Here you go, reading so much about sportbike suspension yer a friggin' expert.  Your friends will be amazed at your arcane knowledge as you spout off at the cafe until they see you wobble around a corner at half their speed.  So put it to good use and get down to the garage.  I know I need to!! 

Although this isn't strictly a riding technique there's a lot of false information about the 'correct' set up for a bike's suspension.The one main thing that most of these articles miss is the most crucial - setting the sag correctly.

The sag range will be the same for everyone but some of you will find that you will need to change springs to get into that right range. Race bikes, generally, need harder setting than road bikes as they ride on smoother surfaces. Road bikes have a huge range of dips, bumps and potholes to cope with so need to set slightly softer.

Most of you will have different preload settings, thanks to your weight but all of you will need to be in the same range for your suspension to work at it's optimum range, namely it's middle third.

Without the sag set right the damping will never be able to work at it's best, as it will be trying to overcome the effects of a badly set spring. Don't forget that your pride and joy has been designed to work with a hugely varying type of rider on board. It has to cope with very heavy riders to very light riders. Aggressive riders to steady riders and all points and weights in between.

A compromise? Damned straight!

It's amazing that people will spend hundreds, even thousands of pounds on suspension components and twiddle with them all day and night but never take the time to set the sag correctly and never get the bike handling any better!

So what is sag? There are two types, Static and Loaded. Static sag is the amount the bike settles under it's own weight. Loaded sag is the amount the bike settles with a rider on board.

To check that you have the right rate of spring you will need to set both the static sag AND the loaded sag, just doing one or the other is only getting half the story.

Here's what you will need to check and set your sag.
A tape measure.
A pen and paper.
Tools to adjust the front and rear preload.
Three strong friends.

Static Sag

Setting the static sag is easier than the loaded but it will still need a set of strong hands to keep the bike off the floor while you take your measurements. Lift the back wheel of the bike off the floor by pulling the bike over on its side stand.

Now, using your tape measure, take a reading from the centre of the rear wheel spindle to a point on the tailpiece that is directly above the spindle. It will be helpful to mark that point on the tailpiece for the future reference.

Write down the measurement and we will call this reading A.

Measurement A will remain the same throughout the procedure so you only need this reading once. Now put the bike back down on it's wheels and hold it upright. You will see the bike settle under it's own weight. Measure the distance from the spindle to the tailpiece. We will call this reading B.

Subtract B from A. This is your rear static sag. Keep a note of it just in case you want to change it back. Ideally you are looking for 5-10mm on a race bike and 15-20mm on a road bike.

Either compress or loosen off the spring to get into this range. The adjuster is normally a pair of rings on top of the spring. One of them is a locking ring and the other is the adjuster. You will need a C-spanner to make the adjustment. If you don't have one then a hammer and drift will do (animal!)

Now lift the front of the bike off the floor using the same side stand method. Measure from the bottom yoke to the top of the stanchion. We will call this measurement C. Like measurement A this will remain the same through out.

Drop the bike back onto two wheels and take the reading again. We will call this D.
Subtract measurement D from C. This is your front static sag reading. Keep a note of it just in case you want to change it back. Ideally you are looking for 10-20mm on a race bike and 20-25mm on a road bike. Either compress the spring or loosen it off to get in this range. The adjuster is the bit wot pokes out the top of the forks. Use a suitable spanner or socket to adjust them.

Loaded Sag

If all is well we have managed to get with in range but this isn't the whole story. We now need to take into account your weight!

Sit on the bike and have one friend steady the front, one steady the back and the other ready to measure. Sit the bike upright. Now WITHOUT touching the front or rear brakes bounce up and down a few times in the seat and then assume your normal riding position. Measure the rear of the bike as before and we will call this measurement E.

Subtract E from A and this gives you your rear loaded sag reading. Try and get between 20-25mm on a race bike and 30-40mm on a road bike by adjusting the preload as before.
Now do the same for the front. We will call this measurement F.

Subtract F from C and try to obtain 25-35mm on a race bike and 35-50mm on a road bike.
If you can't get in these ranges for BOTH Static and Loaded then you will need to change the spring for a harder or softer one. Harder if you're outside the range and softer if you are inside the range.

Now go and ride and feel the difference!

Adjusting Motorcycle Suspension

Here is what "The Racing Motorcycle - a technical guide for constructors", by John Bradley, has to say on the subject of adjusting motorcycle suspension.  A very good book by the way, full of great info.  For more on this great book check out  http://www.eurospares.com

Note: If something needs more adjusting & there's no more adjustment left then it's time for modifications.  Also, the damping adjustments mainly adjust for "low suspension speed" - if "high suspension speed" response needs adjusting you'll need some modifications.

SUSPENSION TROUBLESHOOTING

from Sport Rider Magazine

Here are some basic symptoms of suspension damping problems that you might find affecting your bike. Remember these are only extreme examples - your symptoms may be more subtle. You may have to find an acceptable compromise on either end of the adjustment spectrum. It all depends on how the bike's handling feels to you.

 

LACK OF COMPRESSION DAMPING  ( Front Fork )

• Front end dive while on the brakes becomes excessive.
• Rear end of motorcycle wants to "come around" when using front brakes aggressively.
• Front suspension "bottoms out" with a solid hit under heavy braking and after hitting bumps.
• Front end has a mushy and semi-vague feeling, similar to lack of rebound damping.

TOO MUCH COMPRESSION DAMPING  ( Front Fork )

• Overly harsh ride, especially right at the point when bumps and ripples are contacted by the front wheel.
• Bumps and ripples are felt directly - the initial hit is routed through the chassis instantly, with big bumps bouncing the tire off the pavement.
• The bike's ride height is affected negatively - the front end winds up riding too high in the corners.
• Brake dive is reduced drastically, though the chassis is upset significantly by bumps encountered during braking.

LACK OF REBOUND DAMPING  ( Front Fork )

• The fork offers a supremely plush ride, especially when riding straight up.  However, when the pace picks up the feeling of control is lost.  The fork feels mushy, and traction "feel" is poor.
• After hitting bumps at speed, the front tire tends to chatter or bounce.
• When flicking the bike into a corner at speed, the bike will tend to "porpoise" or wallow a bit, before settling down.  Getting aggressive with the controls makes it worse.  As speed increases and steering inputs become more aggressive, chassis attitude and pitch become a real problem, with the front traction feedback going numb after the bike is countersteered hard into a turn.

TOO MUCH REBOUND DAMPING  ( Front Fork )

• The ride is quite harsh - just the opposite of the plush feet of too little rebound.  Rough pavement makes the forks feel as if they're locking up with stiction and harshness.
• Under hard acceleration exiting bumpy corners, the front end feels like it wants to "wiggle" or "tankslap." The tire feels as if it isn't staying in contact with the pavement when on the gas.
• The harsh, unforgiving ride makes the bike hard to control when riding through dips and rolling bumps at speed.  The suspension's reluctance to maintain tire traction through these sections erodes rider confidence.

LACK OF COMPRESSION DAMPING  ( Rear Shock )

• Too much rear end "squat" under acceleration - bike wants to steer wide exiting corners (since chassis is riding rear-low/nose-high).
• Hitting bumps at speed causes the rear to bottom, which upsets the chassis.
• Chassis attitude affected too much by large dips and "G-outs" - steering and control become difficult due to excessive suspension movement.

TOO MUCH COMPRESSION DAMPING  ( Rear Shock )

• Ride is harsh, though not quite as bad as too much rebound - however, the faster you go the worse it gets. 
• Harshness hurts rear tire traction over bumps, especially during deceleration.
• There is very little rear end "squat" under acceleration.
• Medium to large bumps are felt directly through the chassis - when hit at speed, the rear end kicks up.

LACK OF REBOUND DAMPING  ( Rear Shock )

• The ride is plush at cruising speeds, but as the pace increases, the chassis begins to wallow and weave through bumpy corners.
• Poor traction over bumps under hard acceleration - rear tire starts to chatter due to lack of wheel control.
• Excessive chassis pitch through large bumps and dips at speed - rear end rebounds too fast, upsetting chassis with a pogo-stick action.

TOO MUCH REBOUND DAMPING  ( Rear Shock )

• Very harsh ride - rear suspension compliance is poor and "feel" is vague.
• Poor traction over bumps during hard acceleration (due to lack of suspension compliance).
• Bike wants to run wide in corners since the rear end is "packing down" - this forces a nose-high chassis attitude, which slows down steering.
• Rear end wants to hop and skip when the throttle is chopped during aggressive corner entries.  

Suspension Set-up

Get the Most Out of Your Bike's Suspension

(Text reprinted from Motorcyclist, May 1993, author Will Higgs)

My good friend John had a beautiful GSXR1100, the kind of bike that made me feel self-conscious riding anywhere near it.   One day, when we were planning to work on our bikes together, John arrived grumbling about his suspension.  He got off the bike and, through a series of frantic hand motions and strange sounds, attempted to explain what was going on.

"Have you ever adjusted your sag." I inquired.  From the bewildered look on his face, he obviously hadn't.   After a few quick measurements we figured out he needed more pre-load at the rear.   We cranked down the collar three or four turns and he rode off more than slightly skeptical.  About an hour later John called me up.  "It's ten times better," he raved, "I'm still not sure what we did, but it sure worked".

His bike was always in pristine condition.  He had carefully modified it, bolting on a Yoshimura Duplex exhaust system, Dynojet carb kit and larger rear wheel wearing the latest in radial technology.   But despite all the time and money John had spent improving his bike, he had ignored simple adjustments that could have significantly improved his bike's performance at practically no cost.

John's story is all too common.   It's not unusual to find someone who has spent countless hours and an untold fortune to improve a motorcycle without ever sorting out the bike's suspension.   These riders don't fiddle with it because they don't understand it, or because they decide it won't make that much of a difference.

To many, there is little difference between the multitude of adjusters on modern sport bikes.  If the suspension is soft and mushy, they just click it up or tighten it down without really knowing what they're adjusting, though they reckon they are at least doing something.  Many riders don't realize that each adjuster has a specific purpose; it adjusts one particular suspension function.  It's important to understand these adjustments and what effect each of them has.

A motorcycle suspension's primary job is to isolate the chassis from obstacles on the pavement.  Quality suspension soaks up bumps and ripples, keeps the wheels in constant contact with the ground and makes the chassis feel settled and planted.  The adjustments let you control exactly how the suspension reacts to forces that act upon the motorcycle.

The suspension itself can be broken into two parts: spring and damping.  Put in simple terms, the spring affects how easily the suspension travels and the damping affects the speed at which it travels. - By tuning these variables anybody can create tailor-made handling manners.

PREL0AD

The first and most important step in setting your suspension is dialing in the correct preload.  Preload is the amount of tension applied to the spring when the suspension is fully extended.  Preload sets ride height.  Correct preload helps keep the chassis stable in most conditions.   Most suspension systems are designed to work best when one-third of the travel is used up by the rider.  This gives room for the suspension to load and unload.   Almost all shocks are designed to work best from a given point in the linkage; setting preload correctly puts your spring in that range. Sag is the amount the bike actually hangs (or sags) from the weight of the rider.  It reveals whether the spring is properly set to operate under a particular rider's load.  To measure rear-end sag, measure the distance between the rear axle and a specific spot on the tail section without the rider on board.  It is important that the suspension is completely extended.   This usually means you must lift the rear of the bike because the weight of the bike itself creates some sag in the suspension.  After taking that measurement, take another using the same two reference points, only this time with the rider on the bike.   For best results, the rider should put both feet on the pegs and both hands on the bars, so another person should be on hand to hold the bike upright.  The difference between the two measurements is the amount of sag in the suspension travel. Usually the ideal amount of sag indifferent sizes of spacers until you find the perfect amount of added preload for you.

(Note - when measuring sag, there is a certain amount of stiction, or drag, evident in nearly every shock absorbing system -   both front and back.  To get a completely accurate measurement, have one of the friends that is helping you pull up on the bike as you are sitting on it and let it gently settle, then measure.  Then have the friend compress the spring with you on it, and let gently rise.  The difference is the amount of stiction.  Take the average of the two measurements with the rider on the bike and this is “actual” sag)

After you've made the adjustments go out riding and try to evaluate any difference in feel.  Don't be afraid to change it again, and try to note the differences.  A harsher ride and poorer traction are usually indications of too much preload, though a little more preload up front might let the front wheel track bumps more accurately as you're accelerating out of a corner.   There's a common misconception that adding preload combats excessive front-end dive, but actually, stiffer springs or extra oil fork oil are the cures for that problem.

If your front suspension has adjustments for preload, measure it also.  Secure a zip-tie snugly on the stanchion (sliding) tube flush with the fork seal.  Have the rider sit on the bike again.   Once the rider dismounts, top out the suspension and measure the distance between the zip-tie and the fork seal.  Again, it's very important to make sure the fork is completely topped out to ensure an accurate measurement.  When the rider is off and the fork is extended, the zip-tie indicates how much travel has been used just to support the rider's weight.  Front sag should be a little lighter than rear and should fall between 1.25 to 1.5 inches, the rear should be between 1 and 1.25 inches.

If there aren't preload adjustments available in your fork, it's not too difficult to create the right amount with simple materials.  Most forks have spacers between the springs and the top of the fork tubes.  Increasing or decreasing the length of the spacers affects the amount of preload in the fork tube.  The longer the spacer, the greater the preload and vice versa.  Many roadracers make their own spacers out of PVC tubing because it's light, easy to cut and fits nicely inside most fork tubes.  Using PVC, you can cheaply manufacture several  

Quick notes:

• A few turns of the preload adjusters will reduce or increase preload by raising or lowering an adjustable spacer on top of the fork tube.  Equally spaced grooves are machined along the tubes as reference points for preload adjustment.
• A threaded collar on the rear shock can be tightened to add preload or loosened to reduce it.  Before you can do this, you must dislodge the locking ring.  The ring often locks down tighter with suspension play.  If a spanning too/ won't budge it, try a few well-placed knocks with a screwdriver and rubber mallet.
• To obtain rear-end sag, measure the distance from the rear axle to a fixed point straight above it on the frame.  As with the front, its important to make sure the suspension is COMPLETELY topped out.  Then have the rider mount the bike and measure this distance again making sure you use the same two exact points.

DAMPING

With all the technological advances incorporated in today's motorcycles it's amazing that suspension is still built around that simple age-old spring.  A spring works by absorbing, storing and then releasing energy, and a steel coil spring is perfect for suspension applications because it does this in a very fluid manner.  Since the spring used for suspension duties is preloaded, it does not work in the fully extended or compressed portions of its range but somewhere in the middle.

The point from which a laden spring operates is its "sag point". - If the spring was the lone component of the suspension and the bike hit a bump, the spring would compress as quickly as its coils would allow.  Unless the spring is wound progressively, its resistance grows linearly in relation to the amount it's compressed.  The energy that compresses the spring is met with resistance from the spring, but if the compression force is released, that stored energy is released in the opposite direction.  Unhampered, the release of this energy is quick and forceful, and if enough energy has been stored, it can throw the chassis back up past the spring's sag point.  The weight of the bike (which now contains stored energy because it was pushed above the spring's sag point) then pushes the spring down again and this continues, slowly tapering off until all of the energy has been released.   This means the bike bounces up and down until it settles.  The end result can be witnessed in a '76 Coupe DeVille with worn out shocks.  A bouncing chassis is not a stable chassis.

Damping dissipates some of the energy that the spring is absorbing.  This energy converts to heat.  Damping can be used to control how quickly the spring bounces (compresses) from a bump and how quickly it returns (rebounds) after that bump. - Not only can it hamper bouncing, but damping allows a rider to tune the suspension to handle an array of different conditions. - Most stock motorcycles are under damped to provide a gentler ride and broader range of use.

Quick note:

• It is important not to have too much spring preload in suspension components.  An oversprung shock overpowers the damping system and does not allow it to work at an optimum level.

REBOUND DAMPING

Front rebound-damping adjustments are usually made through screw adjusters on top of the fork and through an adjuster found on the bottom of the rear shock.

If your suspension has only one damping adjustment, it's a safe bet that it's for rebound.  Damping consists of low-speed and high-speed (speed of damper) adjustments.  Generally, only low-speed components are adjustable with external adjustments; high-speed damping is changed by adding or subtracting washers inside the shock.  Without damping there is no pressure other than the weight of the bike acting on the spring during extension.  This means substantial damping is needed to keep the suspension settled.  There should be enough damping to keep the spring from rebounding past the sag point or snapping back too quickly after compression.  You want a smooth return to the spring's sag point, but it must be quick so the suspension can set up for the next disruption.

If you are adjusting rebound damping, it's a good idea to get a feel for how different adjustments affect your suspension.   Try turning the rebound all the way out then quickly compress and release the suspension several times.  Then try it with the damping on its maximum setting.   This gives you a good idea of how the damping works.  The slower the suspension returns, the greater the damping resistance.

When you begin adjusting, start with rebound damping set in the middle of its range and work from there.  An underdamped bike feels loose and imprecise.  Without enough rebound damping resistance the bike tends to pogo.  This is especially evident in transitions between quick turns as the bike rises excessively.  Too much rebound damping and the bike chatters over ripples.   This tells you the suspension is packing down; in other words, the suspension cannot return to its sag point quickly enough between bumps.  Each new bump the suspension encounters takes up more and more of the travel.  Enough of these rapid-fire bumps cause the motorcycle to use up all of its travel.  With no travel left, the chassis takes the brunt of the bumps.

COMPRESSION DAMPING

Compression adjustments are usually found on the bottom of most forks and on the top of, or on a remote reservoir protruding from the top of the rear shock.

Much less compression damping is needed because the spring itself offers resistance to compression.  Without compression damping a bike might bottom out quickly, leaving no travel for the next bump.   Too much compression and the bike stops soaking up bumps and transmits them right through the chassis.  Compression damping is set much lower than rebound because you want the suspension to absorb bumps as quickly as possible.  When dialing in compression damping, start on the conservative side. - Then feed a click at a time until the ride gets harsh.  Once it gets harsh, back it out a couple of clicks and that should be about right.  Too much compression damping creates a harsh ride because the suspension can't compress rapidly enough to absorb large bumps or jolts.  Not enough compression damping and the bike wallows, especially in transitions.

Before you make any changes, learn exactly how your bike feels.  This gives you a good base to work from.  Then head out to your favorite stretch of deserted road with a notebook, heightened senses and quite a bit of free time.  Work carefully, documenting each change and making only one change at time.  Try to distinguish how each change affects your bike's handling.   If something doesn't feel quite right, try to pinpoint the problem.  Ask yourself, "Why would the bike be doing that?"

If your rear suspension does not have adjustments for preload or damping, it may not be a bad idea to replace it with an aftermarket shock that does.  If your front suspension does not come with adjustments. there are easy ways to modify it yourself with alternate fork-oil weights and springs.  If you work on one section at a time, one adjustment at a time, you can systematically tune your suspension to suit your needs. 

Suspension Setup Guidelines

by David Allan Feller

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(Originally published this story in American Roadracing, September 1994.)

The array of suspension adjustments on modern sportbikes leaves many riders bewildered, wondering where to start. Dale Rathwell, Vance & Hines Yamaha's suspension expert, can end their worries. "I have a procedure that I follow for setting-up suspensions," says Rathwell, "which concentrates in four areas: the springs, ride height and the two damping adjustments--compression and rebound. "The first thing you have to do is establish a baseline for your settings. Always start by setting all adjustments to the manufacturer's suggestions. These are usually found in the service manual, but if not, just ask a racer or knowledgeable mechanic. Then make up a little sheet and record all your settings. This is crucial: Always pay attention to what you're doing and record it, or you'll never get anywhere. "That said, the first thing you need to do is set the suspension's sag."

To do this, it's best to have three people. First, lift the rear end off the ground until the shock is topped out, and measure the distance between the seat and rear axle in a line perpendicular to the ground. Then, wearing your riding gear, sit on the bike, have one helper balance the bike and the other measure the distance between the same two points. Do the same for the front suspension, using a tie-wrap on an O-ring around the inner fork tube. Subtract the compressed measurement from the extended measurement and you have calculated you bike's sag.

"There is a rule of thumb," continues Rathwell, "that says one-quarter to one-third of the usable suspension travel should be used up by the rider in sag, which is necessary to allow the suspension to extend after the bumps that it must compress over. Similarly, if your bike is powerful enough to squat the rear suspension under acceleration, then the front end is not going to stay in contact with the ground if there isn't enough sag."

One problem here, notes Rathwell, is that most sportbikes are equipped with too-soft springs and must be excessively preloaded to achieve the proper sag. This results in a suspension that is too firm in initial movement and then too soft through the rest of the travel--exactly the opposite of what you want. If this is the case with your bike and you plan to retain the stock springs, it may be better to strike a compromise, settling for slightly more than optimum sag in order to reduce preload; too much preload can override the suspension's rebound-damping capabilities.

The next problem to tackle is chassis attitude. This determines how much weight is on each wheel, accomplished by adjusting the ride height. Essentially, you want the bike to steer as fast as possible while maintaining stability and not dragging. This is usually achieved by sliding the forks up in the triple clamps and raising the rear ride height with an adjustable-length aftermarket shock. "The last thing you should do," cautions Rathwell, "is the fine-tuning of the dampers because they're a small part of the picture and only come into play when everything else is properly set. The dampers are there to dissipate energy stored in the springs at a controlled rate (preventing the spring, and motorcycle, from continuing to bounce up and down) and can only be properly set when everything else is correct.

"Begin by making notes detailing what adjustments are available and then, again, establish your starting point from the manufacture's suggestions. Or if that's not available, just start in the middle of all the settings and experiment from there. "Next, move the front and rear compression and rebound adjustments up and down individually throughout their ranges and see what they do. Right away you will have explored the range of adjustments possible and as a rider, will be familiar with how each adjuster makes the chassis react."

From this point on, things get complicated: Too much rebound, for instance, can produce responses similar to insufficient compression damping and vice versa. "There's absolutely no substitute for experience here," stresses Rathwell, "but I can give you some clues. If you've got a lot of chatter in the handlebars when you're not braking, or under light braking, then the spring is loaded and the hydraulics won't allow it to absorb it--too much compression damping. Similarly, in the rear, if you feel a lot of wheel skipping--the wheel feels very light--this is another sign of too much compression.

"Too much rebound is evident, especially in the forks, if you've also got a lot of chatter, but the difference is that it is following the bumps--the suspension is absorbing the bump, but then 'packing' and not riding down the backside. Whereas with too much compression it can't absorb the beginning of the bump and 'jumps' off the crest."

Comments to David Allan Feller

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Suspension Setup Guidelines

by Brad Hepler

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I'm not an expert on this stuff, but I've compiled some tips over the years. There are three (3) possible adjustments on the front and rear suspension of a motorcycle: spring preload, compression damping, rebound damping. The spring preload sets the ride height of the bike and determines how much of the total travel will be available for compression and how much will be available for extension. Damping keeps the bike from behaving like an old Cadillac - i.e., still bouncing 10 seconds after hitting a bump. Compression damping slows the shock when it is being compressed. Rebound damping slows the shock when it is rebounding.

Set the preload on each end so that the bike settles 1/3 of its total travel -- look in your owner's manual for total suspension travel. This ("sag") is measured between (a) the wheel suspended off the ground, and (b) you on the bike with all applicable gear, luggage, passenger, etc. Use less sag for smooth roads and fast riding, say 25%. On shock absorbers: get the sag/preload right before messing with the damping. Start at the shock manufacturer's recommendations. Only change one setting at a time, and don't change damping more than 2 clicks at a time. For compression damping, increase until expansion joints or sharp bumps are jarring, then back off one click. Don't forget that when you increase preload, you have to increase rebound damping and vice versa for decreasing preload.

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Front Suspension Symptoms, Likely Causes & Possible Solutions

Symptoms:Not using full travel, feels harsh, poor traction in bumpy corners
Causes: Overly stiff springing or compression damping
Solutions: Lower air pressure; reduce compression damping; softer coil springs

Symptoms: Bottoms, soft throughout travel
Causes: Spring rate too low throughout travel, or too little compression damping
Solutions: More air pressure; increase compression damping; stiffer coilsprings

Symptoms: Bottoms, but compliant over small bumps
Causes: Spring rate not progressive enough
Solutions: Raise oil level

Symptoms: Harsh over large bumps, but good over small ones
Causes: Spring rate too progressive
Solutions: Lower oil level

Symptoms: Excessive sack, feels soft initially but doesn't bottom
Causes: Initial spring rate or preload too low; springing too progressive
Solutions: Add air pressure *and* lower oil level; increase spring preload

Symptoms: Harsh over small bumps but uses full travel
Causes: Initial spring rate or preload too high, springing not progressive enough, or too much compression damping
Solutions: Raise oil level *and* lower air pressure or install softer springs; reduce compression damping; reduce spring preload

Symptoms: Takes first bump in a series well but harsh over later bumps, poor traction in washboard corners
Causes: Too much rebound damping
Solutions: Use thinner fork oil (or decrease rebound damping if adjustable)

Symptoms: Front end springs back too quickly after bumps, poor traction in bumpy corners
Causes: Not enough rebound damping
Solutions: Use thicker fork oil (or increase rebound damping if adjustable)

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Rear Suspension Symptoms & Causes

Symptom: Not using full travel, feels harsh, poor traction accelerating out of bumpy corners
Cause: Overly stiff springing or compression damping, possibly too much preload

Symptom:Bottoms, feels soft throughout travel
Cause:Spring rate or compression damping too low

Symptom: Bottoms, feels harsh, sinks far into travel with rider board
Cause: Too little preload, perhaps combined with too soft spring

Symptom: Wheel chatters over small bumps during braking or downhills, doesn't hook up accelerating out of washboard turns
Cause: Too much preload (perhaps because of soft springs) causing suspension to top out; possibly too much compression damping

Symptom: Kicks over large square edge bumps, but not over large rolling bumps
Cause: Too much compression damping

Symptom: Kicks over rolling bumps and square edge bumps
Cause: Too little rebound damping.

Symptom: Rear end takes first bump in a series but is harsh on later bumps. Poor traction out of bumpy turns or braking on washboard
Cause: Too much rebound damping.

Symptom: Back end extremely springy and shock doesn't respond to adjustments
Cause: Damping is gone because of low nitrogen pressure, tired oil, or damaged internal components.

 

Some of this material came from either Cycle or Dirt Rider magazine & was originally intended for dirtbikes but is also applicable to street."

 

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Head-Shake Theory

by Joe Arcoraci

Some Riders report problems with head shake especially in the 45 - 55 mph range and when decelerating. The head shake may be more pronounced when the Rider takes their hands off the handlebars. Head shake seems to be related to at least four (4) areas.

1. An under inflated front tire
2. Improperly torqued head bearing
3. Damaged head bearing
4. Tire wear and tire design

This article may reflect the most common solution to this problem and was originally posted by Joe Arcoraci to the VFR List and it's reposted it here as a reference for others. It is an excellent post that may be useful to other riders.

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Joe was responding to another Lister about head shake when he wrote ...

"I had a similar problem when I switched from the stock D202s to D204s. At first I thought that the tires where not balanced properly so I took them to my local dealer to have the balance checked. It was just a little bit off so I had them rebalance them using only center weights, not the adhesive type on one side of the rim like it was used previously. That really made a lot of difference (using the weight on the center vs. on one side of the rim), however a little head shake was still happening.

The shaking was nothing to worry about, it only happened between 45 and 50 during deceleration. The dealer told me that it could be that the steering head bearing needed to be retorqued and possibly changed to a tapered one to completely fix the problem for a couple of hundred bucks job. I decided to just lived with it.

After about 4000 miles it was time to replace the tires so this time I chose to install the MEZ1s, front and rear, since the 170s are now available. I went for the racing compound on the front and street compound on the rear. After I had them mounted and balanced with center weights THE HEAD SHAKING WAS GONE!

Moral of the story IMHO. Head shake probably has to do with the harmonics of the bike. When you run a more triangular tire (D204, D207) it makes the bike shake. When you run a rounder profile tire (MEZs, D202) the bike is stable. I don't think it has to do with the head bearing at all but just the way that the bike was designed and the way it responds to tire profile."

Hope it helps,
Joe

Actual tire sizes vary by brand and can affect handling

Do the Tires Suck, Or is it Chassis Geometry?

By David Swarts

Changing tire brands can change your lap times, but not necessarily why you might think. How each tire affects your bike’s chassis geometry may be more critical than a change in rubber compounds.

Imagine that you go pretty good around the track on Brand X tires. Without changing anything else, you switch to Brand Y tires sold for the same bike, go back out on the track, ride exactly the same, lose the front end, and crash.

Some people say, "That’s just how Brand Y is." Others say, "You didn’t give them enough time to heat up." Then someone states, "You’re a cheesy squid who can’t ride on any tires." Ouch!

In fact, changing brands of tires—even in the same labeled sizes—can have a profound effect on the handling of a motorcycle. Despite being labeled with the same width and sidewall aspect ratio, different brands of tires will rarely have the same diameter, or height. And a different diameter tire will change the geometry of a machine and the way it handles. A rider could be left searching for a whole new set-up on a practically brand new motorcycle.

The same thing can happen when switching tire sizes, a good example being the case of GSXR750 riders switching from an OEM Dunlop D207 190/50-17 rear street tire to a Dunlop D207 GP 180/60-17 rear racing tire. The D207 GP’s diameter is 641mm, 13mm greater than the diameter of the D207 street’s 628mm. That change raises the rear axle 6.5mm, making the rake 0.4 degrees steeper and reducing trail 3mm. Depending on the initial set-up, that can be enough difference to make the front end tuck when the rider is off the gas.

In some cases, switching from a dry pattern tire to a rain tire—even with the same brand and labeled size—will cause handling problems because the rain tire has a different diameter than the dry tire. Such a change can increase a tendency for the front to tuck when that’s the last thing a rider needs on a wet track.

Let’s consider the real-world example of Todd Harrington, best known for his 1996 AMA 600cc Supersport win at Road America. For many years, the 4&6 Racing rider was a loyal Dunlop customer even though he wasn’t getting anything more than a discounted price. But the tires were good, and so was Harrington’s relationship with Dunlop.

Coming into the 1999 season Harrington heard good reports about the new Michelin Pilots. "But every manufacturer with a new tire says that," said Harrington skeptically. "Then I saw what Josh (Hayes) was doing."

Hayes, a Team Valvoline EMGO Suzuki rider, qualified third for the AMA 600cc Supersport race at Daytona on Michelin Pilots.

That was all it took for Harrington and 4&6 owner Jim Rashid to decide to try the Pilots. Rashid not only operates the Chicago-based 4&6 Cycles, he recently became the latest owner of a G.M.D. Computrack franchise.

Rashid took Harrington’s 1999 Kawasaki ZX-6R with Dunlops and set it up according to Computrack’s "sweet numbers." The closely guarded sweet numbers are a guideline for chassis geometry settings that optimize handling characteristics. Both Harrington and Rashid realized that small changes in one area can have a relatively drastic effect in many other aspects. Still new with the Computrack system, Rashid decided to consult Kent Soignier of the G.M.D. Computrack Atlanta location for his experienced input.

Soignier had already measured the new Michelins, so Rashid plugged the numbers into the "What If" program of the Computrack computer and transposed them onto the computer model of Harrington’s bike to see what effects it would have on the geometry and what he could do to compensate for the new tires. Computrack’s "What If" program allows theoretical changes to be made to a motorcycle in the computer with a mouse rather than in the hot pits with wrenches and a tape measure.

Rashid entered the new tire diameter, and the computer told Rashid that he would have to lower the front 6mm and raise the rear 12mm to compensate for the different dimensions of the Pilots compared to the D207s. Those are big changes in ride height. "Usually one or two millimeters makes a big difference," underscored Rashid.

With the chassis of his ZX-6R optimized for the Michelins, Harrington was able to lower his best lap time by 0.5-second after just 10 laps. Now, this is not some Novice racer dropping time while learning a track. This is a professional racer dropping a half-a-second from 1:56s to 1:55s on Thursday afternoon at Daytona.

With the chassis optimized for the new Michelins, Harrington knew before he went out that he would have exactly the same motorcycle as before, except with different tires. "I knew that I only had to concentrate on the feel of the new tires," Harrington explained.

When Harrington dropped his times without even going 100 percent, he knew that he had found an improvement with the new tires. In fact, the Pilots allowed Harrington to brake so much harder and later that the team had to add compression damping to keep the front end from bottoming. Harrington was also getting more traction at the rear, so the front was lowered further, and his lap times continued to fall.

A good tuner and rider could have taken the measurements and figured out the proper adjustments by trial and error—but certainly not in just 10 laps. Realistically, it could’ve taken all weekend to find a revised set-up that worked.

Harrington wasn’t the only rider at Daytona to find that changing tire brands required geometry adjustments. When last year’s Formula USA Champion Michael Barnes came over to buy some fuel from Rashid, he mentioned that he was also running the Pilots. "Barnes said that they felt good but not perfect," said Rashid, who then asked if Barnes had altered his ride height to suit the new tires. "After he said no, I told him to try dropping the front and raising the rear of his bike." After making chassis adjustments, Rashid said that Barnes dropped two seconds from his qualifying time.

Even though Barnes was riding a Honda CBR600F4, the ballpark ride-height adjustments helped because Barnes’ Pilots were the same size as Harrington’s Pilots.

The Computrack measurements show the Michelin Pilot 120/70-17 front tire is larger (5.72mm) in diameter than the Dunlop D207 GP despite being badged with the same size. A larger front tire increases the rake and the trail, raises the swingarm pivot height, increases the swingarm down-slope, and increases the wheelbase. So making this tire swap would affect the steering, rear wheel traction, and overall handling of a motorcycle. Then adding the smaller circumference 170/60-17 Pilot rear would increase the above-mentioned geometry changes, compounding the effects on the handling characteristics, plus shortening the gearing.

A rider changing from Dunlop D207 GPs to Michelin Pilots on a Kawasaki ZX-6R without any other changes would have increased the rake by 0.39 degrees, added 3.62mm of trail, lowered the swingarm pivot height by 3.07mm, increased the swingarm down-slope by 0.39-degree, and increased the wheelbase 0.13mm—not mentioning the change in effective gear ratio. These numbers may not seem very large, but to a professional searching for tenths of seconds in practice, these numbers are significant.

Switching from the ZX-6R’s stock Bridgestones (with a 60-series front tire) to the Pilots (with a 70-series front) would result in even more drastic changes. The rake would go up almost a full degree, and the trail would increase by 8mm!

Many riders seem bound by an ignorance-based sense of loyalty to one brand of tire, fearing that switching brands will change the way the bike feels. Now that I better understand why a different tire brand may react unlike a known tire, I can’t wait to try out other tires to find out what is best for me.