Safe Speed Forums

How green is cycling-part 2

A while back I posted a suggestion that Cycling is likely to be rather less green than it is made out to be since cycles need fuel too, in the form of food! And (all other things being equal) the inefficiencies of food production, distribution and eventual physical utilisation by the cyclists body might well mean that each Horse Power at the bicycles wheels might represent considerably more at the well head! (10, 20 50 even!) And that this might actually not really be that much of an improvement over a modern small high efficiency diesel. (if any...)

(This is not to say that there aren’t other advantages in cycling, but I am just commenting on the “greenness” of it for now)

Now, while this might be seen to be a bit tongue in cheek (like the studies claiming that a Humvee is greener than a Prius) the more serious note is that none of these things exist in isolation and that arguments by green campaigners along the lines of “Two wheels good, four wheels bad” are wayyy to simplistic.

So, now for part 2.....

Rolling road blocks. On busy roads the presence of a cyclist is likely to result in otherwise free flowing traffic having to slow down when confronted by a cyclist, travel some distance in low gear waiting for an opportunity to pass before finally accelerating back up to cruising speed (say 40MPH)

This is likely to consume a significant amount of additional fuel over and above what would have been consumed by vehicles maintaining a steady speed.

Now the question is, How many vehicles does a cyclist need to have in his “Rolling road block” (Or how many vehicles/hour have to be compelled to carry out the brake/wait/accelerate manoeuvre) before he is effectivly consuming more fuel than he would have done driving his own car??

I suspect not that many! Indeed I suspect compelling a single HGV to carry out the brake/wait/accelerate manoeuvre will on its own, quite likly consume more fuel than many cyclists would have otherwise consumed over their entire journey in a small car!

Now, I don’t want people to think that this is a simple attack on cycling. Rather the point I am trying to make is that arguments suggesting that national fuel consumption/CO2 emissions (or what ever) are going to be reduced by encouraging people to cycle are unlikely to be realised since (Unless fully segregated cycling facilities are provided) the increase in congestion caused by greater numbers of cyclists obstructing otherwise free flowing roads will increase everybody else’s fuel consumption by more than the cyclists might themselves be saving.

_________________
"The road to a police state is paved with public safety legislation"

On the first point, I can't offer any definitive figures, but my gut feeling is that the pushbike is probably the most energy-efficient way of moving a human being over level ground, yet devised. Put another way, try feeding a car 2000 calories and see how far you get! I'd be surprised if the energy required to even turn the starter motor was less than the energy required to move a person on a pushbike over level ground for (say) a mile!

On the second point, yes, I'm sure the effect you mention exists, but are cyclists the only reason we have to reduce speed and accelerate again? Given that we're doing this all the time (on the kind of roads where we're likely to encounter cyclists) for lots of other reasons - bends, gradients, other motorists, traffic lights, roundabout...

...I'd be surprised if the additional "cyclist effect" was even measurable.

I'll certainly concede that the first point was a bit tongue in cheek (Though it would be interesting to know just how many "Calories" worth of oil it takes to make one "calorie" worth of food in my stomach! on average at any way (I suspect the actual figure varies wildly and that with a bit of care high conversion efficiency nutrition can be easily selected for, though it might be a bit boring!))

However on the second I suspect it could be quite significant. I would not be surprised if a "Passing Cycle" (if you excuse the pun ) on an HGV might consume an additional litre or more of fuel over that of maintaining a constant speed!. A modern HE diesel car could do ten miles on that! And an HGV operator only has to do this a dozen times or so in a shift and he's lost the profit for the day!

And while it is true that there are plenty of other reasons why vehicles may need to slow and accelerate back up to speed, it always consumes more fuel than maintaining a steady speed. And increasing the numbers of cyclists on otherwise free flowing roads can only result in drivers having to go through this cycle more often. Individual drivers may not notice a big increase in their fuel consumption but that doesn’t mean it isn’t there and as far as the “Green” argument go's the important thing is whether or not, overall, increasing the number of cyclists is actually going to reduce national fuel consumption.

(Quite apart from the issue over how reasonable is it to reduce ones own energy consumption at the expense of somebody else’s! I am sure, for instance, that I could cut my home heating bills by drilling through the ceiling of the flat below so his heat ends up in my flat! But would this be an acceptable strategy?? )

_________________
"The road to a police state is paved with public safety legislation"

If I may bring it down to my level Dusty, (‘Being Green for Dummies’). What you are saying is it isn't just about how I can eat a bowl of porridge which will get me from A to B and back, but the knock-on effect that I have on everything else in my environment?

If so, I am on board.

If not, I'll have to think of something else and keep reading...

e.g. When I cycle into work the first thing I do is have a shower I wouldn't otherwise have had and one when I get back home. There's 10 minutes @ 8.5 KW for starters...

Am I warm? I don't mean in the shower of course

_________________
The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

PS re starter motors.

To use a real life example. (LC80 4.2D) 24 volt system. say 20V under load, 100 amp. This always starts first compression (Like old Transit vans used to before they got all techy ) so all you get is a sort of “Chew” noise and its running. I guess 1-200mS operation tops!

So VxAxT, say 200 Joules to start the engine.

Cycling, say 500-800 calories/hour (depending on how enthusiastic you are)

Of course these are “Dietary” calories not physics ones. To get the Physics one you have to multiply by 1000.

Sooo say gentle cycling. 500,000 C/Hr. In SI that’s just over 2Mj/Hr or 550J/s

or 200J/in just under half a second!

Or, at (say) 12MPH (Gentle cycling)

You can cover about 2 metres on a bike on the food energy it takes to start my engine!

Though this seems extraordinarily poor one should bear in mind that before one had starter motors one was expected to start ones engine by hand using a crank handle and that this could generally be easily achieved, this result should not be too surprising.

_________________
"The road to a police state is paved with public safety legislation"

In the old days, when motorbikes had what bikers called a kick starter, I can’t believe my one leg consumed more than a fraction of what my motorbike does now in 2011 now matter how I fudge the physics or biology.

Firstly, I think of the extra weight the heavy lever it carried around all the time and gear mechanism, but there again that’s less weight than a starter motor.

Then I think of the extra weight of the battery I need to operate a starter motor as opposed to the small ones when we used leg power.

AND, back then, it was something called exercise which has yet more benefits does it not? So there are health benefits to using human/biological energy and we all know how much our lack of exercise costs the NHS - don’t we?

I get the feeling this is going to be incalculable on so many levels it’s going to be like Unweaving the Rainbow; but interesting...

_________________
The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

can't see this going anywhere useful but if we're in the business of plucking numbers out of the air...

a petrol car at 40mpg..... around 8.9 miles/litre .... or 14222 metres/litre
petrol at 9.5 kWh/litre or 34200 kJ/litre

gives 4.5 x 10^-4 metres/J

or for comparison with your 200J cyclist managing 2m, the car manages 0.083m.

I've heard tales like that but I have to say I remain unconvinced! Looking at the number of HGVs that will sit there on the speed limiter taking a couple of miles to overtake the next HGV sat on its speed limiter - only to have to do it all again when the first one passes him on the downhill bit, I'd say there ought to be a lot of bankrupt hauliers!


looking at it the other way though, you could argue that every cyclist is a vehicle off the road, so that's one less vehicle that you don't have to overtake or give way to, or stop-start in a queue behind - all of which will also use energy!

Really???! MAN, I am SO going to take up cycling!!!! If I can get rid of over a Mars bar per hour, that's pretty darned good! Where did those figures come from? I keep hearing all the depressing ones about you having to run something like a half marathon to get rid of a Mars bar!



Not with today's compression ratios you wouldn't! Even my mate's old 1.5 litre Riler with a 6.5:1 compression ratio had me working up quite a sweat - especially on damp mornings!

assuming one isn't hanging around.

_________________
Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

That seems somewhat counter-intuitive! If an adult male is supposed to have an intake of 2500 calories a day, can you really burn up to 1/3 of them in an hours furious pedalling? Although I can't support this, I'm told that the vast majority of the calories we burn are just as a result of "living" - even if we're lying in bed. Might it be the case that the 500-800 figure includes the "base demand" too? In which case, the EXTRA number of calories consumed by the furious cycling might be only a couple of hundred at most? I'm saying this because having done more than an hour's hard pedalling on the less-than-flat terrain in Cumbria, on a cheap (and therefore pretty heavy) Halfords mountain bike, towing a trailer with a couple of toddlers in it and propelling my (ahem!) not inconsiderable bulk in the past, I didn't seem to loose much weight compared to doing stuff-all! Of course, I might have been consuming, er, a tad over the 2500 mark but I don't think my intake has changed significantly!

I grabbed the figure from the readout on the gyms cycle/rowing machines. It might be a bit optimistic but it gives a ball park figure. My guestimates are likely to be within the correct order of magnitude. Bear in mind that before we got all podgy and lazy a manual labourer of yesteryear might have got through 5-6000 cal/day (Dragging a sled across the Arctic takes about 11,000, unfortunately you can only digest about 9,000/day so Arctic explorers aim to put on a couple of stone before going on expedition so they have something to draw on)

The KE of a 44,000Kg truck between 8MPH and 40MPH is about 2KWHr. A reasonable electrical generator will give about 4KWHr/Litre. A truck wont do that well under acceleration. recovering the energy of motion is MUCH harder work than maintaining a constant speed (Think about the difference in your accelerator pedal position between cruising and accelerating) An extra litre to get a big truck back up to speed is not an unreasonable estimate.

Consider this from a different angle.

We all know that vehicles might do double the mileage at a steady 50 than on "Urban Cycle". Urban cycle isn’t heavy on fuel because it is slow, it is because of the stop/starting.

The introduction of even small numbers of cyclists onto previously cycle free roads will effectively turn free flowing roads into something more like "Urban cycle" ones with a similar reduction in fuel efficiency for the other vehicles using them.

This is not likely to be particularly beneficial for either drivers pockets or national energy consumption figures and will almost certainly consume far more fuel than the cyclists themselves might be saving by not driving.

I am not arguing against cycling as an energy/pollution reduction policy per see, its just that it is only going to deliver the energy/pollution saving goods if the cyclists use fully segregated cycle paths and do not obstruct faster motorised traffic. If they use the same roads and create stop/start driving conditions any such policy is actually likely to make things worse



That’s basically my point. The truth behind many green claims frequently is!

_________________
"The road to a police state is paved with public safety legislation"

I think that Moley has hit the nail on the head. It is very simplistic to equate the number of calories consumed when cycling with additional food intake. Nearly everyone in Western societies consume substantially more food than is required for their basal metabolism ( 2500-3000KC against a basal requirement of 1500KC). Moderate cycling or walking merely takes up that difference. And training for more vigorous exercise increases the metabolic efficiency.

When you go on a whole day (8 hour) bike ride do you really consume 6,400 calories - 12 Big Macs -over your normal diet? I certainly didn't, though I would loose some weight. Which is why I am getting plump now that i don't ride much.

As for causing motor vehicles to use more fuel. That might be true where a cyclists obstructs that mythical creature, free flowing traffic. But generally traffic only flows freely on roads where a cyclist isn't an obstruction (unless he insists on ridding in the PP ). In cities, where the average traffic speed is 10-15mph - a modest cycling speed - , overtaking cymbalist is an unnecessary manoeuvre .

_________________
When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

and now for some REAL data:

lets take one of my 5min easy intervals from yesterday's ride... av speed 14.7mph, av power 159W at the wheel.
being generous say 80% transmission efficiency gives 200W at the cranks.

that makes it conveniently using those 200J in one second..... in which time i've travelled 6.5m.

i thank you

(and yes now we can debate the efficiency from mouth to muscles)

You know I said I wouldn’t mind riding with you one day at an easy pace Ed? I’ll use my motorbike if it’s all the same to you.

Actually, if the same can be said of bicycles as well as motorbikes you could be losing a lot of energy depending on what bike, of course, but also the condition. I once read that you can lose up to 4 or 5 BHP just through badly worn chain and sprockets! The girl’s bike at work that I park mine next to looks like it has chain Rigor Mortis; I’ve never seen so much rust!


On the motorbike lights thing, I haven’t found exactly when it happened yet but I found this so far which I didn’t know.

“The BMF has slammed a European Commission decision to fit daytime running lights (DRLs) to all new cars from 2011, claiming the move will make it harder to spot motorcycles.
New laws introduced on Wednesday will require all cars and small vans made from February 2011 onwards to be fitted with dedicated DRLs, which would come on automatically when the engine is started. Trucks and buses would follow in August 2012.”

That could be bad news for us on two wheels.

_________________
The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

So its a similar order of magnitude (Which is all "Back of enverlope" first approximations will give you), In any case I was responding to Moles comment thus...





I think we have both satifactorilly blown that one out of the water...

PS



Since I was working forward from an estimate of the dietry calories consumed and you have worked back from an estimate of energy expended at the pedels then, allowing for a biolgical conversion efficiency of 30% then we have both basically got the same awnser.

I Thank Youuuuu!

_________________
"The road to a police state is paved with public safety legislation"

This might need a thread split.


If you could eagerly cycle continuously for that amount of time, then you really should consider being in The Tour. Even those guys don't work that hard (usually sat in slipstreams), for that long (typically 5 hours), and they have really, really nice machines; yet those guys do eat a lot, and none are what anyone could consider bulky!


I think the 2500kcal figure was created to ensure folks (men in that case) get the necessary nutrition, and I think that figure assumed a little something in the way of exercise; wasn't that from the WW2 era?
Regardless, that equates to only 104kcal per hour for the baseline. There are many folks (especially women) who eat far less than the recommended levels, yet they don't wither away; however, they generally don't exercise either.

Some quick numbers:
Fairly eager cycling is say 150W. That's my finger in the air estimate, based on the same mechanical effort as a typical male (85kg) climbing typical stairs at 1 step (18cm) per second. edited to add: I've just now seen Ed M said 200W, so my guess really wasn't a bad one!
1 cal = 4.18 joules.
So 150W over an hour = 129kcal. however, the body doesn't burn that with 100% efficiency. For folks who are trying to lose weight, I guess that efficiency would be quite low, say 25% (again finger in air - it could be lower), so is the intake equivalent to 516kcal.

_________________
Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

We did???

I think you reckoned something like 100A

This chap:
http://books.google.co.uk/books?id=oMSD ... nt&f=false

seems to reckon 150A for a SMALL engine. Add to that the solenoid current (let's say 20A) and you're not too far from 200A @ 12V, which is 2.4kW.

As for the claim that it operates for 0.2 of a second, I think that's a bit short - on average, let's say 0.5 seconds. so 2400W for 0.5 seconds is 1200Ws. Now if "fairly eager" cycling is 150W, let's say that some more gentle cycling is (?) 75W? 100W?

so for the energy to crank the starter, we can get 12 seconds of gentle pedalling. So OK, not a mile's worth, but if we're only aiming for the right order of magnitude, I don't think it was THAT far out to be honest!

Steve. For all your detailed arithmetic the fact remains that most casual cyclists - or other exercises - don't increase their food consumption in direct proportion to the miles they do. I certainly don't. Some days I walk twenty miles, some days I cycle a bit, other days I drive, other days I am entirely sedentary. But I always have the same three meals. So I think that that aspect of cycling not being green is a canard

I will give some credence to the suggestion that obstructing free flowing traffic increases energy consumption but that is largely down to the design of car engines. The fact that small cars are designed to most efficient at 50-60mph when they spend most of their life pottering around at under 20mph seems rather odd. A marketing rather than an engineering decision?

_________________
When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Sounds like it's in the right ballpark. Walking burns off approx 100 calories per mile.

_________________
Only when ideology, prejudice and dogma are set aside does the truth emerge - Kepler