|Pulling other riders||filtersweep|
Oct 5, 2001 1:03 PM
|OK- this might all be in my head, but when others draft me, it actually subjectively feels like I'm pulling something- not the full weight of the rider behind me, but "something." While I've assumed there is a reason it is called "pulling," I can't reconcile the physics behind it. I'm not talking about the difference between drafting and leading- which is obvious, I'm talking about the difference that is felt when someone actually gets on my tire... or am I just imagining things?|
|no expert, but i'd say it's all in your head.||Js Haiku Shop|
Oct 5, 2001 1:16 PM
|pulled a few last weekend through a windy spot. knew they were approaching, but thought they were either just slow to pass or had stopped for a mechanical. when i glanced back, they were coming on strong. ten minutes later i noticed they were on my wheel. was so windy i couldn't hear the squeaking of the single-speed tandem's springy seat three bikes back. :-)
all this to say that i felt no adverse affects of their drafting my big butt, but i ALWAYS feel obligated once i know they're there to keep the speed/intensity up. kinda like an anonymous domestique.
|re: Pulling other riders||guido|
Oct 5, 2001 1:31 PM
|Y'know, I've had this same sensation. It's like a little suction behind my rear wheel when someone grabs it, so to speak. Maybe its the air pressure differential made my my draft. Huh?|
Oct 5, 2001 1:34 PM
|guido: exactly.... so now it is 50:50 (unless I count myself here...)|
|It's all in your head.||look271|
Oct 5, 2001 1:49 PM
|The only thing that actually ocurs is that there is a slight BENEFIT to you by pulling, aerodynamically speaking. (Very slight, I understand.)|
|yup, it helps||Dog|
Oct 5, 2001 2:28 PM
|Someone in your draft slightly fills the negative pressure area behind you, actually helping you.
Sure, the front rider is "pulling," but in a small way, the rear rider is "pushing."
Now, if you had a very strong tailwind, and someone were behind you blocking that wind, that might be different.
Oct 5, 2001 2:40 PM
|blocking the tailwind- the most logical expanation
-but- does a tail wind "push" or does it "suck"- meaning that is it more of a negative pressure?- the absence of a certain amount of resistance in front of the bike? Most wind is slower than a bike's speed anyway.
|oh, we are going to anger the engineers on this one||Dog|
Oct 5, 2001 2:49 PM
|A tailwind, at a minimum, reduces the pressure in front of you and fills the negative pressure behind you. Maybe that's what it's all about, really.
I've had tailwinds that were far faster than I was riding. Last spring in the desert I rode 130 miles at 23 mph sitting up and not even breathing. Had a tremendous tailwind. At one point, I spit, and the spit went forward - I kid you not. I certainly felt pushed.
How does a sailboat work? Isn't it "pushed"? Maybe when you talk in terms of air pressure differentials it doesn't make any differnce, they are two sides of the same thing.
I've read that the wind helps or hinders you by about 1/2 the speed of the wind. So, a 20 mph tailwind would speed you up by 10 mph. Probably not linear, though.
|and the sailors, too||mr_spin|
Oct 5, 2001 3:02 PM
|Sailboats are only "pushed" if you are on a run, which is to say the wind is behind you. That's usually when those big spinnaker sails go up.
Otherwise, a sailboat functions very much like an airplane wing, which is all about making the pressure on the top surface of the wing lower than it is on the bottom surface. I guess in a sense, this would qualify as "sucking." For a sailboat, the outside of the sail is the same as the top of a wing.
|The laws of physics||Elefantino|
Oct 5, 2001 3:10 PM
|Let me see if I've got this figured out: |
If you are pulling and the riders behind you "suck," and you continue to try to pull them, then you will slow down because they "suck."
If you "suck" and you are pulling other riders, you will be passed immediately and they will soon be off in the distance after which you will continue to "suck."
I believe I have experience on both ends of this equation.
|lift and driving force||Dog|
Oct 5, 2001 3:13 PM
|I can see the sail acting as a wing, in that it would provide lift. But, what, then, is the driving force? Doesn't there need to be a push, too? Airplanes have props or jets, sailplanes have gravity or updrafts... I thought for sailboats that essentially they are being pushed, but can go across, even into the wind, as the keel opposes or redirects the force for wind.
I know I'm likely all screwed up on this, just trying to think through it.
I've often wondered on a bike if I'd go faster with a sail with a tailwind. In fact, some wheel makers claim negative drag with their wheels in certain angles of wind, such that they are actually pushing the bike forward.
|call me gilligan||mr_spin|
Oct 5, 2001 4:00 PM
|Essentially by creating a pressure differential, you are creating a "void" that must be filled. The high pressure rushes into the low pressure. Just the act of wind hitting the sail should accomplish this. The key is the orientation and tightness of the sails. The better the sailor, the better they know how to trim their sails to get the highest performance. Planes have engines to force wind over their wings. Boats don't have this luxury.
But wait! There's more.
Unless you are on a run (wind behind), wind is going to be a side force. It pushes on the sails, and because the sails are generally large and stick way up in the air, it tries to push the boat over. Sailboats have keels to prevent this. The keel helps counter balance the boat, and in fact, sailboats are most stable when leaning over on their side. That's also why even when sailboats are motoring, the main sail will usually be up.
Side force does not really drive the boat, however. Sure, it has some small effect, but the primary driving force is the lift action of the sail.
Oct 5, 2001 3:40 PM
|First, having someone drafting you actually helps a tiny bit. The idea is that together a couple bikes take on the aerodynamic appearence of a larger object and the first object isn't getting the full effect of the airstream closing back in around them and generating some the full amount of low presure on the backside (which will slow you down). When flying an airplane in tight formation you can actually feel when your wingman settles in nice and close. Tanker pilots can tell when a refuleing aircraft settles in behind them and their speed increses slightly. All of this is in the sub-sonic regiem. |
You first have to decide where the relative wind is coming from. A gross generalization is that if you ride a closed loop you'll have a headwind on the order of 70% of the time. Really it all comes donw the vectors - direction and magnitudes can really vary so the results vary. having a 20 kt. steady wind over a closed course is definitely a negative effect on the speed/distance/time thing.
Next, the sailboat thing - sure there's always some "push" on the rig, even going up wind, but what really matters is the area of low pressure created on the *other* side of the sail. The typical explanation is the Bernouli Principle. A fluid travelling a greater distance has to go faster if it's going to get to the same place at the same time as fluid travelling the shorter distance, and when velocity goes up pressure goes down. The shape of the sail is critical for the amount and angle of lift produced - just like an airplane wing. Ultimately the "push" alone isn't enough to generate the kind of force required - you need the low pressure area to really make things happen - it's the difference between the two. When a stall occurs it starts with disturbed flow on the low pressure side of things. Go deep enough or adjust the angle enough and you can get stalling on the high pressure side as well. Next we can get into things like induced drag whihc is really the drag directly associated with the production of said lift - not the same as parasitic and form drag which is attributed to just moving something through air without the production of lift (i.e. bikes).
Any negative feelings to having someone on your wheel is all psychological. I've had occasions where I've passed groups of people aonly to later realize that they hopped on my wheel - I didn't even know they were there. i don't really mind having someone on my wheel as long as they're safe - if they're to beat to pull that's fine, but if they just suck your wheel for a while then ditch you as you peter out that's not very nice - then it's more like racing and anything goes....
|Oh Brother....so here is a quiz.||Krusty the Clown|
Oct 6, 2001 5:13 AM
|Suppose you are sailing directly downwind with your sails full. The wind is a constant 20 mph, the water is still. The maximum speed your sailboat could obtain is
A) nearly 20mph, B)somewhere between 20mph and 40mph, C) much less than 20mph D) More than 40mph?
Oct 6, 2001 5:58 AM
|Readers should be advised that although your very impressive reference to the Bernouli Principle, induced drag and parasitic drag has us in awe, airplane wings produce the greatest percentage of lift due to their angle of attack (or angle of the wing relative to the wind).
As a matter of fact, the lift produced by the angle of attack is so much greater than that caused by your Bernouli Principle that airplanes can fly upside down. Also, although it would not be the most efficient design, an airplane wing can be built without any airfoil shape to the wing at all, and it will still fly.
But we are can't wait for you to impress us with stuff like Reynold's numbers!!
Oct 6, 2001 7:53 PM
|A sail on a sailboat works as follows: Most of the time (even when heading into the wind) the wind is coming off either the starboard or port side of the boat-NO SAILBOAT WILL SAIL DIRECTLY INTO THE WIND. The wind pushes on the sail and because the sail is curved and slightly bowed at one end (the end opposite the mast) the wind does two things, it pushes the boat over and it rushes out the curved end. The weight of the boat and the keel counteract this pushing over effect and this is what drives the boat forward. A good skipper will know how much sail to fly in a given wind and trimming is just a matter of pulling the sails in or letting the sails out (and moving ballast around-namely crew) to maximize speed and efficiency.
I may not know much about the aerodynamics of cycling but I know a lifetime about sailing.
|Benefits both puller and pullees||Mick|
Oct 5, 2001 4:48 PM
|I'm pretty sure that the drafter's benefit is in the 20-30% range, depending on wind direction.
I don't have the scientific evidence but the accepted number amongst my cycling friends is that the puller also gets a 5% boost. Someone else remarked that two bikes together make a larger air pocket (or wing effect) and the high pressure air rushing back in behind the second bike's low pressure void helps push both bikes forward.
So, if we have 20 riders in a paceline is the puller getting a free ride and a 100% push? ;-)
(Yes, I can do the math and the answer is no.)