6WD Pros and Cons

[Daniel_LaFleur]

Originally Posted by M. Krass

In the instances above, you're assuming a different center of mass in each case. If the weight is distributed evenly across the robots, the center of mass will be in the center of the robot and it will, thus, turn about that point.

You are correct (as usual ), if the weight is evenly distributed the robot will "hop" and rotate around the CoM. I've generally set our weight distribution to 2/3 weight over the rear axle and only 1/3 weight over the front axle. That allows the robot to provide enough force to have the front wheels break free (sidewards) allowing the robot to rotate around the rear axle.

And by shortening the wheelbase (by driving 36" wide and 28" long, giving us a wheelbase of ~19"), having a high CoF (~1.3, adjustable because they are pneumatic tires) and providing plenty of torque (4 small Cim ... 1 per wheel ... with 16:1 reduction) we've been able to turn without difficulty, been able to push almost anyone, and have been very difficult to be pushed or turned by others (including 6 and 8WD opponents).

** Note: I use the term axle, but we usually power each tire individually and there is no contiguous (sp?) axle across the rear (or front) 2 tires.**

[caraddicted101]

I would recommend 6 wheel with the center wheel lowered just a bit. I have found that the amount at which you should lower the center wheel depends on the type of wheels that you are running and the type of tread that you are using. When you lower the center wheel you want to be careful not to lower it too much because the robot may bounce back and fourth when turning hard. I have found that the optimal height to lower the center wheel is between 1/16 to 1/8 of an inch. This Has worked for me, but if you use thicker tread, or tread that has a lot of grip you might want to change it a bit. My suggestion would be, if you have time, make the center wheel adjustable and let your drivers test out variable heights. Make sure to allow for the extra weight if you are only running a test chassis, as the extra 80+ pounds that you put on top of the chassis will affect the way the robot reacts.

--hope this will come in handy--

[Cory]

Originally Posted by nuggetsyl

The point of having 6 wheels is for traction.

I think most people would disagree with you here. My guess is the majority of teams who move to 6wd do so to improve the turning performance of their robots.

If that's why you guys in specific use 6 wheels, there's nothing wrong with that. But assuming everyone else is using the same reasoning, and making a blanket statement like "dont drop the center wheel" is misleading to people who do not fully understand the physics behind drivetrains.

[Aren_Hill]

I see 25's setup working towards the strategy they depicted, such as 2006 they human loaded then ran in a straight line to the goal and fired minimal turning recquired especially since they had a turret.
Last year however there was quite a bit of turning involved depending on how you played the game. So ultimately to figure out whether you want the center wheel lowered or not you have to anazlyze your game strategy, and what type of driving you'll be doing

[Blue_Mist]

Team 766 has used 6wd with rocker for the last three years and that has worked out very well.

Pros:
- Heavy, which resisted being pushed, and this year we didn't need to go all that far considering the rack in the middle of the field as opposed to 2006, with the goals, defense and ramps (Biased a bit here)
- Low cg for pushing others
- Turning is definitely better than 6wd no rocker

Cons:

- Center wheels wear down much faster than the outer four (This is one problem related to our choice of tread this year; we used this thick, heavy, rough stuff that we screwed to our inch-wide Andymark wheels that would actually wear down to show the screw heads. The center wheels wore down much faster than the outer four and lasted perhaps one regional. Also, not good for PR demos on concrete, asphalt, etc.)
- Heavy, but this is a con only if you need speed (Cheesy Poofs are known for speed, and a very light robot)

We have toyed with the idea of omniwheels, but haven't gotten around to using them.

[Cory]

Originally Posted by Blue_Mist

- Heavy, but this is a con only if you need speed (Cheesy Poofs are known for speed, and a very light robot)

Our robot was actually exactly at the weight limit with the ramps. It's very possible to make a light 6wd drivetrain, without needing to make all your gears out of aluminum like we did.

[Bsteckler]

About steering, why don't you link the left and right sets of wheels on a 6WD robot togeather, then you can use one joystick for the left, and one for the right. I have an RC car that operates on this exact principle, turns easily and on a dime.

[Daniel_LaFleur]

Originally Posted by Bsteckler

About steering, why don't you link the left and right sets of wheels on a 6WD robot togeather, then you can use one joystick for the left, and one for the right. I have an RC car that operates on this exact principle, turns easily and on a dime.

If you mean all the left wheels tied together (with chain or such) and the same with the right wheels, most teams do this and it's called Skid (or tank style) steering.

If you are talking about like a car turns, it's called Ackermann (or car) steering. Most teams don't do this because it does not allow them to turn in place (it has a turning radius above 0), but it will have less (almost) no scrubbing of the wheels when turning .

[Frinkahedron]

Originally Posted by caraddicted101

I would recommend 6 wheel with the center wheel lowered just a bit. I have found that the amount at which you should lower the center wheel depends on the type of wheels that you are running and the type of tread that you are using. When you lower the center wheel you want to be careful not to lower it too much because the robot may bounce back and fourth when turning hard. I have found that the optimal height to lower the center wheel is between 1/16 to 1/8 of an inch. This Has worked for me, but if you use thicker tread, or tread that has a lot of grip you might want to change it a bit. My suggestion would be, if you have time, make the center wheel adjustable and let your drivers test out variable heights. Make sure to allow for the extra weight if you are only running a test chassis, as the extra 80+ pounds that you put on top of the chassis will affect the way the robot reacts.

--hope this will come in handy--

One thing to watch out for, don't try and save weight by using too light of a material on the center axle. Last year, we drove the robot around all day during Space Day at the National Air and Space Museum and the axle and wheel ended up shearing off completely at the end. We replaced them with steel instead of aluminum ones after that.

[Cory]

Originally Posted by Frinkahedron

One thing to watch out for, don't try and save weight by using too light of a material on the center axle. Last year, we drove the robot around all day during Space Day at the National Air and Space Museum and the axle and wheel ended up shearing off completely at the end. We replaced them with steel instead of aluminum ones after that.

What alloy Aluminum were you using?

[lukevanoort]

Originally Posted by Cory

What alloy Aluminum were you using?

And what diameter?

[Frinkahedron]

Originally Posted by lukevanoort

And what diameter?

Sorry for the late reply. I do not remember, as I was a measly programmer/drive team member.

[redbarron]

Go with a 8 wheel drive system

[itsnotabigtruck]

So...now that the challenge has been announced, is anyone going with either of these? Why?

[redbarron]

We are going with the same base from last year with very minor changes, but we are keeping the 8 wheel drive system we had last year.