Shock Tuning Primer - How a shock works

Part 1 (Part 2)

By Marcus McBain

All information, copyright 2012 Marcus McBain/RPSRaceTeam.com

 

 

Welcome. This document is provided to give you a complete understanding of how your shock works. Over time, as you gain experience on the track and begin to become proficient in understanding how to adjust your shock, this overview will provide insight into what the shock is doing internally. Just as important, you will also be able to understand what your vendor is or should be doing to provide support and service for your shock.

 

Most likely 90% of the people that read this don’t honestly know what really comprises the shock’s internal technologies and why it is important to select and then have the shock maintained for optimal performance. This article is designed to provide riders, mechanics, and suspension vendors a solid amount of information so that they can understand how a shock works and what exactly goes on in a shock for better long term success.

 

The following material will be presented and discussed in depth to help customers and technicians have a better understanding of the various products and grasp of why a shock “works”:

 

  1. About this article

  2. Upgrading your rear shock (Ohlins, Penske, JRi, K-Tech, Fox, Elka, etc.)

  3. Basic shock and suspension principles

  4. Velocity, acceleration, and damping

  5. How the spring works with (and against) the shock

  6. The nitrogen chamber

  7. How the damping circuits work together

  8. Key build and maintenance items

  9. How to pick a vendor

  10. Emerging technologies and tuning strategies

 

 

About this article

 

This article is provided for all levels of riders and technicians. It provides an explanation on how many current motorcycle shocks work and insight into RPSRaceTeam.com tuning and building processes. Several specific technical situations are discussed. Many street and track day riders will most likely not need to address issues discussed. Faster riders and seasoned technicians will likely identify the scenarios discussed and be able to digest the views of this article and the writer with their own criticisms and agreement to the content.

 

At a minimum, all readers will be able to utilize the diagrams provided for better insight and understanding of “how a shock works”. There are many different “tuning strategies” that tuners have and will use. The strategies provided have been used for the last 10 years at RPS. RPS was open to the public for 3 years and has worked privately with riders for the last seven. In that time, Over 100 regional and national championships have been won as well as nearly 1000 races with RPS sponsored riders and customers.

 

Most tuners rarely (if ever) share this type of information. It has been provided for riders to understand the shock better and to help those that don’t have a teacher, trainer, and/or mentor. The majority of tuners had someone teach them a tuning strategy that provided a solid base to begin their own approach to developing their methods, yet you will never hear them publicly acknowledge that anyone taught them anything. This article is submitted to assist any tuner or customer as an approach to learn from, criticize, and/or help develop their own method.

 

Lastly, thank you to those that provided me insight, training, and mentorship as I learned the mechanics and art of suspension. These people shared their candid understanding, knowledge, and experience so that I could provide solutions that worked well. These include:

 

Max McAlister (Traxxion) – Thank you Max for providing most of my initial training and skill development.

 

Dave Hodges (GP Suspension) – Someone that has unconditionally shared knowledge and has been a great “sounding board” over the years.

 

Jim Lindemann (Lindemann Engineering) – The late Jim Lindemann never trained me nor did I ever have any discussions with him. I learned more about suspension doing service work for some of his customers. After opening up things he built, observing his work, and concluding that most suspension technicians use technology instead of craftsmanship and skill…and that understanding the “basic principles” will carry a suspension tuner farther than anything else. I arguably learned more from Jim Lindemann than anyone else that has helped me, just by observing how he achieved success.

 

Tim Godshall (JRi Shocks) – Tim helped me tremendously in his days at Penske. and continues to do so as I continue to work with him at JRi Shcoks. Always answering my “what if” questions when I first started. His knowledge with shocks was a great benefit to me and continue to be.

 

Again, thank you to Max, Dave, Jim, and Tim. The information and the way it is presented does not necessarily reflect any of their influence nor do they endorse anything within this article. I am thanking them because even the best “suspension guys” seem to forget there are more than likely several people that helped them along the way.

 

I additionally hope this article assists other suspension tuners in their endeavors. Racers deserve the opportunity to compete with the best opportunity for success. I hope that local “suspension techs” will realize that the customer is more important than their relationships with their vendors and not to continue to succumb to selling what is the easiest and most profitable product. Great work and products are what is best for the customer. Best of luck to all racers and tuners that may read this.

 

Upgrading your rear shock

 

After many people buy a shock and ponder the “real impact” of their investment they may wonder, “Is this really as good as I thought it would be BEFORE I bought it?” Buying any high $$$ aftermarket performance part is stressful and best compared to buying a Rolex watch for most. After you spend several thousand dollars on a Rolex and realize that your old Timex actually kept time just as good as the new Rolex, it is frustrating. Still you put on “your happy face” and let the world know what a great watch your Rolex is. You most likely wouldn’t say anything different as you don’t want the world to know that a big sum of money went towards something that doesn’t work. Your shock and a Rolex are one in the same in that you don’t want your friends and peers to know you just spent a lot of money for something that really didn’t perform like you hoped. Unlike the Rolex, you can easily get your suspension to work at its intended performance level.

 

It is important to clarify though that almost any aftermarket shock (Ohlins, Penske, JRi, K-Tech, Fox, Elka, etc.) is absolutely going to work better than your stock shock. No doubt. What most people buy an aftermarket shock for and hope to achieve is that (1) Lap times drop, (2) “feel”, safety, and confidence improves, and (3) tire wear improves. Achieving all three of these is difficult and can only be achieved when the proper spring, damping rates, and geometry are achieved.

 

I still see many riders with that “frustrated look” right after their purchase of their new shock. The expectations were that shock would “hook up” and deliver the performance, but now they are less confident in the “feel”. Although the lap times immediately improve, riders with a new shock continue to search for the “feel” that provides confidence.

 

What keeps the rider from getting the 100% desired performance is that many times the shock simply is not properly adjusted for them. This is a two-way street though as the rider has to be able to communicate with the tuner what the bike is doing.

 

Although the new shock performance is obviously better, geometry, damping, and the spring provide a completely different feel that sometimes alienates the rider. In addition, many shocks are not specifically built for that rider. Yes, your vendor may have ordered a shock “just for you”, but many times that came off a shelf and if you are fortunate then you even got a spring for your specific weight and skill. At the same time, many vendors are more concerned about “hitting numbers” with their wholesaler and the result is you may receive a shock that is really not what you ordered.

 

LASTLY, experience is the biggest factor in whether your shock will work for you. If you haven’t ridden a bike that is “setup” before, you aren’t going to know what to look for. I generally start customers off with a geometry that is mild (no rear ride height added) and slowly work them up over two or three weekends. What this does is provides feel (it generally feels like you are flat tracking when the tire slides because the rear is a little low) for the rider to give feedback AND (with supervision) it gives the rider a strong learning experience. The rider can know their shock is working well when:

 

  1. The bike is very stable on the fast straights.

  2. The tire provides warning BEFORE it slides.

  3. The bike turns in well.

 

(NOTE: Achieving these goals in the listed order will help the rider learn setup with the best “safety cushion”).

 

 

Basic shock and suspension principles

 

Many people ask, “What does suspension do?” The mostly standard answer is, “…to keep the tires smoothly tracking on the asphalt.” Well, that is only half the answer. Suspension is designed to allow the tires to optimally maintain traction with the asphalt, while providing “feel and feedback” for the rider to have maximum control over the motorcycle. Keeping the tires tracking is only part of the job. If the rider cannot get “feel and feedback”, then they will not have proper control of the motorcycle.

 

Tuners can easily build in “feel” for suspension, but it is also contingent on the rider’s performance. Different levels of riding provide varying performance in regards to the suspension. A track day rider that is 8-10 seconds off the track record could jump on a bike that was the “fastest” that weekend, and have the suspension work terribly for them. The reason for this is the “faster rider” is putting in tremendous load(s) on the suspension in some areas vs. the “track day rider”.

 

To achieve suspension compliance, it is important to realize all suspension components work on simple principles of hydraulics and fluid dynamics. Key items in the performance of your suspension:

 

  1. Velocity: In order to soak “up a bump”, a shock must be able to move. This is achieved by creating the appropriate velocity. If the damping or spring is too stiff, then the shock (tire) will “skip” over bumps and cause an unstable situation. If the shock moves “too much”, then the suspension will create an unstable situation at faster speeds.

 

  1. Damping: The external low speed, external high speed (compression), and internal high speed damping rates need to provide a balance where the shock shaft will move to accommodate a bump, BUT does not allow excessive velocity.

 

  1. Spring: The spring needs to be stiff enough to meet the aggressiveness of the rider, but not prohibit free movement of the shock.

 

  1. Geometry: The geometry needs to provide balance so that the swingarm angle and weight bias enhances the overall performance of the rear suspension rather than create an extra burden and create a poor performance situation.

 

Most likely you have never seen the “keys of suspension” described this way, BUT these are the essential elements of how a shock works and how it has to be built & setup to ensure maximum performance.

 

Velocity, acceleration, and damping

 

Under the perfect situation the shock shaft will be able to move when required (hitting a bump), but at the same time the shock needs to decelerate nearly as quickly as it accelerates. If the shock shaft does not quickly slow down, the suspension will have excessive movement and not perform well.

 

When you realize that the shock shaft may move 3 inches per second or more, and that most of that is in ½” to ¾“  increments, you also realize there can be several movements on the shock shaft during every second. This is a tremendous amount of activity in the shock. If the damping and spring do not limit velocity as well as they create it, then the shock will go into excessive oscillation and performance will degrade.

 

“Low speed” and “high speed” simply refers to how fast the shock shaft is moving. So, if you are adjusting “low speed compression”, you are simply controlling how fast the bike will compress under load when the shock shaft is moving at its slowest speeds. Adjusting the external “high speed” compression adjuster will require you to probably jump up and down on the motorcycle to feel the effect. NOTE: Tuners over 200lbs with practiced experience will be able to observe external “high speed” compression adjustment(s) simply by “pushing on the bike”.

 

It is difficult for most tuners to “push on the bike” and “feel” the internal high speed damping. Again, some tuners over 200lbs with a practiced experience will be able to feel “what is going on”. For most to fully appreciate how Velocity is generated as well as how deceleration in the Shock occurs, they have to work based on rider feedback. They also need to understand the low speed circuit(s), externally adjustable high speed compression circuit, and the internal high speed circuit(s) impact the overall maximum ability to create velocity and what is needed to provide deceleration. When they are working well, it is essentially a balanced equation where the damping circuits complement the others. Here are some notes on the individual circuits and their performance:

 

  1. Low speed circuits - (Rebound and Compression) – Both of these are simple orifices that are controlled by manually adjusted metering needles. The user can literally “close these circuits” by turning the adjusters “all the way in”.

 

  1. External high speed - The external “high speed” compression adjuster is a simple adjustable tensioner that preloads a small shim stack that is in the remote reservoir. By preloading the shim stack in the remote reservoir, you can change when the eternal “high speed” circuit will open. Many users fail to use this as an adjustment as they have been “brain washed” that there is something really “special about this feature”. All it really does is provide a “go between” for the low speed compression damping and the internal high speed compression damping.

 

  1. Internal high speed – (Rebound and Compression) - The internal high speed damping is the actual shim stack on the piston. These are not adjustable, but rather are built to a determined specification by the factory or suspension builder. These are the MOST CRITICAL damping forces as they have drastically more volume capacity than the other “circuits”. If the internal high speed compression and rebound stacks are “weak” or don’t operate properly, then the rider will be constantly adjusting the external adjusters trying to “band aid” the inadequacies of the internal high speed damping circuit. Again, they have the ability to create the most velocity. When and how damping is provided is either a performance feature or a detriment to the performance of your shock.

 

When you analyze the shock damping circuits, you will always realize that the lowest common denominator is the internal high speed compression and rebound circuits. Why this is important is that when these circuits have to excessively open (because the low speed circuits are being overwhelmed combined with the stacks being “weak”), then velocity becomes exponential and this creates additional issues.

 

Even the smallest initial increase in velocity can create a much different curve. In Figure 5 of this presentation, you will see what happens when just a small increase of velocity (because of poor low speed damping) occurs. In this example, a 10% increase in initial velocity created a situation where the shock moved 1.35” instead of only 0.75” for the properly “damped” shock. As a rider, you only noticed that the shock/rear end moved a lot.

 

Although it hasn’t been focused on, the inability to create velocity can be an issue, BUT very, very rarely will that ever occur except on poorly adjusted, grossly built shocks, OR when a experienced builder is “working the fine line” of performance. Even with a “harsh build” (stiff) on the shim stacks, tuners can create velocity fairly easy by “backing out” the low speed (compress/rebound) and external high speed compression adjusters.

 

“So what causes excessive velocity, and how do you fix it? Also why is this such a big deal, no one else is in the industry talks about this?” The main issue that causes excessive velocity (as already alluded to) is an internal high speed stack that opens too quickly and too much. Specifically, if the shim stack opens too early then the shock shaft will gain velocity too quickly AND will not decelerate quickly enough. Velocity issues are generally specific to the rebound circuit as the compression stack is not as sensitive to “excessive velocity”.

 

The most common “fix” for excessive velocity issues is to re-valve the shock piston (modify the shim stack). The shims are nothing more than steel washers. In most shocks, these “washers” are generally anywhere from .004" thick to .015" thick with diameters of .75" to 1.4" wide. Sequentially stacked, they provide “damping force” when oil is forced through the Damping Port on the Piston.

 

Experienced shock builders will analyze the issues prior to the shock being sent in for service/repair/upgrade. Each shim has a specific function. There will be approximately 4-7 sets of different diameter shims. Each shim set can be comprised of one or more shims. The most important shim set is the smallest diameter shim(s). This shim provides for the overall stiffness of the stack. Builders should be careful as making this shim (relative to other shims in the stack) too stiff may cause irregular damping action that will degrade overall performance.

 

It also is important to note that the damping circuits are designed in a manner that as velocity increases so does the damping force(s). The reason this occurs because at a certain point, no more oil can move through the ports than its physical maximum flow design will allow. Adding more velocity only increases pressure.

 

Each manufacturer will have their own “valving strategy” in regards to how many shims and in what progression to achieve designed damping rates. Compression damping requirements are generally softer than rebound damping as the compression circuits usually need to only produce a maximum of 200lbs – 300lbs of maximum damping force, while rebound valving may need to generate 700lbs – 900lbs of maximum damping force.

 

In most cases, the rebound and compression stacks are “linear” when you receive your shock. What that means is that each shim set is the same thickness as the others in the stack. Some manufacturer’s will actually ensure the compression stack will induce velocity on the shock shaft quickly by putting in a “blow-off” shim set in the compression stack. This shim set is designed to be slightly weaker than the remaining shims which in turn will ensure the shock compresses adequately when a bump or track irregularity occurs.

 

When the shock is working correctly, it will soak up the bumps with minimal excess movement in the shock. Many times, a rider can have their mechanic watch the rear shock around the race course and it will be blatantly apparent if the rear is moving too much. As a an example of the physical movement, figures 7 & 8 show what travel looks like with proper damping and how excessive velocity forces a rider to slow down (see figure 8).

 

How the spring works with (and against) the shock

 

Many consider the Shock Spring as part of the suspension. The Shock Spring is part of the suspension, but it does nothing dynamically nor does it actually do anything to “soak up bumps”. The Shock Spring is best described as a “Kinetic energy capacitor” that transfers the energy created from the bumps, irregularities, and other forces affecting the tire and chassis to the damping circuits within the shock.

 

Different bikes use different weight springs, because the shock connects to the frame via a linkage assembly. This “linkage” is nothing more than a lever(ed) mechanism that provides an engineered value of input to the shock from the swing arm. Because manufacturers use different strategies in building the linkages, shock spring weights vary greatly. Linkages can provide linear, progressive, and digressive inputs into the shock depending on design(s).

 

Although it is arguable that a spring does absorb small amounts of energy (which is transferred into heat), it is not of significant amounts to introduce into this discussion. Many times, springs are poorly selected and improperly adjusted.

 

Within the compression stroke of the shock, the spring exists to absorb and hold the energy that is being “loaded” into the shock. The compression stack is there simply to meter how fast it will “go down”. Some of the energy is absorbed within the compression damping circuits, the majority is still held within the spring. Once the spring/shock is compressed and no new load is introduced, THEN the spring tries to unload the remaining energy in the form of the rebound stroke as quickly as possible. This is a violent amount of energy and action that occurs. Again, the typical shock dyno sheet tells this story.

 

The shock dyno sheet in figure 9 demonstrates the tremendous difference in damping rates of the rebound and compression circuits. This dyno run was performed after a CMRA race at Hallett, Oklahoma where the RPSRaceTeam.com GSXR-750 with Danny Kelsey set the fastest lap of the weekend.

 

If you had to explain how damping works with the spring on the compression stroke, then you could describe the situation as someone spotting and helping a weightlifter doing bench presses with 300lbs. The spotter is helping the weightlifter to a small degree, but he is really there to make sure the weight (bar) doesn’t harm the weightlifter and (the spotter) provides some varying degrees of “assistance” depending on what the weightlifter requires.

 

The rebound stroke could also be described as trying to get the cork off a bottle of champagne without it flying out of your hand (and trying not to spill the champagne). The shock being compressed with the spring ready to “rebound” with over 500lbs of force or more is a violent situation. Just like opening the Champagne though, when enough force is evenly applied the release is very smooth.

 

With the analogies over, it is important to note that the spring does provide excellent tuning options. If the rear/shock is constantly squatting with compression adjusters turned in heavily, YES go to a stiffer spring. In that same breath, it is important to note we re-valve the piston rebound stack anytime we make a change of over 25 pounds in the shock spring.

 

CONTINUE TO THE 2ND PART OF THIS ARTICLE

 

 

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Addenda Illustrations

 

These illustrations are provided for comprehensive review of the subject matter discussed. PDF Downloads are provided on the 2nd part of this article.

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