weight distrabution

[Alan Anderson]

I suspect that having more weight on the forward wheels will make it harder to turn. The robot will tend to try to pivot around a point farther forward, making it try to slide the trailer hitch sideways. Weight on the back wheels will tend to put the pivot point farther back, doing a better job of keeping the trailer from resisting the turn.

The TechnoKats' robot has its center of mass extremely far back, and it turns great.

[EricVanWyk]

Originally Posted by Alan Anderson

I suspect that having more weight on the forward wheels will make it harder to turn. The robot will tend to try to pivot around a point farther forward, making it try to slide the trailer hitch sideways. Weight on the back wheels will tend to put the pivot point farther back, doing a better job of keeping the trailer from resisting the turn.

The TechnoKats' robot has its center of mass extremely far back, and it turns great.

I've gotten advice that supports your claim and advice that claims exactly the opposite. A team with waaay front weight swears by it, and a team with waaay back weight swears by it, and a third team with waaaaaay balanced weight swears by it! I think this requires either {rambling science discussions} or {just trying it and seeing what happens}.

[Jon Stratis]

You know, i had a big long post drawn up here explaining the principles behind it all, and then i realized i was explaining it for an ideal, frictionless environment. I'm not a Mech E, so take this with a grain of salt, but here's what i remember from class years ago... The short answer is any time you turn you're going to have some scrub on your wheels. Your ideal turning radius is determined by the angle the trailer can make with your robot (at that angle or wider the trailer wheels will scrub the least. So by this definition, your center of rotation is going to exist outside the robot.

the way your center of mass and center of rotation work is like a pendulum. You have a long arm with a big mass attached to it, swinging around a central point. The longer the arm and the bigger the mass, the more effort it takes to rotate a certain number of degrees around the center. In those terms, the center of mass for the robot doesn't really matter.

What does matter, however, is how much scrub you generate on the robot's wheels. A robot that has a center of mass on the front edge will have a tendency to "power slide" in a turn - the trailer will whip around and pull the back of the robot with it. Similarly, having the center of mass at the extreme rear will give you a similar experience, as the trailer imparts a rotational force to the robot. A balanced center of mass would give you more direct control.

So, what's better? That's up to your drivers. The examples i gave were for extreme examples, where you have a bare chassis with a 100 lb weight on one edge. Most likely with most robot designs this year, you won't be able to artificially shift the center of mass enough to make a noticeable difference (by adding a 15 lb steel plate, for example).

[Cory]

I don't really see where the debate is. It will turn best if the weight is on the back, because it's helping to lower the scrubbing forces on your front wheels.

look at it from the perspective of a dropped center 6WD. If you have the center wheel on a 38" long robot at exactly 19" and your CG is forward of the center wheel, then your wheelbase is 38" long, plus the distance from the trailer axle to the back of your robot.

If you have the same setup but the CG rear of the center wheel, then your wheelbase is effectively 19" shorter (assuming the front wheels never touch the ground).

The same thing applies to a 4WD robot, or any robot with all wheels coplanar, it's just less exaggerated.

Shorter wheelbase=less scrubbing force=easier turning.

[spc295]

thanks for your help, our robot has 34lbs of counter wieght, and our drivers insist that we should move it.