Best Form Of Six Wheel Drive?

[kramarczyk]

Originally Posted by Otaku

Hmm. I've not studied these sort of things (I want to though), but that seems to make sense, except I would imagine that traction is much more than the weight pushing down on top of the wheel, although that would have a noticeable effect. You also have to keep in mind the material used as tread (Say, a plastic/perspex "tread" would be much less efficient than normal traction tread) and the driving surface matters too (such as carpet, hardwood floor, concrete, cobblestone, sand, etc.).

How then, would you calculate the traction of a robot whose weight is mostly on their right side, and the rest is evenly distributed front to back? It's a 4-wheel bot, though. (I'm asking because 675 is using 4 CIM's/4 56mm Gearboxes @ 12:1 and we can push another bot AND the rack with normal traction tread on carpet, without losing traction)

You are on the right track

How to calculate traction... well, traction is friction

(Ff) and Ff = mu*Fn

; where mu is the coefficient of friction (CoF) and Fn is the normal force acting against the friction surface.

You are correct in stating that the traction is dependant on the surfaces involved and that is what the CoF charaterizes. This information must be arrived at through testing, fortunately some wheel vendors have done the testing for us and provide those numbers along with the rest of thier product data (Thanks guys!) Keep in mind that a material may be directional and have more friction in one direction than another. Think omni-wheels.

The normal force is something that can be calculated, and for a simple case of a robot sitting on level ground the Fn is equal to the weight of the bot. If the bot attempts to climb a ramp (not sure why they would want to do that ) then the

Fn = Weight * cos (Ramp Angle)

(more detail and pics at http://www.ac.wwu.edu/~vawter/Physic...PlaneMain.html )

OK, so now what about if you have different styles of wheels on the bot?
In this case address the situation on a wheel by wheel basis. First figure out how much of the bot's weight is being carried by each wheel (possibly CAD, or several matched scales) and the appropriate Fn for the wheel. Multiply the wheels Fn by the wheels CoF to get the contribution for that wheel. After this is done for all of the wheels then add up thier contributions to determine the traction limit for the bot.

So has anyone noticed that area didn't come up? Friction, and therefore traction (for our purposes) is independant of area. For those that are getting ready to bring up dragster tires and such I suggest that you read this first. http://www.newton.dep.anl.gov/newton...ysics/PHY2.HTM
cliff notes version... dragsters use wide tires to limit wear, deal with thermal expansion, and take advantage of the adhesive like properties of the rubber compounds.

wheew...