4wd vs 6wd?

[Yov]

Hello, I have a question that I haven't been able to find the answer to anywhere...
Why would a team use 6 wheels on their robot? What are the advantages and disadvantages?
Also, if using a 6wd, what wheels are controlled and how?
Thanks!

[Cory]

A big reason teams have been using 6 wheel drive lately is because they can drop the center wheel a very small amount, which will allow them to turn easily on the carpet, yet afford similar traction to a 4WD robot. This is because when the robot goes to turn it is essentially pivoting on two wheels, rather than trying to drag 4 wheels across the carpet. Turning issues and current draw are two big problems with 4WD and 6WD helps with that. One disadvantage is that it's heavier than a 4WD bot due to two extra wheels.

In all the 6WD bot's I've seen, all the wheels are driven, each side by one or more motors, and driven like a tank, in the same manner as a 4WD robot.

There are many teams who built similar drive systems last year, and some over the summer. Im sure they'd be more than willing to give you detailed information

[Camerzn]

The best example of 6wd I've ever seen did have the dropped center wheel (about 2 cm). The center wheels on each side were powered directly from the gearboxes, and belts drove the other two wheels on each side; even if all four belts broke, the robot would have still been powered. The robot wasn't terribly fast, but it had excellent power and traction on the low pile carpet. It was definitely a snazzy 'bot, and if you want to try it, go ahead. Like Cory says, however, remember that six wheels weigh 1.5 times as much as four.

[Joe Johnson]

This has been covered in many many other threads.

I have not been on the forum for most of the year, but if you search "Joe Johnson six wheel" you will find 2 pages of threads.

In my opinion, six wheel drive should be the standard drive system by which I mean that if you don't have a good reason to use something else, that is what you use.

Bottom line: good balance between weight, traction, and turnability.

Read the old threads to learn other view.

Joe J.

P.S. I promised myself that I'd put in a shameless plug for the "NBD: Nothing But Dewalts" whitepaper whenever I posted in the 2005 Season, so here it is: I believe this instant classic will take its place as one of the most important How To's FIRSTers have access to. JJ

[Yov]

dropped down?
meaning the two middle wheels are lower than the others?

[Cory]

Originally Posted by Yov

dropped down?
meaning the two middle wheels are lower than the others?

Yes, like I said in my first post, if you have the center wheels lower, when you turn, the robot will pivot on them, which allows you to have to overcome less friction, which means turning is easier.

by lowering them just a slight amount, you still at any given time have the robot's mass over 4 powered wheels that are on the floor which means no significant (assumption, haven't done a study ) loss in traction

[Andy Baker]

Originally Posted by Yov

dropped down?
meaning the two middle wheels are lower than the others?

Yes, in most cases. For many FIRST robots that have used 6 wheel drive bases or tank tread drive bases, the middle wheel (or tread roller) is about 2-3mm (1/8") lower than the other wheels.

This has been the case if the robot has it's wheels positioned along the long dimension (36 inches~914mm) of the robot. On the other hand, if the robot is wider than its length and the wheels are positioned along the short dimension (30 inches~762mm), then that middle wheel may not need to be lowered at all.

Lowering the middle wheel allows the robot to turn easily. The front and rear wheels skid sideways easier when there is a lower middle wheel (some folks call this middle wheel a "bogey wheel").

Andy B.

<edit: wow... 4 posts, all written at the same time, all giving Yov the answer he needed>

[Bill Gold]

<edit>
Darn. Andy and Cory beat me...
</edit>

[Yov]

ok, thanks for the help

[Conor Ryan]

also 6 wheel drive has more traction and is less likely to be pushed since it naturally has more friction than a 4 wheel design, but current draw is a big problem with 6 wheels.

[Yov]

how can there be better traction if only 4 wheels are on the ground at a time?

[Max Lobovsky]

cdr is probably referring to designs without the middle wheel dropped down (though he is still partly wrong). It does not inherently have more traction, and it should actually have less current draw because it has a higher ratio of wheel width to wheel length.

[greencactus3]

Originally Posted by cdr1122334455

also 6 wheel drive has more traction and is less likely to be pushed since it naturally has more friction than a 4 wheel design, but current draw is a big problem with 6 wheels.

this has been discussed in a couple threads already, so if you want more arguement about if 4wd or 6wd or treads have more traction, please refer to the search thing.

[Mike Ciance]

ever since 2001 we have had 6 wheels. our beloved Evil Machine from 2003 had no drop for the middle wheel, but it turned perfectly. 6 wheels is a great benifit, and the wise choice for nearly any design

[Conor Ryan]

to clear some stuff up from earlier, i was talking about if all 6 wheels are making contact on the ground at once. But i raise this new question, with a dropped wheel is it pushed around easily?