[thefro526]

There seems to be a bit of conflicting information in this thread, although most of it's on the right track...

In it's most traditional (Pure) sense, a T-Bone is a situation where a defensive robot it's pushing against another robot in such a way that the defended robot cannot actually pull away defending robot. This is most often due to the fact that the robot being defended has a high enough lateral CoF to keep itself from being moved laterally allowing a significant amount of pushing force to be transmitted through the contact surfaces making them appear to lock together. There are also a number of situations where it would seem like the robot being T-Boned should be able to 'spin' off of the pin, but more often than not, it's actually a physical impossibility.

Looking at the bumper to bumper contact first, you've got a fairly straightforward friction problem, where the amount of force required to make the two surfaces slip against one another is going to be proportional to the effective* CoF of the two surfaces and how hard they're being pushed together. I don't know the numbers off the top of my head for Cordura to Cordura interactions (Or any other fabrics for that matter) - but for the purpose of this discussion, lets just say it's somewhere in the neighborhood of .5. Now say the force between the two surfaces is somewhere around 150lbs (not an unreasonable amount) you'd need approximately 75lbs of force (really 'thrust' from the drive) to break the friction between the two surfaces and start pulling away. 75lbs doesn't sound like a lot, but remember that your drive wheels are also being loaded laterally during a t-bone as well, so it's very likely that putting an additional 75lbs of force through them would cause them to spin, meaning that it's very likely that your robot will start to spin it's wheels in place, rather than actually moving out of the T-Bone.

The other half of the T-Boning problem are the forces (mechanics?) that keep you from just 'spinning' out. Most FRC Drivetrains are skid steers of some kind, and have some sort of "Traction" (not omni) wheel - and these usually are going to pivot around some consistent point. In a 6WD, this point is usually approximately centered left and right on the robot, and somewhere between the middle and one of the outer wheel pairs, and in an 8WD robot, this point is usually centered left/right, and in between the two inner wheel pairs. When either a 6WD or 8WD robot attempts to 'spin' out of the T-Bone, you'll notice that one of the ends of the robot will appear to move 'away' from the Defender, while the opposite side will attempt to move towards the defender. Because of this, you'll find that it's nearly impossible to 'spin' out of the t-bone since the drive's maximum turning torque/force will be significantly lower than what is required to over come the lack of mechanical advantage.

Now in the case of Omni Drives, or even a robot that's 4WD with Omnis on one side and traction wheels on the other, you'll see that they're very rarely, if ever T-Boned. Part of this is because of their relatively low lateral CoF - they'll most often be pushed sideways, rather than locked in place - and another part of this is that their more likely to be able to turn out of a situation like this since their effective center of rotation is much less constrained than a traction drive. If you think about why a robot with omnis on one end can just do a cool 180 to turn, you'll see that it's point of rotation appears to be approximately centered between it's two traction wheels, and it's omni wheels appear to be moved laterally relative to those when turning.

Back to traditional drive setups for a bit - say you've got a 'regular' (no tricks) 6WD or 8WD and you need to get out of a T-Bone? One thing you can try is to 'spin' the defending robot quickly enough that their contact force appears to decrease, and you can eventually 'slip' out of the pin. All you have to do is apply force in one of your fore/aft directions and wait until the defending robot starts to turn, once they start to turn (you'll turn in an arc with them) you can either keep spinning them in that arc while gradually attempting to turn out of - or rock the defender back and forth in that arc and eventually you should have an opportunity to wiggle out. The other method is to apply force to the inner (closer to the defender) wheel set in some amount that's greater than the outer wheel set, which should result in your drive attempting to go in a smooth arc that's approximately tangent to the contact between the two machines, and will more than likely result in you slipping out of the pin - if I remember correctly, this is the method that our driver used to breakout of T-Bones this year, and I don't remember him getting caught up all that often.

Also, it's worth mentioning that bumper heights (relative to one another) can have a bigger effect on a contact scenario than some people think. In cases where the pushing robot has bumpers that are significantly lower than the 'pushed' robot, you may notice that the pushing robot gets under the robot being pushed, which results in a change in normal force in favor of the pushing robot. In scenarios like this, you may find that there is little hope that the robot being defended gets out without help, since their normal force has decreased to the point where they can't actually apply a significant force to counter the pin. (Our 2013 robot had the tendency to get under other robot's bumpers, and usually once it did, the pushing match almost always went in our favor, assuming that our breaker stayed closed....)

Anyway, some of these concepts are really hard to grasp without sketching out a few diagrams of different scenarios, so if you're trying to wrap your head around this, it might be a worthwhile exercise.

*As others have mentioned above, CoF and calculating Force of Friction are rarely as straightforward as is assumed when using the equation Ff = CoF x Fn. There are microscopic interactions between two surfaces - along with visible interactions between to surfaces (think about wearing cleats to play soccer) that will cause something to appear to have a higher CoF than what's published - in these cases, you may find that Ff increases with surface area, which is contrary to what is taught in most physics classes.