Team worried about robot tipping while crossing rail.

Hello everyone

My team is finished building the prototype chassis but we are worried about the robot tipping while crossing the rail. Since our middle tire is about 3/4 of an inch lower than the front and rear wheel it raises the whole chassis up pretty high. One idea was to make the middle wheels smaller and put omni wheels on either the front or back wheels to aid with turning. How ever we can’t put omni’s on the front because the may break from being rammed into the railing. We also can’t put them in the rear because the robot would lose traction driving onto the ramp.
Also with all of that weight coming down at the same I wondered would this cause excessive weight and stress on the rims causing them to break. :confused:


IMG_20120114_162651.jpg
IMG_20120114_162704.jpg
IMG_20120114_162729.jpg
IMG_20120114_163054.jpg



IMG_20120114_162651.jpg
IMG_20120114_162704.jpg
IMG_20120114_162729.jpg
IMG_20120114_163054.jpg

Your middle wheels should be about an 1/8" lower than the outer wheels not 3/4".

Why are your middle wheels lower?

yea I must admit that I was not really paying attention while measuring the wheels in order to post this but I do know we decided to use custom spacers to raise the chassis up a several inches off the ground to clear the rail which is what threw the measurements off some.

6 wheel drive.
you’re doing it wrong.

but on a more serious note:
It’s very weird how you did your 6 wheel drive; it looks like you leveled the wheels all out evenly?

Middle wheel should be ever so slightly lower so it can grip the floor and provide the actual movement by a motor, back and front wheels should be dead. Wither front or back will touch the floor depending on movement, so no more than 4 wheels will be on the floor at all times (middle and either front or back).

Standard 6WD trick to keep maneuverability. It shortens the wheelbase, making turning easier.

But 3/4" of an inch is WAY too much for a standard 6WD drop. Those are more often in the 3/16" range. With 3/4" inch, there’s a little bit of a tipping worry even when you aren’t crossing the rail–all it takes is a hard bumper hit when you’re rocking while in acceleration, and your robot is no longer on its wheels.

Wait why only have 4 touching at once? This seems to be counterproductive. Why not just use a 4 wheel drive at that point?

Because if you’re using a 4WD, your robot bounces all over the place when trying to turn.

The 6WD gives six wheels either contacting or skimming the ground with a shorter wheelbase, which allows for much better turning.

Humm, I see. I did not know that. That is very interesting. We have never attempted a 4 or 6 wheel normal drive, except for lunacy and that doesn’t count. We used mechanums fro 2010. I will definitely keep this in mind for the future.

4 wheels will only be touching at one time. HOWEVER, that does not mean that the 2 other wheels wont be used. Throughout the match all 6 wheels would have been used. Lets say you are acceleration forward, so that means that your middle and back wheels will be touching. The you stop, now your robot wants to keep going, so your front and middle wheels are now touching instead of your back. The middle wheels also act as the pivot point. With 4 wheel drive it is harder to turn if not done properly. If you have all six wheels on the ground at once than it is extremely hard to turn.

Hope that helps explain it :slight_smile:

In addition to what others have said, ideally you would have all 6 wheels powered. In addition for better results in pushing matches, I don’t see making it over the barrier to easily without them all powered.

The think tank has some good articles, and videos. This one explains the turning problems with having your driven wheels far apart, center of gravity and other useful information. I would suggest giving it a watch it is worth it.

http://thinktank.wpi.edu/article/8

I don’t know if it would help but it seems that you could possibly lower the base with electronics on it, lowering the center of gravity. That would probably make you a little harder to tip.

In the pictures above, it looks as the guy is leaning the Kit-Bot backwards to make it look like the robot was going over the barrier. Is this true, or is the robot actually going over it?

All of the wheels will soon be powered the pictures were just taken before we finished the drive-train.

Its a little bit of both he was holding it like that in-order to simulated it going over the railing since the drive-train was not finished.