OAT Vs. turn over

[tom8658]

What I was thinking about with gravity's torque at the time was the top. With a large circle like the top shows, there is quite a bit of force pulling the top down because the center of gravity is so far out. Causing a torque?

The torque in the picture of the top is caused because the gravitational force and the normal force (-F_g) are not applied at the same place, creating a lever arm and thus a torque.

You are 100% correct about apparent wobble actually being precession - it is not possible for a spin-stabilized object to react to a torque without precessing.

What I really want to know is why that precession causes the disc to turn over - i.e. behave as if the advance ratio were lower than it actually is.

 

[tom8658]

This seems backwards to me. There is no way on a true sidearm throw (not a 3/4 throw like a baseball) that you are getting more velocity and less snap then a backhand throw. You are almost certainly getting more velocity and less spin backhand at least this is true if we are talking about 2 very identical throws in distance. Someone help me say this in sciency terms.

Well, you can compare throws at different power levels by looking at the advance ratio - a ratio of spin to forward speed, essentially.

As far as I know, no one has ever tried to measure the spin of a disc after it leaves the hand, so we don't know which kind of throw produces a higher advance ratio.

In the absence of direct knowledge, we have to look to the model for answers. The model says that spin stabilizes the disc - therefore, because forehand shots tend to turn more, they have a lower advance ratio (less spin and more forward speed) than backhand shots, which is why forehand throwers tend to prefer overstable plastic.

Then we can consider the new "hammer pound" technique at DGR, which is supposed to produce more "snap" - such throws are characterized by a high advance ratio (more spin), and indeed the reports are that forehand throwers using the technique are able to flick more understable discs with success, and most throws are very straight with little fade when thrown low (characteristic of a high advance ratio).

That's why I concluded that forehand shots tend to be dominated by speed effects rather than spin effects, which seems counter-intuitive, but fits with all of the other facts. Until we have direct knowledge (which would be relatively easy to obtain with a high speed camera, a sharpie, and a couple of test subjects), we have to rely on the conclusions drawn from things that we do know, even if they oppose conventional wisdom.

Now, the part that I left out is that OAT tends to make a disc more understable, and it seems to me that forehand throwers suffer from a lot of OAT problems, and this is perhaps why we see such a drastic difference between the "hammer pound" technique and a traditional sidearm - they are getting the same amount of spin and armspeed, but the hammer pound produces much less OAT. But this is still consistent with a model in which OAT acts to decrease the advance ratio of the disc, even if I can't explain the mechanism, because the underlying fact is that the higher the advance ratio, the more stability the disc will exhibit.

Make sense?

 

[tom8658]

Hey, it's the snap guy! I love his videos and how he explains the concept of 'snap.' I've been thinking about this lately and what is the main physical concept behind it. My thinking is that 'snap' is nothing more than a preference for rotational as opposed to linear motion during the final part of the throw and that this is a more efficient type of movement and thus creates greater disc speed. If u watch the first two 'snap' videos, it really seems that that is what he's getting at. I have more thoughts on this, but I don't want to hijack the thread here. Should I start a new thread?

Not quite. Spin acts only to stabilize the disc. The actual distance the disc flies is predicated entirely on the forward speed of the disc as it leaves the hand and the aerodynamics of the disc itself (how much lift and drag it produces).

Of course, this is only true for shots that don't nose up and die, or turn and burn, so the disc has to be spinning enough to keep it within some margin of stability.

My thinking is that "more snap" means both "more armspeed, more spin, and higher advance ratio".

For example, if you are throwing with 2 units of spin and 2 units of armspeed (for an advance ratio of 1), and you do the hammer pound drills and start applying "more snap", what you have actually done is learned to apply 4 units of spin and 3 units of armspeed, for an advance ratio of 1.33. This will tend to lead to longer flights, but also more straight flights.

Though the latter, of course, depends on the disc itself - getting more snap on a putter will make it flip because it's exceeding its rated (forward) speed, while you would be lucky to keep your firebird on a straight line at all because of it's high rated speed and inherent overstability.

 

[bigt]

My head hurts......LOL

 

[dodgeball]

Not quite. Spin acts only to stabilize the disc. The actual distance the disc flies is predicated entirely on the forward speed of the disc as it leaves the hand and the aerodynamics of the disc itself (how much lift and drag it produces).

Of course, this is only true for shots that don't nose up and die, or turn and burn, so the disc has to be spinning enough to keep it within some margin of stability.

My thinking is that "more snap" means both "more armspeed, more spin, and higher advance ratio".

For example, if you are throwing with 2 units of spin and 2 units of armspeed (for an advance ratio of 1), and you do the hammer pound drills and start applying "more snap", what you have actually done is learned to apply 4 units of spin and 3 units of armspeed, for an advance ratio of 1.33. This will tend to lead to longer flights, but also more straight flights.

Though the latter, of course, depends on the disc itself - getting more snap on a putter will make it flip because it's exceeding its rated (forward) speed, while you would be lucky to keep your firebird on a straight line at all because of it's high rated speed and inherent overstability.

Hey Tom, thanks for the reply. I'm afraid I wan't very clear at all in most previous post. I was trying to say that a preference or utilization of rotational motion of the thrower him(her)self will lead to a greater net disc speed. I wasn't even thinking about disc spin, and reading your post just now, it makes sense that the spin of the disc doesn't really affect the distance the disc will travel, but rather just affect it's stability. Regarding the thrower wanting more rotation, it just seems that using rotational motion is inherently more efficient when talking about a disc golf throw. It seems this really comes into play during the last part of throw, or the 'snap' portion of the throw, as the thrower turns (quickly hopefully) his shoulders and forearm, releasing the disc. The first part of the throw seems more linear as you 'pull' the disc on your reachback. My trouble is in converting from linear motion of the first part to the rotation of the last part. After some thinking it seemed that the principal of conversation of momentum would be a good way of connecting the two, but I'm a loss of demonstrating it mathematically. The best I could come up is to equate the linear momentum of the first part with the angular momentum of the start of the rotation or 'snap' part. To simplify, the mass would be considered a point mass, equal to the mass of the disc and located at the center of the disc. So then, how does that affect disc speed? After equating the linear and angular momentum of the disc, I get that v(2)= v(1)/r, where v(1) is the initial velocity of the disc and r is the radius (here the forearm approx). Of course this assumes no outside forces or torques. Does this make any sense? As you can tell, I don't have much background in physics, although I did take a course in high school and then one in college. I think my equation probably doesn't really work, lol, but do you think the theory is on the right track at all? It seems you have a good grasp of this stuff--are you an engineer?

 

[CwAlbino]

Not quite. Spin acts only to stabilize the disc. The actual distance the disc flies is predicated entirely on the forward speed of the disc as it leaves the hand and the aerodynamics of the disc itself (how much lift and drag it produces).

Of course, this is only true for shots that don't nose up and die, or turn and burn, so the disc has to be spinning enough to keep it within some margin of stability.

My thinking is that "more snap" means both "more armspeed, more spin, and higher advance ratio".

For example, if you are throwing with 2 units of spin and 2 units of armspeed (for an advance ratio of 1), and you do the hammer pound drills and start applying "more snap", what you have actually done is learned to apply 4 units of spin and 3 units of armspeed, for an advance ratio of 1.33. This will tend to lead to longer flights, but also more straight flights.

Though the latter, of course, depends on the disc itself - getting more snap on a putter will make it flip because it's exceeding its rated (forward) speed, while you would be lucky to keep your firebird on a straight line at all because of it's high rated speed and inherent overstability.

Well said, I don't know the technical terms like you do, but I know how to throw each and what I need to do in order to get more distance just by experience. I'll give a few examples of my throws and how I adjust each. Fit this into what Tom just said when explaining the physics behind it.

I max out at 400-420 forehand, 425-450 backhand.

My distance disc ia an E* Destroyer. If thrown backhand, it turns just a little, and will S. If I throw it forehand, it will turn more, and reach the destination faster.

That is what is really really hard about trying to putt forehand, have you ever tried it? You cannot lob putt with a forehand, because it's going too fast. It will fade and not drop, because the spin isn't stabilizing it.

In order for me to forehand a buzzz, I really slow down my forward pull on the disc, to slow the speed down. If I throw slow and smooth, I can get the buzzz to 300' without turn and burning it. It will still turn over more than a backhand, but I just cannot get the spin higher while maintaining accuracy.

I have a huge problem throwing tomahawks nowadays. They don't turn over, they throw like a backhand sky roller. Now if you were to throw a backhand like you are throwing a sky hyzer, but don't put as much spin on it, it would flip over like a thumber. Since you are getting spin on it, it holds the line and won't flip due to the "advance ratio". It's the same for my "tomahawk" which is why I don't use it anymore. I've increased my spin on my forehand enough so that it will hold the vertical like and not flip over due to >velocity and <spin.

Most people don't throw mids forehand, because they don't have very much spin on their forehands. It's all velocity or what we call "arming it". I can do a small reachback and do as I said with the buzzz, but when it comes to a putter, I have almost no reachback at all. I focus solely on the snap, yet I still get quite a bit of spin on it. My whole hand moves about a foot, and I can get the putter to ~200' without turning it over. This reduction of arm movements makes it really accurate for me, and is why I forehand upshot primarily. To compare it to a backhand, it's like the beto pec drill, cut in half. Very very very small arm-movement, focusing all on spinning the disc to let it glide. Boosting the spin and lowering the velocity, changing the advance ratio.

 

[tom8658]

Hey Tom, thanks for the reply. I'm afraid I wan't very clear at all in most previous post. I was trying to say that a preference or utilization of rotational motion of the thrower him(her)self will lead to a greater net disc speed. I wasn't even thinking about disc spin, and reading your post just now, it makes sense that the spin of the disc doesn't really affect the distance the disc will travel, but rather just affect it's stability. Regarding the thrower wanting more rotation, it just seems that using rotational motion is inherently more efficient when talking about a disc golf throw. It seems this really comes into play during the last part of throw, or the 'snap' portion of the throw, as the thrower turns (quickly hopefully) his shoulders and forearm, releasing the disc. The first part of the throw seems more linear as you 'pull' the disc on your reachback. My trouble is in converting from linear motion of the first part to the rotation of the last part. After some thinking it seemed that the principal of conversation of momentum would be a good way of connecting the two, but I'm a loss of demonstrating it mathematically. The best I could come up is to equate the linear momentum of the first part with the angular momentum of the start of the rotation or 'snap' part. To simplify, the mass would be considered a point mass, equal to the mass of the disc and located at the center of the disc. So then, how does that affect disc speed? After equating the linear and angular momentum of the disc, I get that v(2)= v(1)/r, where v(1) is the initial velocity of the disc and r is the radius (here the forearm approx). Of course this assumes no outside forces or torques. Does this make any sense? As you can tell, I don't have much background in physics, although I did take a course in high school and then one in college. I think my equation probably doesn't really work, lol, but do you think the theory is on the right track at all? It seems you have a good grasp of this stuff--are you an engineer?

I'm a software developer and perpetual student.

If you want to model a normal backhand throw (i.e. not the bent elbow technique), you've got four levers - the centerline of the body to the shoulder, the shoulder to the elbow, the elbow to the wrist, and the wrist to the center of mass of the disc. All four levers play a part in generating linear speed on the disc, and to a smaller degree, spin, though spin is mostly a matter of using the first three levers to make the wrist to disc lever swing as fast as possible, thus ripping the disc from the fingers.

Thus the technique of the hammer pound drills and working from the hit backwards - start with learning the feel of the motion of the most important lever, then integrate motion at the other levers to strengthen the hit.

 

[CwAlbino]

The torque in the picture of the top is caused because the gravitational force and the normal force (-F_g) are not applied at the same place, creating a lever arm and thus a torque.

You are 100% correct about apparent wobble actually being precession - it is not possible for a spin-stabilized object to react to a torque without precessing.

What I really want to know is why that precession causes the disc to turn over - i.e. behave as if the advance ratio were lower than it actually is.

I've thought about it a lot and I think I've figured out why, and it's actually really simple. It all deals with the advance ratio.

Precession slows down spin speed, causing the difference in advance ratio. Forced OAT to create a turnover is used to get the same speed out of the disc, but to reduce the spin speed on the disc so that it turns over like a forehand acts normally (more velocity to less spin ratio)

Outside of air/wind friction causing deceleration, I'll just do the precession.


------------------------------------------------------------------------------------


To put it into our language. Assuming advance ratio = spin speed / forward velocity

A clean oat free throw has:
velocity = x;
speed_of_spin = y;
advance_ratio = speed_of_spin / velocity;

Throw with oat has:
amount_of_OAT = a; (<-as a force)
speed_reduction_on_OAT[]; (<-increments of 2mph)

velocity = x;
speed_of_spin = y - speed_reduction_on_OAT[a];
advance_ratio = speed_of_spin / velocity;


Okay so our made up test data, assuming >=1/1 is a stable flight, <=1/1 would be an understable flight..
velocity = x = 60 mph
speed_of_spin = y = 60 mph

advance_ratio = 1.0
-------------------------------------------------------------------------

velocity = x = 60mph

amount_of_OAT = 3
speed_reduction_on_OAT[3]= 6 mph

speed_of_spin = y = 60mph - 6mph = 54 mph

advance_ratio = 54/60 = 9/10 = .9

-----------------------------------------------------------------------

This would also work on a clean OAT free putter throw, where it's going slower, with the same snap

velocity = x = 40 mph
speed_of_spin = y = 60

advance_ratio = 3/2 = 1.5

 

[CwAlbino]

Rameka seems to think precession is the cause of fade, not OAT. I think, though, that when he says precession, he means torque-free precession. So after the disc corrects itself from torque-induced precession, torque-free precession takes over and begins the normal flight and fade of the disc.


Fade: Fade is not as easy to explain. Fade happens because of a phenomenon called precession. Did any of you play with gyroscope toys when you were kids? Or even things like tops? Precession is the change in the direction of the axis in rotating objects. Precession is not off-axis torque, or in any way associated with it. Precession happens because of a pressure gradient, just like turn. You see, pressure that builds up in the underside of the disc due to slower air flow isn't radially symmetric. There's a point (which differs for different discs, obviously) called the center of lift. To reiterate, the center of lift is generally in a different spot from the center of gravity. This creates an unbalance on the horizontal plane of a disc as seen from behind, just like turn.

Figure 5: 3/4 side views of disc, disc is flying to the left. Air flow is shown in blue again.

Since air slows down in the pocket of the underside of the disc, pressure builds up in this initial area of the pocket, where air is slowest of all. In other words, the leading half of the disc experiences the most pressure; this should seem intuitive. This is the center of lift. Since the disc is spinning clockwise (looking down from above the disc), this pressure's force acts, and then continues to act, for a large portion of the spin, shown above in pink. This causes the starboard wing of the disc to lift up, and, consequently, the port side to dip down (also shown in pink). This causes the disc to fade.

 

[sillybizz]

Off Axis Torque isn't real, it's just like crop circles or the moon landing.

 

[CwAlbino]

Off Axis Torque isn't real, it's just like crop circles or the moon landing.

u so silly.

 

[bikinjack]

Rameka seems to think precession is the cause of fade, not OAT. I think, though, that when he says precession, he means torque-free precession. So after the disc corrects itself from torque-induced precession, torque-free precession takes over and begins the normal flight and fade of the disc.


Fade: Fade is not as easy to explain. Fade happens because of a phenomenon called precession. Did any of you play with gyroscope toys when you were kids? Or even things like tops? Precession is the change in the direction of the axis in rotating objects. Precession is not off-axis torque, or in any way associated with it. Precession happens because of a pressure gradient, just like turn. You see, pressure that builds up in the underside of the disc due to slower air flow isn't radially symmetric. There's a point (which differs for different discs, obviously) called the center of lift. To reiterate, the center of lift is generally in a different spot from the center of gravity. This creates an unbalance on the horizontal plane of a disc as seen from behind, just like turn.

Figure 5: 3/4 side views of disc, disc is flying to the left. Air flow is shown in blue again.

Since air slows down in the pocket of the underside of the disc, pressure builds up in this initial area of the pocket, where air is slowest of all. In other words, the leading half of the disc experiences the most pressure; this should seem intuitive. This is the center of lift. Since the disc is spinning clockwise (looking down from above the disc), this pressure's force acts, and then continues to act, for a large portion of the spin, shown above in pink. This causes the starboard wing of the disc to lift up, and, consequently, the port side to dip down (also shown in pink). This causes the disc to fade.

This precession could also be described as the conservation of angular momentum. This conservation causes the described disc to tilt to the left, because of the center of lift imparting a torque on a different axis than the axis of rotation of the disc. It's the whole force vector cross multiplication thing.

 

[CwAlbino]

This precession could also be described as the conservation of angular momentum. This conservation causes the described disc to tilt to the left, because of the center of lift imparting a torque on a different axis than the axis of rotation of the disc. It's the whole force vector cross multiplication thing.

Okay so would you agreed based on the definitions here: http://en.wikipedia.org/wiki/Precession

that torque-free = conservation of angular momentum and the cause of fade
and that
torque-induced = OAT, one cause of turn. (based on what I think is slowing down the spin speed)



I think I just hit ace member again, it's 2k posts right?

 

[bikinjack]

I've yet to read a satisfactory explanation for what causes the disc to turn (RHBH, flies to the right at high[er] speed). Nor can I offer one myself. The fade of a flying disc seems pretty straight-forward to me.

Your explanation of how more angular velocity makes the disc fly straighter makes perfect sense to me, however. This would be in line with other things I've read about how discs with more mass on the rim fly straighter, all other things being equal. Think Wizard compared to BB Aviar, or the way the Ion was developed, for instance.

 

[dodgeball]

I'm a software developer and perpetual student.

If you want to model a normal backhand throw (i.e. not the bent elbow technique), you've got four levers - the centerline of the body to the shoulder, the shoulder to the elbow, the elbow to the wrist, and the wrist to the center of mass of the disc. All four levers play a part in generating linear speed on the disc, and to a smaller degree, spin, though spin is mostly a matter of using the first three levers to make the wrist to disc lever swing as fast as possible, thus ripping the disc from the fingers.

Thus the technique of the hammer pound drills and working from the hit backwards - start with learning the feel of the motion of the most important lever, then integrate motion at the other levers to strengthen the hit.

Yep, a good DG throw puts those bio-mechanical levers to good use! I would say that big ones are the hips and shoulders using those core muscles and then the forearm lever that propels the disc out. I think if there's any doubt that you need to utilize these during a throw, then just watch some videos of the pros driving. In almost every video u can see them spinning after they release the shot. You always hear to shift your weight forward (which is true), but if u do it right, u will end up continuing to spin after the shot. You made a point of saying a normal backhand throw is different than the 'bent elbow' technique. Do u think there's a big distinction? I kind of think that most backhand throws will devolve into this at the end of the throw.

 

[Pwingles]

I've yet to read a satisfactory explanation for what causes the disc to turn (RHBH, flies to the right at high[er] speed). Nor can I offer one myself. The fade of a flying disc seems pretty straight-forward to me.

Your explanation of how more angular velocity makes the disc fly straighter makes perfect sense to me, however. This would be in line with other things I've read about how discs with more mass on the rim fly straighter, all other things being equal. Think Wizard compared to BB Aviar, or the way the Ion was developed, for instance.

really? we've established spin keeps a disc flat and creates an aerofoil type phenomena that acts as a wing or similar to a wing on a plane. less spin means less straight flight and more fade sooner or left turn(rhbg) so if this is the case more spin means more turn or right hand turn since it creates the opposite effect. When the disc slows down the left wing dips accelerating the the drop of the disc and slows forward momentum. This is what we call fade. Some discs have more or less fade. Basically when the disc is spinning faster than it is supposed to it will drift to the right, or the right wing will dip similar to what happens during "fade" but along a long enough timeline "fade" will still occur given that the disc has long enough to slow down before it hits the ground.

 

[bikinjack]

really? we've established spin keeps a disc flat and creates an aerofoil type phenomena that acts as a wing or similar to a wing on a plane. less spin means less straight flight and more fade sooner or left turn(rhbg) so if this is the case more spin means more turn or right hand turn since it creates the opposite effect. When the disc slows down the left wing dips accelerating the the drop of the disc and slows forward momentum. This is what we call fade. Some discs have more or less fade. Basically when the disc is spinning faster than it is supposed to it will drift to the right, or the right wing will dip similar to what happens during "fade" but along a long enough timeline "fade" will still occur given that the disc has long enough to slow down before it hits the ground.

I got the fade thing down cold. I understand it well. It's the high speed turn that I don't completely understand the cause, aeronautic-ally speaking. Also, I'm of the opinion that more spin does not cause more high speed turn, rather that more spin (higher angular velocity) would cause the disc to turn less, or hold the line better, or fly straighter, all other things being equal. Fade is caused by the differential between lift at the front of the wing and the back, or in other words, the center of lift is in front of (in regards to the velocity vector) the center of mass, causing the front of the disc to rise in relation to the rear (causing a torque), thus the disc fades left (for RHBH) due to the conservation of angular momentum, or in other words, precession.

 

[CwAlbino]

I got the fade thing down cold. I understand it well. It's the high speed turn that I don't completely understand the cause, aeronautic-ally speaking. Also, I'm of the opinion that more spin does not cause more high speed turn, rather that more spin (higher angular velocity) would cause the disc to turn less, or hold the line better, or fly straighter, all other things being equal. Fade is caused by the differential between lift at the front of the wing and the back, or in other words, the center of lift is in front of (in regards to the velocity vector) the center of mass, causing the front of the disc to rise in relation to the rear (causing a torque), thus the disc fades left (for RHBH) due to the conservation of angular momentum, or in other words, precession.

Spin doesn't cause turn, it has a part in it, but it's not all of it. That's why Tom corrected me when I quoted Rameka's post, because it's explaining it, but not in the best way. It all deals with the advance ratio regarding how a disc will fly.

Also wrong. What we call turn is more properly called negative roll (though that depends on convention) and is caused when the differential between the center of gravity and the center of lift creates a pitching moment (that is, a torque about the pitch axis - pushing the nose either up or down). Because the disc is a spin-stabilized wing, gyroscopic effects dictate that moments on an input axis normal to the angular momentum vector also cause motion on a mutually perpendicular output axis.

http://www.discwing.com/research/aerodynamics.html
http://en.wikipedia.org/wiki/Gyroscope

The only variables that truly matter to disc flight that are not the product of the aerodynamics of the disc itself are release angle, angular velocity / directional velocity (more properly, a quantity called the advance ratio), and and off-axis moments applied to the disc at release.

The velocity and the spin both create the pressure needed to force down the starboard side, while the spin's gyroscopic effect also helps to stabilize it.

 

[MattMan]

Wrong, Here is the correct definition of turn.

Turn: When a disc is fired off, it carries a lot of speed in its initial path. This causes the disc to act like it ignores the friction of air for a short period of time. Because we can count air friction as having a negligible effect during this period, all that factors into the tendency of the disc is torque. The direction of torque on a RHBH throw is clockwise (if seen from above). This means that the disc has a speed about its center of gravity which is different from its speed at its port and starboard wing.

God, I hate classical mechanics, so I won't chime in much. I feel like I've seen this several times, that the torque is going around the disc, which is not true. The torque is a vector pointing downward in this case. Unless of course you're treating it as a bivector coming off the exterior product, in which case the clockwise motion is the orientation given along with the orthogonal direction.

Off axil torque if taken literally in this case, which is apparently what is being argued (I don't really know, I skimmed 85% of this thread), just implies as you apply your force to the disc (the one that makes it go spin), you don't keep the disc in a level plane (what ever that angle plane may be is irrelevant).

 

[Apothecary]