Could you elaborate?

I am a big fan of four wheel drive. Too many times have I seen teams use a two wheel drive with casters, and not be able to control it. Either they turn about too spontaneously, or they can't focus enough power in one direction to get up a ramp... ick :ahh:

Four wheel drive really isn't as hard as it seems. The key is to make sure that your wheels aren't super, super sticky because you need them to slip enough to turn. With four wheels, you still get a zero turn radius... but you also get pushing power ;)

actually, i believe that they used a 'tricycle' steering system, where they powered the steerable back wheel. this would allow them to have a turning radius of zero, but not have to worry about side slip like a 4- or 6-wheel drive robot would have to.

I may be wrong about this, as i'm going on what i've seen in the pictures.

Also with 4 wheel drive you can acieve asymetric sp? drive whinch is a major major advantage.

my fav design
 

for multiple arenas I always liked what I call the "rocking horse" design,lol though I am sure all you future MEs know the real name for it :) Basically at any given time you have 4 wheels touching with 2 in the front and 2 in the back, so it allows to body to "rock" while still maintaining "4 wheel stability". I attacked a really rough draft of the basic concept.

Any support system that has more than three points of contact is indeterminate. Think of the crappy table at the restaurant where one leg is too short and the table rocks back and forth between two different determinate conditions.

If you have six wheels, for instance, the platform has to be exactly machined and the surface flat (or compliant) in order for all six wheels to be on the ground at once.

If the tires wear or the frame gets bent or the surface changes such that the there is about 1/16" to 1/8" difference between the highest tire and the lowest tire, the machine will settle on a support of three or four tires.

The way to avoid this is to put all six wheels on a suspension where the wheels have 1/4" or more travel. The spring in the suspension will allow the tires to accommodate to the variations in wheel diameter and surface.

Technically, the carpet is a "suspension" in that it has a surface springiness. Also, unless the robot frame is really, really stiff, there is some compliance in the frame. This is usually enough for the four wheeled robots to have all four tires on the ground.

Re: my fav design
 

This is the way that the Technokat track drives have been for the past few years. It's always been a very slight curve, but when the robot is on flag ground you can barely tip it back and forth. It seemed to help out a ton in 2001 when we had the metal tracks, and we reduced the curve a little bit in 2002-2003 with the rubber belts. I wish i still had all my Technokat pictures, but my PC locked up right before I left for college and I didnt feel like fixing it.

Two wheel has in many cases solved alot of younger teams problems they had with 4 wheel. Easier to move around simpler too build. However the 4 wheel and up class have won the comp. alot more as vet teams have perfected these designs. Probable the 2 most extreme wheel systems I've ever seen were at the Final round last year. Wild stang with their 360 degree swivel 4 wheel. and That 6 wheel monster that was getting air off the ramp, 26 I think. Usually 2 wheels have that agility that 4 wheelers don't come close to.

A few ideas to point out...

-Last year 1064 had an articulated tread system where they had 4 mini-treads in place of 4 normal tank drive wheels. The cool thing is, these treads could rotate. On one position all 4 treads could be flat on the ground, giving supreme traction. At another extreme, the treads could rotate at an angle so that only the two closest ends were touching. This would cut down on the turning friction (although I think the real reason was for the ability to drive over other robots).
Pictures here

Fundamentally, the design makes a lot of sense. As your wheels are closer together (wheel base is greater than the distance between the wheels), you would experience a lower turning friction. Unfortunately they didn't perform well at the regionals they attended...


-Team 16 had a 3-wheeled design in a somewhat triangular shape. The two wheels in the front were powered by drill motors, and then they had one rear wheel that could pivot, but it was powered by two CIM motors. This type of radial system does have advantages, because you have the ability for radial movements (circles, etc), and tank style.