will cause the turn. The steeper the angle, the more the turn.
Weight-transfer is achieved by the rider moving their weight inward or weighting the inside footpeg, causing the bike to also fall into the corner. To exit the corner the opposite proceedure is followed. Remember Penny when I got you to hold the wheel of Jason's bike and spin it, then try to move the wheel off it's axis. Well that weird force you experienced was gyro effect..*S*. At very low speeds, steering is achieved by simply turning the front wheel in the desired direction of travel (dare I say it, as per a car), as the wheels aren't spinning fast enough to allow for enough gyro effect. But at anything above this, all cornering momentum is achieved by the above mentioned effect, combined with weight-transfer. The Suspension OK..in a perfect world with perfect roads, we wouldn't need suspension...in fact, bikes would corner much better if they didn't have suspension, but obviously with things the way they are, we NEED to have some form of suspension. First of all to comfort the rider, but more importantly, to keep the wheels in contact with the road at all times (except over railway lines..) and the steering free from lateral shocks. This is why racebikes on smooth racetracks can afford to run much harder suspension settings. This saps less of the cornering momentum, which helps increase corner speed. Harder suspension also greatly increases rider "feel", because he/she is more directly connected to the wheels. Unfortunately, when a bike is leant into a corner, suspension performance will always be compromised, because the forces caused by road irregularities are now not acting vertically upon the suspension components and in turn not all the force is being absorbed by the suspension. Now the art (and sometimes it can be a bit of a "black art") to correct suspension adjustment is to marry enough suppleness to ensure the wheel follows all the contours of the road and is not subject to "bounce", with adequate hardness to stop excessive suspension travel and spring "bottom out" (i.e fully compressed, thus returning to a no suspension situation). This balancing act can be achieved though a number of suspension settings....spring rate, preload, compression and rebound damping and linkage adjustment. But before we go there, just a quick foray into unsprung weight and steering geometry.

Unsprung Weight Often forgotten, but an important factor in suspension performance. Unsprung weight, as the name suggests, is all the components which are not sprung ..i.e. everything "below" the suspension.....such as wheels, brakes, mudguards, fork legs, swingarm and suspension linkage (incidentally this is why "upsidedown forks"..as per ummm let's say an ST4, are popular at the moment..there is less fork on the unsprung side of the spring, thus less weight. Another positive aspect is there is less flex, because the lower section of an upsidedown fork is shorter, which results in less leverage acting upon the fork seals..therefore a more free seal can be used, resulting in less stiction and more fork suppleness.. phew...what a mouth-full ( btw racebikes and some more upmarket sportsbikes use gold titanium nitrided fork tubes as it slides better through the fork seals than chrome ). Unfortunately the tradeoff is the overall weight of an upsidedown fork is usually greater and as a general rule they cost more...sorry class, went off on a bit of a tangent there... back to unsprung weight). So the lighter you can make these unsprung components, the less inertia they create when in motion, resulting in better wheel control. Lighter springs can also be used to gain that all important "suppleness" over smaller bumps I mentioned earlier. This is why racebikes use those super expensive Marvic or Marchesini wheels... yes, they do add to overall weight saving, but mainly it is for their reduction in unsprung weight and quicker steering response.
Steering Geometry
Just while we are here..a quick explanation of some of it's terms. Steering geometry is complex and very interrelated, so I'll just touch on it here. For this, it's a classic case of, "a picture tells a thou...........", but I shall do my level best to make it coherent. OK..for the purposes of this exercise I want you to picture looking at the bike in direct side view.
Rake Run a centreline through the tube of the steering head ( I'm sure you all know where that is, but just in case..it's the part of the frame, where the forks pivot ), continue that line until it reaches the ground..now run another line downward from the middle of the steering head perpendicular to the ground...those two lines intersect and form an angle, that angle is the rake, typically around 24 degrees. Rake is necessary to give the bike inherent stability. Because the forks need to be free to turn, they need to be angled to give some stability, by virtue of the wheel now acting through two planes. The more the angle, the more the stability, but the slower the steering. If the forks where at 90 degrees to the ground ( 0 rake ), you'd be over the handlebars and on your bum before you could blink.
Trail For this we need that centreline through the steering head again. From the spot where that line meets the ground..to the point where the wheel contacts the ground, is the trail...it's normally around 90mm. The less the trail, the quicker the steering. Triple Clamp Offset The triple clamp is the device which attaches the forks to the steering head. OK..back to that good ol' centreline through the steering head again....now draw a centreline down the fork tube. Remembering we are in side view here and the front wheel is not turned. The distance between these two lines is the offset. Offset effects the trail, the more the offset, the less the trail. I realise I didn't go into much detail here, but this lesson is about suspension after all..so let's get back to it......
What I'm about to say applies to both wheels equally, except when we start talking about suspension linkage.
Spring Preload There seems to be some misunderstanding out there about preload. Some are under the misnomer that preload can be used to make the overall suspension performance, softer or firmer...this is not strictly the case. In fact the only way this can be achieved is to replace the spring with one of a different gauge. What preload does do, is allow adjustment of the amount of suspension sag the bike is subject to, after the mass of the bike ( with rider ) is applied. Sag is important in the scheme of things, it negates suspension "top out" and thus allows the wheels to "fall" into holes and keeps the front wheel on the ground under acceleration ( yeah right, who am I kiddin' ! )..etc.
It's optimum to have both ends of the bike with the same preload, to test this, see if the bike dips equally when downward pressure is applied to somewhere near the centre of the bike..the seat perhaps. Most modern sports bikes should run around 30mm of sag. The typical treaded collar (and lock nut) on the rear spring and the large hexagonal nut on the top of the fork, used to adjust preload ( and without going too much into spring dynamics here ) only work on the first part of spring's compression, that part being approximately equal to the mass of the motorcycle. Sure, if you could wind that adjustment way down, it would make the suspension appreciably harder, but all you are really doing is reducing the amount of travel and the ability of the suspension to cope with small bumps, normally the domain of that first section of spring, now lost to the preload adjustment.
Damping There are two forms of this..compression and rebound. But first a brief explanation of damping per se. If the spring was allowed to work unhindered in it's movement, we would all end up doing an animated "pogo dance" down the road. Some sort of control on the spring was needed, enter the "shock absorber "or "damper". When a spring is compressed, potential energy is created, this is in turn converted into kinetic energy as the spring rebounds. Left unchecked, it leads to an oscillation of the spring. The damper slows down and smoothes this movement. This is achieved by passing oil through two small valves in the body of the shock or fork as the damper is compressed orextended. As oil is viscous, passing it through this valve absorbs some of energy produced as the suspension moves. Unfortunately as the oil is forced through the valve, it creates friction, which of course leads to heat... the bane of all shock absorbers. Excessive heat causes the oil to thin, leading eventually to shock "fade"( this is more typical of rear suspension, which has to handle power loadings also ). Heat also makes the oil more conducive to frothing.
To try and alleviate the heat problem, shock manufacturers now add remote reservoirs and sometimes even finning to their products. The reservoir adds to the volume of oil in the shock and also provides a cooling function, thus helping to bring down the overall temperature.They also discovered that by pressurising the interior of the shock body with a gas ( just plain air at first, but now mostly nitrogen ) it stops the frothing from occuring, because the oil is now under constant pressure.
Just a small proviso to what follows....the damping adjustment available on the exterior of the shock, only pertains to the low speed adjustment....i.e. when the piston inside the shock is at low speed..for example..at the initial movement of the suspension.
For high speed adjustment, shims ( thin washers ) inside the shock need to be physically replaced with ones of a different thickness.
Compression Damping
Probably the lesser of the two adjustments ( some tourers and less exotic machinery don't bother with compression adjustment...course, on Harleys you get no concession to damping adjustment period....2010 model release, maybe??? ).
Adjustment for compression damping at the rear is normally found on the remote reservoir, in the form of a knob with graduated "clicks" and on upsidedown forks as a screw adjustment at the base of the fork leg, near the axle.
This adjustment allows some compensation for the variables the suspension is likely to encounter....pillions (huh..pillions, what are they ? ), gear carrying ( please..don't make me laugh ), different riding styles and locations, etc.
Also if the spring is substituted for one of another rate, the damping may need some tuning. The factory settings are normally somewhere in the middle ( this goes for rebound too ) and the adjustment allows the damping to become either harder or softer ( also sometimes referred to as "slower" or "faster" ).
As explained earlier, damping helps control wheel movement, the result of such things as road irregularities, cornering, power delivery and braking. Compression damping deals with the initial movement, as the wheel movesupwards and the spring is compressed ( unless you ride a Buell...and do it backwards, they use the spring like a rubber band..got some funny ideas those Yanks..seems to work though ).
The idea here is to have the wheel's movement slowed down enough to be under control and therefore not likely to bottom out, but not too slow so as the suspension does not have time to react to different loadings. If this was to become the case, the whole bike would tend to "kick in the air", as most of the energy involved is being absorbed by the bike and not the suspension. When replacing the springs of either the forks or rear shock with harder items, it is common practice to soften the compression damping..and vice versa applies for a softer spring.

Rebound Damping
Rebound damping, yes..you guessed it, controls the spring in it's rebound cycle. An important one this, unless you enjoy being launched skyward every time you negotiate a bump of any substance. As you can imagine, a spring when compressed has an abundance of energy just waiting to burst forth, after the load is released. This released energy is contolled by the rebound damping and again it slows the downward moving wheel, bringing itback to position in one smooth, controlled motion. Not enough rebound and you are faced with that "pogo dance" ...too much and the suspension can' t react quickly enough to keep the wheel in contact with the road. Too much rebound also contributes to "squat" or "pack down"....this being a situation where the suspension can't extend to it's original position with enough speed, causing the rear of the bike dip. An excess of rebound, in combination with too little compression damping can lead to the dreaded and infamous, " fork chatter" over series of small bumps (usually under brakes).
Adjustment for rebound is provided to enable the suspension to cope with the same variables mentioned above for compression damping. For upsidedown forks it can be found as a screw adjustment on the top of the forks and on the rear it is usually at the base of the shock, just below the spring. Unlike compression damping tuning, when changing to a different gauge spring...if it is a harder spring, the rebound damping is normally increased. For conventional forks the location for damping adjustment is reversed...ie...compression on top and rebound at the base of the forks. OK....almost done, just one more to go.....

Rear Suspension Linkage
Most modern sportsbikes offer some sort of adjustment in the linkage which attaches the rear shock to the swingarm and frame.
Back in the dim dark days of "twin shock" and "cantilever" rear suspension, the spring and shock absorber were attached directly to the swingarm and frame, thus as the swingarm pivoted upwards it worked linearly on the shock and spring. The further the swingarm rose, the more it compressed the spring, but only at a constant rate. The best they could manage back then was to use a progressively wound spring, where the softer section of the spring copes with small road irregulaities, giving a more supple ride. You can still sometimes find progressive springs fitted to front forks, where a linkage system is unable to be used because of the forks need to pivot.
So in the quest for more compliant and sophisticated rear suspension, designers discovered by adding a linkage between the shock and it's mounting points, they could take advantage of the laws of leverage..."rising rate" suspension was born. As the swingarm rises it's movement starts to compress the spring, but by adding a pivoting T-link ( the most common these days ) to the top of the shock, the link also starts to compress the spring, only this time in an exponential curve... thus compressing the spring at an ever increasing rate.
Some manufacturers have taken this system even one step further by adding another linkage to the bottom of the shock where it attaches to the swingarm... Suzuki's "Full Floater" for instance ( yeah P..think you got one of them..*S* ). With this set up the shock has no direct mounting, so hence the name.
Another advantage of rising rate suspension is that you can use a softer spring..soft in the initial stage, where there is little leverage, which is required for small bumps, but when a heavy load is applied, the ever increasing rate of compression allows the spring to reach a point where it is hard enough to cope with the load ( hope that makes sense...
it's difficult to explain without a diagram ). Using a linkage system also has the added benefit of longer travel suspension... a little increase in the length of the shock, equates to a large increase at the wheel. By adding a number of offset holes at the top of the "T" where it mounts to the frame, it allows for adjustment to the amount of rising rate inherent in the system. Having a linkage system also makes ride height adjustment easy. By simply making the pushrod arm adjustable, ride height can be varied by increasing or decreasing it's length. Adding to the ride height at the rear increases ground clearance but also effects the steering geometry, by effectively steepening the angle of the front forks, thus decreasing the rake.
So it would seem the best approach to this myriad of adjustments, is to start at the factory settings ( noting them down ). If you are not comfortable with these, use incremental variations, both alone and in combination, till you achieve something that feels right for you.
Ok well there it is. I don't pretend to be any sort of "guru", it's just stuff I've picked up over the years. Suspension tuning is a bit of a black art, a lot of it is subjective...ask ten different people and probably get ten different answers. A lot of it you may already know, maybe all of it.... it's difficult to know where to start, but anyway I just hope you enjoyed reading it.
Chris


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