M4 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.
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, it appears a Park Tool SPA-2 (red) pin spanner will work for both tasks.
Surprisingly, the M4 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 M4 forks would be shorter, if anything.
Only the left side fork tube contains a spring, M4 part number 402A30275. No optional spring rates are mentioned in the user manual.
M4's 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 M4's service manual.
An SAE 5-weight fluid is used on the spring side. M4's part number is 402A0500.
An SAE 20-weight fluid is used on the damper side. No part number was apparent.
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 uses 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 M4 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 fork is 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 M4 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 M4 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 M4's 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, M4 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 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 M4 manual says the spring-side fork tube contains 325 ml of SAE 5-weight. I never go by volume. I use level instead. But M4 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.
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 M4 fork so that gives another tuning variable.
Kinematic Viscosity Comparison
M4 specifies a 5-weight fluid for the spring side. 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 M4 fork was identical to Maxima's 85-150 (5-weight).
But in retrospect I don't think it's all that important. I don't think there there are any damping circuits on the spring side, and the oil is just there for lubrication. Probably any 5-weight would be just fine.
However, the damping side (where 20-weigh is specified) would be far more import to consistent operation.
Timing flow to judge relative viscosity
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.