# Speed of a Tenser's floating discmobile

So Tenser's floating disc "follows you so that it remains within 20 ft" if you are more than 20 ft away.

So let's get a bit of carpentry skill together, and (whilst keeping the total weight under 500-1000 pounds) rigging up a cart that is 25 feet long. The underside of the carriage is 3 ft tall, and the wheels are 3 ft diameter.

The wizard casts Tenser's Floating Disc, and the party's barbarian picks up the carriage, and puts the rear end on top of the floating disc (let's strap it in with some ropes for good measure).

The wizard then climbs on the carriage, and moves towards the front.

As soon as he gets > 20 feet away, what happens? What speed does the happening occur at?

Bonus question: Assuming the above answer is that the carriage moves, at a high enough speed to be useful, any ideas how to steer such a device other than the wizard moving back within 20 feet to stop it and turning it manually like a peasant?

To alleviate concerns that the carriage is an obstacle to the disc, an alternate construction could instead use a 50 ft plank, not touching the ground, with the disk in the middle, the wizard at the front and an equal weight counter balance on the other end. That way, the disk is carrying things, but it not touching anything other than the things it is carrying.

• I swear, in every edition people put 10 times more skull sweat into abusing Tenser's Floating Disc than they do for any other spell. Commented Jul 12, 2015 at 13:46
• @ObliviousSage You say that like it's a bad thing. :) TFD cheese has been amusing players for 4 decades. I think it should be encouraged. Commented Apr 4, 2018 at 22:21

# Nothing Happens

From the PHB, page 282

The disk follows you so that it remains within 20 feet of you

Given that it cannot move to within 20 feet of you, it won't move. It would consider the cart as an obstacle.

Your best bet for your "tenser's physics defying discmobile" would be for the wizard to get on a horse and let the disk follow. Everybody else can sit on the disc.

But if it does some how work, and you need to steer...

Let the barbarian hold the wizard out on a stick 25 feet long. He can steer the cart by changing where he hangs the wizard from. You may need a counterweight so he doesn't get tired.

• This way the Wizard gets great glory for the new way of using low-level magic, followed by utter humiliation of being used like a carrot on a stick for a donkey. Commented Jul 12, 2015 at 15:57
• Your reading seems to imply that TFD somehow "knows" if its motion would bring it closer and then moves or not accordingly. Let's accept that. What say you, then, to barbarian on disc, 25' pole, 5' rope suspending wizard? Would TFD "know" that moving toward the wizard would (momentarily) bring it closer, as the wizard swings back (as viewed in disc-frame)? High enough cycle speed and I think we've got the TFD Segway!
– nitsua60
Commented Jan 2, 2016 at 3:36
• As @nitsua60 noted, this answer requires the disk to be sentient and know whether its action or not moves it closer the wizard who is suspended in front of it. Commented Oct 2, 2019 at 16:08
• @Praxiteles Some spells have this effective property (very narrow situational awareness), though TFD isn't the clearest case of it. I mean, the disc already "knows" how far away the Wizard is and in what direction. It's not unlike Enlarge "knowing" not to expand a creature's size beyond what its surrounding area can contain. Commented Oct 2, 2019 at 17:52
• @NeilSlater and then, the wizard realizes he has a ring of Spell Storing. The barbarian who made such elegant arguments for why he had to do it for the good of the party isn't laughing now... Commented Dec 23, 2021 at 4:05

# The cart accelerates incrementally until it falls apart

Disclaimer: This is a very literal, and some would say "munchkin" way of reading the rules

Seems weird at first, but let's look at the spell:

The disk is immobile while you are within 20 feet of it. If you move more than 20 feet away from it, the disk follows you so that it remains within 20 feet of you.

So this infers that the disk accelerates at a speed slightly faster than yours, enough to overcome its inertia and remain within that 20 feet.

What this means is that, as you take the step that would put you >20 feet away, the following scenario occurs:

• The disk (including what it is "carrying"... i.e. the cart and you) accelerates with enough force to overcome its inertia and keep up with you, which leads to you accelerating forward as well.
• This means that the disk accelerates to attempt to keep up with you.
• Which means that the disk accelerates to attempt to keep up with you.

ad infinitum... Except at some point the restraints that are keeping the disk lashed to the cart give out and the disk is free to continue forward at its current speed.

• Next step : add a spike in front of the wizard, and enjoy a high-velocity siege engine. Commented Jan 12, 2017 at 13:53

## Nothing happens.

From the spell description (PH p.282):

If you move more than 100 feet from the disk (typically because it can’t move around an obstacle to follow you), the spell ends.

Any attempt to harness the disc to provide motive power counts as an obstacle and prevents it from following the wizard.

• The load on the disk is not an obstacle. The fact that the load might have supporting wheels to the side does not make it an obstacle. And the fact that the wizard is moving does not make the load an obstacle. Commented Apr 12, 2018 at 2:21
• The body of the cart sitting between the caster and the disk, which prevents the disk from getting any closer to the caster, on the other hand... that can be reasonably interpreted as an obstacle. Commented Apr 12, 2018 at 12:00

It's going to be a lurching ride

The interpretation of the obstacle might be correct, but I'd rather instead follow the continuous acceleration interpretation (also mentioned by Kira).

The disk is immobile while you are within 20 feet of it. If you move more than 20 feet away from it, the disk follows you so that it remains within 20 feet of you.

The question, then, becomes: how fast is the disk going to accelerate? This is not specified in the spell. I find this quite unfortunately since the disk can be up to 100 ft. from the Wizard, blocked by an obstacle, and thus as soon as the obstacle is removed the disk should come to within 20 ft.

Now, in the absurd case of an infinite, or very high acceleration, any object on the disk would in this situation be ejected either at the start of the acceleration or the end of the deceleration. It does not seem very useful, a disk which would lose everything onto it at the slightest obstacle, so I'll assume that in the case the disk accelerates and decelerates moderately enough that anything on the disk stays on the disk (though it might topple cups and the like)

All right, so the disk starts to accelerate smoothly, the players might feel very clever...

... right until the Wizard starts having difficulty hanging onto the ledge of the cart as the winds push him back, and indeed everything onto the disk is pushed back by the wind until it topples off the disk. But let's imagine the players solve this issue, they might feel very clever...

... right until the Wizard cannot keep his eyes peeled to direct the disk because of the wind. But let's imagine the players solve this issue, they might feel very clever...

... right until air starts to heat around the Wizard (friction creates heat after all). But let's imagine the players solve this issue, they might feel very clever...

... right until the Wizard's reaction time starts to be quite insufficient to dodge obstacles. But let's imagine the players solve this issue, they might feel very clever...

... well, I could continue for some time, but the ultimate issue of reaching relativistic speeds has already been explored by Randall Monroe, of xkcd fame, in its What If 1: Relativistic Baseball article.

Oh, and of course, let's also talk about deceleration. There are two ways to decelerate should the Wizard suddenly be within 20 ft. of the disk:

• smooth deceleration, aka inertia
• immediate deceleration, aka catapult

The latter might be a little violent, and contradicts the smooth acceleration thing, so I would instead advise a smooth deceleration. That is, a disk that does not stop until it hits an obstacle or is given sufficient time to return to a null speed (like, proportional to the time spend accelerating).

The main advantage of smooth acceleration and deceleration is that it does reward players' creativity, as long as they manage to avoid infinite acceleration and handle the inertia. At the same time though, there is no stable speed here. The disk is either accelerating because the Wizard is more than 20 ft. or decelerating because the Wizard is less than 20 ft.; that makes for a lurching ride, not everybody might appreciate.

Oh, and I seem to remember something about the disk hovering close to the ground; hope your Wizard pilot avoids the cliffs!

• Err, that is not correct. The PHB states that the disk speed is 30, which puts an upper limit of about 5ft/s or 3.4mph. If the disk is at 60ft of you and the obstacle preventing it from coming to you disappears, it won't get to 20ft of you in one turn, it will take two turns. Commented Apr 12, 2018 at 2:19
• @FranciPenov Disc's speed is not stated anywhere in TFD's spell description. Commented Apr 12, 2018 at 14:10
• Ugh, yeah, I misread the range as a speed... :-/ Commented Apr 13, 2018 at 17:18
• A reasonable approach for how the disc governs its position / speed / acceleration would be to imagine a servo feedback loop that proportionally controls its speed, based on how far past 20 feet away it is. So if the wizard is separate from the disc and moving very fast (e.g. hasted Fly and dash, or Wind Walk), the disc will keep up but will be more than 20 feet away, maybe 40 when really going fast. (Subject to the DM's decision on an upper speed limit, either based on exceeding 100 ft, or separate.) Commented Dec 23, 2021 at 7:12
• Whatever model of control loop you decide on determines what happens when disc motion moves the wizard. If it controls its speed, the wizard's position sets the speed directly. (With acceleration determined by how fast the wizard moves past 20ft). If it controls acceleration, then as you say the wizard being slightly past 20 feet leads to acceleration with speed increasing over time. If it controls force, opposed by wind / wheel resistance, speed would stabilize. (But "obstacle" is absolute in the spell description, so there's no provision for it pushing through things or not.) Commented Dec 23, 2021 at 7:15

# The disk moves. The DM decides how fast.

The spell states that the disk moves to within 20 feet of the wizard.

The wizard is more than 20 feet away. Thus, the disk moves towards the wizard.

Unfortunately, the spell does not define how fast the disk moves. Previous editions did specify in the WoTC 3.5 FAQ (Pg. 91) but the link is now broken. Here is the quote:

Can you ride your own Tenser’s floating disk?
No. While you could command your Tenser’s floating disk to move close enough for you to sit upon it, it has no ability to move under its own power. It can follow you only at a maximum rate equal to your normal speed.

Thus the DM has to decide whether to use the previous rules or not.

To stop or turn the disk - the wizard would have to move within 20' of the disk - or move to the left or the right in the cart to change the direction of the disk's movement. Notice that the disk's turn radius would be very large which could make dodging obstacles challenging.

# Your idea could change areas of the fantasy world everywhere

Given the ease of your idea to use - presumably this approach could be used commonly to cross areas of mud, boggy ground, deserts, shallow puddles of acid, lava or anywhere the ground isn't easy on cart wheels.