Another chapter in the drive train story
 

Now, in all the threads that have been discussed rescently, no one has mentioned a thing about weight distrubution...

So we know how to figure out optimum power and what type of material works well for wheels and treads, but we haven't discussed why these different materials have worked well for certain robots.

The difference that has to be considered is how the weight is distributed among the drive wheels/tread. What was the positioning of the drive train of everyone's robots? How did you go about keeping a low CG and still keep the weight in the optimal position to help the drive train? Was this solved by four wheel drive or by other means?

More preseason thinking.....

Our gearbox was at the "rear" of the robot, however the robot can be driven with either end the front. It drove the "rear" axles and sprockets and chains conected the front and rear axles for 4 wheel drive. Everything else on the robot was very semetrical and balanced so to counteract the rear gearbox, the battery was placed more near the front (opposite end). Everything is kept relatively low as our robot is only a total of about 14 inches tall.

we mounted our battery on the bottom of the robot to help keep a low CG. we also put our air storage tanks down there. plus 5 motors, gears, sprokets, and chains. we had 2 drive wheels in the middle and 4 castors slightly raised at the ends. this kept most of the weight on our drive wheels.

Our robot was pretty low to the ground (only 20-some inches tall) and everything but the grabbers sat on the main deck inside. We put the compressor in the front right over the wheel and the battery in the back left over a wheel. The arms went down the middle, but everything else was on a plywood sheet at the mid-wheel level. Our center of gravity was VERY low, unlike last year's bot (which I won't get into.... 13' arms, 26" wheelbase....ugh)...

13 foot arms:eek: :eek: :eek:

Pictures please

pictures pictures pictures pictures pictures

Well, the arms are not quite 13 feet. The robot with arms extended straight up is 13 feet tall. The arms themselves are 8 feet, the robot is 5, 8+5=13. So I cheated a bit....

So, without further adue.... I present Tippy, our stability-challenged, much-cursed 2001 bot:

Thanks for the great picture. I joined the FIRST program for the 2002 season so I hadn't really seen those giant lifting arms before.

I have a question. If you look at the robot in the left of the picture, the wheels are on the outside of the frame. I've seen a few battlebots like this too. Is there any advantage to having the wheels on the outside?

I forgot to mention a couple things.

First of all, I think all the threads involved with "the drivetrain story" are GRRREAT. Kudos to Patrick Wang for getting us going on something important. I was even inspired enough to go buy a CAD program.

Anyway, what I really posted for is I was wondering how many robots use treads instead of wheels? Does it give that much more traction? I would personally use them for the coolness factor alone but the pulleys are between $80 and $120 apiece!!! How hard would it be to make our own. Could it be done on manual mills/lathes or would we need CNC.

As the Pontiac commercial people will tell you, Wider is Better. To see the effect on stability, take it to an extreme, and imagine placing the wheels very close to each other in the middle, or on the very far edges far apart from each other.

Our robot this year could pull a lot. (don't remember how much though) We had 4 wheels with this kind of elevator belt that has a lot of grip. Personally, tracks weigh more and are harder and more $$ to make. But if you can have the weight, $$, and time then sure, use tracks.

PS i included a pic of our fancy wheels w/ the tread.

What CAD program did you get? Also, while an NC machine isn't required to do belt pulleys, it would save time and effort. You could use a thing called a dividing head on a manual mill. It could also be used to make gears.

The CAD program I bought is made by Swift software and is called 3D CAD. It was $2.88 on the clearance table at Office Depot. The program is worth nothing for scale drawings or precise measurements but it works excellently for laying things out for initial concepts. It is also very useful for showing other people your ideas. For $3, it can't be beat.

most of our bots from the last couple of years were EXTREMELY LOW to the ground!! 2001 we had 3inches of ground clearence, this year we had 3/8ths of an inche!!! we use wheels that we make ourselfs, chassis are low to the ground and not alot about the cg.

pics:

Our robot had an intriguing design. To ease zero radius turns, we had only two drive-wheels mounted in the center of the robot and had castors on the front and back. Most of the weigh was centered low around the chasis plate and the battery was the lowest point on our robot (beside the wheels) which lowered the center of gravity substantially. The wheels were powered by two drill motors and a 200:1 gear / chain reduction. Thus, to keep the weight on the wheels, we just balanced the wheels. However, I do suspect that if we had used more drive wheels, we could have gotten more traction, which always helps, but we wanted to tread lightly on the carpet.

We've used the two-wheel drive with castors setup before. It's worked pretty well for us. It turns well, and can push okay, but sometimes you just need the power and traction 4-wheel-drive offers. Plus, if there is any kind of incline, that setup won't work. You'll high-center your 'bot when you try to go up the ramp. It's not perfect, but it's surely simple and effective.