What a shocker.

By Rob Bradford

OK so we went to the trouble of getting our Armstrong Shocker done up and upgraded by the best in town and after a little while, well 20 thousand Miles, we are sick of the crashing noise from the rear bump stops when touring with a full load of shoes and hats.

Time I say to consult Mr Google and likewise every expert known to man and see if there might be a home grown solution which does not entail removing and sending said Armstrongs to experts who are going to charge like wounded bulls and not likely to divulge their secret recipes.

Armstrong lever arm shocks were originally available in a variety of damping grades. The variations being made primarily in the valving but they were originally designed to use hydraulic oil in the viscosity range of ISO 30 to ISO 100 (equivalent to SAE 10W to SAE 30W) which according to the TR manual does not need to be changed.

The way these devices work is that there are two pistons pushing oil back and forth through an orifice. This orifice is small, so there is resistance to flow due to the viscosity of the oil in the piston chambers.  If the size of the connecting hole was not allowed to vary, the damping would be TOO strong for sudden jolts, so to accommodate sudden jolts, the oil is allowed to force its way through 2 larger spring-loaded valves, one to control the upward jolts and another to control the downward return of the suspension to its rest position.

When these units were new, there were alternate compression springs available, however these are not readily available so in order to adjust damping today, we can tighten the nut holding the rebound spring and insert spacers (washers) under the compression spring.

My research found 2 sources that pretty much agree on what modifications to make, which were to tighten the nut on the rebound spring and install a 0.040” to 0.080" spacer under the compression spring at the bottom of the bore.

Armed with this Google expertise I decided to remove the devices and investigate what my performance enhanced units had to offer. I suspected that the energy input to a small amount of fluid during operation was so intense that it could not be possible for the shockers to maintain performance without maintenance which is contrary to the TR manuals advice.

First job, drain the oils. Black and burned is what we find. Now compare this with some fresh Fork oil.

Imagine allowing your engine to operate on this sludge? How long will it last?

Let us have a look at my high performance upgrades and see how they compare with Dr Google.

The first thing to do was to compare what performance was dependent on the oil, so I conducted some base line tests on the original shockers using a weight and a timer to determine how long it took for 1 stroke.










The next thing was to have a look at what upgrades had been done:

As it turns out, spot on, what our local race technologies had done was as per the famous TR Kastner recommendation. 2 x 0.039” copper washers under main spring. Maximum 0.080”






 Now I happen to have a destroyed Shock absorber to compare. This one had totally failed and had stripped splines between the main shaft and the bellcrank.








The reason?

Well: Both relief valves were suffering from coil bind before they were half open which would have created massive stress on the splines which of course failed.









Notice the difference in the 2 valve designs. Something to watch for if you decide to pay an expert to overhaul your shock absorbers.
Ask for confirmation that they are a matched pair.

So now all that is left is the oil. Much investigation and assistance from Greg Parker provided me with a smorgasbord of inside information.

I had previously bought some Harley Davidson Screaming Eagle Fork oil after interviewing a Harley workshop Tech who advised me that this was the best of the best and they used it in every Harley.

No information is readily available for Harley about the oil spec as this is secret.

Greg advised:
“Harley Type E is 5 wt
Harley Type B is 10 wt

Harley Type H/D is 20 wt
Screaming Eagle is 15 wt
Screaming Eagle Heavy is 20 wt 

In the motorcycling world, we have different fork and rear suspension fluids. 

As a broad rule in the fork oils we have 5 wt. 10wt, 15wt and 20 wt (in years gone by here was also a 30wt) 

Note: 30 Wt was recommended for higher performance by Armstrong but these oils are not readily available today.

My tests used the 15 wt standard screaming Eagle oil and I got a 25% improvement on rebound and a 50% improvement on compression however the 2 units were not the same? Inspection found that the valves were different from left to right. I happened to have a spare which matched so it is worth having a look at the size of the notch in each valve as the performance upgrade and overhaul clearly does not include testing and matching performance.






This shot also clearly shows  the reason for failure. DO NOT go there.



  1. Change the oil every 10K Miles with 15 grade standard screaming Eagle for standard performance.
  1. Use 20 Wt screaming Eagle heavy on standard shockers to get a 50 % improvement.
  1. Shim a max of 0.065” washers and use 20wt oil for a high performance rally race type outcome.
  1. The cost: $19.00 for 1 Pint of oil which will suffice for 3 oil changes in both rear shockers.
  1. Before sending your shockers off to the experts, have a go and save enough to take the navigator out for dinner.

So HOW do we go about it?:

Unfortunately this means removing the shock absorbers but well worth the effort.

I often hear tales of woe with regard to keeping the bolts tight and the need for special spanners, however this simple modification has worked well for me. (Weld a bar onto the head of the bolt to stop it turning.)

Inserting the bolt from the outside with no chance of it turning allows a washer and locknut to be easily fixed with a socket from the inside and torqued accordingly.










Simply remove the plugs and drain from both ends while moving the lever back and forth.

Flush the system out with paraffin or hydraulic oil until there is no residue then fill the bottom and top using the same actions to get all the air out. Make sure there is no air left and then finally fill to the bottom of the plug and refit.

A 4000 Mile trip to Targa Tasmania fully loaded with navigator and a multitude of hats coats and shoes proved highly successful with no crashing against the bump stops and great rear end control... of the car of course!

Rob Bradford 2015

If you would like a PDF version download from THIS LINK