Tech 39mm Forks

Tech 39mm Aluminum Forks

As received, I was not satisfied with how the front end worked.  The bike did not turn as well as my EFI OSSAs.  I was not using all the fork travel, and it was harsh near the end of the stroke.

I improved the turning behavior by sliding the fork tubes up so that 10mm protruded above the clamp (this is roughly double the standard protrusion) but it still does not turn as well as my OSSAs.

The ePure's previous owner is a big guy who had installed stiffer springs at both ends.  I replaced them with standard and set the preload at the minimum to get the sag in the ballpark.  I still did not use all the travel, and the ride height of the bike was now such that the side stand became useless (causing the bike to be too upright for safe parking).  The rear seemed to be working pretty well, and I increased the front preload to mid-scale to get the sag closer to ideal.

EM claims the front has 175mm of travel. I have only been able to measure 165mm, and am only using 150mm.  A 165-mm travel is more in keeping with other modern trials bikes.  Chris at SplatShop informed me that GasGas also states the Tech fork has 175mm of travel.  He guessed the 175 number is from “metal stop to metal stop” and that the top-out spring will provide the missing 10mm.  He added that you'd have to run a very stiff spring with a lot of preload to get anywhere near 175mm. 

The Race's front forks are 39mm Tech made by Formula in Italy.  Tech forks come in 3 grades.

All have adjustable rebound damping and spring preload.

Credit: SplatShop, adjustable compression damping valve

I was planning to do a suspension fluid change and happened upon a kit to retrofit adjustable compression damping to the mid-range Tech fork.  SplatShop in the UK sells this kit quite reasonably.  

It's really too bad Tech did not provide variable damping with the mid-range aluminum fork.  The cost difference at the time of manufacture would have been insignificant.  But if they had, Tech would not have had three different grades of forks to sell.

Six(!) manuals were produced by Formula for servicing their Tech forks.  You can download them from the website.  SX and DX refer to the right and left sides.

Tech recommends an initial oil change at 20 hours and then every 6 months thereafter.  An informed estimate is that my forks had about 50 hours on them.  The photo below shows the oil I got out of each side along with virgin 5-weight Maxima.

The rightmost container is from the right fork (damping side).  It looked like used fork oil.  The center container is from the left (spring) side.  It was very dirty, probably from the spring sliding against the ID of the tube.  The photo does not show the true difference in color.  The leftmost container is virgin 5-weight Maxima.

Left: virgin 5-wt Maxima.  Center: used fluid from spring-side fork.   Right: used fluid from right damper-side fork

It is interesting to note that I got pretty nearly exactly 250cc out of each side (which is the volume Tech says to put in prior to setting the level).  Initially, I only got 180cc out of the right (damping) side.  The cartridge contained the rest and could only be drained by removing the M14-1.0 bolt at the bottom of the fork leg.  I could not find a torque specification for that fastener and simply measured the break-away torque during removal at about 200 inch-pounds.  (Later I read the spec is 16.9 - 23.5 Nm.)

Prior to draining, I measured the air gap at the top of each fork tube.  The right side oil level measured 75mm with the stanchion collapsed, the fork cap removed, and the cartridge rod at the bottom.  In the Tech service manual, the range is 55mm to 75mm for various trials bikes.  75mm is the spec for the 2018-2019 Electric Motion (latest listed).

The left (spring) side oil level measured only 144mm.  The manual says 130mm for 2018-2019 Electric Motion.  The range is 100mm for older Montesas to 130mm for most of the newer bikes.   It's possible that some oil was lost when my springs were exchanged, twice.

Comparative viscosity evaluation tools

Comparative Viscosity Observation

I next made a comparative viscosity evaluation by measuring the time it takes to flow 10cc of fluid through my standard burette at 61 degrees F.  The adjacent photo shows the apparatus.

The left (spring) side took 52 seconds.

The right (damping) side took 32 seconds.

Recall, the left side looked much darker.  I expect there were a lot of metal particles contaminating the fluid, and this could explain the apparently higher viscosity.

Tech specifies the correct oil as “Formula O J RACING TYPE 01” which is not readily available in the US.  I learned it has the following specifications:

Two oils stand out from Peter Verdone's excellent suspension fluids table as candidates:

 My go-to suspension oil is Maxima.  Their 5-weight viscosity is very close at 15.9 cSt.  (For forks, I only care about the 40-degree C spec.)  I hate to be cheap with suspension, but I doubt there is any noticeable difference between the Yamalube and the Maxima.  Long ago I had an excellent exchange with the president of Maxima – he was actually doing their technical support at the time.  I came away with a high regard for their products and his knowledge.  I suspect most of the higher cost of the Yamalube 01 is just profit and getting the product into the USA in small quantities.

Again, I used my timed flow test and got the following results for the oils I had on hand.

In thinking about the oil that came out of the left side, the viscosity is certainly more than even a fresh 7-weight, maybe nearly 10-weight.  I can see where that would have made the bottoming behavior harsh.

Compression-side cartridge disassembled (above), variable compression kit parts (below)

Cartridge Disassembly

The adjacent photo (upper row) shows the parts that came out of the fork.  

Below that, are parts from the variable compression damping kit.

Unfortunately, when I took the photo there was still a red cup stuck inside the fork leg (which I only discovered when I tried to reassemble everything).  

You must use the red cup that comes with the kit because it has holes where the OE one does not.

It's really too bad Tech did not provide variable compression damping with the mid-range aluminum fork.  The red M14 x 1.0 bolt at the bottom contains a needle valve to change the damping.   That's really the only manufacturing change, other than a few holes in the red cup.   The cost difference at the time of manufacture would have been insignificant.  

Note that a 2mm hex key is needed to make the compression adjustment.  It has slightly less than 5.5 turns of range.

Although not visible in the compression-side cartridge photo, the circlip had some plating that chipped off as I was removing it.  It would probably be fine if left alone, but is the kind of thing that can foul a shim stack.  I used a wire wheel to remove all the plating.

As well as the circlip, there is a tiny setscrew that must be removed.  The photo shows a 1.5mm hex key in that setscrew.

End-stroke cones (left one showing holes for variable compression damping)

End-Stroke Cone

The adjacent photo shows the red cup (properly called the end-stroke cone) at the bottom of the cartridge.  The one on the right is original and the one on the left comes with the variable compression damping kit.  

The original shows wear in the anodizing.  I'm not sure how that happened but obviously, there was some movement in that area.  That's likely why it was stuck at the bottom – not coming out with the cartridge initially.

This next photo shows the damper rod on the spring side.  The gold anodized bolt is also the bottoming adjustment which is just a needle valve.  It has about 7 turns of total range and can completely shut off the flow.  Tech calls it the “end-stroke governor.” It requires a 2mm hex key for adjustment.

Quoting the Tech manual: “Below the left leg, you find the end-stroke governor.  This setting works only over the last 45 mm of the stroke.  Usually, we use this adjuster completely closed, but in case of very thin riders who can’t reach the end-stroke position, it is possible to have a better feeling unscrewing it.

Left-side damper rod showing top-out spring.  (Formula calls this assembly the pumping unit.)

Close-up view of End-Stroke Governor in damper rod

End-Stroke Governor shown with 2mm hex key for adjusting


The Tech forks have some advantages over the excellent Marzocchis that came on my OSSAs.  For one thing, they are still being manufactured!  The Techs are much easier to work on.  They have no propensity to shoot a geyser in your eye when bleeding the cartridges.  And, the bottom-out adjustment is nice – assuming I can get it to work.  Plus, a design using a single spring is very tolerant of fork tube height discrepancies during installation.  They are probably lighter, too.

Unfortunately, it's winter where I live so I won't get a chance to do any testing for a while.

Although I could attribute the poor bottoming performance to the very dirty (high viscosity) oil, it's possible the air spring is too stiff even though the left side had a lower oil volume than standard.

My suspension guru reminded me to leave the air volume large.  I kept the air gap at 144mm on the left side and increased it to 89mm on the right side. It's far easier to add fluid than to remove it.  While the fork is on the bike just by unscrewing the cap, I can use a syringe to add fluid.

Changing the fork oil level on the right side requires 0.66cc / millimeter change in level.  On the left side, it is 0.74cc / millimeter.

I'm a bit puzzled about the oil viscosity on the right side being more than that of the virgin 5-weight.  Usually, I think of oil shearing down (losing viscosity with use).  But I guess it's equally possible that contamination will increase the viscosity. 

I'm also considering polishing the OD of the fork spring so that it slides easier inside the fork tube.  This should help keep the fluid cleaner as well.  But I'm not sure if I'll want to use a softer spring and would prefer not to go to that much trouble.  So I did 50% of the job with 10% of the work by running the spring against a wire wheel.