Mechanicals

Gear Reduction Ratios

Motor speed reduction is accomplished via a primary toothed belt where 20 teeth drive 44 teeth for a 2.20:1 speed reduction.

The secondary is a 428 chain with 128 links where 9 teeth drive 57 teeth for a 6.33:1 speed reduction.

This yields an overall reduction of 13.93:1. The rear tire has a measured circumference of 82” (when weighted rolling on the ground). This results in a speed of about 5.58 mph per 1000 motor rpm. So the claimed top speed of 34 mph would require a motor speed of 6000 rpm.

Primary Pulleys and Belt

The 5.7's motor output shaft is different than what Golden Motor sells to the general public. Golden's specifications say the output-side shaft is 7/8" (22.2mm) in diameter and has a 5mm wide keyway.

The motor on the bike has a splined shaft, 6 x 16 x 20 (6 keys, 16mm ID, 20mm OD). This is also called 20 x 16 x 6T.

While attempting to disassemble the motor for a peek inside, I was unable to remove the drive pulley and wondered how much one would cost if I destroyed it. Unfortunately, the US importer's parts website does even list the 2015 pulley (or the belt for that matter). They do show the 44T driven pulley (P/N OPL10-0844-16-300) being available at a cost of $155.99. But it's not a stocked part.

Parts are listed for the 2019 EM Sport. That drive pulley is $83. A belt is $153, and the driven pulley (appearing to have an integral shaft) is $195.

The belt on the bike says Gates GTX 480 8MX. Its measures 27mm in width. But I could not find any belts that width. The pulleys are 30mm wide. The closest available belt I found is 480-8MX-30 (480mm length, 8MX pitch, 30mm wide). This is Gates product number 9327-50040. Because the belt is in fine condition, I did not pursue it any further.

Front Fork Oil Change

Prior to adapting the Marzocchi forks for use on the 5.7, I tried to improve the Olle forks.

While searching for a fork oil specification, I found an excellent manual for the EM 5.7 on Manualslib.com. It is vastly superior to what's available on the US EM website. (It even gives detailed disassembly instructions for the superior Tech cartridge forks that came on the EM 5.7 Sport.)

The manual says to use 310cc of “OJ 01” fluid in each leg. A bit of searching revealed that this is Formula brand oil that has a kinematic viscosity of 15 cSt at 40 degrees C. The EM manual also said the air gap should be 260mm, which was obviously a typo as I measured the air gap prior to disassembly at 157mm. I got roughly 300cc of filthy black fluid out of each side.

Progressive Spring Rate Calculation

To calculate the spring rate, I treated each of the five progressive coils as an independent spring by assuming it has 1 active coil. So, for my spreadsheet, I say each of these springs has 2.5 turns. The spreadsheet then subtracts 1.5 from the total number of turns to get the number of active turns (1 in this case). This results in some very stiff single-turn springs - on the order of 700 to 900 pounds per inch (which is as it should be).

Then I used the formula for springs in series (reciprocal of the sum of the reciprocals) to calculate a combined rate. This results in a slightly weaker overall spring rate (say 16 pounds per inch) than the majority of the spring. This makes sense to me as using more turns makes springs softer.

These springs are really soft compared to our other trials bikes (none of which are progressive). Personally, I would opt for a single-rate spring and control the bottoming behavior with a change in fluid volume. So far, all of the changes I'm made have improved the bike for both Cindy and myself. However, the forks may be a different story. Cindy does like my Montesa 4RT's front end, so maybe I'll head in that direction.

Motor Cover Repair

The motor cover, P/N TL01J-70204-00-00, was not available in the US. Although I was able to purchase one out of Europe, it's not ruggedly built, and I felt a repair of the damaged original would be worthwhile.

My first thought was to hammer a new cover out of soft aluminum but the more I looked at that, the more it seemed like an excessive amount of work for the potential gain.

A better way would be to rebuild/reinforce the original cover with fiberglass. (Interestingly, fiberglass has much better impact strength than carbon fiber.)

There is a wealth of information about working with fiberglass on the internet. Others are vastly more qualified to offer instruction, so I'll just outline what we did. What was left of the plastic pieces was joined back together with Super Glue and baking soda. After a lot of research, I ended up just buying a 3M Bondo 420 kit from a home improvement store. Cindy and I worked together on the project because she is really good with fabrics.

We did everything by weight using a gram scale. A low percentage of resin-to-cloth yields better mechanical properties. Starting with 60% fiberglass / 40% resin by weight results in less than 40% resin after application. But working with such small batches (there was a lot of waste), we found that was insufficient resin. The recommended hardener-to-resin ratio is 0.75% to 2% by weight. We shot for the middle of the range.

We used 3 layers of cloth on the outside and 2 layers on the inside. Strand orientation for the layers was perpendicular. The inner layer of the 3-layer sandwich is at 45 degrees.