Physics of T-boning

I’ve been curious to know what T-boning is, precisely, from a physics standpoint, and what factors are involved.

In a situation where Robot A (with, say, a standard four rubber wheel drivetrain) is getting T-boned by Robot B (but not pinned), is it appropriate to define the T-bone as a situation where Robot B’s drivetrain is applying a force that causes Robot A to lose traction with the floor (therefore only having the benefits of the coefficient of kinetc friction) and therefore not being able to generate enough force to escape laterally?

Does this imply that Robot A’s drivetrain would be more resistant to T-boning if it had a higher coefficient of static friction?

T bones occur when the usual dynamic of friction in FRC is switched. Usually, friction between the ground and the wheels is static, or rolling, and the friction between bumpers is dynamic (as robots slide off each other, etc). When the bumper-bumper friction becomes static, and the wheel friction of the robot being defended becomes dynamic, then theres a T bone pin going on.

The pinned robot can’t escape because they can’t move sideways relative to the defending robot (because the bumper friction outweighs the sliding friction on the tires) and it can’t move forwards of backwards relative to the pinning robot because it’s wheels don’t move that way. Its wheels are in constant dynamic friction because it is being pushed from the side, so it’s always sliding (whether or not it’s wheels are rotating). As soon as the pinned robot stops getting pushed sideways, it can usually get out of the pin.

Interestingly, one of the things we look for in tread (low sideways COF) which makes it easy to turn makes it particularly difficult to get out of T bone pins. We found VersaWheels were just terrible in this regard this year.

Now, omni wheels do not slip when pushed from the side, so they would be good for these situations, right?

That is correct, but in most cases* an all-omni drive would just render you powerless against your opponents since you have almost no traction. A traction + omni mix, a butterfly drive, or drop down casters fixes this by adding an area of high friction to rotate around while the low friction spins out of the pin.

*Ellen Green and 33 are exempt from this

I’ve seen other robots “roll” around defenders with all omni drives. See 9973 in the 2013 offseason, for example.

If it’s a head to head pushing match, you’re right, a omni drive would get creamed, because the rubber on omnis is no match for roughtop. On the other hand, if you’re a robot getting defended, you shouldn’t really be getting into many head on pushing matches, especially if you’re an all omni drive (or butterfly drive). I’d argue that all omni drives are better for anti-T bone pinning purposes than half omni half traction (or drop down casters, etc.) as they’re even less sideways friction, and give the defended robot more degrees of freedom to roll off in.

Vex gives these values for CoF:
Omni: 1.1
Traction: 1.1
Versa: 1.2
Versa DT: 1.0

There isn’t that much more traction with any of them. I don’t know what roughtop would get using the vex method, but seeing as when you push sideways on a robot with omnis you don’t get much normal force or friction (unless you are pinning, which is different), I would think they would work pretty well.

That implies that bumper construction and design would be critical in a T-bone situation.

How would

  • Relative Bumper Height
  • Bumper Material
  • Robot Center of Gravity

theoretically play into a T-bone situation?

That is only in the direction of rotation, assuming movement is all in a straight line. Problem is because omnis have rollers on the wheels which makes them slip and slide and rotate, which makes it extremely easy to move an omni bot sideways, or rotate it from head-on.

When you are trying to get out of a t-bone, you are trying to move forwards. With omni wheels, you should be able to do this pretty easily, with some extraneous sideways movement. Unless you’re trying to do something like hang on a 2007 goal, I think this is better than being pushed halfway across the field.

As you may have already seen, many teams are starting to shape their robots so that their bumpers assist with getting out of pins. 971 is a notable example with their octagonal frame this year, where the majority of their robot frame is angled so that they are more likely to be pinned from one of the angled parts where they can get out of instead of from the side or front where it’s harder to get out of the pin.

The height of the bumper itself doesn’t matter - it’s the point of contact. Bumpers that contact each other more have more friction between each other. If both teams have their bumpers at the lowest possible point, then there is more friction between the bumpers. If one has their bumpers at the highest point, and another at the lowest point, there is less bumper friction and is therefore more difficult to pin solely due to bumper friction. What this also does is makes it easier for the robot with the lower bumpers to get under the bumpers of the robot with the higher bumpers, thereby lifting the pinned robot off the ground lessening their normal force (and their friction), and increasing their own normal force and friction, making their pinning strength a lot more powerful (and it’s completely legal since it’s not within the frame perimeter).

Bumper material definitely does play into the pin, since the coefficient of friction between two bumpers is dependent on the two materials. Teams have recently started making smooth leather bumpers to decrease this coefficient of friction and make slipping out of pins a lot easier.

A robot’s center of mass will change where a robot rotates around when getting pinned, or if it will fall over when hit too hard. It is an option to put your center of mass off to one side making rotating out of pins through rotating that side a very viable option.

You are correct that the lack of horizontal traction allows you to keep moving forward, but you don’t just slip by because of the friction between the bumpers. What ends up happening is the pin starts moving in a circle due to the bumper friction pushing perpendicular to where the robot is driving away.

There’s a famous quote out there that goes something like, “Now you’re thinking with portals.” Along those same lines, think in terms what each of those variables would do to the friction between the two bumpers. In other words, start thinking in terms of vectors.

Relatively Bumper Height won’t play too big of a role in prevent t-bones. Friction is a function of the surface material and the force involves. Limiting the surface area wouldn’t do anything. However, mounting your bumpers too high could let a particularly low defender get under your bumpers, which isn’t good either.

Bumper Material is definitely an interesting idea, and something that immediately came to mind when this thread popped up. The going theory is that if your bumper cover reduces friction, then you’re on the right track. However, in my opinion, you’d need to not only show that your material is (a) low-friction on most other bumper material used in FRC and (b) makes a significant enough difference to actually matter.

COG is another interesting point. If you look on the three axis, “up and down” positioning should not matter in a pin situation (aside from the obvious instability issues). When considering where on the base your COG lies, this could be a difference. You’d have to consider the moments involved, which include the moments caused by your pinner, your wheels, etc. Ultimately, there’s two things to note about this; you have to balance your free performance with anti-pinning performance and if a defender pins you head on your COG, you’re both gonna get to know each other well for a few seconds.

  • Sunny G.

Watch 33 in MSC Finals (https://www.youtube.com/watch?v=iTV77XLXB0Q). both 67 and 74 attempt to get them stuck in a t-bone multiple times, and the most successful attempts, 67 at 2:14 and 74 at 2:25, result in them getting turned a bit, but not dragged into a circular pin. Maybe it’s just amazing driving, but I think their omni wheels play a pretty big role in it.

I’ve actually seen this happen a lot on robots with good, traction setups. However, an all-omni setup goes to the other extremely and lets the robot slide out easier.

It is my understanding, that’s why team 33 was successful with their DT this past year (along with some fancy software).

  • Sunny G.

Remember my disclaimer - 33 is an outlier because of incredible driving. I’m talking in regards to the average team and average play. If someone is driving as well as the 33 drive team this year they understand all there is to know about pinning.

254’s 2014 bot was pretty good at avoiding t-bones like that as well as t-bones in general. They used an alternative bumper material (I believe it is sail cloth, but I’m not certain) and is at least 20 pounds under the 120 pound limit. In most t-bone situations it either bounces off of the other robot, slips off of the other robot, or does a combination of the two. It’s not quite as un-t-boneabe as a 33 or other butterfly/butterfly-esk drive trains out there, but it does appear to have an advantage over other traction setups I have seen.

Not quite, the exact reason why 33 is able to get out of friction pins is because with all omni’s you are not applying any resistant force back against the robot that’s pushing you which greatly reduces the friction between bumpers. Increased friction causes the opposing robot to “lock” onto the side of your robot and if they just keep driving forward they get pulled in whatever direction you drive. On the other hand, with a low amount of friction between bumpers your robot doesn’t apply enough force to rotate the robot pinning it, and simply slips off the side of the pin instead of locking bumpers with them.

Omni’s are the best thing in the world for getting out of a friction pin, you just drive forward and you slide right off of them. Traction wheels are only good for pushing in the direction of travel and resisting pushing (relatively isotropically). You can only spin your wheels forward and backward, so they’re really only good for applying force in that direction, and a high coefficient of friction in all directions makes you a little but harder to push sideways (although this advantage tends to have very little application and is generally more disadvantageous than advantageous).

For maneuverability on the field, and getting around defense, the ideal wheel for most FRC games (specifically this year and last year) would be an omni wheel with a very high CoF in the direction of rotation, and very low CoF perpendicular to their direction of rotation.

A similar effect can also be achieved by making your bumpers out of butter.

IIRC it was sail cloth. They switched after SVR (as well as I believe strengthening the bumpers to better stand up to Arial Assault), and from watching their videos, I agree they did see performance increases.

One dirty little secret about bumpers is that the best teams in the world have been using “alternative” bumper fabrics for years. Just take a look at some 67 or 2056 bumpers from the last few years, they clearly aren’t the recommended fabric (Cordura?). It just seems like 971 and to a lesser degree 33 have precipitated a conversation about pinning this year, and suddenly these innovations have come into the light.

With Swerve there is the rotate out maneuver. It does take a driver allot of practice to learn the timings of this action.

Seems to me then, that an optimal drive train for avoiding a t-bone friction pin may in fact be a swerve drive with omni wheels, as counterintuitive as that sounds.