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Unread 18-09-2003, 15:20
Mercutio Mercutio is offline
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Question center-wheel drive

hey there! i'm surprised so many people are on the forums so far from the FIRST season -- why all the buzz?

anyway, i'm just wondering if anyone has ever used a center-wheel drive, with two drive wheels in the middle and casters on each corner like this:

o--------o
|--------|
|--------|
()-------()
|--------|
|--------|
o--------o

where the 'o's are casters and the '()'s are drive wheels.

my team seems heck-bent on using center-wheel drive, so i'd like your input. does it work? are there any problems/quirks/limitations we should account for in our design? would you reccomend center-wheel at all?

thanks for your help!

~ aaron
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Unread 18-09-2003, 15:29
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once

way back in '97 we built a center wheel drive......i don't recommend it.

in my opinion drivetrains have been a major deciding factor in every FIRST contest. You can develop new systems in the off season and repeatedly develop designs year after year, so many veteran teams bring a lot of drivetrain experience to each new game. With small bumps or ramps being the biggest obstacles to overcome most teams can adapt their highly advanced drivetrains to new challenges.

Center wheel would not be my first choice. It does offer turn-on-a-dime capability, but it offers an opportunity to get high centered easily. We experienced this in '97 near the lip of the spinning tree. This is potentially true with any two-wheel drive vehicle though.

Just my opinion!
Put four on the floor, or spend some time developing a more diverse and adaptable drivetrain.
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Unread 18-09-2003, 16:05
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this year's combbat robot used this type of drive train.
while it performed better than most other two wheel drive robots (the average configuration of 2 powered wheels and 2 casters) when it was on the hill, i did see it get stranded once when it ran up onto another robot, which lifted its drive wheels off of the ground.
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Unread 18-09-2003, 16:56
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Non-powered wheels

Any time you have non-powered wheels (i.e. casters) supporting any of the robot's weight, you are not getting as much power to the carpet as you possibly can. Therefore, if the game requires pushing force to gain an advantage, I would recommend that any surface that touches the carpet should be powered. If power is not an issue, than it doesn't matter much.

-Chris
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Unread 18-09-2003, 17:38
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Casters = a big no

Our first year we had a four wheel drive bot that was overweight by about 10 pounds. The only way to drop the weight was to drop two of the wheels. We figured we would be real smart if we simply replaced them with two casters. With the casters driving the robot became more of a chore. Instead of focusing on the game the driver had to focus on just getting the robot to drive straight.

A lesson learned on our team is that all points touching the ground will always be powered.
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Unread 18-09-2003, 18:00
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depends on your reqts

Aaron-

There are some great attributes to center drive designs. The two biggest imho are simplicity and maneuverability. If you want to be able to spin in place it is a great system because you have very little scrubbing. Because you only have 2 drive wheels it is simple and lightweight.

But.... like the others have said, it won't work if you are going up ramps or over obstacles. You get high-centered. If you are in a pushing/pulling contest you HAVE to make sure you get weight on the drive wheels and NOT on the casters. Casters will kill ya if you don't watch out.

Every year our team has done center drive (1997, 1998, 2000), we have made it to the national semifinals, so it *must* be good....) BTW, in 2000 our center drive was "enhanced" with shift-on-the-fly rear-wheel-drive wheels to get over the ramp, so you can't count that one as simple.

Ken

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Unread 18-09-2003, 18:25
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ah the infamous 2 wheel drive topic... my high school team, team 263, has used 2 wheel drives ever since 2001. In 2001 it was a 2 wheel front drive, 2002 was 2 wheel center drive, and 2003 was 2 wheel back drive. The difference we did was, no casters... we used teflon skids. In each place you would put a caster, we put a curved peice of teflon since that was the best material we found with low friction that didn't get scratched up easily. The skid system won us a Delphi award in 2001 and a Motorola award in 2003, and our 2001 robot also won the long island regional in 2001. 2 wheel drives provide you with a much greater ease in turning, but thats pretty much the only advantage you can get from them. With the concept of having a caster or skid on one end or both ends, you will have weight resting on these skids or casters, thus casuing less weight to be on your wheels which ultimately leads to a lower max pushing power. Some recommendations I give if you are set on making a 2 wheel drive would be:
1. keep as much weight as possible over the wheels, ie. battery, compressor, add iron under the axel to bring the robot up to max 130 pounds when done building, etc.
2. find a low friction non-caster material for the skids, avoide casters at all costs. especially if we get a field element like the ramp mesh from 2003.
3. add a wheel front and back of each center wheel to make it a 6 wheel robot

In all unless you get a game like 2002 where you can grab 2 goals and balance your self out over the 2 wheels, I would not recommend a 2 wheel drive robot. However, it can lead to some very interesting design ideas and innovative awards, with the added bonus of easy driving.
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Unread 18-09-2003, 20:00
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Casters are the worst thing to use for a driving component of a robot. Use tank drive, it is more effective and very easy to build.

Basically, attach a motor to a chain that is connected to the wheels on the left side of your robot, and then connect an identical motor to a chain connected to the wheels on the right side of your robot. Very easy, I am sure there is a whitepaper somewhere with more detail =)
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Unread 18-09-2003, 20:05
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Quote:
Casters are the worst thing to use for a driving component of a robot. Use tank drive, it is more effective and very easy to build.
It depends on what castors the robot uses. Ball transfers probably would make the best castors since all they are is giant bearings.
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Unread 18-09-2003, 20:05
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I will alert some one from Team 166 about this thread, they used center wheel drive last year with moderate success.
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Unread 18-09-2003, 20:19
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Do you want to make a sports car or a truck?

It all depends on the game. For 2002, there were no ramps, so there were a few very successful 2 drive wheel bots, 95 included. In that game, we decided speed and maneuverability was more important then traction. In addition to giving us great handling and letting us devote the whole front end to ball handling, the drive train was almost an after thought in terms of design and work time. It let us focus on the much bigger problem of the ball pickup.

I'll second the 'skids in place of casters' idea. We used something along those lines, and it worked out very well. I would also suggest trying not only to put weight over the wheels, but to balance the weight as close to 50/50 (front/back) as possible. This way you put as little weight on the skids as possible when accelerating and slowing down.

It has it's very real advantages, and its very real drawbacks. But, like I said, what kind of bot do you want to make? That depends on the game, which I'm convinced depends on the mood Dean's hair is in the day they actually start working on the game.

So, if you don't want to build the typical pushing 'bot, then two wheels are a real fun way to go. You'll have the chance to move your focus away from drive train design and onto all the other stuff that sometimes gets ignored. If nothing else, you'll have fun driving it. Two wheeling is just a pleasure to drive.

You might also consider a 6 wheel design, with the center two wheels set slightly lower then the outside 4. This gives you great climbing and traction with (hopefully) much lower scrubbing when turning then a traditional 4 wheeler. Its a good mix, but is a bit more complicated.

-Andy A.
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Unread 18-09-2003, 20:29
Mercutio Mercutio is offline
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thanks!

wow! thanks for all the feedback -- i can't believe you all responded so quickly!

i noticed a lot of people are saying that four-wheel drive is the way to go. how do you do it? we tried four-wheel in FIRST last year, and it was a *disaster*! we couldn't turn at all until we covered the rear wheels with slippery plastic to decrease traction, and even then turning nearly burned out the drill motors. in the end, the only way to get decent manuverability was to run each wheel off a seperate motor. what did we do wrong?

p.s. what does "high centered" mean? i'm afraid i've never heard that term before

again, thanks for all your help!
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Unread 18-09-2003, 20:50
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Re: thanks!

Quote:
Originally posted by Mercutio
i noticed a lot of people are saying that four-wheel drive is the way to go. how do you do it? we tried four-wheel in FIRST last year, and it was a *disaster*! we couldn't turn at all until we covered the rear wheels with slippery plastic to decrease traction, and even then turning nearly burned out the drill motors. in the end, the only way to get decent manuverability was to run each wheel off a seperate motor. what did we do wrong?
From the sound of things, I'd bet you had two problems: your gear ratio was too high, and the treads on your wheels were too grippy. Those two things together can cause big problems for any robot. However, if you pick the right tires and the right gear ratios, 4WD can be many times better than 2WD for pushing power, especially.
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Unread 18-09-2003, 21:06
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Here's what you gotta do:

Take the 4 wheel drive and the center wheel drive systems and combine them into a 6 wheel drive system. Your turning problem will be solved by slightly lowering the center wheels. You'll have no worries about castors messing you up, either.

Just power the center wheels directly and connect the outer wheels to the center ones with belt or chain.

This is the most effective of the simple drivetrains. (I consider holonomic, swerve, ball, and tank drive to be complex.)

George
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Unread 19-09-2003, 01:44
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in 2002 we went to championships and we had serious drive problems with our tank drive 4 powered wheels on two motors .. at some point we decided to go with caster wheel dead center in the back of our robot causing two of our power wheels off the ground and leaving two on the ground to move around with.. we were fast robot to beginning with .... once we ran with the caster we went wild on the field we spun around about faced and out maneuvered the other teams in such great way tho if u do remember the 2002 game was a flat field game with only goals as the only real structures on the field so we had alot of running space and made good use of it

you might think that caster would make your robot go out of control but with the lifted wheels on each side of the caster every time we made turn the wheels feel to the ground and made contact with the ground breaking the turn and keeping the robot from going all over with no control so if done right it suggest the use of caster when "battling" and out maneuvering other robots is involved (it also makes great lil hat trick when the other teams trying to pin you to wall with super easy and quick about faces ) and when speed is needed over power cuz with caster u might end up with the short end of the stick everytime u push against others

Casters are great !!! just look at my thing below lol
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