The desire to make our own shock absorbers was borne out of the fact that 1) they are no longer in production and not available via any world-wide parts store and 2) reconditioning old shocks was notoriously unreliable and just didn't have that new-bike smell. We started with the reservoir hose, then the reservoir and then we moved onto the shock itself and in between, we made our own springs...

Reservoir

What is it for? The reservoir is a complex system of parts that is essentially a bottle that stores gas on one side and oil on the other. When the shock absorber is compressed, oil inside the shock body is forced into the tank. Along the way, the oil passes through a hollow shaft located inside the reservoir seal head on the oil side. The shaft has a series of concentric holes of different sizes located at the end of it which, when rotated into any one of 4 spots using the plastic dial, will let the oil pass into the tank (see image below). The size of the holes dictates the speed at which the oil moves through them during compression, thus enabling a variable switch for compression damping of the shock absorber.

Reservoir Shaft and Seal Head. The different sized holes on the left hand side of the shaft control compression damping.

Neat huh? It gets better - the oil and gas are separated by a plastic free-floating piston. Although the shock is completely full of oil (and approximately half of the reservoir), when the shock is compressed, the shock shaft and valve stack push through the oil. What happens during this process is the shaft length that is inside the shock body increases. This creates less volume for oil to occupy within the shock body which in turn increases the pressure on the sealed system. The free floating piston moves backward as the shaft volume increases in order to maintain the original volume and pressure. 

Why does it need gas? The variability of the gas chamber volume is what enables the shock absorber to function correctly - oil is a non-compressible fluid and when the shock compresses, something's got to give. As described above, the gas keeps the oil under a constant pressure however as the piston inside the reservoir moves up or down, the volume of the gas chamber increases or decreases accordingly and so the pressure of the gas changes with each movement. Gas is compressible and thus enables the shock absorber to move through its actions. Additionally, the constant pressure ensures that the oil does not "foam" due to the constant movement and squeezing of the oil through all the small apertures within the shock. In this way, the oil can be maintained in a good working condition for a long period of time. 

Why is the gas Nitrogen? Due to the constant expansion and contraction of the gas reservoir volume as well as the heat generated by the compression and rebound cycles, the gas wants to get hot and expand. Normal air gets hot and expands very quickly which reduces the effectiveness of the gas to maintain that constant pressure that the system needs. If the pressure on the system increases, compression resistance will increase with it, oil viscosity will be reduced due to increased heat, and rebound damping will conversely be reduced. In addition, air has impurities including water which are incompressible. We use Nitrogen because it has a higher working temperature range and is therefore less prone to expansion and contraction due to the effects of the heat produced by the repeated shock movements.

Making the Reservoir Piston

Reservoir Piston

A seemingly simple looking part was in fact one of the most challenging to make. As pictured above, the piston has a dark grey/green/khaki look similar to the colour of Teflon on a saucepan. Our initial thoughts where that this material was Teflon (PTFE) and we made the pistons using this material. This material turned out to not be correct. Not only is PTFE white in it's purest form, but we discovered quite quickly that PTFE is highly sensitive to temperature change and so during testing, the pistons were locking up inside the reservoir.

It was at this time that we fully measured the OEM pistons for density, hardness and thermal expansion and then searched for products in the market that were compatible. We discovered a product called PAI GF30 (commonly known as Duratron T5530). It is a Polyamide-Imide reinforced with 30% Glass Fibre. To give you an idea how good this material is here is a comparison with PTFE:

Characteristic	Target	Teflon (PTFE)	PAI GF30
Density g/cc	1.6-1.7	2.2	1.62
Hardness Shore D	>80	50-65	87
Coefficient of Thermal Expansion (x10^6)	<25	>200	15

 

To put this into context, the CTE of aluminium is around 21-24 so the piston needs to expand and contract at a rate similar or slightly less than the aluminium body of the reservoir. We had a winner.

The next challenge was trying to figure out the clearance for the piston to move inside the bore throughout its entire operational temperature range. We estimated this to be around 10-110 DegC. Like all piston and bore situations, the piston clearance is critical to performance. We tested free piston movement for different piston clearances and decided on a minimum clearance at room temp of 0.1mm. To account for machining tolerances, we set a range of 0.1mm-0.15mm. The clearance at maximum operating temperature (room temp +100 DegC) would be 0.136mm-0.186mm (the aluminium expands slightly more than the piston material). We then pressure tested the cylinder up to 2.5 bar (250kPa) to be sure the piston o-ring would remain sealed at maximum operating temperature.

All good so far however ensuring production stays within the required tolerances was our next problem. Pistons all measured between 40.04mm-40.07mm. We set the target diameter for the machining of the bores to 40.16mm-40.20mm. This would produce worst-case clearances of 0.12mm-0.16mm for the smaller pistons and 0.09mm-0.13mm for the larger pistons. Although this tolerance range would theoretically produce out-of-spec clearances, the smaller pistons could be matched to the smaller bores and visa versa. In this regard, each piston had it's size stamped on the inside and each cylinder had its size written on it with a marker pen. At the time we measured and stamped the pistons, it was 15 DegC. So, machining had to also take place at a similar temperature. We then wrote a computer program to optimise the matching process thus ensuring that every piston had a matching cylinder. Using this process, we were able to achieve a 96% sample pass rate on the pressure test bench.

Although we succeeded in the end however the pursuit of the correct material and production tolerances caused delays and some units shipped with OEM pistons with clearances set to 0.1mm. Worth noting at this point that future KX80 shock absorbers (and current modern designs) use a bladder instead of a piston which is a far better solution.

Compression Adjuster

Making the compression damping adjuster assembly was one of the fun parts of the project. As mentioned earlier, a shaft rotates inside the reservoir head with different sized concentric holes at the end to allow for varying oil flow rates when the shock compresses. To get the shaft to "click" and thus help the user find each selection point, a spring-loaded ball bearing, pushes against the shaft, seating in an opposing hole to which the oil passes, thus creating the audible click that is heard as the shaft it turned. The difficult part here was the spring. These were not "off the shelf" springs. Suffice to say, we are now experts at measuring spring rates and making springs...

Shock Absorber

Shock Shaft and Valve Stack

Making the main shock absorber assembly had many challenges, chief among them was the shaft assembly. This is a complex hollow shaft that houses the rebound rod. We decided early on to try and reuse the OEM shafts complete with the valve stack in order to keep the overall costs within budget. Ultimately, we try to make all our parts for less than the original cost charged by the OEMs. To begin, we purchased several shock absorbers, stripped them down and salvaged the shaft assemblies. The shafts were reground and hard chromed to original factory spec however we soon discovered that the original Kawasaki design did not really suit this approach- the rebound adjuster rod has to be removed from the shock shaft in order to re-chrome it. As a result, we observed inconsistent performance in the rebound damper assembly as the damper rod quite often failed reattach properly to the selector disc inside the shaft. So, back to the drawing board- we decided to make the whole shaft assembly complete with the piston, ring and valve stack.

Once we had committed to making the shaft assembly, we decided to redesign certain aspects of it so that we could avoid the issues we encountered with repurposing original units. Chief among the improvements was redesigning the way the damper rod connected at each end of the shaft. This would make the shock absorber 100% totally rebuildable and reliable.

This image shows the shaft base connection with the clevis. The rebound damper shaft (blue) has been redesigned to have a gear end. The original Kayaba unit has a square end. The gear end enables a trouble-free assembly process, making it easy to achieve the correct alignment of the rebound damper shaft and the main shock shaft (see below image).

This image is of the piston end of the shaft. The selector disc is now integral with the redesigned damper rod. Kayaba had previously had the disc as a captured, sliding lug that connected to the damper rod via a spline and involved a complicated assembly process to ensure that all the holes aligned properly. Our design is 100% reliable compared with the original units. 

We also simplified the valve stack, reducing the number of components from 20 down to 14. In the image below, the highlighted items show the 3 components of the valve stack. Kayaba's original design utilised 9 separate washers.

3-piece Valve Stack

Of course, we couldn't help ourselves and went one step further by optimising the material types based on modern technology. For example, modern CNC machining practices enabled us to make the piston from aluminium as opposed to the mild steel used in the Kayaba units, thus reducing unsprung weight. The valve stack components have been made from an exotic hardened stainless steel alloy for better heat and corrosion resistance and the shaft itself is made from 1050 cold-drawn stress relieved steel bar.

Is the shaft important? Yes! The clevis/ shaft assembly is the only moving part in the whole shock absorber assembly. It is mounted to the clevis (the U-shaped aluminium piece at the base of the shock absorber) and on top of the shaft sits the valve stack. In between, is the seal head through which the shaft moves through with each compression and rebound event. 

Shock Shaft Assembly with (from top to bottom) Seal Head, Packer, Bump Stop, Bump Stop Cup, Clevis. The rebound clicker is visible at the base of the clevis assembly. The rebound "hole" is visible at the top of the shaft.

Ok, so how does the rebound damping work? We've already talked about how the valve stack and shaft push through the oil during compression. When this happens, a small amount of oil (which has been displaced by the increased volume of the rod inside the shock body) is forced into the reservoir but more importantly, oil also moves through the valve stack to the bottom of the shock absorber. When the shock absorber rebounds (remember the main shock spring is working to force the shock absorber back down), oil, which is now sitting underneath the valve stack now has to make it's way back to the top of the shock absorber. Importantly though, the oil does not pass back through the stack (there is a collapsible washer in the valve stack) - it passes instead through a small hole at the top of the shaft. It is this hole that creates the possibility for variable rebound settings. The collapsible washer enables a separate oil circuit for rebound and compression.

The green outer washer slides up and down the inner washers with aid of the star-shaped spring. When at the top of its movement, this washer seals the underside of the piston (not shown for clarity), forcing oil on the underside of the valve to be forced through the shaft during rebound.

Rebound "Clicker"

The hole at the top of the shock shaft helps to control the speed of the oil as it returns to the top of the shock absorber. The hole size is altered by turning a small shaft inside the main shaft using the 4-position selector located underneath the clevis. The selector turns a rod that runs almost the entire length of the shaft. The rod is connected to the smaller shaft with concentric holes of different sizes that line up with the hole in the shock shaft. It works the same way as the compression adjuster- the smaller the hole selected, the slower the oil moves through it and thus increases the rebound resistance.

The valve system allows oil to pass freely through the piston/ valve assembly during compression events. During rebound, oil is forced through the hole in the shaft (blue) and exits out the end of the shaft, bypassing the valve stack entirely. The damper rod can be turned to one of four pre-set positions, each with a different sized hole. This is what enables variable rebound!

The rebound "clicker" operates much the same way as the compression clicker described above. A spring-loaded ball bearing seats into grooves in the rod as it is turned. One of the challenges we faced with the rebound clicker assembly, was making a small felt washer located on the reverse side of the clicker knob. It took us several months to locate a supplier that could actually make these washers in the desired size and thickness and with an adhesive backing...

Bump Stop

What is the bump stop? It is the small rubber stopper located at the base of the shaft that you see visible in the shock absorber photo below.

Is it important? Yes! The bump stop is the last line of resistance for the shock absorber during a compression event. It's size, shape and hardness are extremely important to help cushion the shock absorber when it "bottoms". Without a bump stop, there would be nothing to stop the shock base (clevis assembly) coming into contact with the shock body each time the shock bottoms.

We opted to use a genuine KYB bump stop as they are readily available in the market.

Seal Head

We opted to use a readily available aftermarket seal head to complete our shock assembly.

Shock Absorber Frame Mount Bushing

We opted to use an OEM Kawasaki unit here and pressed it into the shock body.

Top Shock Mount Bushing

Shock Spring

As for most 80cc motocross machines of the 1980s, the KX80 was designed to suit the median age, weight and height characteristics for the relevant class (boys 9-16 years of age) and median weights of 40kg/90lb (Small Wheel) and 50kg/110lb (Big Wheel). Nonetheless, we got to work and engineered stiffer springs for the guys that still ride them. 

Kawasaki listed the stock spring rate at 4.2 kg/mm (4.6 kg/mm for Big Wheel model). Our testing indicates that these spring rates are good (with some additional preload and less than optimal Static Sag) for up to 70kg/150lb. So we engineered three spring rates to choose from: 5.5, 6.0 & 6.5 kg/mm that will suit riders from 70kg (150lb) and up depending on your riding style, age, skill level and the type of riding you will be doing.

Each spring has been blasted, zinc-primed and powder-coated in original matt-silver.