What's This 45 Mph Talk?

[Hal the Elder]

Hey Quenton:

The dealer I bought Oscar from is not a Whizzer dealer. He's a Used Car dealer who occasionally sells a Whizzer Motorbike as a sideline, and does not stock any Whizzer parts.

So what do I do now? You or Mike Simpson should know...

Thanks...
HAL

 

[Quenton Guenther]

Hi Hal, Contact Mike for the warranty exchange.

Have fun,

 

[WZ507]

How Fast Should My Whizzer Go?

Motor bikes and fishing have a lot in common. The stories never diminish in any detail with repetitious telling, but quite the opposite, the salient details tend to creep ever greater with repetitious telling. If we tell the stories often enough we actually start to believe them after awhile. And that is where I want to bring us all back to reality.

I'm talking about Whizzer speed here, and want to set the record straight regarding what a person might realistically expect. I feel this is important to air out so that any new Whizzer performance enthusiasts have realistic expectations of what sort of speeds well prepared Whizzers might achieve. I wouldn't want to see someone labor for years trying to figure out why his flathead bike won't go 65-70 mph, when no one else has either. I questioned people for years trying to figure out how fast a flathead Whizzer will actually go and the more I asked the more confused I got. Below are my opinions on the subject.

There have been reports in various threads here of flathead Whizzers going up to 70 mph on a dyno, and I have personally seen OHV Whizzers run up to 84 mph on a dyno, but in both cases this is way outside the performance envelope of what these bikes are capable of in terms of real road speed, and the dyno in these cases was simply a test fixture employing a large squirrel cage fan for resistance, that does not relate well to real road speed. I own a OHV Whizzer, and 60 mph on the real road is being pretty generous on the best of days.

No one has said anything untrue on these pages, and my intent is not to question what others have said, but rather to simply bring us back to the "real road" of reality. I think we've stretched the truth about as thin as it can be stretched without breaking. Perhaps the thing least appreciated by readers here is the extrapolation from "dyno speed" to "road speed". We tend to think of them as one and the same and they certainly are not. Dyno's vary from home grown devices to highly sophisticated machines that provide highly reproducible results. But even among high quality dynos reproducibility from one brand to the next can always be an issue. And certainly a quick pull on a dyno is not a good predictor of real road speed.

For those toiling in the Whizzer performance arena here are my opinions regarding performance benchmarks for well prepared flathead Whizzers running on level road, in calm air, with the rider sitting upright (26" bike, 7.5:1 gearing).

50 mph - you've done a great job and should be really proud of your project
55 mph - you've outperformed 90% of Whizzer performance enthusiasts
60 mph - you're in an elite group exceeding 99+% of Whizzer performance enthusiasts

In the same performance ballpark as mentioned above, the July 2008 Whizzer Newsletter reported ¼ mi speeds of 3 different OHV Whizzers that ran at the Four Ever Fours Antique Drags event in June 2008. The speeds? 55-62 mph.

The bottom line - you get your flathead Whizzer going 60 mph and you've really done something. Hope this helps.

 

[Hal the Elder]

HEY WZ507: (wish you had a name)

As a retired Aerospace engineer, I know something about hydrodynamics as applied to air resistance vs speed, and I know that horsepower requirements vary as the square of air resistance on a frontal surface area.

Thus, if a motorized vehicle is capable of a given limiting speed through the air, with air resistance being the limiting factor, then to double that speed it will require FOUR TIMES the horsepower.

To illustrate, if the limiting speed of a 3 HP motorbike is 40 MPH THROUGH THE AIR, NOT ON A DYNO, then to achieve double that speed (80 MPH), it would require 12 HP.

I'm personally not interested in any speed over 30 mph, but it would be nice to know that I had sufficient reserve power to aid in accelleration from a stop and to handle some grades in my High Desert area, NOT for any extra speed potential.

I get a bit nervous when I go past 30 MPH on my stock Whizzer, and feel more secure in the 20-25 MPH range!

Thanks for your technical dissertation, Mr. WZ507!

HAL

 

[WZ507]

Aerodynamic Drag Vs HP Requirements

Hal - Your comments about aerodynamic drag, as it relates to HP requirements, are not correct, and under estimate the HP requirements by a factor of 2. Your comments are correct for relating wind drag to changes in speed, but not HP.

The drag, not the HP, increases as the square of the speed, i.e., if the speed doubles, the drag increases by the square of the speed, or a factor of 4 (2^2 = 4). The HP requirements are also affected by speed, but in this case even more so, as HP = drag force x velocity, or restated, as speed increases the HP required is now the cube of the change in speed. So, if the speed doubles the HP required increases by a factor of 8 (2^3 = 8).

Let's make an assumption and apply this to some of the numbers I provided earlier. Assuming it takes 5 hp to go 50 mph, how many HP does it take to go 60 mph? The speed changed by a factor of 1.2 (60 mph/50 mph = 1.2). Cubing the speed change gives a factor of ~ 1.73 (1.2^3 = 1.73), and applying this factor of 1.73 to 5 hp gives ~ 8.6 HP (1.73 x 5 = 8.6).

Think about that for a moment. You've done lots of work on your engine and say you've increased the power from 2.5 HP to 5 HP (100% increase is a huge change!) and moved you from 40 mph to 50 mph. Now you want to go 60 mph which will require 8.6 HP. Where in the world are you gong to find an additional HP increase of 72% on top of the 100% you've already gained? That's why I said previously that 60 mph is a very challenging target.

 

[Hal the Elder]

You're Absolutely Right, Mr. Nameless WZ507!

I was confusing aerodynamic drag with horsepower requirements!

You are correct: Horsepower requirements increase as the CUBE of the speed increase ratio, therefore to double the speed from 40MPH supplied by a 3 HP engine to 80mph would require 2^3 (2 cubed), or 8 times the horsepower, or 24 HP, not 12 as I had erroneously stated earlier.

Doing the comparison on a Dynamometer, where wind resistance does not enter the picture at all, doubling the Dyno speed would require only a modest increase in horsepower, NOT eight times!


Thanks, John, or Bill, or Tom, or Bob, or Frank, or...?

Cheers...
HAL