Calculating Wheel Size To Climb

[Yan Wang]

It seems like one prevalent method that will be used this year to get onto the platforms will be to utilize big wheels. I tried to calculate how big such a wheel would be needed using a 130lb robot but didn't come up with a way to do it. Can someone with a deeper physics background explain the method?

[Cory]

Generally, I believe that you need 2/3 of the wheel to be beneath the step. Anyone with a physics background care to confirm/deny this?

Cory

[Matt Adams]

It seems like one prevalent method that will be used this year to get onto the platforms will be to utilize big wheels. I tried to calculate how big such a wheel would be needed using a 130lb robot but didn't come up with a way to do it. Can someone with a deeper physics background explain the method?The short story is that any 4-wheel driven drive train could climb up the 6" lip assuming the coefficient of friction is high enough and the motor torque was high enough. However, the true problem is in "bottoming out" ie where the bottom of your robot or some component will actually scrape along the edge of the step before your back wheel climbs the back step.

To ensure this doesn't happen you can just choose a wheel size that has a radius larger than what you wish to climb, and mount it such that the bottom of your frame is taller than the 6" step- you're able to let both wheels stay in constant contact with the ground.

However, one should note that there exists a very real fear of tipping. A robot with a high center of gravity could potentially tip while attempting a feat such as this. If you would like some calculations for when tipping occurs, please let me know.

I hope this (briefly) points you in the right direction.

As to Cory's comment about 2/3's... I've never heard of such a thing, though I know that a wheel with a radius higher than the step size will have a mechanical advantage, since you simply need to rotate about the point, as opposed to climb the wall.

Good luck!

Matt

[Cory]

Matt, you are right, we tested our robot out last night, using a piston to lift up the front of the robot. The one we used only got us to about the midpoint of our wheels, but since we had so much torque, we were able to drive right up. If you do it all in one continuous motion, and you have enough torque, you actually wont high center the frame. If you stop midway, however, youre stuck.

I believe the reason I heard the 2/3's thing was so that you could know exactly how large of a wheel would get you over, no questions asked. I very likely am wrong though.

Cory

[Matthew_H]

I would like to see the tipping calculations.

[ngreen]

Our robot from last year was able to climb a six inch step with a slight modification. We added two non-driven wheels on the front to lift the front up to the first set of wheels. This was with a four wheel drive system with 8.5" wheels that have lots of traction (go-kart racing slicks). We are planning on going to a 10" pneumatic tire though. We want to smoothly make our way onto the platform, not ram our way up.

[Joel Glidden]

There are more variables to consider than just wheel size. MIT's 2.007 course has some very good info on this subject online. There's even a handy Excell spreadsheet for calculating the ability of wheeled vehicles to climb obstacles. Link follows:

Wheel Climb.xls (http://pergatory.mit.edu/2.007/software_tools/Excel/wheel_climb.xls)

-Joel

[ahecht]

As Joel said, wheel size is not the ultimate determining factor for climbing stairs. One good example of this is the Personal Rover Project's Rover1 (http://www-2.cs.cmu.edu/~personalrover/Rover1/), which can climb a 7 inch step with 2 inch wheels.

[Eric Thompson]

My team is wanting to get up the platform on the side in order to bypass the "small" space where the goals are. Is it possible to have a two wheel drive in the front with casters in the back and still be able to get up the incline? One of our team's teachers said that it was possible but im having doubts about it bottoming out. The person that designed the drive system or body says that the robot can and/or will be 6in or below from the floor. Im just the Arm Captain, but am I freaking out or what?

Eric Thompson
Northside High
Team 1415
Columbus Georgia

[Anne Shade]

I would really advise against having castors in the back if you want to climb. The big problem with castors is that you can't control their orientation so it will make it very difficult to control the direction of your robot with your back end spinning away. Having skids or non-powered wheels would be better than castors. Even better would be a 4 wheel drive. If you have any questions about how to do this, email me. The GT FIRST group in GA is here to help all the GA teams as much as we can. You can also IM me if you need to at Astroanne10.

[KenWittlief]

You dont need big wheels to climb a step

ever see a kid go UP a curb on a skateboard?

with a little momentum your wheel doesnt have to hit the face of the step at all - if you get creative :^)

BTW - did I ever mention that when I was a kid, "Speed Racer" was my favorite saturday morning cartoon?

[ngreen]

We know their is much more than wheel size but is does help in several ways. Higher clearance and more lift by an indivual wheel. We are testing 8 wheeled designs. 3 driven and 1 not. From are test we might be able to use eight but it is much smoother with 10 for us. and a inch in height will not through our center of gravity off much. Using four wheel drive you could climb the step with two wheels. The robot would have to have a lot of clearance and large wheels, say bigger than 12". Two wheel drive with casters would be hard, it might be possible but I wouldn't reccomend it.

[Jedi Padawan]

I beleive the numbers one of the engineers on our team from a tire company decided you would need somewhere in the ballpark of 18 inches to "comfortably" climb the 6-inch step. We quickly decided on a better method.... :]