Monday, September 05, 2005

Apples and Oranges

Someone asked what kind of mileage my bicycle gets going back and forth to work. I said I’d never considered it, though I noted that the bike seems to run well on beer and Italian food. But the question kind of rattled around in my head for a while.

I like poking around on Usenet. You never know what you might find, so yesterday I was pleasantly surprised to stumble over this. Please note that MS Word has graciously cleaned up the spelling and grammar:

I am a fourteen-year-old freshman who just started my school year. I was fortunate enough to get into my dream elective, intro to industrial technology. This is the first time I have had access to a full metal shop and I am considering building a small jet engine. I only want enough to get up to 30 mph on a bicycle. How many pounds of thrust should it have and what kind of design would be best for a beginner? Right now I'm thinking a pressure jet would be the best. Pulsejets have moving parts that wear down to fast, ram and scram jets have to be moving really fast to even start, and turbo jets have complicated oil and cooling systems. Pressure jets sound like the best of each design and simple for a beginner to build.

Bret Cahill replied, after doing the math:

The thrust should be the same as the force delivered to the road from your tires when you are pedaling 30 mph. You can get an idea of that several ways…. power increases with velocity cubed. For example, if you can only pedal 15 mph then the power required to go 30 mph is 8 times more than what you can generate.
…This is the thrust necessary to go 30mph.

For comparison Armstrong goes just about 28 at 0.8 hp or 440 ft'lb/sec. Divide power by velocity to get thrust. 440/(28 X 1.47) = 10 lbs.

The reason you see very little jet engine propulsion of vehicles and boats is because air and gases are very light. The only way to get enough thrust is to:

1. Move a lot of air, which requires a propeller, which requires a shaft drive. If you have a drive you might as well run the wheels or a water prop.


2. Move the air fast, which uses a lot of energy. Kinetic energy is 1/2 mv^2 and yet vehicle propulsion only increases with velocity. The vehicle moves forward slowly while the jet exhaust mostly gets wasted as heat in turbulent friction, stirring up air for no reason at all.

Why a jet engine? Well, why not? I can think of some practical reasons to avoid a jet. They’re extremely noisy, for one, and they have some undesirable throttle characteristics, for another. A jet is so noisy it would be impossible to hear other traffic in the vicinity. Throttle lag is a big problem too. When you increase or decrease power, the turbine lags behind for a short time. This is not conducive to safe operation on a city street.

Still, there’s a guy in New Zealand who experiments with jet-powered vehicles - His neighbors are not fond of the noise, to put it mildly.

A more practical approach would install a small piston engine into a bicycle frame, gearing it to provide assistance at higher speeds. Before you accuse me of re-inventing the moped, consider that a moped engine works at lower speeds to accelerate the rider rather than maintain a higher top speed, and I’m thinking a practical top speed would be in the 35-45 mph range. This could be a good choice for a commuting bike. The rider would pedal to accelerate up to about 15mph, then the motor would assist in going faster.

For that matter, such a machine would be similar to a motor-pacing derney.

Once upon a time, Honda produced a 50cc motorcycle, a ‘tiddler’ in Britspeak. If I recall right, these bikes were capable of 70-80mph, and in fact, similar bikes are still available for racing and street use. Honda still lists the Dream 50R and the NSR50, though they’re strictly racing bikes in the US. The Dream 50R is way retro. I love it!

The reason I’ve brought this up is fuel prices. Mopeds and tiddlers deliver tremendous gas mileage. But one of my co-workers asked an intriguing question when I told him about running my bike on beer and Italian food. How can you compare fuel costs between a motor vehicle and a cyclist? Bicycle riders derive power from carbohydrates – calories – and it should be possible to calculate the ‘fuel’ consumption on an ordinary commute with the gasoline consumption of a hypothetical ‘average’ motor vehicle.

If I recall right, given my weight and average speed, a 10-mile commute burns about 150 calories. That’s one 12-ounce glass of beer. Rolling Rock longnecks cost about $0.67 each when I get a pack of 18, so my daily commute ‘burns’ less than $1.40.

By contrast, my Ford gets about 24mpg, and at today’s gasoline prices, the commute costs $2.58 in fuel alone. The difference is almost enough to let me have a third beer! But the ‘savings’ is wiped out if I have Guinness rather than Rolling Rock.


Blogger hereNT said...

The roadies @ bikeforums were just debating this HERE.

Personally, I'd stick with the Guiness - lots more carbs.

I was looking for a link to a fixed gear motor-pacing track bike that some company made with an electric motor, but can't find it. It was really cool how they had the power input adjusted for speed and cadence... Oh well.

12:51 PM  
Blogger Mikhail Capone said...

You should do a search on for electric bikes.. There are tons, many of them you can pedal at the same time as the electric motor is in action.

4:15 PM  
Blogger Mikhail Capone said...

"By contrast, my Ford gets about 24mpg, and at today’s gasoline prices, the commute costs $2.58 in fuel alone. The difference is almost enough to let me have a third beer! But the ‘savings’ is wiped out if I have Guinness rather than Rolling Rock."

Don't forget that you are also buring calories just sitting there in your car doing nothing.

Also add to that the cost of owning a car (lease/payments), repairs, maintenance, parking, etc.

The $2.58 figure is pretty low..

4:17 PM  
Anonymous Anonymous said...

yes, it's cool, and useful for me
Bicycle Frame Italian

2:20 PM  

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