Traction Questions

[Jared]

Originally Posted by Ragingmain

I guess one more way I can put it is this. If you had to move a piece of 4x8 plywood and could not carry it and the only two options you had were to
1. Lay it on the ground flat and try to push it?
or
2. Lay it on its edge and push it?

Same amount of weight. one has a bigger contact patch then the other. Which one will be easiet to move?

If I'm pushing on a carpet, number 2 is much easier than number one. If I push on a smooth floor, then the second may be easier. It depends on the surface you're pushing on.

[Ether]

Originally Posted by Ragingmain

1. Lay it on the ground flat and try to push it?
or
2. Lay it on its edge and push it?

On a rough surface I'd stand it on edge lengthwise and pull it, lifting slightly so the corner does not catch.

[sanddrag]

I've always wondered about the traction of a larger diameter wheel compared to a smaller diameter wheel of the same width. I'd speculate it is more, since the curvature of the wheel approaches a straight line as the diameter approaches infinity. On squishy surfaces such as rubber and carpet, I can definitely see this making a difference.

It would be cool if AndyMark and VEX would step up to the plate to put this debate to rest once and for all, with some real scientific testing and published results

In the end though, the real limiting factor in pushing is the internal resistance of the battery.

[themccannman]

Originally Posted by Jared

If I'm pushing on a carpet, number 2 is much easier than number one. If I push on a smooth floor, then the second may be easier. It depends on the surface you're pushing on.

Precisely, it depends on the surface, hence why modeling this can get complicated, different surfaces interact in unique ways.

Here's some reading that went way over my head that may help understand friction between rough surfaces: https://workspace.imperial.ac.uk/med...20surfaces.pdf

Originally Posted by sanddrag

It would be cool if AndyMark and VEX would step up to the plate to put this debate to rest once and for all, with some real scientific testing and published results.

Why make them do it? Most teams have the resources to test this ourselves. Perhaps we'll do a study of this and publish our results.

Originally Posted by sanddrag

I've always wondered about the traction of a larger diameter wheel compared to a smaller diameter wheel of the same width. I'd speculate it is more, since the curvature of the wheel approaches a straight line as the diameter approaches infinity.

You can simulate this without buying huge wheels, just use a pneumatic wheel and test it at varying levels of inflation, the under inflated wheel will likely have better traction in accordance with your theory.

[sanddrag]

Originally Posted by themccannman

Why make them do it? Most teams have the resources to test this ourselves. Perhaps we'll do a study of this and publish our results.

Because aside from banebots and colsons, they have all the wheels.

[T^2]

Originally Posted by sanddrag

I've always wondered about the traction of a larger diameter wheel compared to a smaller diameter wheel of the same width. I'd speculate it is more, since the curvature of the wheel approaches a straight line as the diameter approaches infinity. On squishy surfaces such as rubber and carpet, I can definitely see this making a difference.

I'd speculate it might be less, because a tighter curvature on the wheel will spread open the geometry of the tread surface, allowing more carpet between them.

[EricH]

Originally Posted by themccannman

Why make them do it? Most teams have the resources to test this ourselves. Perhaps we'll do a study of this and publish our results.

Cyber Blue tried that about 3-4 years ago, IIRC. I don't recall seeing any results, though I do think I heard a rumor about "'interesting' data" or something of that nature.

[Karthik]

Some testing from 2003:

http://www.chiefdelphi.com/media/papers/1381
http://www.chiefdelphi.com/media/papers/1382

[Chris is me]

Originally Posted by Ragingmain

I guess one more way I can put it is this. If you had to move a piece of 4x8 plywood and could not carry it and the only two options you had were to
1. Lay it on the ground flat and try to push it?
or
2. Lay it on its edge and push it?

Same amount of weight. one has a bigger contact patch then the other. Which one will be easiet to move?

With your mechanical engineering degree, you should absolutely know why this situation is totally different than traction of a wheel applied to the ground.

First of all, you're attempting to make an argument not based on physics, observation, or empirical data, but on intuition. You're basically saying "doesn't it seem like this would be harder?". You're creating a contrived example and implicitly asserting that the reason one technique is harder than the other is *entirely* friction. Intuition is not how to make an engineering argument.

Second, I'm going to challenge the implicit assertion that the differences in force are due to friction entirely. A thin sheet of plywood is not flat - it's going to dig into the likely deformable surface it is under, such as carpet. For example, an edge could get caught in some carpet fibers. Putting a piece of plywood on its edge helps in part because there is so much less area for the plywood to "catch" the surface on. On a perfectly smooth surface, with smooth plywood (equal surface treatment on the edges and the flats), I don't believe there would be nearly as dramatic a difference.

Finally, the scenario you described does not even relate to wheel traction! The contact patch of an ideal wheel that is not slipping is stationary relative to the ground. Static friction is the phenomenon in play in this case. When pushing a flat piece of wood across the floor, dynamic friction is a factor for a vast majority of the time you're pushing the wood. Dynamic friction can depend on surface area / contact patch much more than static friction does.

[themccannman]

Originally Posted by Karthik

Some testing from 2003:

http://www.chiefdelphi.com/media/papers/1381
http://www.chiefdelphi.com/media/papers/1382

Very interesting results. It looks like you guys concluded that pressure has a pretty direct relationship with traction and surface area. I would guess that the increase in pressure causes the studs from the tread to dig deeper into the carpet whereas tread under lower pressure tends to "float" on the surface of the carpet instead therefore actually decreasing the effective contact area between the wheels and carpet and preventing the tread from digging into the carpet to make an interlocking profile.

[Ether]

Here's an interesting thought experiment:

On a dry sandy beach, which has more rolling resistance:

1) a bicycle with skinny racing tires, or

2) the same bicycle with fat balloon tires?


Which has more traction?

[EricH]

Originally Posted by Ether

Here's an interesting thought experiment:

On a dry sandy beach, which has more rolling resistance:

1) a bicycle with skinny racing tires, or

2) the same bicycle with fat balloon tires?


Which has more traction?

Is this question posed at t=0, or at some positive time? Living near the beach, and having plowed through some sand on a bike (mountain tires) on one or two occasions, that does (kind of) change the answer. (Note: I usually stayed on the bike path. Did hit a few drifts, though, in addition to the time I took a shortcut off of it.)


If it's at some positive t, I'm going to be going with the skinny tires having more rolling resistance and more traction. But...

Spoiler for But:

... the reason that's the case is that they're rim-deep in sand, so the more effective traction goes to the balloon tires. On the other hand, mountain tires are going to get you dug in even faster due to the knobs.

[Chris is me]

Without hard data, this post really isn't worth jack... but I thought I'd add something I'd heard from teams before. For the specific case of roughtop tread, when using larger wheels, increasing width does not noticeably increase tractive force. As wheels get smaller, approach 4 inches diameter (sound familiar?), widening tread does begin to increase pushing power. My baseless conjecture is that as wheels get smaller, fewer rows of "spikes" on the roughtop tread are in contact with the carpet at the same time, putting more pressure on each individual row. At a certain point, the increased pressure really causes a row of spikes to "dig in" to the carpet. Adding more width to those wheels, while decreasing the pressure per unit area, allows more spikes to dig into the carpet, and once interlocking forces come into play surface area really does start to matter.

Someday, when there's time and money to be spared by whatever team I'm on, I'd love to test this hypothesis / idea / rumor with several tread types (roughtop, wedgetop, Colsons, AM HiGrips)