Motorcycle Suspension Tuning & Tire Wear

When it comes to motorcycle suspension tuning, there is more misinformation than time in the day to even attempt to address it all.  It seems no matter where you go, there's always a guy with an opinion on how you need to set up your bike.  However, if you request any further information past his one-liner advice, or if you ask why, or how, that conclusion was drawn, you are accosted with some of the most elaborate non-answers imaginable.  My favorite is when you are told something like "You just need to set it up for comfort."  What does that even mean?!

"Opinions are the cheapest commodities on earth.  Everyone has a flock of opinions ready to be wished upon anyone who will accept them." -Napoleon Hill

To achieve your appropriate suspension settings, we must forgo opinions and align the riding physics with the correct metrics.

On the street, the metrics are:

• Tire Wear
• Fork Travel
• Shock Travel
• Handlebar Effort

On the track, we employ an additional, more readily usable, metric:

• Weight Transfer Behavior

Despite the unintelligible and incoherent ramblings of the "one-liner advice" guys, the physics are rather straightforward.  When your tire comes into contact with the pavement, a deformation occurs on the tire surface and through the tire carcass.  The surface deformation that interfaces with the pavement is referred to as the "contact patch".  

Congruence of riding style, suspension stability, and tire pressure are the irrefutable variables that procure (or hinder) a consistent and uniform contact patch.  A consistent and uniform contact patch is the fundamental element for riding stability, rider control, and nominal tire wear.  

When there is dissonance among these variables, the contact patch deformation fluctuates in diameter, depth, and uniformity.  This constant fluctuation is retransmitted through the delinquent suspension and into the chassis where the rider is compelled to combat the instability by administering continuous corrections to steering and speed, especially in turns and corners.

Motorcycle suspension settings are as unique to you as the memory seat setting in your car.  Unfortunately, motorcycle suspension differs from the memory seats in that you can't just set it up once and have it be good forever.  Your suspension and tire pressure settings are only able to accommodate a fixed set of environmental variables.  Additionally, the forks and the shock have oils that age, wear, become contaminated, and must be changed, no different than engine oil.  

Your suspension settings can only be singularly matched to a given alignment of rider weight, riding style, and tire pressure.  In pragmatic application, this just means that you will have 1 unique suspension and tire pressure setting for any single combination of:

• 1 up riding
• 2 up riding
• Regular commuting
• Spirited riding
• Canyon/Mountain carving
• Racetrack 1 riding
• Racetrack 2 riding, etc.
• Street tires
• Track tires
• Hot ambient temperature
• Cold ambient temperature

Old fork and shock oils behave very differently than new fork and shock oils.  As the oil ages (like all oils), it breaks down through thermal and compressive hydrocarbon decomposition, oxidation, and contamination.  In the forks, contamination primarily comes from the springs rubbing along the inner fork tube surface causing metal flakes to become entrapped in the oil itself.  Secondary fork contamination comes from dust and dirt making its way past the fork seals during fork travel.  In the shock, primary contamination comes from dust, dirt, road debris, and slung chain lube making its way past the shock seal during shock travel.  

As the fork and shock oils age, the viscosity becomes quite unstable.  Specifically, new fork and shock oils have a typical viscosity range of 4 centistokes (at 212°F) to 20 centistokes (at 104°F).  Old, worn oil viscosity can be expected to have an even lower cSt (centistoke) value when hot and an even higher value when cold.  In short, oil is thick when cold, and thin when hot, but, old oil is even thicker when cold and even thinner when hot.  This is where suspension adjustments must be occasionally revisited even after it has been initially set to your weight and style of riding.  After the fork or shock oil has degraded to a point of inadjustability for your weight and style of riding, it is time to change the fluid and start over.

All of this just to say, it is important to understand that environmental conditions directly affect the contact patch.  With this in mind, we will target a setting that best suits the manner in which you ride.  

Check your owners manual.  Depending upon the year, make, and model of the bike, the fork and shock oil service intervals may, or may not, exist in the owner's manual.  If you've lost your owner's manual, you can usually find it for free online.  

If there is no documentation for these service intervals provided by the manufacturer, we can use some industry standard guidelines.  

Street Riding - Mild to Aggressive:

• Hydraulic adjustments (compression and rebound/tension) should be checked every 800 to 1,000 miles
• Fork oil should be changed every 5,000 to 7,000 miles
• Shock oil should be changed every 8,000 to 10,000 miles

Track Riding:

• Hydraulic adjustments (compression and rebound/tension) should undergo  continuous evaluation
• Fork oil should be changed around every 10 days
• Shock oil should be changed every 1 to 2 years

All Riding Conditions:

• If any of the hydraulic adjustments exceed 80% of the range of operation, the oil should be changed 
• Example: If rebound damping has a total click range of 0 to 25 and the click number required to attain  a consistent and uniform contact patch is 20 to 25, then it is time to change the oil

There are a few behavioral and evidential characteristics that your bike may exhibit which indicate an imbalance in the suspension settings, tire pressure, and riding style.  Many of these characteristics lend themselves to become more evident during track riding, but certainly, can manifest on the street as well.  We must consciously be on guard against these characteristics as the unconscious mind will always attempt to adapt to any mild progression of such inferior performance behaviors.

Capricious Tire Wear

• Flat spots
• Cupping
• Scalloping 
• Raised tread edges
• Chamfered tread edges
• Radial wave pattern
• Dark black spots or craters 
• Rubber ball build up on tread edges
• Liquid rubber mountain ranges
• Cold tears
• Hot tears

Cold vs. Hot Suspension 

• Bumps feel very harsh when bike is cold
• Bike feels very loose or sloppy over bumps when warmed up

Hard Turning

• Bike is hard, or feels "heavy", to turn
• Handlebars require constant, increased, or excessive pressure to maintain the line

Undesired Corner Lines

• Runs wide
• Runs narrow
• Falls into corner
• Does not flick into corner

Brake Chatter

• Handlebars oscillate vertically (up and down, not side to side) under hard braking

Wallowing (rear tire dancing around)

• Corner exit
• Partial to full throttle

Weight Transfer Instability

• Mid-corner instability
• Brake to throttle transition instability
• Throttle to brake transition instability

As your riding skills progress, suspension settings can frequently become a balance or compromise between two or more things.  In example, we can set up your bike such that it becomes an unadulterated wheelie machine or have it withstand the braking forces of a freight train (being slightly facetious).  However, if your suspension is set to handle either of these two conditions specifically, it would also traverse corners about like a tricycle.  

If you find that your riding skills have progressed to a point where these compromises are now hindering your continued skill development, then it is time for a suspension upgrade.  Your bike should always be better than you.  If you are better than it, the hardware deficiency must be addressed.  

Let us not forget that tires account for 20% of your suspension system.  Old, worn tires also behave very differently than new tires.  As the tires age, they wear and break down through natural biodegradation (rubber is an organic material), natural vulcanization (sunlight, friction, and heat), and oxidation.  These degradation processes cause the tires to become more rigid and brittle thereby inducing a reduction in available traction.  Because of this, many motorcycle tire manufacturers recommend that tires be replaced around every 5 years regardless of indicated wear.  The 4 digit code molded directly onto the side of your tire indicates the week and year of manufacturing so you can calculate age.  

In evaluating suspension performance, or triaging a root cause of any of the aforementioned behavioral symptoms,  we must also evaluate tire condition.  Brittle and worn tires can easily masquerade as delinquent suspension settings.  

Without question, any rider can adjust and calibrate their own suspension.  The owner's manual itself typically provides very clear instructions on how to perform the mechanical adjustment actions of preload, compression, and rebound/tension.  So why would you have me do something you can do on your own?

What your owner's manual omits, is which combination of those adjustments yields a consistent and uniform contact patch for your particular riding.  If you have consumed the information contained on this page, you understand why the owners manual is genuinely incapable of providing such information.  

My objective is to increase your riding safety margin and set you up for success in your riding adventures.  I broach this by reducing your risk of a fluctuating contact patch and teaching you how to maintain your own reduction in risk for your continued riding endeavors.