Gear Reduction

Motor speed reduction is accomplished via primary gears in which 27 teeth drive 75 teeth for a 2.777:1 reduction at the countershaft sprocket.

The secondary utilizes a DID 428 NZ gold chain with 128 links in which 11 teeth drive 57 teeth for a 5.182:1 speed reduction.

This yields an overall reduction of 14.39:1.  The rear tire has a measured circumference of 82" (when weighted rolling on the ground).  This results in a speed of about 5.39 mph per 1000 motor rpm.  So the claimed top speed of 43.5 mph would require a motor speed slightly in excess of 8000 rpm (which seems surprising based on the motor's performance graphs ending at 6000 rpm).  I would be really impressed if the SiliXcon is performing “field weakening” to get the extra rpm.

2024 Gear Reduction

The 2024 Race and Comp have different reduction ratios than earlier models.  The primary is now 25 teeth driving 75 teeth for a 3.00:1 ratio.   EM's literature says, The transmission case on the 2024 Epure Race is lighter than in years past, but still plenty strong.”  Was this new case design needed to accommodate the smaller primary gear?  Or is there enough range in the eccentric adjuster mechanism to accommodate it?  If so, it would seem the 25T gear could be retrofitted to earlier models.  But I am unsure of the benefit.

The change in the primary reduction ratio means the clutch basket will be turning about 8% slower (25 / 27 = 0.926) than in earlier models.  As a concrete example, say the motor has a maximum rpm of 8200.  With the old 27T primary gear, the clutch basket would be spinning at 2952 rpm.  With the new 25T primary gear, the clutch basket would be spinning at 2733 rpm.  This slower-turning clutch implies 8% greater torque multiplication, making life more difficult for the clutch plates.

Additionally, the clutch basket itself is also a flywheel of sorts because it has inertia.  The energy stored in any rotating system is a function of the square of its rotation speed.  This means the slower-turning clutch basket will store less energy than earlier models.  

The secondary ratio is different too, at 10 teeth driving 53 teeth for a 5.30:1 ratio.   Quoting EM, “[ This ] supplies a smoother chain effect for better behavior of the rear shock, resulting in improved traction.  The smaller rear sprocket also provides better clearance from rocks and other hazards than its larger predecessors.

Thus the overall ratio gear reduction changes to 15.9:1 from 14.39:1 for about 10.5 percent shorter gearing.  I feel this is a step in the right direction too.

2024 EM Comp with 10T front sprocket.  Note circlip is different from models that use an 11T sprocket.  If EM sells this circlip as a spare part, I have not found a number for it.  McMaster Carr calls this style of circlip “speared ends external retaining ring.Their model number 95524A115 has an ID of 23.11mm ID and is 1.57 thick.  Although it provides some much-needed clearance, it looks like it could easily fly across the workshop during removal - or worse, across an open field.

Ten Percent Shorter Gearing

The 2024 Comp's 10-tooth sprocket is part number OP422-25428-10-00.  This provides 10 percent shorter gearing when replacing the 11T front sprocket fitted to the earlier models.  This is equivalent to using 63T on the rear (and the stock rear sprocket is already excessively large).

Ten percent shorter gearing yields 10% more wheel torque and 10% slower ground speed.  This seems to be an improvement for me, but there is a complication with the snap ring.

The 10T sprocket has very little metal between the splines and teeth.  I attempted to use the OE snap ring for the 11T sprocket, but it contacted the chain.  This was concerning, so I performed a bit of surgery by grinding away excess metal near the pin holes.

Ultimately, I found a snap ring in my collection that was preferable.  See below.

After a bit more testing, I feel there is more acoustic noise generated because the motor is spinning 10% faster.  It also seems I am using more watt-hours/mile (resulting in decreased range).  However, this is difficult to accurately access because I would have to ride the same lines, conditions, and speeds with each sprocket to compare. 

Snap Rings: Left modified original EM.  Center and right from my spares collection.

Countershaft Snap Rings

Left: EM original, modified by grinding near pin holes.  ID 23.25mm, 1.22mm thick.

Center: Unknown, but likely from a Japanese OEM.  Modified by squashing in a vise.  ID 23.0mm, 1.3mm thick.

Right: Same as center, but before squashing.  ID 23.95mm, 1.3mm thick.

Front Brake

I like the front brake!  It's more powerful than any of my other trials bikes.  The caliper is a Braktec Monobloc with a large pad surface area.  Unfortunately, my favorite pad supplier, EBC, does not make anything that size so I had to look elsewhere for spares.  Galfer's FD511 G1805 “red” compound is probably OE.  Their G1395 compound is the more aggressive sintered compound.  Chris Pearson at SplatShop recommends the “red” compound saying that the more aggressive sintered compound would only be better if one were riding in very mountainous terrain and it certainly would be harder on the rotor.