Theory and Practice

[Wheelman-111]

Greetings:

In Theory, theory and practice should be the same. In Practice, they are not.

All this talk about 200 MPH Sprees got me doing Math. Ugh!

I actually went out and measured my 2.75x10 wheel and tire this morning. Just under 16" diameter, 48" and change circumference. Let's call it 50 to compensate for the tire's centrifugal expansion at the prodigious speeds we're about to discuss. I'll ignore tire slippage for now.

If the Axle spins at 1 RPM, Flash moves 50 inches. In an hour at that rate, he goes 3000 inches. that's 250 feet, or 0.04735 miles. This will be our Working Factor.

Now assume Flash's engine is capable of 10,000 RPM. The Service Manual says the Variator can "overdrive" the front Drive shaft of the tranny by 0.8:1 so at that engine speed the "D" shaft will spin at 12,500. Let's ignore fudge factors like clutch and belt slippage for now too.

Flash's stock gears are 12/34 Primary and 12/47 secondary, for a ratio of 11.097:1. I counted them. I also believe there is lots of mis-information on this Forum and elsewhere when ratios are discussed. The only way to be sure is to count the teeth and do the math.

Back to my tranny, which is abuzz with 12,500 RPM in and -let's see divide by 11.097, carry the 4 - I get an Axle speed of 1126 RPM.

Remember the Working Factor? 1126 times .04735 = 53 MPH and change.

Figure in those slip factors now, maybe 5-10% wasted. Call it 50 again.

You can plug in your own numbers to see what yours'll do...

[Wheelman-111]

Greetings:

Back in 1981, the fastest production motorcycle tested by one of the Glossy Bike Mags was the Kawasaki GPZ 1100. They got it going a radar-checked 144 MPH. Then they tried again but this time they removed the mirrors The result was 146!

This is not unusual. Especially at higher speeds, a little aerodynamic improvement can yield significant gains in speed. Y'see the reason things can't just go faster and faster - no matter what gearing is selected - is mainly the Wind. Horsepower can challenge the wind - never conquer it - because the drag it applies increases as a function of the square of velocity.

This means if you want to go twice as fast, you need not double your pony count but four times more horsepower. Modding a 7-HP 35 MPH scooter into a 14-HP fire-breather won't get you to 70, no matter what gears you're running. Now, I'm fairly certain that lots of Forum members have managed to extract substantially more than 14 HP from their stroked - big-bore - hot-piped - massive-carbed - ported 125cc Elites and Dios. I also believe some of them have actually achieved the elusive 70 or more in so doing.

But 100MPH? Maybe. But not for long... See Part III.

[Wheelman-111]

Greetings:

The first Honda S-2000 roadster reportedly produced a whopping 240 HP at something like 9000 RPM. This from a normally-aspirated (No Turbo or Supercharger...) liquid-cooled four-stroke. It sucked so bad for torque at the lower RPM ranges that Honda-san quickly increased its displacement to 2.4 liters to compensate. With the same output, that's about 100 HP Per Liter - aka Specific Output.

In the motorcycle world, the strongest per-liter outputs are found in the 600cc inline 4s - approaching 120 HP at the rear wheel on Dynamometer testing. This figure of 200 HP Per Liter of displacement appears to be the upper limit of (sustainable) performance in production today. In the 4-Stroke world, even that requires sneaky tricks like Sodium-filled exhaust valves.

There's always a But:

In this case, the Butt's name is Heat. I used to believe my shop class teacher when he said the engine derived its power from the explosion of the fuel/air mixture in the combustion chamber. I recently discovered that it is not so.

The power comes simply from the thermal expansion of the gases involved in internal combustion. Simply put, set something on fire and the heat excites the molecules. This causes any gaseous mixture to want to expand, pushing, not slamming the piston back down the cylinder. So Heat can be our friend. To a point...

If you want more power from a given engine size (and who doesn't? ), you can do several things:

1. Make it spin faster - bigger carb, freer-flowing exhaust supports more combustion cycles per second.
2. Make it spin harder - expansion chambers, resonance tuning of the intake size increase the mixture "preload" to put more molecules of fuel and air together just in time for the Main Event. Fooom!
3. Make it bigger: Bore kits, stroker cranks...

In any case, a byproduct of power production will be heat. If you do it hard enough and fast enough, sooner or later you need to get rid of the heat before reaching the melting point of the material you're using to make it go. Anyone know at what point Al turns into liquid?

Fan-forced air cooling works great in lawn mowers and weed-whackers, but keeping a highly-modded 2-Stroke engine at an output level of 200 HP/Liter without upgraded cooling is sure to lead to the dreaded Meltdown. This means a 125 might make 25 HP, but for how long?

Because water can absorb great quantities of heat (calories), liquid cooling is a more efficient way to whisk away unwanted heat from the combustion chamber. Despite all the expense and complexity associated with water pumps, radiators, seal problems, etc. all the highest performance hardware out there today is liquid-cooled for this reason.

[steffen707]

Is the overdrive .8 a function of the ratio between front variator pulley and rear clutch pulley?

[Wheelman-111]

Greetings:

Yes. According to Honda-san with the front sheaves fully compressed, the belt path radius is bigger in front than out back by that 0.8:1 factor. Think 10-speed bike.

[steffen707]

Greetings:

Yes. According to Honda-san with the front sheaves fully compressed, the belt path radius is bigger in front than out back by that 0.8:1 factor. Think 10-speed bike.

Alrighty, now I understand how its all calculated. Thanks wheelman.