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Originally Posted by M. Krass
I've thought about this for a bit and, operating under the assumption that all the wheels remain on the ground all the time, I can't understand either how such an arrangement is helpful.
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I don't know if it is a joke or not. I have heard that back when he was a competitive bodybuilder, the current Gov of CA used to psych out his opponents by leaking crazy stories to the press about the "secret" to his success (e.g. eating huge amounts of some strange food or sleeping 20 hours each day rather than the hard work of actually bodybuilding).
Between this "larger wheels from front to back" posting and
the NASCAR posting from Neil Parikh, it is quite possible that Team #25 is pulling our collective chains. But...
...this is how it may work and work quite well.
Put all axles in a line. Have the front/middle/rear wheels with radii X/X+a/X+a+b respectively where a and b are small positive numbers (both with respect to X and in absolute terms) and b<a.
This does provide a similar effect as lowering the middle wheel; on a flat carpet, only the front and middle wheels OR the middle and the rear wheels can touch the ground at any one time (I leave this as an exercise for the reader -- but trust me it is true). This changes the aspect ratio of the width/wheelbase and makes it easier to turn.
BUT... what else does it do?
It also means that the "ground speed" of each tire is slightly different (assuming that the wheels are driven by a single chain and the sprockets are the same size).
What difference does that make?
The first thing it will do is increase rolling resistance. When you are driving in a straight line, with two wheels on the carpet, it means that one wheel must be slipping on the carpet. This, in turn, means that the straight line rolling resistance will be higher than it would with the same size wheels. This sounds bad, but, when you have 3 motors per side like I understand Team #25 typically has, you have power to burn, especially when you are driving is a straight line.
BUT... ...what happens when this same robot turns? Now one wheel is already slipping on the carpet.
I remember a prof. in college beating one idea into us in his class, "Friction is a VECTOR and the MAGNITUDE of the vector is at MOST muXN." This sounds like a stupid and obvious idea, but it is surprisingly important. He would give us examples of where this is important like pulling a cork out of a wine bottle (by twisting the cork as you pull it out, you point some of the muXN Vector in a direction that is 90 deg. from the direction you are pulling... it is easy to pull out) and a rear wheel drive car on snow (if the driver guns the gas, slipping the rear wheels, very small forces toward the back of the car can easily move the car from side to side).
How does this apply to this case?
Well, it may mean that by having one set of wheels already slipping on the carpet, turning ability may be improved because there is a smaller component of friction available to resist the scrubbing of the tires on the carpet.
I am not sure that I have convinced myself that it is a good idea, it would take some noodling to figure out the right values of a & b for example, but I am not sure that it is as far fetched as I had initially thought.
Joe J.