abikemechanic

https://www.youtube.com/watch?v=HOsQBrtD80g

at 1:36 they say that FLO has experienced negative drag on their wheels. Has anyone else heard of this, is it real or just some marketing hoohaa?

gregf83 

It's not uncommon for deep profile wheels tested in isolation without a bike and rider at the right yaw angles. Doesn't mean you won't still need to pedal

gerundium

please tell me how you can have negative drag? That would mean that the wheel is ADDING energy to the flow of wind. Seems like an impossibility.

nathan.johnson

It's only impossible if you follow the laws of physics. The laws of marketing are a different story...

Triguy

I think it's more appropriate to say thrust(or lift), not negative drag but it is possible. Hed has seen "negative drag" with their lenticular discs as far back as the early 90s in some circumstances.

Dan The Man

Isn't negative drag the basic idea behind wind turbines? Or am I confusing what they are talking about. Also, lift and drag are at perpendicular angles. So one should not be confused with the other. I suppose for a spinning object, drag might be aerodynamic resistance in the opposite direction of rotation, but for a moving object, like a bicycle, drag aught to be opposite direction of motion.

gerundium

As far as i know wind turbines work because the air is moving. This causes them to flow past the airfoils, and those are designed to cause a net pressure difference between the two sides (also known as lift) this makes them start to spin. That spinning motion causes a rotation in the center axle that has a generator attached to it and energy is produced.

Drag is in the direction of air flow, and is always a loss term. If the net energy of air flow is increased after a wheel you have created a generator and you should get a patent ASAP. The only way i can think of is if they tested these wheels in a situation with very low air speeds and a very fast spinning wheel. Combining those factors might cause a positive energy balance after the wheel because of the rotation accelerating the nearly stagnant air that passes by. (and that still wouln't be negative drag in the general perspective, because energy from the wheel is still being lost)

gregf83 

The same way a sailboat can tack upwind. If the wheel changes the flow of wind there can be a component of force in line with the wheel which results in negative drag.

Dan The Man

Yeah, so if your bicycle wheel was say, a propeller, and you spun it really fast while riding sideways (only possible in a test condition), that wheel might experience negative drag, aka thrust. Apparently these wheels have a similar effect while riding forwards.

However, a bicycle already has a really good mechanism for turning wheel rotation into forward force, it's called contact with the ground. Any thrust generated by wheel rotation would simply be a less efficient form of propulsion. In other words, these wheels suck more energy out of your legs.

Dan The Man

So they are saying that under certain crosswind conditions the wheels can generate thrust? That also sounds kind of bad, because as a cyclist you have no control over the angle you are tacking into the wind. If sailboats had to stay on a road, or within a tiny sea lane, they would be tacking back and forth like mad under most conditions.

gerundium

Still not true. Boat sails use lift and pressure differences between sides to create a propelling force. there is still positive DRAG!

For another way to look at this, the force created by the sails for tacking upwind follow from the Impulse balance of F1 + F2 + K = 0 (vector addition as you crudely pointed out)

But the flow of wind on the wheel also has to answer to the energy balance: H1 = H2 + dH (H is energy height z + P/rho*g + U^2/2g or also known as h+U^2/2g)
and dH >= 0 ALWAYS.

if you want to call a force generated because of lift negative drag be my guest, but technically it's wrong.

Dan The Man

Sails are essentially vertical wings. It's the lift/drag ratio that they are exploiting to generate propulsion.

pgjackson

In surfing, there are fins called "thrusters". They are the two small fins on either side of the big fin in the middle. The idea is that based on the shape and angle of these thrusters, they actually help propel the board...kind of how the shaped wing of an airplane provides lift. At least that's what they tell you.

Inertianinja

You are are misquoting the video.

The video said
"flo reported that in some of their prototyping stages, their wheel shape was actually achieving negative drag. this was without hub and spokes, but impressive nonetheless"

so nobody's claiming that their wheel produces negative drag.

Dan The Man

But so far, all of the examples are based on lift/drag ratios. There is no way to create negative drag without some sort of power source. Even without a hub and spokes, a negative drag object would be of significant interest to the military and aerospace community as well as the free energy nutjobs.

Consider this, something with negative drag will push itself into a headwind. The faster it goes into the headwind, the higher the relative headwind is. The higher the headwind, the more negative drag is has pushing forward. The cycle continues until relativistic effects take action.

pgjackson

Favorite quote: "what's special about these wheels is that Chris and John Thornum, founders of FLow Cycling, are both engineers...so they poured a lit of time into designing the shape of these rims so they can be as fast as possible." Then it goes on to explain how they simply copied other brands.

Translation: they were designed by engineers, so they have to be good, right?

pgjackson

I'm sure that under certain wind-tunnel environments, that these shaped rims (without spokes and hub) could push itself forward the same as how an airplane wing works or the sail on a boat. But it means nothing in reality. What I gather from this video is that they simply copied the shape of other wheels and are now going to offer similar wheels for a much lower cost.

nathan.johnson

Physics has no place on BF! An "actual" negative drag wheel would be a perpetual motion machine.

Seattle Forrest

I saw something about negative drag on their site a month or two ago (not in the video). I was incredulous, but wanted to start a thread one of these days and make sure I wasn't wrong to write it off.

Dan The Man

Boat sails do not have a negative drag. They have a lift force which is perpendicular to the wind, and this force can push against the keel to generate a net upwind force. But they still have positive drag. Similarly airplane wings do not push themselves into the wind. They also have a lift force that pushes them perpendicular to the wind so that the whole plane can stay in the air. There is still a positive drag on the wing The propeller is what pushes them into the wind.

On a somewhat interesting note, there is a "car" that managed to drive downwind faster than the speed of wind using the wind as its only power source. It involved coupling the wheel drive to a propeller drive. Still not negative drag, but the inventor got laughed at quite a bit before he proved himself right.

DScott

gregf83 

Semantics. They're saying that the wheel (or more appropriately rim) will generate a force in the direction you are travelling under the right wind conditions. So from the riders point of view it doesn't take any effort to push the rims through the wind, in fact, they propel themselves. Note this is not the same as a perpetual motion machine since you are using energy from the wind.

I think everyone is getting a little carried away with this. The reality is it was a few grams of negative drag for their rims similar to what Zipp has observed on their disc wheels.

pgjackson

I think the important thing to remember is that according to them, all they did was replicate (copy) more expensive wheels at a cheaper price by matching up a CF deep dish with a cheap AL rim, spokes and hub. Essentially what you get is a wheel that LOOKS like an expensive CF areo wheel only heavier and less expensive. But the owners are engineers, so they must know what they are doing.

rpeterson

Ya know, if it's nearly as aero as the expensive CF wheel then I'm sold.

pgjackson

I agree. I'm all for making things less expensive. But despite what the video implies, there is nothing new or revolutionary about the wheel. Essentially it's a cheap knockoff of expensive CF wheels.