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 Post subject: Aftermarket Gears
PostPosted: Sun Nov 08, 2009 2:45 pm 
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Joined: Wed Oct 22, 2008 4:09 pm
Posts: 10865
Location: Deepinnaharta, Texas

A Forum member requested a primer on Gearing Up. There are probably a lot of better-qualified Forumistas than myself, but I said I'd give it a shot. Here goes:

Aside from their many decimal places, gears aren't all that mysterious. For comparison, you can start with the earliest form of reciprocating self-propulsion, the steam engine. Great clanking cylinders were attached directly to the drive wheels by means of an external connecting rod. One "RPM" at the piston/cylinder = one spin of the drive wheel, which effectively was the crankshaft for the steamie. Works pretty well when the engine's maximum torque occurs at Zero RPM.

Early examples of Internal Combustion featured atmospheric intake valves and massive crankshafts that didn't chug/rev much higher than the steam engiines they would eventually replace. Early scooters from Indian and Harley-Davidson ran leather belt drives directly from a small pulley at the crankshaft to a very large one near the rim of the drive wheel. In order for the modest engine output to be useful, engineers knew that it would take several power "strokes" from the engine for each turn of that drive wheel. The different-sized pulley wheels achieved that goal with a minimum of machine development costs and utilized readily available Rawhide.

Forward progress meant developing engines that produced their best performance higher and higher up the RPM scale. Turning that high-revving power into a usable form eventually required a method to de-multiply those high RPMs to an axle speed that the vehicle could use. Clockmakers had long before solved the problems of altering rates of rotation. Thus the reduction gearbox was born.

I've always found it remarkable how well a production vehicle is sorted out from its very first year on the market. Engineers have to consider multiple factors - including the vehicle's power output and intended RPM, Aerodynamic drag characteristics, effective drivetrain frictional losses, overall size and load capacity among others - when deciding what gear ratio to install. I just find it amazing how close to perfect they always seem to be. Of course, decades of past experience and observation is a bit like cheating...

Anyhow, this carefully-chosen ratio no longer makes sense once the end user modifies some critical characteristic. A Big Bore Kit, for example, can more than double the stock power output. Now the stock gearing fails to give the more-powerful engine sufficient "legs". Once the BBK-equipped engine accelerates blazingly fast through its powerband, it reaches an RPM dictated by its internal characteristics to be maximal. However at that groundspeed - dictated by the vehicle's gearing - there's not nearly enough aerodynamic drag developed to keep the engine occupied. Single-speed devices like the Spree and Elite "E" must make the biggest compromises between acceptable rates of acceleration and top speed. A shift into second, third, or more is indicated. In our scooters' case, enter the Variator.

Actually this thing just confuses the heck out of a lot of people. If we just assume its role is to reduce the pain associated with getting the scooter up to near top speed, then ignore it altogether, it clarifies the role of the Final Drive gear selection.

This is because once the variator has "shifted" to its maximal contribution to overall gearing - which Honda says is a factor of 0.8:1 - then it plays no further role in the power-Top Speed dance. I will now invoke THE RULE:

The more power you have, the lower number you can use.

Stock AF16E is 12.1:1
Stock AF05E is 11.097:1
Stock bore/power can handle 10:1 easily, raise top speed potential by a sixth, or about 7 MPH

Stock bore/power can handle 9.1:1 with a little more noticeable decline in acceleration, raise top speed by about 25% or about 9 MPH.
A tuned BBK running this ratio will be RPM-limited on top speed; (not drag-limited), and be at risk for over-rev conditions any time a long downwind/downhill run is encountered.

BBK or stock bores with substantial exhaust/intake help can run 8.4:1 secondaries. A BBK can power throught the acceleration issue, but a stocker will pay a noticeable price in pickup.

I ran a BBK AF05 with 7.83:1 primaries. The top speed potential was pretty high, but at 65cc it was infrequently able to exploit it without a stout tailwind or a good downhill to get things going. Once underway, speeds approaching 60 were attainable.

From reading alone, there are available ratios well below these, with the anticipated penalties in acceleration. Only well-tuned big-displacement monsters need apply.

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, 58MPH


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