rec.sport.disc FAQ (1/5)

* Indicates new or vastly revised questions

Ultimate is a fabulous, high-energy sport that can be enjoyed by people of all ages and disc-skills who don't mind a little running and a lot of fun. The description below applies to the outdoor version of the game. The indoor version, being on a smaller field, is somewhat modified (a slightly smaller field and fewer players) but mostly similar.

Picture, if you will, a playing field (usually grass, but desperate teams will play on almost any surface) as follows:

     <- 25 yds -> <--------------- 70 yds --------------> <- 25 yds ->
   ^ +-----------+---------------------------------------+-----------+
   | |           |                                       |           |
   | |           |                                       |           |
   | |           |                                       |           |
   | |   End     |                                       |   End     |
  40 |           |                                       |           |
  yds|   Zone    |                                       |   Zone    |
   | |           |                                       |           |
   | |           |                                       |           |
   | |           |                                       |           |
   | |           |                                       |           |
   v +-----------+---------------------------------------+-----------+

On this playing field are two teams of seven players each. The object of the game is for a team to pass the disc from player to player, all the way up the field, and catch the disc in their end- zone, which scores a point. Players cannot run with the disc, but must plant a pivot foot (as in basketball) and throw the disc to a teammate. When holding the disc, a player gets ten seconds to throw it to a teammate (five or seven seconds indoors), which is counted off by the defender guarding the offensive player (known as "marking" the thrower.) If the disc isn't thrown in time, it's called a "stall" and the defense takes over.

If the offensive team drops the disc, catches it out of bounds, or failes to complete a pass because a defender somehow blocks the pass, the other team picks up the disc where it lands and works to score in the other direction. Defenders gnerally play either a man-to-man or zone defense in their attempt to block a throw.

The game is non-contact - it's a foul to hit the other player, or to hit the disc while it's being held. (Blocking the disc right after it's thrown, known as a "point-block", is a very hot play!) Nor can a defender be "picked" off the player being guarded. Any play carried out with the main intent to prevent another player from having a fair chance at catching the disc or making a defense is considered a foul; in other words, you have to "play the disc, not the person!"

Probably the most important part of Ultimate is known as "The Spirit of the Game". This catch-phrase is used to describe the respect that every player in the game has for his fellow players. No referees are used in the game. Instead, each player does his best to make an honest call if necessary, and trust the calls of his fellow players, with the implicit assumption that nobody in Ultimate would try to cheat.

This principle is what makes Ultimate special to so many people, and all Ultimate players try to keep the Spirit alive by maintaining this high level of trust, no matter how competitive the game gets. If people cannot resolve their differences, people usually say "back to the thrower", which allows play to continue on without forcing the issue one way or another.

The best way to see how Ultimate is played is to go watch a local tournament. Ultimate players share a great comraderie, and LOVE to introduce new players to the sport. So come on out and watch!

Disc golf is a great sport for everybody that relies on one's ability to throw a disc with power and accuracy. People of any age, ability, and gender can excel and enjoy disc golf immensely.

The object of the game is to traverse a course from beginning to end in the fewest total number of throws of a golf disc. Similar to the traditional golf game, a course is composed of a number of holes, in which each player begins by throwing from the tee, and completes the hole by landing in or striking the target.

The total score for a course is determined by totaling the number of throws made on each hole. The winner is the player who completes the course in the fewest number of throws...or whoever has the most fun!

Disc golf courses exist in many different terrains. Often they are laid out among wooden areas, with water hazards, large elevation changes, and difficult throws. Other courses are mostly flat, with few natural obstacles. The obstacles should be considered part of the course, and not tampered with (even when a tree eats your disc!)

The average course is 18 holes, but 9 hole and 27 hole courses exist as well. The average hole is around 425 ft (130m), but some are as short as 150 ft (45m) or as long as 1000 ft (300m). Courses usually have a listed par, for pro or amateur players. Of course, people practice disc golf all the time by just aiming for an object a hundred yards away, which is the kind of disc golf one will often see being played on university campuses or urban parks.

Terms:

Tee -

this the area where the player starts each hole. Some courses have multiple tees for each hole. The material on the tee surface varies from concrete, asphalt, dirt, crushed stone, or wood chips. In general, any flat non-slippery surface is good.

Target -

The target is where the disc must land in in order to complete the hole. The target is usually a "pole hole" which is specially made to catch the golf disc. Courses that do not use pole holes are usually known as object courses. A typical "object target" is a tree trunk, 4x4 or pipe.

Golf disc -

a "golf disc" is a flying disc made especially for the sport of disc golf, although some players use Wham-O type frisbees. Golf discs vary in weight and size. They are usually harder and denser than Wham-O type frisbees. Special models exist for driving, putting and "up shots" (not as far as a drive, but more than a putt) much like different golf clubs exist in ball golf. However, players are not required to use a "driver" as a driver or a "putter" as a putter. Some players throw a putter as their first shot from the tee. A golf disc generally costs anywhere from US $5-7, depending on how many are bought.

A professional PDGA tour exists, currently has about 5000-7000 active members, some of whom play on a professional level for money, and some play on a amateur level for non cash prizes. The top money winner last year won over US $16,000.

The UPA Top 25 is calculated by Eric Simon and distrbuted weekly. However, the Top 25 isn't accurate unless college tournaments call in their scores! So, please, all college teams and tournament directors should send in their scores to Eric or the UPA (see FAQ.2 for a contact list.)

The most basic explanation of the Top 25 rating system is this: for each game a team plays, the team gets rating points. These rating points are then averaged.

The next level of complexity is how to compute the points for a given game, and how to avereage them. The points for a given game is given by this formula:

        pts = opp_rate + (400 / x)                           (1)

        x = max(.66,(2.5*(losing score/winning score)^2))    (2)

So, suppose team A has played in 4 games, and each individual game rating is 1298, 913, 1410, and 1103. Well, we simply average them together, and team A has a rating of (1298+913+1410+1103)/4 which is 1181. But, actually, the averaging isn't quite that simple, either. We actually take a weighted average. In the above example, each game had a weight of 1, in actuality, the weight depends upon how recently the game was played. This formula is:

        wt = min(1,1/(((today-gamedate+4)/7).4))            (3)

Finally, whatever the weight it, it is doubled for games at Regionals, and tripled for games at Nationals. After all, teams are usually at full strength during those tourneys, and the games are more important. Finally, it is hoped that the winner of Nationals will come out as number one in the rankings. Luckily this has always happenned, although one year a team that lost in the semifinals almost finished first.

But that's not all! Suppose the ratings of the teams you play change. An underated team you lost to in the first round ends up winning the tournament. Should your rating reflect that teams' victories, in other words trying to take into account that the other team was a really good team. Of course it should. Suppose your team's rating went up during the course of the tourney, too; shouldn't other teams, in turn, get the benefit of that?

This is done in an interative process. On Monday, every team gets re-rated. That is, we recompute every individual game rating, based on the previous week's ratings, and the new date. Then, each team gets a new rating for the current week. Then, we re-rate every team again, using this week's ratings, to get a new set of ratings. We do this 20 times (this is why a computer is indispensable). Eventually (usually after only about 8 interations) the ratings reach some sort of equilibrium. It's kind of a neat process to watch. If some team does really well, and the rating goes up 250 points, then, on the second iteration, all teams that have played the first team goes up by a smaller amount, and on the third iteration, all the teams that have played the teams that played the first team will go up by a small amount, and so on.

The biggest problem with the system is that in some areas of the country not everyone is calling in scores. Let me give a classic example of how an entire region can be adversely effected by this.

Suppose the best team in Region X always calls in their games (and, in fact, more winners than losers call games in). So, suppose this team "State U." calls in 13 games of theirs, all victories. None of the other teams had called in any scores. This team beat, say, team B in the finals of two other tournaments. Obviously, team B must've been pretty good to make it to the finals, but to the computer, team B was simply 0-2. In fact, to the computer, it looked like the 13-0 team was playing a really wimpy schedule because every team that had played was winless! So what happens? State U doesn't get a very high ranking. Now, weeks later, the other scores are called in. It's too late, State U is already ranked lower than they should be, and all these other schools are, correctly, ranked lower than State U is. So, the whole region gets ranked lower than they should be.

A MAC, also known as a "mack", actually stands for Midflight Attitude Correction. In the sport of Ultimate, it usually happens by mistake, but here's how to do it on purpose.

To MAC a disc effectively, one needs to be aware of the direction of spin the disc has. The two possibilities are clockwise (originating from a standard backhand throw from a right-handed player) and counterclockwise (a sidearm throw from a right-handed player).

The best throws to MAC are hard with lots of Zs (spin). The technique is to allow the disc to be throw at you very hard, allow the disc to pass you, the MACer, on one side of your body or another, and just as the disc is perpendicular to the throw line, touch the side of the disc very briefly.

If the throw is clockwise, allow the disc to pass your right side (as you are looking at the thrower) and tap the top of the disc's platter, near the outside of the disc at the point closests to you. The disc will then take a MAC, climbing upward. If the clockwise throw passes on your left, that tap will send it straight into the ground.

Switch everything around for a counterclockwise throw. Disc passes you on the left, tap the side of the disc, it takes a MAC and climbs up. Disc passes on your right, tap it on the side, the disc dives down into the ground.

There are some neat variations of the MAC, like the foot MAC, which takes some extra practice. Hitting the disc on the outside edge from the thrower is also harder. If you want to see one of the best MACers alive, watch Dan (Stork) Roddick sometime. He is amazing! (he is also the Sports Director for Wham-O). And no place is better to MAC than on a California beach somewhere.

Apparently, the patent applications from Wham-O in the late 1950's are interesting reading material. See the patent section of any well-stocked university library for references in this area.

frevi@athena.mit.edu did work as an undergrad (MIT) involving the visualization of flow around a rotating frisbee using dry ice vapor as the tracer aerosol and stroboscopic and conventional photography. In particular, a number of photos were taken of vapor flowing around a disc mounted on a motor in various orientations, the trajectory of a frisbee throw through a sort-of stationary flow field stopped stroboscopically, and various multiple exposures of throw/release motions. The results of the flow studies seemed to indicate that a rotating frisbee induces lift independent of a trajectory vector; i.e. the disc doesn't have to be going someplace to generate lift, just spinning.

medf214@chpc.utexas.edu (Aaron Altman) did some interesting work analyzing the behavior of a disc in a wind tunnel, with specific regards to the so-called airbounce. He examined the effects of windspeed and angle of attack [alpha] on a disc. From his messages, slightly edited:

After performing many wind tunnel tests on an old, wasted Wham-O, I measured the effects of varying angle of attack and windspeed. It was difficult to determine the rate at which an average disc is spun, so this part of the experiment is very much "up in the air". There was also no way to simulate the initial "throw", or accelleration of the disc, so all of these results examine the disc under a constant windspeed, which ignores all of the interesting things which happen to the airflow around a disc as it is thrown.

The simplest visualization for the results is to draw the analogy between an airplane on approach to landing, and a disc at high alpha. Increasing the angle of attack increases the induced drag (or resulting drag force), but enables the disc to fly slower while still flying in the same flight path. The airflow on the top of the disc is usually not "attached" fully, inplying a turbulent, vortical, unsteady, non-laminar flow. The same is true for an airplane on approach to landing. The airplane reduces its speed, but the flight path is maintained (within a certain range) by increasing the alpha of the plane. In an airbounce, some extra lift is generated from the so-called "ground effect" as well.

This experiment gave no data on the limits of the ground effect. However, the limits are determined by the amount of wing loading, so one can guess from experience with other flying objects. For example, the ground effect for a Cessna 172 tends to be approximately 1/2 the span the wing, which is s approximately 20 ft. This causes the airplane to float above the runway at speeds lower than would normally be possible for this airplane. From discussions of the ground effect with pilots of larger airplanes, such as the Boeing 727, this equation (1/2 span) doesn't hold as well, and the ground effect tends to be between 1/2 and 1/3 span. At any rate, that brings us back to the disc...If we took 1/2 the span of the disc, this would place us somewhere about 6 inches off the ground.

So, what does this all mean? In terms of flight dynamics, the small displacement given to the disc by the thumb at the last second causes the leading edge to rise. This, in combination with the large, instantaneous, simultaneous forward force of the throw, (called the impulse,) creates a high angle of attack flight regime, with the possibility of an increase in altitude, depending on the actual angle of release from the hand (angle the arm makes with horizontal upon release of the disc, not the same as displacement given by the thumb.

Of course, if all of this is true [and it may not be!] it should be possible to throw a disc at high alpha, without the angle imparted to the horizon by the arm, and still have an airbounce. This would result in a disc flying without a change in height, but with a large angle of attack. This would imply that, not only is it necessary to apply thumb pressure, but the angle of release is also quite important in establishing a true airbounce.

Given all of this, one can describe the physics required to throw a disc that goes down and then up. The follow-through on the throw would be downward, but the force applied to the disc in that instance is applied just below the horizontal (say, between -3 to -8 degrees). This force will cause the initial trajectory of the disc to be slightly downward. After some distance, the component of lift generated by the forward motion of disc (at alpha) overcomes the initial slight downward component given in the initial release of the disc. In studying the problem as a thin airfoil (using thin airfoil theory) this can be shown to be possible quite trivially.

When a disc is thrown, it undergoes an initial acceleration that is quite large. Once released, the speed decreases as a result of viscous losses due to the friction of air. This change in speed results in a highly unsteady problem (which changes as a function of time). By analyzing the lift force and drag force (Cl, Cd respectively) at many different windspeeds and alphas, a profile of the behavior of the disc as its thrown can be examined, with Cd and Cl increasing essentially linearly with alpha.

In addition, I would like to comment on some info included in the most recent version of the FAQ. As determined by my experimentation, the component of lift generated by a stationary disc, spinning, is extraordinarily small when compared to the component of lift generated by the forward motion of the disc.

This is with reference to the work done by frevi@athena.mit.edu. The information that he obtained was strictly qualitative, and the quantitative data that I obtained tells me that a spinning disc (without a directional component) generates very little lift. A good physical analogy would be to say that if this were true, than this aspect of lift would be exploited in modern lifting bodies, lending creedence to the possibility of flying saucers!

Overstable -

a disc that turns left when thrown flat with a right handed backhand throw

Stable -

a disc that goes straight when thrown flat with a right handed backhand throw.

Instable -

a disc that turns right when thrown flat with a right (understable) handed backhand throw.

Hyzer -

The disc is tilted towards the ground to make the disc turn left when thrown with a right handed backhand throw.

Anhyzer -

The disc is tilted to produce a curve from left to right when thrown with a right handed backhand throw.

The Greatest -

Shortening of the phrase "The Greatest Play in Ultimate". I.e., When a player on the offensive team jumps from in bounds to catch a disc, going out of bounds, and before he contacts out of bounds he throws to a team-mate in bounds. Bonus points if it is also thrown for a goal.

This is part one of the rec.sport.disc FAQ [Frequently Asked Questions list]. This file, and its companion files, are posted bi-weekly to rec.sport.disc, news.answers and rec.answers. The posting is designed to answer questions which are commonly asked by new readers of the group, as well as to provide a reliable source of information for regular readers.