Quote:
Originally Posted by spc295
what makes a wide base better than a long one?
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"Better" is a relative one. The ease with which a base with tank steering and 4 wheels touching the ground turns is related to the ratio of wheel track (width between wheels) and wheel base (front-to-back distance between wheels).
A robot with a long wheel base and narrow wheel track (in FIRST, the 28" wide 38" long configuration) prefers to drive straight, whereas as robot with a short wheel base and long wheel track will turn easier, but have a harder time driving completely straight.
The reason for this is because a 4wd tank steering system requires all of the wheels to skid sideways slightly when turning. If you imagine the robot trying to turn around its centroid, the wheels ideally want to be rolling along the circle circumscribed by their radius to the center of rotation (aka the wheels would prefer to be rolling tangent to the circle described by that radius) But because the wheels are all oriented in the same direction (which is not tangent to the circle), it's easy to see that they will all be sliding sideways slightly in addition to rolling forwards or backwards.
Now go back to the long and narrow versus short and wide base. The wheels on the long and narrow base are pointed at a steeper angle away from the ideal tangency than those on the short and wide. The extreme case for the long and narrow is when your frame gets so long and narrow that it becomes a line, with one wheel at each end oriented to roll along the line. Trying to turn around the midpoint of this line will cause the wheels to only skid sideways and not to roll forwards or backwards. Thus we see that the higher the ratio of wheel base to wheel track, the more skidding is necessary and the harder it is to turn. In addition, because the wheels are laterally closer to the centroid, the torque they can apply to the center is less. In the extreme case, the wheels cannot apply any turning moment to the midpoint of the line, because the applied force is going straight through the point of rotation and thus the lever arm is zero. Conversely, if powered, these wheels will drive ONLY in a straight line, because they cannot turn.
In the extreme case of the short and wide base, it again becomes two wheels on the endpoints of a line, but this time the wheels are oriented to roll perpendicular to the line. In this case, the wheels ARE rolling along the line of tangency, and therefore are not slipping sideways at all. This is the theoretic best case for turning, but the wheels are much more susceptible to being rotated, since they do not resist the rotation at all.
In the context of Lunacy, a while wheel base provides a better turning ability. When you have a trailer attached, it becomes easier to drive a wide base in a straight line because the trailer is trying to straighten out the robot, and it also becomes easier to turn because your wheels can provide the maximum torque about the point of rotation.