4wd vs 6wd?
 

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!

Re: 4wd vs 6wd?
 

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

Re: 4wd vs 6wd?
 

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.

Re: 4wd vs 6wd?
 

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

Re: 4wd vs 6wd?
 

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

Re: 4wd vs 6wd?
 

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

Re: 4wd vs 6wd?
 

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>

Re: 4wd vs 6wd?
 

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

Re: 4wd vs 6wd?
 

ok, thanks for the help ;)

Re: 4wd vs 6wd?
 

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.

Re: 4wd vs 6wd?
 

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

Re: 4wd vs 6wd?
 

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.

Re: 4wd vs 6wd?
 

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.

Re: 4wd vs 6wd?
 

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

Re: 4wd vs 6wd?
 

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?

Re: 4wd vs 6wd?
 

If you have a 4WD bot that is made for pushing, and you add on two more wheels, and the middle two are dropped, you're not going to be pushed around easily by the same people you could push before.

Re: 4wd vs 6wd?
 

Team 42 (P.A.R.T.S.) has used a 6-wheel drive system for several years now. The center wheel is dropped (as I recall) 1/4" and we drive all 3 wheels on each side.

If you don't drop the center wheel enough, then the extra friction from the outer 4 wheels can make the current draw really spike up when turning.

We have found that your choice of wheels also can make a big difference. A robot that would barely turn at all on carpet without browning out a couple years ago turned just fine after we changed the corner wheels back to solid core instead of the pneumatics we had mounted for extra traction. There can be too much of a good thing...

For the last 2 years we have used slightly deflated pneumatics as the center wheels and solid cores on the corners. That gives us a nice balance between traction and turning ability.

Re: 4wd vs 6wd?
 

Few questions. How many teams that have done 6 wheel design only dove 4 wheels. which ones did they drive and why. Also a nice 6 wheel design from last year is 233S roccobot very nice. I am only saying this because I got a up close and personal look at there components and transmission. o second question how much weight does an average 6 wheel design run? Any teams that have done 6 wheels for a while.

Re: 4wd vs 6wd?
 

This, I think, is the most pertinent and useful information you'll find in this thread so far. There is nothing intrinsic to the use of 6WD systems that make them superior in turning. Instead, current draw during turning is affected by friction and wheelbase dimension, essentially, and 6WD systems with a lowered center wheel are preferred over a shortened 4WD wheel base only because they are more stable. The 6WD system is, after all, nothing more than than two shortened 4WD systems that share a common wheel.

The other benefits that people seem to associate with 6WD systems, such an increased traction, seem to be related only anecdotally. For once, though, I'm going to leave the beating of that poor horse to some other people.

This Whitepaper by Chris Hibner is a great resource when trying to figure out if you're robot will turn.

Re: 4wd vs 6wd?
 

In 2003 we had a 6 wheel bot with only 4 of them being driven. The front two wheels had steering. We did this becuase we thought it would allow us to accomplish many tasks. Looking back it would have been nice to have had all the wheels driven.

Here are some pics:

1

2

Re: 4wd vs 6wd?
 

2 years ago, we had a woefully underpowered 4wd bot that just didn't want to turn (as the breakers kept popping). The wheels were essentially in the corners of the base, with long chains connecting the 2 wheels on each side. Our problem was that we had such a long wheel base that the drill motor we were using on each side just wasn't powerful enough to get the job done. To turn, we finally took the wheels off the bot and took them to the router, giving a rounded face to the edges of the wheel.

Last year, we went with a 6wd, drill/CIM drive. We got lawn-mower wheels (the hefty black rubber ones with the white steel hubs) and used those. We trimmed the corner wheels with the router as we had the previous year, but we left the middle wheel alone, and we never dropped it. We never really had issues with being pushed around, or any turning problems.

What I really want to know is: what were the heaviest things in a 6 wheel drive train that caused the most weight headaches, because they were essential?

Re: 4wd vs 6wd?
 

are you asking any 6wd heavy component? or heavy compared to others such as treads or 4wd?
because many heavy components would be necessary for any kind of drive system. ex) batteries.
well if you're comparing, lets start a list
extra wheels (compared to 2 or 4 or 3 or 5?? wd)
more sprockets or w/e you use to power the exra wheels
chain or belt or w/e you use to tranfer power to all the wheels
bearings/bearingblocks
axles
bolts nuts of course


please add on

Re: 4wd vs 6wd?
 

Team 492 http://www.titanrobotics.net has been using 6wd with lowered center wheels since 2000 and team 360 (the first team I was associated with ) used it in 1999. AFAIK we were the first as I didn't see any 6wd robots at either San Jose or Epcot, that year. Since then, they have become pretty popular.

Benefits: 100% traction no matter what the contact is (hills, valleys, flats, going over barriers), fluid, low power turns. In 2004 we were able to run 6 finals on one battery. The motors only got "warm", never hot.

Disadvantage: more complex, somewhat heavier, *harder* to drive straight and easily knocked about: the reason? Because the robot is pivoting on two wheels, there is very little restraining force. That is why 4wd robots drive so straight: they resist mightily any turn! They also burn up the motors...

In 2004 team #492 solved the disadvantages by using active servo to control wheel velocity. We used very grippy tires (3" beadlock) and used torque estimation to limit current draw when in a shoving match (no need to smoke motors!). Finally, for autonomous mode, we used a gyro chip for heading reference (the encoder feedback from the servo for distance) and never failed to hit the target ball - even after being hit by another robot one time!

Typically we aim for a 15-18 fps top speed, but use the servo to limit speed to something less (say 10 fps). We use the servo for control rather than gearboxes. It hurts our ability to push, but, fortunately, 2004 wasn't totally about pushing. A maneuverable robot that was fast & controllable could still dominate.