Going over 6 inch step with 12 inch wheels

[Joe P]

Our 128 pound robot can go over the six inch steps using the 12 inch wheels in the kit. The set of front wheels go over with no problem but the back set of 12 inch wheels get hung up and start spinning a lot. We eventually get up to the top of the platform but it takes great effort for the driver to get up there. I was thinking that maybe lowering the pressure on the back tires would help. Also, I was thinking that adding 2 pounds of weight behind the back tires would also help since most of the weight would be in front of the back wheels after the front wheels climb the 6 inch step. Any comments or suggestions would be appreciated.

[Paul H]

Well when you think about it, when the front wheels pushing up, all your weight is on 4 wheels pushing up. When the back wheels are going up, you are pushing with 2 wheels. I know you probably can't do it, but a powered "climbing wheel" in the center that is set higher than the other 2 would help tremendously. When we were driving around our drive module without the articulating chassis on the front yet, we could still get up pretty well with 4 6" wheels in the corners and 2 wheelchair wheels in the middle linked to our main drive system.

[KenWittlief]

if you have 4 wheel drive then when the front wheels hit the step the bot wants to do a wheelie, due to the torque on the rear wheels

but once the front wheels are up, and the back wheels hit the step, the bot wants to do the opponsite - the torque on the front wheels wants to push the back wheels down harder on the suface, making it harder for them to lift up onto the step

[henryBsick]

It takes finness. Our bot has the same setup as you described. 4 kit wheels. In the beginning all the robot did was bounce for the back 2 wheels to get up. Practice was neeeded (during practice rounds lol) to climb the step. I start off slow with the front 2 wheels on then gun it as the back two wheels are about to hit the paltform. That or the wieght distribution, or the "bump" wheels in the middle.

[ngreen]

How I see it adding the 2 lbs behind the back tires might not help but rather hinder more. When you climb stairs you want your center of gravity to shift forward to make it the easiest. Your best bet would to get it so those front wheels will come down and be touching and have enough traction to pull the back wheels up. That how I see it. Reduce pressure may give you enough traction to drive up the step but it will still be hard for it. Or add an arm that you can throw forward with weight on it so that it will put you center of gravity more over your from wheels when you climb. But I don't think the adding weight behind the back wheels would do you a lot of good.

We actually started with an eight wheel design with the center wheels like paul descibed. We ditched the center wheels and are instead using two plastic "sleds". It makes it up a lot smoother and they weigh like half a pound together. We still use non-powered wheels on the front to bump us up onto our front wheels. The sleds lift the robot so that the back wheels hit the edge 2 inches higher. It still takes momentum but is a much smoother transition. I would reccommend trying it by taking two pieces of a strong plastic and clamping them on between the two drive wheels . We found it's better if the slope is fairly flat or even a little more sloping downwards to the front wheel. It should take a couple minutes to try out. And if it doesn't work just change it.

[Arefin Bari]

we are using the wheels from the kit... and we did have a problem getting up the 6" steps first... then we took some pressure out of the tire and it looks like the robot climbs real easily... if you want to see it... watch the last qualifying match @ midwest regional... i wish if i knew where you could find the video and i would post a link here too but unfortunately i am not sure... you can try robotics.nasa.gov ... if you need help come get me from 108's pit @ nat... or there is always something called AIM...

thanks to wildstang for bringing their practice field at midwest regional for which we were able to test how can we get up on the platform...

[jjdebner]

This may sound silly. But if you reduce the pressure in the tires to get more traction then if it is a four wheel drive system won't it jerk and jump when turning alot more. So it is basically a trade off between turning and climbing. Unless you have "special wheels". So just make sure you can still turn when you have some pressure taken out.

[tenfour]

Our team dropped some caster wheels off the back of the robot in such a way that they lift the rear of it of the ground. As the front wheels go up, the distance between the casters (attached to the rear of the frame) and the floor changes. Resulting, the robot rides on the casters instead of the back wheels when it straddles the step. This raises the height of the back wheels and allows them to climb easier. We also put HDPE "Sliders" on the front so that we can hit harder and carry ourselves up.

PM me if you need any clarification. Its a really easy process.

-Tenfour

[ngreen]

We also put HDPE "Sliders" on the front so that we can hit harder and carry ourselves up.

These are what is in the middle. You need to have a smooth movement up the stairs (can't stop and go, stop and go) because you need a little momentum. We chose to use non-powered wheels in front in hopes of not riding on balls really easily.

[Ryan Albright]

WEll if your robot has a arm what you could do is swing the arm forward. Thats what we had to do we get our front forks up and over and then we get closer pull up are back forks and swing are arm forward which puts more weight on the front giving the front wheels more grip

[Max Lobovsky]

We have the same situation and the two best pieces of advice i could give you is defenitley lower the pressure of the back tire significantly (like down to 15) and use the following technique when driving: get the front wheels up, backup as far as you can without the front wheels falling off, throw the joysticks forward. Be careful when using this technique for the second platform as it is easy to go to fast and go over the other side.

[Biff]

We had the same issue, with more mass in the rear to counter the Big ball when on the arm. Our bot can best be described as looking like a Bobcat. Arm piviots at the rear. The only way up (and we never did it in a match) is to extend the arm after the front wheels are on the platform, shifting some mass to the front tires. Then it climbed very smooth. BTW we are using the 3" tires on the kit wheels which makes a 14" wheel when all is done. This is a tire and tube that comes from Skyway.

[Al Skierkiewicz]

Originally Posted by Joe P

The set of front wheels go over with no problem but the back set of 12 inch wheels get hung up and start spinning a lot. We eventually get up to the top of the platform but it takes great effort for the driver to get up there.

Joe,
I think if you watched closely, the robot front tires are actually being pushed into the step edge. The resulting force combined with the turning ability of the tire results in the tire climbing the edge. The robot front wheels would climb even if they weren't powered. (Think of a bike going up a curb.) There is just more point friction on the front tires. Lowering the air pressure in the rear tires should help quite a bit. Some teams have added some friction enhancers, i.e. teeth or grippers, to the tires. These grip the edge during climbing and do not allow the tires to slip on the platform edge. I have seen grippers mounted right on the tire and on a wheel attached to the tire.

[Grommit]

I'd like to point out that the 12" wheels are actually 12.5" wheels, which makes a slight difference. But more importantly, it requires a very low center of mass to drive one of these things. I watched Kehillah Jewish High School, Team 1043, with Patrick as driver (over two years of experience), in practice, and even he had problems with the robot tipping over. The robot had a large arm mounted upon it to hang, and it wasn't particularly heavy, but consider the following:

Diameter 12.5" implies a maximum wheel base of under 24 inches, probably 20 after the protection in the front. Under these rather optimal conditions, with one wheel up on the step and the other down, we have an angle of nearly 18 degrees. With a center of mass that is high enough, the momentumof the base might flip you over before your other wheel reaches the step.

[Matt]

We actually have the 9 inch wheels. We have a belt drive system. When going up the step our belts are being pushed on the plat form and pulling us up the ramp. With this we have the ability to climb the 6 inch step with no power and with no ramming speed.