1 banana = 3.4 miles

[patricktaylor]

MPG is just a way to compare one car with another. No-one believes the actual figures.

Incidentally, it has been proven that you have to cycle a very long way to justify eating a Milky Bar. This was demonstrated many years ago.

[ANTONISH]

[quote="hungrydave
When cycling, the energy demand per unit distance from rolling resistance (in kJ/km or whatever units you want) are pretty much constant. But the demand from air resistance increases with the square of the speed.

Wind resistance increases with the square of the speed. Power is this force multiplied by velocity so the demand is proportional to the cube of the speed (roughly).

I may have missed someone else's comment but when climbing a hill one acquires potential energy. Not all this is converted to kinetic energy as some will be lost in overcoming an obviously higher wind resistance on the descent - so there is not an equal energy consumption between going up and down a hill and covering the same distance on the flat.

[hungrydave]

So how many Bacon Butties per mile or MPBB?

If dem dat is gud wif maffs work deese fings out den de rest ill ave bin lessoned in dis stuff

Well, by my calcs, assuming a bacon roll with two rashers is 300 kCal and you average 40 kCal/mile:

7000 miles = 280000 kCal burned. = 933.33 butties.

7000/933.33 = 7.5 miles per butty

or

933.33/7000 = 0.13 butties per mile

[snibgo]

Wind resistance increases with the square of the speed. Power is this force multiplied by velocity so the demand is proportional to the cube of the speed (roughly).

Yes.

Power is energy per unit time. I was looking at the energy per unit distance. This is (if we think about the units) power multiplied by time divided by distance, or power divided by speed. Hence it is proportional to the speed squared.

[karlt]

[quote="hungrydave
When cycling, the energy demand per unit distance from rolling resistance (in kJ/km or whatever units you want) are pretty much constant. But the demand from air resistance increases with the square of the speed.

Wind resistance increases with the square of the speed. Power is this force multiplied by velocity so the demand is proportional to the cube of the speed (roughly).

I may have missed someone else's comment but when climbing a hill one acquires potential energy. Not all this is converted to kinetic energy as some will be lost in overcoming an obviously higher wind resistance on the descent - so there is not an equal energy consumption between going up and down a hill and covering the same distance on the flat.

It's not just that though; even if there were zero air resistance, you've still got the fact that if going up the hill reduces your speed at a given power output more than 50% it becomes impossible to cover the ground in the same time as if it were flat no matter how fast the hill is descended. The transfer of energy from kinetic to potential and back again tells you that in ideal conditions you'd gain as much velocity going down as you lost going up, but it doesn't tell you everything. For one thing, it's primarily telling you what happens when an object moving without a force applied to it (i.e. zero power output) in a frictionless environment does as it goes up and downhill. The physics is a bit more complicated with a cyclist who is not in a frictionless environment and is applying an accelerative force through pedalling most of the time. If it were just about kinetic/potential energy transfer, the cyclist would slow down more and more as he approached the summit*, which of course is what happens if you stop pedalling. Due to friction, of course, this also happens on the flat, and there it has nothing at all to do with potential energy, but rather transfer of kinetic energy to heat and sound.

To put it another way, from a physics viewpoint it's a very complex system and frankly I wouldn't fancy trying to factor enough in to get anything vaguely realistic out of it

ITYFIABMCTT, as we say.

*I know we often do that anyway, but it's because we're getting tired and our power output is dropping. If we do exert a constant power on a constant gradient, our speed will be a constant.

[Ayesha]

If your entire mass is 400 N, ...

Newtons measure force (or weight), not mass. 400N would be a very small person.

I'm a skinny bloke at 67 kg. Plus bike etc 17 kg makes 84 kg, or about 840 N.

Appologies. Got mixed up with the conversion factor KCal to Joule and Kg to N. kg to N is 9.81.

[Ayesha]

The mpg number on a new car's sticker is gained from a specific laboratory emissions test. The test is strictly controlled so every manufacturer performs the same test. Then mpgs can be directly compared.

Yes, I understand that. It's the best system, doesn't make it right though.

Pick two cars with identical mpg figures. One could have a bigger engine or one could weigh more than the other but constant speed mpg lab tests are the same. In the real world with hills and traffic and loads of short distances could make the two cars very very different indeed. Overall economy can vary from car to car even though they may have the same publicly advertised figures.

I don't know how it could be done, but some figure for fuel consumption per mile per hour per load weight per acceleration per cold starting could be better. There are too many variables to make the publicised mpg figures any use at all.

There was something on the radio a few weeks ago about the way that the manufacturers test their cars, and one lady went into an argument with her car's manufacturer because she wasn't getting the mpg figure they specified. She lost the case because they said it was "her driving style". She disagreed, but the big boys won anyway.

It didn't make them right though.

There was an emissions test proposed which started at minus 3 Celcius. If this had become law, there would be no cars in the <100 g/km bracket and a 100><120 g/km car now would cost double in RFL if it was tested through the -3 C test.
A top emitting car would have had nearly and estimated £1000 per year RFL.

We should think ourselves lucky the European test is as it is.

The max speed on the first part of the Euro emissions schedule is 50 kmh. I strapped my race bike to a motorcycle chassis dyno and tried to follow the trace. Wow, my rear end emissions were.....