Jerl

gang-
Any physics-types out there?

I try to draft busses and trucks, etc as much as possible on my daily ride to work (yes dangerous, but too tempting, fast and fun). Yesterday I manged some serious speed behind a bus accelerating on flatland. Upon telling my significant other we got into the following debate:

-- does the eddie behind the bus actually push me at all? significantly?
-- or was the speed just me in the ~vacuum?

whaddya think?

Helmet Head

I'm not a physicist by any stretch. Physics (and enjoying the pure logic of computers) is what drove me from electrical engineering to computer engineering. Having said that... I think it's all about the speed and direction of the air molecules you are riding in.

When you are riding alone, assuming there is no wind, the air molecules are just sitting there. You have to push them out of the way so that you can proceed. But when you're riding behind a bus, the air molecules are rushing to fill in the space that the bus just occupied and vacated. Those molecules, moving towards the vacated space, and away from you, are easier to push out of the way than are air molecules just sitting there, so there is less resistance. Less resistance means you can go faster with the same effort. But I don't think you can say anything is actually pushing you.

djbrod

Not a vacuum exactly but an area of lower pressure behind the bus. Less pressure means less molecules to move out of the way. No push is involved.

Very dangerous, very fun

norton

Very hated by bus drivers......"If you can't see my mirrors, I can't see you".....


Of course.....now that I'm an EX-bus-driver.....& a cyclist......very dangerous, very fun.....


Try to hold a position where you can see the right side mirror.....Some sporting drivers may think you might know what you're doing.....& govern their speed so you can keep up......Don't count on it unless you can crank at least 30 MPH & have that Bike Messenger "handling" look....In other words......"Crazy guy....likes to play in traffic"....


Be aware that the bus driver might actually hate dare-devil cyclists so much that he's going to slam on the brakes to smash you like a bug on the back of the bus....This is the bus driver version of....."Going Postal".....







I didn't say any of the above.....

PainTrain

Does this explain why Dave could hit 60mph behind the semi in the movie 'Breaking Away'? I originally thought 'Chuh, right' but maybe it could be done.

genec

Also bear in mind that Dave or any other drafter is not having to break the wind directly in front of them.... so all their energy is going strictly to drive the wheels.

I remember hearing something like 70-80 percent of your power over 20MPH goes simply to pushing the air out of the way.

Helmet Head

Actually, the faster you go, the higher the percent of your power goes to pushing air.
That's why the power difference going from 5 to 10 mph is nothing compared to the difference of going 25 to 30, or 35 to 40.

slvoid

Air resistance goes up exponentially. Everytime you double your speed, it's like a squared or cubed increase in power.
After I shower off my stinkin body I'll dig up my old text book and type out the formula for you.

DogBoy

I believe its squared, but physics drove me from physics and into business so I wouldn't bet more than a $1 on it. You are not getting push, you just aren't having to overcome air resistance, so your power increase to speed increase is linear (rather than exponential) when in a perfect draft. I think a pro cyclist could easily hit 60 mph in a draft so long as their gearing is appropriate and the vehicle gave them an appropriate lead-out (and they are going downhill ).

my58vw

Nothing ever just "stands still" . Air is made up of many elements that are in constant motion. That motion is not motion like the wind and generally their is very little net displacement of the air itself. If that was true then things like difusion would not work.

The bus does indeed push the air molecules out of the way. When they are pushed to the side they follow a path because air is fluid. When the molecules hit the back of the bus they do not instantly change direection but continue to flow in the same direction for a short time. Think of air as a wave, water does not move laterally but are displaced in one direction as the wave proporgates. The net motion of each water molecule is small but in the macroscopic world we see a wave. Air works in the same way.

norton

Isn't the world record for a bicycle drafting behind a specially modified-for-drafting race car something like 168 mph.....I believe it was done at the salt flats in Utah some years ago.....I have a vague memory of a picture of a welder's hood-like contraption on the back of the race car.......The bike was extra-long wheelbase with of course a humungous chainring & tiny rear cog......It had a kind of bumper-thing out front.......Can you imagine touching wheels at 168 mph?.....I don't remember the name of the rider.....


Alas.....maybe in a hundred years, Lance will be forgotten, too....

JavaMan

With a 53-11 he would need a cadence of just under 160 in order to hit 60 mph. I could never do that.

slvoid

Drag = (Cd*Area*density*Speed^2)/2

Purely in terms of force needed to overcome aerodynamic drag:
For example, with completely made up numbers of Cd=.25, A=.5, density as 1, speed initial = 1.4m/s (5km/hr). Force to overcome drag is .1225
At twice the speed, drag is .49
At 4x the speed (20km/hr) drag is 1.96
At 8x the speed (40km/hr) drag is 7.84
To go from 6mph to 12mph, it's 2x faster but it takes 4x the amount of force.
So to go from 6mph to 24mph, it's 4x faster but it takes 16x the amount of force.

Compared to 6mph, to sprint to 36mph, you would need roughly 36x the amount of force.

JavaMan

Maybe you should Google it, but here is the way I understand it:

The thing that causes air resistance is a pressure differential. The air pressure at the front of a vehicle is higher than at the back. Why? The air at the front moves smoothly and exerts a given pressure. The air at the back is turbulent, becomes "detatched" from the surface, and exerts less pressure on the back of the vehicle. That's why so much attention is spent on streamlining and making things "aerodynamic". If you could keep the air from becoming turbulent as it moves around a vehicle, the air would exert virtually the same pressure on the front and back, and there would be no air resistance. By the way, most cars on the road today (wedge in front and blunt in back) would be more aerodynamic if they were driven backwards!

When riding behind a bus (I hate busses) you are riding in a mass of air that is being somewhat dragged along by the bus.

krazyderek

It was Fred Rompelberg....
Buses are ok, but stop way to often to be safe, dump trucks are my favorite to chase, and it sure feels like i'm getting yanked along when i don't hear one coming behind me and it passes me. Not sure if it's dead space, vortex, less pressure space, i just call it fast

slvoid

The turbulence and low pressure behind the rider also slows down the rider slightly. So if you're 2 people riding together, the rear is drafting the person in front but also helping "push" the person in front slightly.
They have that a lot in nascar heh.

BTW, my favorite cars to draft, SUV's and minivans. Big enough to provide a draft and most of the time you can see thru the rear window and windshield so you see whats going on in front. Busses, you have no clue when they'll suddenly brake.

cyccommute 

Because there is a slight low pressure area behind the bus, the higher pressure air would flow into that area and exert some force in the forward direction. It would never be noticed by a 30000 lb bus but for a 200 lb bike and rider, it should be significant.

TrevorInSoCal

Don't be so sure. My personal best is 173 RPM (36 MPH on a fixie w/ a 42x16 gear). Though I can't claim to have been exerting any actual power to the pedals at that point. It was all I could do to *keep up* with them. .

On a bike with a freewheel though, you could probably manage short bursts of a super-fast cadence to just *barely* maintain enough speed to stay in the draft...

-Trevor

hombrerana

I'm not sure of the physics involved in draftign a bus, I do know the physics involved when you hit one. I was sprinting to pass when a car pulled in from a cross street and cut me off. At the same time the bus was coming to a stop and I looked up just in time to see it coming right at me. Broke the lugs at the intersection of the top head and seat tubes and just bent the rest of the frame all to custard. the front wheel of course just folded in half and the bars rotated almost 90 degrees downward in the stem, with little ribbons of metal shavings poking out. I was lucky, multiple skull fractures, broken nose, face and right arm. That was 22 years ago and I've done a lot of incredibly stupid things since then, but that one I will not repeat.

2manybikes

If you have ever been on a fast boat like a ferry that has a square stern you can see the spray being sucked into the slight vacuum caused by the boat. The entire stern deck of some fast ferries gets soaked because of this. Don't put your bike on the stern deck unless you like salt water all over your bike!

There is a slight rush of air to fill the space the bus was just in as it leaves creating a slight suction, yes. You have to be very close, if you are not the turbulence is very bad. When drafting a semi on my motorcycle at 70 mph ( I would not consider this now) the force almost pulls you into the truck. You have to hold yourself up against the bars as it pushes your back forward.

If you are in the right place behind a bus you can feel it. I think it is too dangerous to do however. If the bus has a panic stop you must slam into the bus with your bike.

biodiesel

the whole conversation again makes me wonder why fairings have not happened. So far i've only seen one or two companies that make fairings and noone is making wind friendly bags or attatchments even though i've seen wind tunnel tests quoting the % friction from the shape of handlebars or brake cables... ( a ridiculously small percent...)
And most tri bikes are heavier than road racers, most tour bikes could care less about weight... and even thought the the fastest a bike has ever gone was drafting and the fastest a non drafting bike has been was a faired recumbant...
it's still fredville.
wierd.
no real point, just rambling...

Merriwether

I don't have more to add to the comments above re: forward air pressure. Some of the comments are quite helpful.

I will say that drafting a bus or other large vehicle is possible without obscuring your forward vision. A position just to the left of the left rear corner of the bus still provides the draft, but allows you to see the road ahead. This I know from experience.

norton

Yes indeed.....the average bus weight starts at about 19 tons & goes up from there depending on how many people are on board & how heavy they are.....

recursive

Well, remember, he was in the small chain ring. Probably 42-11 at the most.

norton

Yeah.... I remember spotting that.....movie hokum.....Not that it couldn't be done....They just didn't do it in that scene of the movie....


One of my all-time favorite movies.....perfect casting....."RE-fund!.....RE-fund!!...."


I believe it was low-budget as well....