off center 6WD help

[ajlapp]

Check out the .step files on my site near the bottom of the page.

Universal Chassis

This chassis has many of the features you're interested in. Just looking at the cad models may give you some inspiration for how to design your own custom version. Good luck.

[Gaurav27]

Originally Posted by EricH

Oh, and you can easily have a 6WD that turns without bouncing or drop. Stick omni wheels on one end.

Eric mentioned omni wheels. Just thinking aloud here...
Should omni wheels be on the end where CG is greater or less than the other end? The end that has a greater CG would remain on the ground for most of time where as the end with less CG. So the end with omni wheels will help in turning and thus should be on the end with a larger CG.

[Madison]

Originally Posted by Gaurav27

Eric mentioned omni wheels. Just thinking aloud here...
Should omni wheels be on the end where CG is greater or less than the other end? The end that has a greater CG would remain on the ground for most of time where as the end with less CG. So the end with omni wheels will help in turning and thus should be on the end with a larger CG.

When using omniwheels, you'd typically eliminate the "rock" from your 6WD -- and so both ends are on the ground at all times.

Assuming that you have four traction wheels and two omniwheels, you'll want for your center of gravity to live above the traction wheels, not the omniwheels. Increasing the total percentage of the robot's weight acting through the traction wheels increases your tractive force compared to placing it over typical omniwheels because the omniwheels have a lower coefficient of friction.

[joshy1323]

in 2006 my team used 6wheel drive with no omni and no lowered center wheel and we learned that when turning our robot bounced around too much. in 2007 we again used a 6wheel with no omni but we did lower the center wheel. in that robot our weight was dispersed outwards horizontally and our robot didnt have any rocking or bouncing problem. however, it turned slowly because the weight was too far from the center of gravity. in 2008 we used the same setup as 2007 but this time we put all our weight as close to the center as possible. we still didnt bounce when turning and we turned much faster, but we did have issues with too much rocking. this year we plan to use 6wheels that are all the same elevation but this time we will use omni for the front 2. i saw someones post that said this will make us too easily turnable. with a 6 wheel drive with a lowered center and no omni the force of static friction is only present on 4 wheels at a time because 2 wheels will always be slightly off the ground. that means we should have the same static friction ratio with a level elevation wheel setup and 2 omni wheels in the front.

[NickE]

On Team 254's 2008 robot, there was almost no weight forward of the center wheels, due to the open front on our frame. It only really went on the front wheels at all when rapidly stopping or rapidly reversing. We had good performance, and almost never had to change tread on the front wheels.

[lenny8]

Thanks everyone for all the help so far.

keep it coming.

[Stu Bloom]

Originally Posted by Madison

When using omniwheels, you'd typically eliminate the "rock" from your 6WD -- and so both ends are on the ground at all times.

Assuming that you have four traction wheels and two omniwheels, you'll want for your center of gravity to live above the traction wheels, not the omniwheels. Increasing the total percentage of the robot's weight acting through the traction wheels increases your tractive force compared to placing it over typical omniwheels because the omniwheels have a lower coefficient of friction.

Not necessarily ... especially on carpet - the edges of the rolling elements on the omni wheels could "dig in" to the carpet more and actually give you increased traction for "driven" omni wheels. Lateral/turning, of course, is another story completely ...