Correct me if I'm wrong

[EricH]

Originally Posted by BigWhiteYeti

I've only seen two robots with fans and both were pointed horizontal, so they add thrust but not traction. If I remember right, one of the updates had a clause outlawing "mass manipulation" to gain a traction advantage. I guess it is up to the inspector to judge whether a fan is mass manipulation.

On a separate note, why the heck to they have this rule in the first place? The possibility of increasing traction got our entire team talking physics forced everyone to come up with creative designs. Why limit creativity??

There's more than 2.

The mass manipulation was in the Q&A; a team asked if they could, say, move the orbit balls for the sole purpose of traction. Fans are NOT mass manipulation; they simply move air. As they are explicitly allowed by the Q&A (given a horizontal direction so as not to violate <R06>), then any team can use them.

Why the rule? Why which rule? <R06>? It's part of the game challenge. However, you can't add traction by altering the reaction with the ground under <R06>. There only remains the chance of adding a reaction with air to increase speed, acceleration, and/or pushing power. Fans provide that, and are legal.

[Lil' Lavery]

Originally Posted by BigWhiteYeti

I've only seen two robots with fans and both were pointed horizontal, so they add thrust but not traction. If I remember right, one of the updates had a clause outlawing "mass manipulation" to gain a traction advantage. I guess it is up to the inspector to judge whether a fan is mass manipulation.

The point he was trying to raise is that if the fan isn't transmitting force directly through the center of mass of the robot then it will create a torque (rotational force) on the robot as a whole. This torque will apply additional force to one set of wheels (the front if the fan is above the center of mass) and lessen the force on the other set. He was asking if this change in force distribution would be considered to be changing the traction properties of the robot.

Think of a car. When it accelerates the weight of the car shifts towards the rear wheels. When it brakes it shifts towards the front. Same reason motorcyclists do wheelies while they're accelerating and decelerate to bring the front wheel back down.

Now, I don't think this violates the rule, because the total normal force does not change. It is just shifted. Beyond that, if this violates that rule, so do any drivetrains where they don't apply force through the directly through the center of mass, which is likely every single team in FIRST.

[Tristan Lall]

The real problem is that FIRST has been equating normal force or frictional force with traction. They're not the same, but are obviously related: traction is a motive force derived from friction between surfaces and normal forces on the scale of interlocking surface features, while the overall normal force and frictional force are related to any downward components of force (e.g. from weight, downward thrust, momentum over time, etc.).

This has led to Q&A posts that do not frame the response in terms of the stated objectives and context of <R06>—to restrict devices other than rover wheels that would increase traction by interacting with the arena.

Only the craziest interpretation of <R06> leads to the conclusion that downward-thrusting fans that interact with the air are traction devices. It's crazy, because if you're going that route, the rules don't give any leeway to ignore the traction-increasing effects of weight and momentum. As far as the rover wheels are concerned, all downward force increases traction, no matter whether it is a result of gravity, or a fan pushing air upward.

And of course, because this information was released in Q&As, a team might quite reasonably object on the grounds that it is a non-binding statement, rather than an enforceable rule.

Note, also, that the statements about vacuum cups and similar devices stem from <G29> and <G30> issues (i.e. field damage). Even then, there is no prohibition; just a stern warning and the threat of being restricted from using the robot.