M4 39mm Forks
I became interested in M4 forks because they are being used on the Mecatecno Dragonfly. (I later learned that the latest incarnation of the Dragonfly uses a mechanical spring fork. I'll leave this information because it applies to some EM models.)
The M4 is a gas fork that does not use a mechanical spring. This is partly what makes it so lightweight.
Unfortunately, there is very little information available about them. I was surprised to learn that EM used M4 forks for one year (2020) on the ePure Race and Sport. So, much of what I know about the M4 fork comes from Electric Motion.
M4 forks are manufactured by the Spanish company METRIC4. They have a website in Spanish and French. The site also has a short video showing the forks being manufactured as well as them being used on a GasGas. Sadly, I can't embed their video here because there is no direct URL. If you search METRIC4.COM and click on their “M4 Racing Parts” tab, the video will play.
Prior Art
There's nothing new about using gas pressure as a spring. In fact, all forks use air and oil volume as a spring to control bottoming behavior. The only difference is that the air is initially at atmospheric pressure.
A variety of KTM motocross and enduro bikes utilize gas forks manufactured by WP. This type of fork is fairly common on mountain bikes as well.
Cleaning
The 2020 EM manual says to clean the fork tubes after each use. No harsh chemicals or pressure washing is allowed. The tubes should be lubricated lightly with silicon-based lubricant after cleaning. It's important to keep the tubes clean as that would affect seal wear.
Advantages/Disadvantages
METRIC4 claims theirs are the lightest trials forks available, weighing a mere 3784 grams. For comparison, the Tech forks on my ePure Race weigh 4526 grams (of which 465 grams is the spring).
Another advantage is that, in theory, the fork may be tuned for a variety of different rider weights and abilities without purchasing different springs.
But that reduction in weight does not come without a cost - especially around maintenance time. The only place I could even find M4 parts online was via Electric Motion. Let's start with the fork oil, it is part number TC01O-20101-27-00 and costs $150 per bottle (not a typo). No viscosity is specified. Likewise, fork seals (TC01O-20101-17-00) are $129 each. They supposedly measure 52 x 39 x 7.5, but I suspect they are not standard items. And, if you happen to need a couple of O-rings for the internal cartridges, the retail price is $87 per leg.
On the plus side, M4 calls for the forks to be refreshed every 100 hours or 1 year, whichever comes first. This recommendation is under “amateur” usage and is a longer service interval than a typical wire-spring fork. (A lot of fluid contamination in a conventional fork occurs due to the wire spring rubbing against the inside of the fork tube.) However, for “sports” (perhaps they mean competition?) usage you only get 40 hours between services.
Speaking of costs, TrialsWorld.es has the M4 forks available for 1390 euros (which may include the 20% European VAT as I have sometimes seen the price show up as about 1149 euros). As of March 2023, this latter price converts to about $1222 (USD) which is half what EM wants for a set of M4 forks.
One of the other “costs” of an air fork has to do with increased stiction - especially if you are a heavier rider. This manifests itself as a reluctance to move for smaller bumps. There is a public Facebook post from a 2020 EM rider who weighs 230 pounds (105 kg). The M4 forks did not work for him and he replaced them with a set of 39mm Tech forks. He also complained about the impossibility of finding maintenance or repair instructions for the M4 units.
Setup
EM provides some setup guidance for various rider weights (dressed in full riding gear). I have reprinted their chart below and added Imperial units. Nitrogen is recommended, but air is acceptable.
Because the Dragonfly is ~15 kg lighter than a typical trials bike, the M4 fork may be suitable for a heavier rider than if it were used on an EM.
Left Fork Tube (Gas Spring)
M4 says that for each pressure adjustment, the fork should be totally free (maximal extension, no weight on the front wheel).
And, after each pressure adjustment, you must move the fork slowly for the first 10 mm of the stroke to equalize the internal pressure and then check again.
A standard Schrader valve is used to introduce the gas.
The adjacent photo slows a No Air Loss (NAL) chuck. I use it for setting the nitrogen pressure in shocks. It would be my preference for setting pressure in the M4 forks. However, it would be quite inconvenient to use outside the shop because the nitrogen bottle is large and heavy.
Portable Air Pump
Shown below is a portable air pump by Fox Racing. Many similar pumps exist. I don't know if such a pump accompanies the bike or fork itself.
Credit: Fox Racing. Pump model 027-00-007 is available on Amazon for $62.
Rebound Damping, Early Version
Apparently, there are two versions of the M4 gas fork. The initial version was produced in 2019 (and documented in the 2020 EM manual). It had a Schrader valve at the top of the right fork tube. The standard pressure is 0.5 bar (7 psi). You can adjust this pressure from 0.5 bar to 3.0 bar (45 psi). This will change the fork behavior as described below:
Lower pressure = Softer in compression and slower in rebound.
Greater pressure = Harder in compression and faster in rebound.
The standard setting for the rebound adjuster (bottom right leg) is 10 turns out.
Rebound Damping, Later Version
The later version of the M4 fork (and presumable what is coming on the Dragonfly) has a rebound adjuster at the top of the right fork tube. No gas pressure is used on this side. The rebound adjuster requires a 3mm hex key. Faster rebound is counterclockwise (CCW), slower is clockwise (CW).
Although the fork cartridge is statically pressurized at 6 to 10 bar, the fork tubes themselves are not pressurized above atmospheric. Hence, an O-ringed screw is provided to release excess pressure. A 2.5mm hex key is required. The manual says to do this every 30 hours. It might be prudent to do so for a large change in altitude or ambient temperature as well.
Fork Oil Volume
I am not 100% certain about any of the oil volumes. Will update this information if I learn more. Measure everything upon disassembly!
Left fork tube: 40cc (early and late).
Right fork tube: 325 - 375cc (standard is 350cc). However, the early version likely only used 60cc.
Internal cartridge: 225cc ? (early, and late?). This maintenance is to be performed only by authorized agents.