4WD Turning Difficulties

[MrForbes]

Originally Posted by amos229

Center drop wheels arent the answer this year since because you have the wieght of the trailer attached to you then your always going to be on the back four wheels.

Why is that a problem? You can locate the center wheels towards the front or rear of the robot to take advantage of this.

[Larry Lewis]

Originally Posted by squirrel

Why is that a problem? You can locate the center wheels towards the front or rear of the robot to take advantage of this.

But you will still most likely only be driving on four wheels due to the weight in the back of the robot. If that is the case why not just put 4 wheels on your robot but place them closer together. You would get the same effect I think. But I am not sure if that will help out the driving situation.

Has anybody out there built up a 4WD chassis and tried it out on the game surface? I would like to hear about the why things handle if so.

[Racer26]

The front wheels would just be there to stop the front edge of the chassis from slamming the ground when CG shifts (due to collisions/abrupt deceleration/whatever else), you are correct that the vast majority of the time, you'll be on your wheels closer to the trailer.

[sdcantrell56]

Basically in a 6wd this year with a dropped center wheel, the front 2 wheels will be similar in functions to casters, just to keep the robot from slamming forward like has already been mentioned. The advantage of powering is that now when the robot rocks forward, the robot will still be distributing power to 4 wheels, versus if the front wheels were unpowered, only 2 wheels would have power but the friction force on those wheels would be half of the friction force that 4 powered wheels have to work with.

[Dominicano0519]

Originally Posted by amos229

Center drop wheels arent the answer this year since because you have the wieght of the trailer attached to you then your always going to be on the back four wheels.

your right wow

i almost forgot the trailer

[MrForbes]

The trailer wheels are near the center of the trailer, so it won't have a lot of tongue weight. You can position the wheels on your robot wherever you want to, they dont need to be exactly in the center. You can position heavy parts of your robot wherever you want to, so the center of gravity need not be in the center of the robot. What all this means is that you can balance the robot how you want...you can have a 6 wd robot that rides on the front 4 wheels almost all the time, if you so desire.

[computerboi1503]

We slightly modified our fal prototype base so it is 4wd, skid steer. It does not turn as well as we would think, but it does turn, but slowly. If you drive it forwards a bit, then start turning it turns much better. We have not driven it with a trailer as of yet, but on the playing surface it may not be a huge problem, besides the fact that the weight of the trailer may pull the back end of the robot a little bit.

Oh, and the robot drifts. A lot.

[Josh Goodman]

Thanks for this post nick! Hmm....now what other designs could we find?

[AdamHeard]

Originally Posted by computerboi1503

We slightly modified our fal prototype base so it is 4wd, skid steer. It does not turn as well as we would think, but it does turn, but slowly. If you drive it forwards a bit, then start turning it turns much better. We have not driven it with a trailer as of yet, but on the playing surface it may not be a huge problem, besides the fact that the weight of the trailer may pull the back end of the robot a little bit.

Oh, and the robot drifts. A lot.

We had the same result in similar testing. Our base was being driven from a battery, so no speed control. We think some of our initial slowness during turning was because we were "Flooring it" and breaking traction.

Can't wait to get more wheels in so we can test with 6 wheels, and most importantly, with at trailer.

[jerry w]

Originally Posted by squirrel

The trailer wheels are near the center of the trailer, so it won't have a lot of tongue weight. ....

You have not looked closely at the drawings. The wheels are infuriatingly far to the rear of the trailer. This will make it follow without jack-knifing. more importantly, it will increase the arc needed to turn around. Short turning is what we are trying for.
Tongue weight is also important and will increase with the number of rocks in the trailer.

[rwood359]

Originally Posted by Josh Goodman

I also ran this scenario through a drive train simulator and came up with similar results.

What simulator did you use and is it generally available?
Thanks

[spc295]

yesterday we were driving our 2007 robot with the rover wheels on the regolith, the results were interesting, we found that the inline CoF is higher then .05 inline, more like .12. the robot wieghed 110lbs and had more then 50newtons of pull (it maxed out a force sensor we borrowed from a physics lab) also when we were driving at our max speed we could stop one of the drives so that we would be dragging the left or the right, and we would continue moving in a straight line with very little pull. (newtons first law)

once we got a second force sensor we were reading a pull of approx. 60 newtons.

[Josh Goodman]

Originally Posted by rwood359

What simulator did you use and is it generally available?
Thanks

Yeah, I have to get it from one of my friends but when I do I'll put it up!

[MrForbes]

Originally Posted by jerry w

You have not looked closely at the drawings. The wheels are infuriatingly far to the rear of the trailer. This will make it follow without jack-knifing. more importantly, it will increase the arc needed to turn around. Short turning is what we are trying for.
Tongue weight is also important and will increase with the number of rocks in the trailer.

You're right, I didn't look close enough..the wheels are back a bit. This helps being able to maneuver backwards, but does adversely affect forward turning.

You should still be able to balance the robot the way you want to with clever placement of parts.

[Joe Ross]

We had far more trouble turning from the lack of traction then from too much transverse friction.

http://www.youtube.com/watch?v=u8aqjM1mmEs

The surface we were running on has about double the coefficient of friction as the real surface.