Varios and Bosses and Shims, Oh My!

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Varios and Bosses and Shims, Oh My!

Post by Wheelman-111 » Mon Feb 15, 2010 1:18 am


I'm in the process of assembling Flash IV (see in Scooter builds), using a new Kymco ZX rear pulley, once adequate space was carved from the belt cavity. This transmission is designed to work with an 18mm wide belt due to the rear pulley sheave spacing. Stock length is described in the ZX trans thread, but consensus is that a longer 669mm length is better for the quick takeoffs for which this conversion is generally done. Be advised that everything here applies equally to the SE/05 blocks, which use the same stock belt size.

This process got me to thinking about the experimentation I did with various oddball Polini belts purchased at excruciating cost from Europe. My best success was with a 16.25mm wide x 654mm long 248.006 - a belt designed for the Derbi and Suzuki scooters only on the Continent. This longer-length belt rode very close to the edge of the stock SA/16 rear pulley. I could really appreciate the difference on take-off, but I could never extract as much top speed as I could get from the stock belt, probably due to a small-ish 90mm Polini variator. The reason was that I never quite dialed in the optimal shim dimensions I intend to discuss here. It would have helped to pull the engine for a thorough look-see in the comfort of the Boudoir.

Fortunately, the ZX 18x669x30 (30 is the angle of the side faces...) belt is both ubiquitous and cheap. I paid under $10 shipped on eBayfor mine, although I expect it may not last very long. However it will at least serve the purpose of measuring in anticipation of getting a decent one later. The length I measured around the outside of the belt was actually closer to 684mm, as evidently Belt Manufacturers use some sort of Theoretical belt thickness center to measure. It's simpler to order by the nominal size.

The ideal is to select a boss/shim length that gets the belt as close to the center of the front pulley as possible, while avoiding slippage. Longer drive Bosses and/or shims separate the movable face from the fixed one more, allowing the belt to drop all the way to the boss. Image
The extra length of the belt is then completely available to round out the rim of the driven pulley.

Careful here, though; If the belt isn't squeezed firmly with the movable face fully retracted, it will spin and slip on take-off until the variator "shifts" and puts pressure on the belt. This obviously won't give those quick take-offs you wanted, and will likely result in very rapid belt wear. Not to mention those burnt-rubber smells behind you. :sad: Furthermore, at full speed the variator "stroke" may not take the belt up very far on the Drive Face - since it started off with slack. That will mean that the belt won't go as far toward center of the rear pulley either. In other words, too much shimmage is slow too.

Shorter bosses result in the front pulley faces coming closer together.
However if you don't use enough boss length, the belt will be tightly gripped at idle but will come to rest some distance from center. This will prevent the belt from wrapping as close to the edge of the rear pulley as possible, and give that "second-gear start" feeling to your take-off. In addition, the full excursion of the variator will now try to force the belt up to the very rim of the fixed pulley sheave, where the conical profile flattens to the starter Bendix teeth. Not a good place for a rubber belt to live...

The key to remember is that a remarkably small change in shim/boss length will produce a big difference in belt position, due to Laws of Geometry, Circles and Cones and other Science I don't fully understand and never will. No matter, trial-and-error will substitute for actual Theoretical Skill.

So what can you do to adjust to different belts, or even compensate for belt wear? I got a 33.15mm stock SA/16 boss, the ZX boss that's a whopping 4mm bigger at 37mm. A trip to a couple of Big-Box hardware stores found Metric (14mm crank) washers and bushings in various thicknesses as shown.
I"ve even used a flattened-out spark plug washer, which can be crushed down to under 1mm. Some of these nominally "14mm" washers needed a bit of center-hole grinding to walk onto the crank, but I now have a variety of choices.

With the cast-off ugly Elite S variator it needed 2.45mm added to the SA/16 boss for the belt to approach center. Of course, every variator may produce a different effective "thickness" which will also change with the ramp plates, so it's best to check where the belt is with your finger. I Highly Recommend you stop the engine first, however. :wink:

As stated, the downgear effect of a longer belt with a larger rear pulley helps take-off a lot. However BOTH pulleys will need to be larger in diameter if the compromise between quick take-off and high top speed is to be avoided. Larger-than-stock (87.5mm) variators include the

Malossi (DIO) = 90mm
Keli = ?
OKO = ?
Polini for SE/SA = 90
Ruckus = 93mm
Polini for Ruckus = 94.66
Malossi-Ruckus = 97mm
Brand X For Ruckus = 97mm
Bear in mind that the stock outer sheave angles out at the 15-degree slope to match the 30-degree belt. However near the rim it flattens out, somewhat shy of the diameter of the larger variator faces here. This may prevent the belt from taking full advantage of the variator's extra "upgear" effect.
At some point, a larger or more conical fixed sheave would likely help, but of course it has to fit in the belt cover. Daytona makes such an outer pulley in Aluminum. Pricey though - around $50 last time I looked. (May 2011)

Before tackling this project beware belt-snagging narrows in the belt cavity, and also kickstarter castle fouling problems as the drive face fixed pulley moves too close to the cover. Thick gaskets, shimming the belt cover, or using a Kymco or SYM ZX cover assembly can address this second issue. And remember, Your Mileage May Vary.
Most of my money is spent on scooterparts. The rest is just wasted.
Still Seeking Red PCX-150
Flash 9: 2001 Elite SR Contesta 72 ZX Tran, 9:1 Gears, Stock Airbox, Carb and Pipe 58.8 MPH
Punkin: 2010 Vespa/Malossi S78, 99KPH


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