Clutch / Gearbox

Removal of the clutch cover reveals a cassette-style gearbox system.  

This has the hallmarks of being an XiU-rdi Engineering design.  Notice the bright blue anodizing along with the stylized “X” and part number in silkscreened white paint.  The fact that the gearbox assembly is held in place using shoulder screws is also telling.  That type of screw was used on the EFI OSSAs, which had a gearbox designed by Josep “Xiu” Serra.  

The clutch itself has 3 friction plates and 10 clutch release arms (also known as fingers).  It is very similar to the GasGas Pro and Mecatecno Dragonfly designs.

The primary drive has 33 teeth on the pinion gear.  A standard clutch basket has 75 teeth, which yields a 2.272:1 ratio.  This makes for a faster-turning clutch basket with lesser torque multiplication than an internal combustion trials bike. 

The secondary drive ratio is 4.1:1  (10-tooth front sprocket, 41-tooth rear sprocket). 

The gearbox shift pattern is the typical 1 down, 3 up with neutral between 1st and 2nd.

Gearbox ratios were estimated from the ground speed achieved in each gear and other known factors.  

1st gear 3.52:1 (overall ratio: 32.86:1)

2nd gear 2.57:1 (overall ratio: 23.96:1)

3rd gear 1.61:1 (overall ratio: 14.91:1)

4th gear 1.17:1 (overall ratio: 10.87:1)

Regardless of whether you feel a gearbox is necessary with an electric motor, two important benefits become apparent: Selecting 1st gear will help keep the bike from rolling away when parking on an incline.  Selecting neutral will make the bike much easier to push.

The next photo is a drawing from the 2025 parts book.  

From counting gear teeth visible in the drawing, we know one ratio for certain: 14T drives 36T for 2.571:1.  Generally, the gear pair nearest that end of the transmission is 2nd gear.  This knowledge was used to help estimate the other ratio.

We can also count teeth on the next-in-line driving gear, at 18.

The gearbox is only available as a complete assembly with clutch and primary gear.  It is part number TC03R-50801-00-00.  For comparison, in 2013 a complete OSSA transmission was priced at $1125 USD, and it lacked anything outboard of the clutch basket (discs, pressure plate, and so on). 

Clutch & Gearbox Fluids

The FACTOR-e User Manual is available under the Downloads section of the EM France website.  Its filename indicates version 1.0, which suggests there may be revisions in the future.  The MOTUL logo appears on all the pages, so it comes as no surprise that is the recommended brand of lubrication products. 

The manual is a bit confusing, so I will give my interpretation.   The clutch and gearbox fluids are separate and have different requirements (this is smart in my opinion).  Both clutch and gearbox require an initial change after 10 hours of use.   Both require 200 ml of fluid. 

The recommended fluid for the clutch is MOTUL ATF VI, changed every 20 hours.  According to Motul's datasheet, this is equivalent to a Dexron VI automatic transmission fluid.  Ignore the part about it being 75W.  

It is a common misconception that “W” stands for “weight” when referring to lubricating oils - it does not.  “W” refers to the  “winter” or low-temperature viscosity rating.   Also, gear oils and motor oils are classified differently.  A 75W gear oil has about the same kinematic viscosity as a 15W motor oil. 

The recommended fluid for the gearbox is MOTUL Transoil Expert 10W-40, changed every 60 hours.  To me, the important point here is to use an oil optimized for a gearbox.

Gearbox Revealed

Based on Xiu's involvement and how the bike shifts, I have suspected EM's 4-speed gearbox may utilize only 3 gear-pairs, following the idea of GasGas's patented Four/Six System.  The photograph below seems to support this idea. 

Utilizing the estimated gear ratios from the section above, it would seem that either 1st gear or 4th gear is being synthesized. 

If synthesizing 1st gear, power would flow though 2nd gear to the sprocket shaft, then back through 3rd gear to the clutch shaft and finally back through 4th gear to the sprocket shaft.  2.57 * 1.61 / 1.17 = 3.53

If synthesizing 4th gear, power would flow though 2nd gear to the sprocket shaft, then back through 1st gear to the clutch shaft and finally back through 3rd gear to the sprocket shaft.  2.57 * 1.61 / 3.52 = 1.17

We will have to wait until some diligent researcher actually has a gearbox in hand to  know for sure.

In either case, three gear pairs must be involved to maintain the correct direction of rotation.  Every pair of gears produces a change in direction of rotation.  In a typical gearbox, the primary drive reverses the crankshaft's rotation, and a single pair of gears in the gearbox reverses it again.  This “double reversing” keeps the sprocket shaft rotating in the same direction as the crankshaft.  

With GasGas's patented system there is a “quadruple reversing” which keeps the sprocket shaft rotating in the same direction as the crankshaft.  Of course, there is an additional power loss in all these gears.  Each meshing of gears typically results in a  loss on the order of 2%.

Four/Six System Patent and Demonstration

Below is the original Spanish patent (number 2160528) assigned to GasGas Motos in 2002.

But, frankly, a video is worth 10 000 patent words.  YouTuber gravityisnotmyfriend 3D-printed a working model.  In under 4 minutes, you can gain a very good understanding of its operation.

An Apology

When I first learned of GasGas's patented four/six system I ridiculed it.  The gear ratios were published as: 2.966, 2.572, 2.187, 2.112, 1.125 and 0.081.  Notice that there is very little difference between 3rd and 4th (about 3.5%).  Many times I mentioned this publicly, saying that that a 3.5% difference was not useful in trials and I'd rather have a conventional 5-speed without all the monkey motion.

No one ever disputed it.  No one ever said, “That's not how it feels.” 

Only recently did I figure out that 4th gear is really 1.878:1, rather than the 2.112:1 that has been perpetuated in the documentation from the introduction of the Pro model until at least 2023.

That actually makes 4th gear useful, as I suppose anyone who owned the bike understood.

All these years I have been disrespecting Xiu because no technical person ever proofread the manual.

I'm sorry.