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Steel Thunder Custom Cycles
3418 Teagues Road
Marshfield, MO 65706


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Definition of Terms :

Rear suspension on a motorcycle is comprised of a damper and a spring. This combination is commonly referred to as the shock absorber.

Damping is divided into two parts: Compression Damping and Rebound Damping.

Compression Damping comes into play when the shock is compressed and helps absorb the impact when a bump is encountered and the wheel goes up.

Rebound Damping controls the return of the shock absorber and wheel to its original position. If this occurs too quickly, the wheel may bounce back (pogo). If it returns too slowly the shock absorber may not allow the wheel to return to its correct position for the next bump causing the tire to lose contact with the road surface. Control is the key word. Rebound damping controls the speed is which the spring and damper combination return to its original position.

Spring rates are selected by Progressive Suspension to match each motorcycle application. Leverage ratio (wheel travel vs shock travel), wheel travel and load determine the required spring rate for your motorcycle. On those applications where you have a choice of a standard spring or a heavy duty spring, the heavy duty spring should be selected for a load weight over 200 lbs, not by how fast you ride. There are four common types of springs.

Straight wound springs of 100 lb/in. require 100 lbs of force to compress each inch of travel. A progressively wound spring will usually have coils closer together on one end producing a progressively stiffer spring rate. At the beginning of compression, less force is required. As the spring is compressed, more force per inch is required to compress the spring. Two springs are stacked to produce a "dual rate". When the softer rate is used up, the springs jump to the higher rate (i.e.; 70-100).

Air spring When air in a shock absorber is compressed the pressure increases. This increase in air pressure acts the same as a spring rate. The purpose of adding air to the shock is to easily set the correct ride height for changes in load. For safety reasons, Progressive air shocks feature a combination of a steel spring used for the standard load capacity, air is then added to adjust the correct ride height.


Technical Issues:

Types of Shock Absorbers

There is a shock absorber available for each type of riding and every type of motorcycle. Determine which type of shock absorber will best fit your riding style.

Conventional Steel Spring - The rider who has a fairly consistent load from one trip to another and is looking for outstanding performance will choose Progressive Suspension's conventional steel spring shocks.

Adjustable Damping - The high performance rider will want to choose the Progressive Suspension adjustable damping shock for outstanding performance, damping adjustability and a reasonable price.

Air Shocks - A touring rider with widely varying loads from one trip to another should consider our famous Progressive Suspension air shocks. The correct ride height is easily adjusted by adding or subtracting air pressure. These shocks offer exceptional control and comfort.

Ride Height

Progressive Suspension's spring shocks have a mechanical spring preload adjustment feature. This adjustment is used for setting the correct ride height when you have changes in load. When the preload adjuster is raised to a higher notch, it will increase the beginning spring load and raise the ride height of the motorcycle. Ride height is very important. Most complaints of a harsh ride can be traced to incorrect ride height adjustment. If the motorcycle is sagging too much, there will not be enough wheel travel to absorb big bumps and one will get the feeling that the suspension is too stiff but in reality it is too soft. Stability and safety of the motorcycle is impaired when excessive bottoming occurs. Tire wear and damage is also effected by excessive bottoming. When the shock bottoms out the tire must try to absorb the remaining force of the bump. This causes excessive side wall flex in the tire which can weaken the tire and possibly cause premature tire failure.

Lowering Motorcycles

If one needs to lower a motorcycle it must be done by choosing the correct shock and spring for that particular motorcycle. There must be sufficient clearances of the wheel and other components. Choosing a shock that is designed for another motorcycle can be dangerous. Generally when you lower a motorcycle by utilizing a shorter shock you will lose the equivalent amount of wheel travel (or more depending on the ratio of shock travel to wheel travel). For example if you lower the motorcycle 1" you will lose 1" of wheel travel. A motorcycle with 4" of wheel travel that is lowered 1" will lose 25% of the ability to absorb bumps. Decreased wheel travel will increase bottoming and decrease ride quality. Lowered shocks will generally have stiffer spring rates and damping to help compensate for the loss of travel. Initial cornering clearances will also be affected and care must be taken in making turns at any speed. Lowering motorcycles that are heavily loaded is not recommended.

Lowering Blocks

We NEVER recommend lowering blocks on any motorcycle. The use of lowering blocks changes the angle of the shock. This can cause shock failure and the spring and damping requirements will be incorrect due to a change of leverage of the shock. The use of lowering blocks voids most aftermarket shock warranties including ours.

Front Forks

Front forks should be taken apart, inspected, cleaned and fresh oil installed every 20,000 miles or once a year for optimum performance and minimum wear. Many complaints of a harsh ride can be traced to the front forks, including weak springs, worn internal parts, misaligned aftermarket fork braces (which causes binding) and fork tubes that are bent or misaligned to each other (causing binding).

Fork Oil Volume

When installing fork oil we recommend the stock volume of oil listed in your shop manual.

Fork Oil Viscosity

Most motorcycle manufacturers recommend ATF (automatic transmission fluid) for fork oil. This has excellent lubricating and cleaning qualities but we do not recommend ATF oil for one main reason. When it is manufactured it has no viscosity specification other than it must fall between 5 wgt and 17 wgt. One time you buy ATF it might be 5 wgt and the next time it could be 17 wgt. We do not feel that this is acceptable for motorcycle forks. Therefore we recommend most major brands of motorcycle fork oil. As a general recommendation 10 wgt works well for most motorcycles.

Fork Braces

Aftermarket fork braces are another problem. There are good and bad braces on the market. A good quality brace, properly installed, will help stabilize the forks, especially at slow speeds. A poor quality brace, or one that is improperly installed, will cause many problems in fork performance, ride quality and handling.

Things that affect suspension


Wheel bearings, swing arm bearings and steering head bearings all affect the handling of the motorcycle. Adjust steering head bearings and swingarm bearings to the factory specifications. Do not over torque the steering head bearings. Over torquing bearings to try to correct a wobble is a common mistake that well meaning mechanics make. Under no circumstances should this be done, it will cause even more handling problems and premature wear. If your mechanic suggests this, immediately take your motorcycle out of there!


Good quality tires of the correct size and profile are very important. Below are some of the more common tire problems that adversely affect handling.

• Wrong tire size (width) for the rim size

• Tire not sealed on the rim correctly.

• Incompatible front and rear tires (mixing makes or profiles is not recommended).

• Incorrect tire pressure - This is the #1 cause of excessive tire wear and bad handling! Do not try to improve your ride comfort by letting air out of the tires! Under inflated tires overheat, cause handling problems and are more likely to blow out!

• Worn or poor quality suspension will cause excessive tire wear, especially cupping.

• Weight - Some accessories when bolted to a motorcycle may cause handling problems. The most common causes of the problem are: Added weight of additional accessories Overloaded travel trunks and saddlebags Overloaded trailers Trailers with the wrong hitch ball height and/or weight Incorrect distribution of weight (too much weight to the rear of the motorcycle.



(Q) What is the advantage of air over steel springs?
(A) For motorcycles such as touring bikes, where the shock is hidden by the bags and/or is inaccessible, air is a convenient method of adjusting the sag. Steel spring shocks usually require the adjustment of a cam collar or threaded ring to set the sag. Air is a spring and has its own pros and cons. Too much air can cause "sticktion" (friction) problems with seals and the spring rate of air progresses at a very high rate upon compression. But again, it is very convenient for touring bikes.

(Q) What is preload?
(A). Preload is used to set the suspension ride height (or sag) of the motorcycle. The motorcycle should sag about 20-25% of its total travel when the rider and passenger/load (if applicable) are on the bike. Not enough sag (too much preload) and the suspension will tend not to want to move initially, causing a rough ride over small obstacles such as stutter bumps. Too much sag (not enough preload) and the bike loses valuable travel, bottoming out easier and can cause stability problems. Any suspension (stock or aftermarket) can benefit from proper ride height adjustment. If a bike will not adjust to the proper sag after exhausting all adjustment settings, a different rate spring my have to be used.

(Q) What are spring rates?
(A) The rate of a spring is the amount of force necessary to compress the spring, usually measured in one inch increments. A straight rate spring will take the same amount of force for the entire travel of the spring. For example a 10 lb rated spring will take 10 lbs of force to compress it one inch, another 10 lbs (total 20) to compress it the second inch, and so on until the end of the spring travel. Now a progressive rate spring changes the force requirement as the spring is compressed. A 10-15 lb rated spring will require 10 lbs of force to compress the spring the first inch, another 12 lbs (total 22) to compress it the next inch all the way to the last inch where an additional 15 lbs of additional force is required to compress.

(Q) Why progressive rate?
(A) A progressive rate allows a plush, comfortable ride in the initial travel but since the rate shifts higher during compression, it can control the "diving" the bike wants to do under heavy braking or when you hit the big bump. A straight rate spring can't do it all like a progressive rate can because it is a compromise.

(Q) Why should I buy your 420 Single Shock?
(A) Our shocks are designed for the real world of street or dirt riding without the unneeded complexity of racing shocks. Our shocks are engineered and built to perform to a high level of excellence for many, many miles. And as a bonus, they perform without busting the budget by being value priced, sometimes hundreds of dollars less than the competition! Progressive offers true value for your hard earned dollars.

(Q) Why should I upgrade my suspension?
(A) Motorcycle manufacturers have a big job in choosing such things as suspension. They must consider many factors including cost, a soft and comfortable ride, various rider and load weights and a wide range of road conditions. In the final analysis manufacturers supply a suspension system to fit the "average rider", including road and load conditions.

(Q) Why do OEM shocks wear out so fast?
(A) Cost is a major consideration for manufacturers. Keeping within budget is mandatory and long term quality and life expectancy of high wear items such as shock absorbers and springs often suffer. Over stressed components such as internal valving parts, oil and springs are generally the first to wear out. The motorcycle manufacturers have to produce a product at an affordable price for the "average" rider. To achieve this some cost cutting must occur. Automotive manufacturers have the same problems, however, they offer optional suspension upgrade packages to improve handling, increase load capacity or offer adjustability. These options are expensive, sometimes costing several thousands of dollars but are quite popular because of the improvements they offer in handling stability and load capacity. Motorcycle manufactures do not offer these options so we must turn to quality aftermarket manufacturers for these improvements.

(Q) Why do I need aftermarket shocks?
(A) Most stock suspension's are setup for maximum plushness down the interstate, especially on touring motorcycles. However, this may cause problems when heavier loads are carried or more rugged and twisty roads are traveled. You're probably not "average". You may carry heavy loads, or pull a trailer (or sidecar) which taxes the stock suspension. Or you may just want a more controlled, stable ride, especially over rough or winding roads.


5th Element® FAQ

(Q): On my 5th Element Air Shock, is the air valve on the small end of the shock "negative air"?
(A): No, the air valve on the small end of the shock is where you pump up the IFP chamber. IFP stands for Internal Floating Piston and regulates the sensitivity of the control valve. On the air shock, this valve controls the same pressure as the air valve on the 5th Element Coil Shock.

The control valve controls position sensitive compression damping. In other words, the shock senses how far it is compressed and responds with varying compression damping throughout the shock’s travel. The higher the IFP pressure, the sooner the control valve starts to close as the shock is compressed, increasing compression damping.

The correct normal range of pressure is 50psi – 100psi for the 5th Element Air Shock IFP chamber and 75psi – 150psi for the 5th Element Coil Shock IFP chamber.

(Q): Why does my shock seem to have lost air every time I pump it up?
(A): In reality, your shock has not lost air. The chamber of the shock pump, including the hose, dial gauge, pump body, requires pressurized air in it to register a pressure. This air of course comes from your shock when you install a pump. Further, the amount of air actually required in a 5th Element shock (particularly the IFP chamber) is so small that most of the air in the shock empties into the pump. A 50psi IFP pressure may read 0psi-10psi when the pump is installed! The only way is to pump the shock back up to the desired pressure, remove the pump and leave it alone until the next time you check or change your pressures.

To really tell if the shock is leaking air, you can dunk the shock underwater, just like testing for a puncture in a tube.

(Q): Why does my 5th Element Air Shock seem to be leaking oil?
(A): The oil that you may see on the damper strut (skinny end) of your 5th Element Air Shock is actually lubricating fluid used when the shock is assembled in manufacturing. The consistency of this lubricating fluid is like syrup and is not a grease. Inside the air can just behind the seals is a micro cellular foam, or MCU, top out bumper. This foam becomes soaked with the lubricating fluid on assembly. Over the first few weeks of the shocks life, the extension or topping out of the shock will wring the fluid out of the foam like water from a sponge. The more riding is done, the more the MCU is "wringed out" of this lubricating fluid. The fluid is not needed in the foam. The fact that the foam is soaked upon assembly is strictly due to where it is in the shock and how the shock is assembled. In fact, the only oil actually contained within the 5th Element Air Shock is the damping fluid sealed inside the damper cylinder. So, for that oil to actually leak to the point it is visible, it would have to leak past the main shaft seal or seal head, past the main air piston seal into the negative air side of the air spring, past a glide ring, past the main damper seal, and then past the dust wiper.

©2003 Progressive Suspension, Inc. --- All Rights Reserved.    

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Steel Thunder Custom Cycles

3418 Teagues Road

Marshfield, MO 65706


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