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Unread 26-05-2014, 18:45
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Re: Physics of T-boning

Quote:
Originally Posted by brennonbrimhall View Post
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?
This is most of it, but the friction generated between the bumpers is also important as others have said.

Quote:
Does this imply that Robot A's drivetrain would be more resistant to T-boning if it had a higher coefficient of static friction?
The only way this would be true is if the wheels were so grippy, the defending robot could not push them sideways. In which case the pin is more escape-able.

In the real world, where our drives exert enough force that we can make most traction materials slip, a higher CoF wheel actually makes you *less* resistant to T-boning. The force the pushing robot exerts to slip the wheels becomes the normal force of the bumpers against each other. As a robot becomes harder to push, the normal force between bumpers becomes greater, resulting in more friction force between them.

Quote:
Originally Posted by Andrew Lawrence View Post
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.
This is just straight up not true. "Omni tank" drives have far fewer problems with T-bones than you let on. The biggest reason is that by not resisting sideways motion at all, the normal force between the bumpers (and the compression of the pool noodles, etc) is much lower. In addition, omni wheels can be driven forward while pushed sideways without compromising traction as much. The sideways "traction" in an omni wheel comes from rolling resistance in the rollers, while the forwards traction comes from the rubber of the rollers which aren't slipping.

Finally, it is very hard to "square up" against an omni drive to begin the maneuver. This isn't driver dependent, but a good driver helps maximize the advantage in this situation.. Unless the pushing robot pushes an omni wheel robot normal to the exact center of rotation, it will just spin out rather than stay with the pusher. This gives the driver the opportunity to get away. A good driver will predict defender actions and position their robot to make each contact either incosequential or even beneficial by displacing the robot away from the defender with the contact.

Also, butterfly drives for the most part don't drop a set of traction wheels to spin out of T-bones. Usually they just stay in all omni mode and never get pinned in the first place. I think this was one of the design intents of the system but as the butterfly drive has been iterated people don't even gear traction and omni wheels for the same speed anymore. There are situations where 2 traction 2 omni becomes the better option, but strictly in terms of avoiding T-bones an all omni drive does just as well.

Quote:
Originally Posted by Andrew Lawrence View Post
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.
That's the point. Because the wheels are easy to move sideways, there is less grip between the bumpers. Being spun is the best possible outcome here for this specific situation.

Quote:
Originally Posted by Andrew Lawrence View Post
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).
The point of contact is determined by the height of the bumpers... And the height does matter. For both administering and avoiding T-bone pins you want your bumpers as close to the ground as possible. For administering T-bone pins, you'll be able to get under higher bumpers as you've already laid out. This reduces the robot's normal force which is what it uses to be able to drive away at all. Under the right circumstances you can almost completely disable a robot this way.

It's less intuitive why low bumpers would help with avoiding T-bone pins, when raising them reduces the contact patch of the bumper and thus (to a small extent, due to the deformability of the pool noodles) reduces friction between them. The main reason is that force being applied below the centroid of the bumper will almost always result in some part of the pinned robot being supported by the pinning robot. This changes the robot's normal force from the ground. Even if the pin isn't as dramatic enough to noticeably raise the robot off the ground, the higher bumper robot is at a slight disadvantage due to the lowered but non-zero normal force.

Quote:
Originally Posted by Andrew Lawrence View Post
Have you ever heard of wider wheels having more friction? Like why would teams use any wheels larger than the smallest amount possible since surface area doesn't matter? The reasoning lies behind the nature of the two materials in this equation: competition carpet, and rubber tread on the wheel. The difference is these materials aren't perfectly flat - there are small bumps and ridges on each that catch on to the small bumps and ridges on the other. It's essentially how velcro works. This velcro under a microscope shows how the bumps and ridges of the materials get caught together. When the surfaces interlock, you get more grip on the wheel to push forward with, creating more traction. Of course the standard model doesn't account for this, because the math would be insane and extremely difficult to measure.
There's a little bit more to it than that. Wider wheels wear less quickly, which means that at any given instant less of a wider wheel is being worn down - this wear decreases friction.

It's really important to note that this effect you're speaking of has only been observed in 4" wheels with roughtop tread. Larger wheels with roughtop tread tend to have about the same traction with width. Test it yourself some time if you wanna.
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