how important is ground clearance?

[Zippiot]

i see pics of all these bots that are millimeters off the ground, our bot is using 9 inch solid rubber tires and we have 4 inches clearance, but with the arm all the way extended carrying a tetra i have lots of trouble knocking it over (in fact it is quite difficult). how necessary is ground clearance when our center of gravity seems to be quite low already?

[Alex Minot]

not that important, remember there are no obstacles on the field, only loose tetras that can easily be pushed. However, the lower you can get, think you have a lower center of mass and less weight stress

[Billfred]

I beg to differ.

If your robot has no ground clearance, you are not going to be able to climb onto the vision targets to get into a tetra loading zone.

Additionally, what if your opponents decided to dump a bunch of tetras inside the goals? Wouldn't you want to be able to climb into the goal and run them out?

A little ground clearance never hurt anybody.

[Jeff Rodriguez]

Also, what if your opponent decides to flood your endzone. If they put 2 or 3 of their robots in your endzone, your not going to be able to fit in your endzone, unless you can climb into a goal.

[Swampdude]

Originally Posted by Alex Minot

remember there are no obstacles on the field, only loose tetras that can easily be pushed.

Actually there will be 5 moving obstacles, and rather quick and powerful obstacles. When you collide with one, the high CG will rock toward it's momentum. These 5 obstacles will be controlled by adrenaline filled teens with few things in mind other than to win. If one of these catch you on your heels while your CG is hanging beyond your perimeter - over it goes. Only a few teams will sacrifice the match to attempt to upright you. Most likely you will become a swinging gate while laying on your side. Then toward the end of the match, your teammates might attempt to plow you into the end zone for the 10 points.
Anyhow - generally a low ground clearance only helps to lower the overall CG. But you could have a 1/16 ground clearance and a 3' high CG. Then you accomplished nothing by the 1/16 clearance. Unless you have a very narrow wheel base, having the chassis out on the perimeter to touch down in the event of a tip won't help. If anything it's going to get caught on stuff. Also opponents can get on top of you and pin your chassis to the rug. I think the rug is going to get torn up from these. Some machines might flip because the chassis will catch on loose rug while at high speed. I wouldn't attempt to go under an inch from the floor, as the fields at competitions are always getting shredded.

[ChrisH]

Originally Posted by Billfred

I beg to differ. (snip) A little ground clearance never hurt anybody.

I'll go farther and say a little ground clearance is essential. If you don't have any then you don't go anywhere. The question is not so much "how low can you go" but "how high is too high?"

I have done studies on the arm/base/object relationship. It seems that if you keep your combined weight of arm and object close to 25% of your total system weight (including object) and your base CG is within 1 ft of the ground then you will have a very hard time tipping. But once you pass the 25% threshold staying upright gets a lot harder.

The relative weights of the base, arm and object are more important than the absolute number. So if you have "extra" weight you want to make sure that anything you add goes into the base, to increase its fraction of the total weight.

Your base CG is probably lower than the top of your wheels and hence lower than 1 ft. There is no reason to work hard to get it lower than it is. There are plenty of reasons to leave it where it is.

First of all, the carpet is only as flat as the arena floor. Sometimes the floor is really flat, other times it is not. It would not surprise me to see bumps of 1/4" in a floor. If you should build your robot with only 1/8" ground clearance, then you would have a problem.

Second, there is always the possibility that your robot might get bumped so that one or more wheels get pushed into a goal. If you have really low ground clearance, then you could wind up high centered on the goal. With a couple of inches of clearance, this ceases to be an issue.

It is often easier to have your wheel axles go into your frame than to have the frame hanging from them.

There are probably other reasons I can't think of right now. Extra Ground Clearance only becomes a problem when there is too much. How much is too much? only you can decide

[Conor Ryan]

i'd say that you want a smart engineering design, that would cap it would be good to keep a low ground clearance and thus a lower center of gravity. Because with these tetras going up so high, you don't want to tip over now do we?

[Max Lobovsky]

Ground clearance is mostly unimportant in and of itself beyond being able to clear the 1/4" polycarbonate triangles this year. As people have stated, the only reason it matters is for the sake of lowering CG. But, this does not mean putting pieces of your frame very low to the ground will do much. Moving your 10lb frame up or down 1 or 2" is pretty much a mute point. Where you mount the rest of your heavy stuff is what really counts. A low clearance is pointless if you don't make sure your electronics, battery, pneumatics, drive train, etcetera are as close to the ground as possible

[Zippiot]

the poll shows that the majority have less than 1 inch of clearance (not majority but .5 to 1 inch got the most votes) but is that high enough to not get stuck under a larger bot, or on top of a tetra/goal? our bot seems like a truck towering over a little japanese car (i dont mean to offend anyone, i drive a mitsubishi). will the bots with .5-1 inch have any potential advantage over my bot with 4 inches?

[CJO]

Our particular ground clearence (.8670 inches) had more to do with our drive train design than with anything else, still, it gives us an excellent wheel base. Currently, we can have a tetra on our arm, fully extended to cap the tall tetra, and our center of mass is only a couple inches off the ground. Our design is not good for entering the big tetras, but we can have our arm fully extended, and are still virtually untippable.

On a side note. If you have a good animator, you can set up reactor (the physics engine in 3DS Max) with a properly weighted and dimensioned robot, and then do testing with read out about various stresses. This actually saved us from a claw design that although spiffy, was prone to failure at certain angels (i.e., we input the torque from one of the motors, and discovered that in certain situations we could not rotate all of the way)

[ChrisH]

Originally Posted by Zippiot

the poll shows that the majority have less than 1 inch of clearance (not majority but .5 to 1 inch got the most votes) but is that high enough to not get stuck under a larger bot, or on top of a tetra/goal? our bot seems like a truck towering over a little japanese car (i dont mean to offend anyone, i drive a mitsubishi). will the bots with .5-1 inch have any potential advantage over my bot with 4 inches?

There is no inherent advantage in having a very low ground clearance. Much depends on how you distribute the weight. Last year, for example, we had a fairly tippy robot. It wasn't too bad when our lift mechanism was fully retracted, but that 9lb slider motor/gearbox for sliding the bar was a real liability when the lift was extended. At least until we hooked the bar, after that it didn't matter.

The potential problem was that we had a fairly concentrated weight that was pretty high. We got away with it because we were only in that condition for a few seconds.

So a robot with a very low ground clearance that has two chip motors mounted high on the robot to drive their arm, may very well be worse off than a robot with 4" of ground clearance but has all the arm motors mounted low.

Four inches of ground clearance is enough to ensure you won't get trapped in a goal. If you keep everything heavy close to the plane of the wheel axles you should be fine. We have about 2" over most of the frame so that we can climb out of a goal should we wind up in one. We also have areas of low clearance to prevent getting in that position.

Looking at all the robots here on CD I can see why you might be concerned. There does seem to be a great number of low slung robots. But following the herd is not always the best course.