6 Wheel Drive vs. 8 Wheel Drive

[akoscielski3]

Originally Posted by LeelandS

Don't be sorry for correcting me I put this out as I was informed. I like being told I'm wrong, actually, because it's a chance for me to learn. So, first off, thank you!

Secondly, what I think I was trying to say (and I was kinda in a rush when I typed it out), is when you have a drop center 8 wheel drive, you have the two center sets of wheels, and the 2 outer sets of wheels. The robot sits on the 2 center sets, if I'm not mistaken. So that's what the robot is supported by. As long as those wheels sit on the bridge, the robot will not fall over (again, depending on weight distribution). On a drop center 6 wheel, the robot rocks. Depending on where your weight is distributed, when one set of wheels comes off the bridge, the weight of the robot starts leaning in that direction. Thus, the robot falls over. Is this correct? Maybe I'm mistaking what I'm visualizing as an 8 and 6 wheel drive.

As for the rest, thank you for this info. I love the opportunity to learn from those with experience!

OH! Okay I miss understood what you were talking about for what wheels the 8WD sits on during the bridge balancing. I thought you meant a set of Center wheels and a set of outside wheels. Which would obviously make it fall off the bridge.

I can see where you are coming from for the 6WD. However it is not true. As long as you have your COG "on" the bridge, AND have your middle and 1 set of outer wheel on the bridge you will not fall off. Example. This is our practice bot hanging off the bridge 17". Though we had our COG as far back as possible, we had our front wheel WAY off the bridge and stayed on, without falling.

I have No problem helping you learn, but i feel bad sometimes when I have to tell a person they are wrong :$ I dont want to be a "know it all" or act like one. But I guess thats what FIRST is all about right?

[Gray Adams]

I'm curious, has anyone built a robot that can either be 6 or 8 wheel and tried driving it? Maybe you guys are a lot better at balancing your center of mass, but it seems pretty hard for us to compare wheel numbers and center drops because all of our robots are so different in the superstructure. It gets really hard to tell what is causing the different behaviors.

[Alexa Stott]

Originally Posted by Gray Adams

I'm curious, has anyone built a robot that can either be 6 or 8 wheel and tried driving it? Maybe you guys are a lot better at balancing your center of mass, but it seems pretty hard for us to compare wheel numbers and center drops because all of our robots are so different in the superstructure. It gets really hard to tell what is causing the different behaviors.

Well, technically our 2007 robot was built with the option to take out the 4th wheel on each side and drive around on 6 wheels.

I think the best comparison between two robots has been our 2011 and our 2007. They were incredibly similar.

We experimented with the 8WD for one season and didn't really like it or think it was worth it, so we went back to 6WD.

[PAR_WIG1350]

Originally Posted by akoscielski3

6WD chassis do not spin on the middle wheels and what ever else wheel/s it is on. They spin only on the Center wheels, and (if you picture this it will help) "rock" back and forth from the back and front wheels. Making the middle wheels the "Axis" for the turning. In 8WD it is a little different how it moves. It moves with the 4 center wheels, and eliminates the "rock". And makes it a little smoother chassis. But the rock may or may not be a problem depending on how much it rocks. For example this year a 6WD could have made your shot off, by the slightest little bit. 8WD might (most likely) Eliminate that problem.

This is not always true, it depends on how the robot is designed. Actually, a well designed 6WD will NOT rock while turning. unless it is being driven in a jerky manner. The ideal situation for a six wheel drive is to have the COG off-set to the front or to the back to eliminate the rocking effect since that would interfere with scoring and stuff. Typically, the COG is off-set to the rear so that it won't rock back during sudden accelerations and and to counter balance an arm if one is present, but there is no reason that the off-set couldn't be to the front if it provides some advantage specific to your design. This means that if all wheels are identical and the amount of drop is just enough that the set of wheel "in the air" has a negligible effect on driving, It will rotate about a point roughly mid-way between the two pair of wheels in contact with the floor. This is'n as noticeable as one would think, however and might seem like it is pivoting around the center wheels. In a few case, such as low traction corner wheels or a small enough drop that the "raised" wheels cause some scrub effects, the pivot will move closer to the center.