M'4 Spring Forks

The Dragonfly's forks are manufactured by Metric 4 in Italy. The name M4 Racing Parts is also used in the maintenance manual. (I recently noticed the apostrophe in their name, and henceforth I will write it that way.)

Forks allow a lot of room for creativity among manufacturers. This fork has some idiosyncrasies. For example, a pin spanner is required to remove the fork caps and also to disassemble the forks for seal replacement. Based on the Hell Team's excellent video, a Park Tool SPA-2 (red) pin spanner will work for both tasks.

Surprisingly, the M'4 forks are about 10mm longer (axle center to top of fork cap) than Tech's aluminum forks. Because the Dragonfly so diminutive, I expected the M'4 forks would be shorter, if anything.

Only the left side fork tube contains a spring (M'4 part number 402A30275). No optional spring rates are mentioned in the user manual.

For reference, the M'4 air-spring fork is presented in detail under the EM Race's Mechanical section heading on this website.

Left (spring) side, preload adjustment

Right (damping) side, compression adjustment

Fork Service Info

Two different fluid viscosities are specified in the service manual.

The majority of the fork fluid is specified as an SAE 5-weight. The M'4 part number for this is 402A0500.

Additionally, a small amount (~30 ml) of SAE 20-weight fluid is introduced separately on the damping (right) side via the bottom. No part number was given for this fluid.

Fork seals appear to be part number 402A5239. (I have heard that Tech fork seals fit, but have not verified this.)

Fork bushings appear to be part number 402A4339.

Right (damping) side, rebound adjustment

Measuring Spring Rate

My fork spring measuring apparatus is shown below.

It comprises a bathroom scale and a drill press. The drill press is capable of moving 4.75 inches (120mm) so I can only measure the spring's initial rate.

However, the results validated my calculations.

Setup to measure a fork spring's rate

Spring Discussion

Mecatecno says the M'4 forks have 170mm of travel. With the forks still mounted to the bike and the spring removed, I could only achieve 158.5mm of travel. I used 92mm of fork travel while doing a conservative test ride. I'm certain my wife used even less travel. Compression and preload adjusters were at minimum for the test.

So, as received, the forks are extremely stiff. It's no wonder they work for the US importer, who is a big guy weighing 220 pounds (100 kg). For riders who weigh the same (or less) than the Dragonfly itself, the forks can be improved.

I had heard that Tech springs will fit in the M'4 forks. They will, but won't help a lightweight rider. I was quite surprised by my spring-rate calculations and validation measurements.

For reference, the M'4 spring is progressively wound and has an OD of 1.225 inches, a wire diameter of 0.178 inches, and a total of 41.50 turns. Sorry for the US customary units, but that's how my spreadsheet is setup. This gives an initial rate of 31.4 lbf/inch (5.6N/mm) and a final rate of 40 lbf/inch (7.1 N/mm).

By “initial rate” I mean the rate before any coils go into coilbind. Progressive springs provide at least two different spring rates. As the coils that are are wound close together touch each other, they are no longer a spring but form a solid spacer instead. This allows the remaining coils (spaced wider apart) to provide a higher spring rate because there are now fewer active coils. Remember, with other variables held constant, the fewer the turns a spring has, the higher its rate.

This is exactly the behavior we want in a trials fork. The soft initial rate handles small bumps and keeps the fork action compliant for optimal control. But big obstacles create large impact forces and a higher spring rate is necessary to deal with them.

Although my testing apparatus does not have enough travel to measure it, the final spring rate rate calculates to 40 lbf/inch (7.1N/mm).

Since Tech springs have an initial calculated rate of about 42 lbf/inch (7.5 N/mm) they would not help a lightweight rider.

As an aside, I have been provided the following information on Tech fork springs that I'll leave here for reference:

Tech No. 090473902 (Prog. 1) 7.2 - 8.8 N/mm

Tech No. 090473903 (Prog. 2) 7.2 - 10.2 N/mm

Tech No. 090474151 (Prog. 3) 8.0 - 12.7 N/mm

Upper spring, Tech “standard” used in EM ePure Race. Lower spring, M'4 used in Mecatecno Dragonfly.

Air Spring

So I turned my attention to the air spring. All forks possess an inherent air spring (by virtue of the trapped volume of air being compressed inside the fork). This air spring acts in parallel with (thus adding to) the wire spring's force. The air spring's rate is tunable via changes in oil volume. More oil yields a stiffer spring. But it's highly non-linear and a small volume change can produce a large change in force near the fork's maximum travel in compression.

The M'4 manual says the spring-side fork tube contains 325 ml of SAE 5-weight. I never go by volume. I use level instead. But M'4 does not specify a level measurement, so I needed to get a baseline reading.

Unfortunately, I spilled some oil while making the measurement, so am not 100% certain about the number. But I think the factory level was about 85mm. (Later determined from the M'4 manual to be 75mm.)

I decreased the level (to soften the spring effect) to an educated guess of 165mm. It's easier to add more oil if the spring is too soft than it is to remove oil if it's too stiff. This measurement is made from the oil inside to the top of the tube (with the fork tube fully collapsed). Also, remember that the compression damping is adjustable on the M'4 fork so that gives another tuning variable.

Even after decreasing the fluid level again to 194mm, the air spring was still too stiff. I'm now trying a fluid level of 250mm.

Kinematic Viscosity Comparison

M'4 specifies a 5-weight fluid. Since there is considerable variation from one manufacturer to another regarding what exactly constitutes a 5-weigh oil, I prefer to go by kinematic viscosity.

But when I don't know the kinematic viscosity of the original fork fluid, I must make a relative comparative with a know fluid. This comparison is only valid if both oils are at the same temperature.

The bottom line is that the fluid I got out of the M'4 right fork was identical to Maxima's 85-150 (5-weight).

I have not looked at the kinematic viscosity of the fluid in the left side, but the M'4 manual indicates it is also 5-weight.

Note that a small amount of 20-weight fluid is additionally specified for the left side, but is filled via the bottom (see manual for details).

Timing flow to judge relative viscosity

Compression Damping Surprise

I concentrated on the air spring in the left fork tube but eventually wondered if the right side had an air spring that could be adjusted as well. The short answer is no. The spring function (both air and mechanical) is strictly on the left side, and the damping function is strictly on the right side - a nice separation of duties.

Although the M'4 service manual leaves much to be desired, it eventually dawned on me that the oil level is specified (but in an unfamiliar manner). When describing the fill volume, they have parenthetical remarks specifying “75mm chamber” for the left side and “140mm chamber” for the right side. As a quick review, oil level is measured in millimeters with the spring removed and the fork tube completely collapsed. The distance is measured from the top of the fork to the liquid level inside.

Now here's the surprising part, the oil level measured 142.5mm with the fork tube fully extended. The adjacent photo shows where this oil level would be relative to the compression valving assembly.

I wondered how this could possibly work since the chamber above the piston would fill with oil and hydraulic lock before reaching the specified 170mm travel.

As it turns out, the right-side oil level is a constant 142.5mm through the entire range of the fork's travel. There's no air spring on the right side at all.

Compression valving assembly with a 142.5mm “air gap” shown for reference

Close-up of M'4 compression piston with needle valve for adjustment

Note that there's a steel shim both above and below the compression valving piston. These shims cause fluid to flow slowly through the valve when the forks are compressed, yet bend to provide little resistance to flow when the forks are extending (due to the availability of much bigger holes in the piston during extension).

The adjustment clicker at the top of the fork opens and closes the needle valve thereby changing the resistance to fluid flow.

With the the clicker fully clockwise, the flow is maximally impeded, thus providing the slowest fork movement during compression.

In Summary

If you don't ride like Oriol Noguera, the fork may be too stiff for you.

I use a zip-tie around one fork tube to measure the maximum travel while riding. Despite the 170mm specification for fork travel, there appears to be only ~160mm available. If you are a lightweight rider and feel the fork is too stiff on compression, remove oil from the spring side.

The ID of fork tube is 32mm. This yields an internal volume of 0.8 cubic millimeters per millimeter of level change. This is equivalent to 1.25mm of level change per per milliliter for fluid change.

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More Suspension Tuning

The adjacent box is an embedded link to my other trials-related website (which is about EFI OSSAs). You will find information that's specific to the tuning of 40mm Marzocchi forks, but there is general information there as well.

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