Quote:
Originally Posted by brennonbrimhall
I'm pretty sure that I'm misunderstanding something here. Ff = mu*Fn. While I could see that more bumper-bumper contact would make the t-bone more stable (greater chance of maintaining contact as Robot A wiggles and tries to escape Robot B), there should not necessarily be a frictional force increase as the bumper-bumper contact area increases.
|
The common force of friction model taught in most high schools isn't false....all the time. It's a good way of predicting generalities of friction, but it makes a lot of assumptions about the two materials in contact - mainly that each are perfectly flat and that there will be no catching on one another - the big difference here. We make the assumption that all materials just perfectly slide past each other, but this isn't true in many cases.
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.
The same applies to two bumpers in contact. Bumpers aren't extremely hard materials, nor are they perfectly flat due to the pool noodles and the nature of the cloth. When two robots push against each other, the bumpers squish together and deform to take the impact and protect the robot from harm. But by deforming inwards, a ridge of sorts is made that the pushing robot can use to get extra grip from. The point of contact is important as well, especially with the shape of our bumpers. By using two cylindrical pool noodles, there is an open area between the cylinders that can be pushed inward upon impact if the contact point is right. If that happens another indented ridge is created that the pushing robot can use to interlock bumpers against (this is why I've always been a fan of square pool noodle). Remember a lot of this is on the small scale of contact, so you won't be able to see all of it with the naked eye (though you can see the bumpers deforming). Cloth material is equally important. Rough cloth like the cordura suggested by FIRST has lots of bumps and ridges in the design that can grasp onto the bumps and ridges on the opposing robot's bumper cloth. As Roger suggested above, some teams combat this by using smoother, flatter material in their bumpers that have less bumps and ridges that could interlock with the opposing robot's bumpers.