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Hey I was wondering what kind of chassis, wheels, gears are for a robot with a lot of torque. Please help. I have decided on a trapezoidal chassis, and I know that the servo connects to the small gear and the wheel connects to the bigger gear. But I do not know what the optimal gear ratio should be and size of the wheel. Also should I have 4 wheels (two in the back and 2 small ones in the front.

The competition is to see if you can pull the other robot in a tug of war. It is going to be happening on a typical gym floor at a school. We are provided with only two servos, and we can make any design that can move and work for the tug of war.

The robot has to be 8" x 8" x 8" and should not exceed 2 lbs. There is no budget, and I am building this for real, not fiction. I'm planning to laser cut the chassis but I don't know how long and wide it should be. We also can laser cut wheels (I don't know if I should use 4 wheels or 3). If I am going to do the tracks (like on a tank), how would I do that? Does it require 4 servos and 4 different gear boxes? Also, for the type of gears, which one would be the best?

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  • $\begingroup$ gear ratios will depend on final robot speed desired, diameter of wheels and motor speed & characteristics just to start with. This is a very broad question as it stands. $\endgroup$ – Solar Mike May 15 '17 at 6:20
  • $\begingroup$ Please provide additonal information, the size of the robot, what is the underground the competition is held on, what materials are you using? $\endgroup$ – Mech_Engineer May 16 '17 at 14:37
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I would like to commented this on the question (but I can't due to the 50rep requirement)

I don't know if wheels are obligated or there a other options?

Tracks should give more friction to the ground, so you will be able to have a smaller chasis with more traction than a long one with wheels.

If the design must be wheel based, this is also a option. Wheel tracks

Also keep in mind for your design to have a low as possible center of gravity. The higher it is, the faster it tips.


Additional question(s)

To answer your additional question in the comments I would like some more info:

Are you really building this in real life, are we talking about a scale model or is this just fiction?

Is there a budget limit?

There are various ways to build "tracks" or something that gives a similar effect.

For example if there is a large budget for this, buying or making tracks would be the number one solution. But in many cases as in hobby projects there are no big budgets available. A cheaper solution that would give you more friction are snow chains, this will also work in mud.

This technique is often used in by logging companies (link in swamp loggers discovery channel)

Tire chains

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  • $\begingroup$ good points re CoG and tracks, but track technology is interesting and a real pain when they come off in 18" of mud... $\endgroup$ – Solar Mike May 15 '17 at 14:04
  • $\begingroup$ Do you mean tracks as in the ones in tanks? that would be an amazing idea. I thought of that but i don't know how to build it. Would you be willing to shed some light on that topic? $\endgroup$ – elf on the shelf May 16 '17 at 0:48
  • $\begingroup$ Tanks, crawlers, all sorts of agricultural stuff - even small snow blowers and look at the picture provided above by Mech_Engineer $\endgroup$ – Solar Mike May 16 '17 at 14:03
  • $\begingroup$ @Mech_Engineer i think i am allowed to do tracks. can you please sketch your design so I can get a better idea? Also, would there be 4 servos? $\endgroup$ – elf on the shelf May 16 '17 at 22:58
  • $\begingroup$ As you edited your answer the tracks are not efficient. Not at all, tracks only generate friction if they can be pressed in the ground or mud, the gym floor is too hard for your robot weight to crush the floor and generate friction. Best you go with slick tires. and alot of them to generate more contact area. Or you could go with rubber band track that could generate some friction something like this in the link. (bigtyre.com.au/images/tracks/asphalt-paver/…) $\endgroup$ – Mech_Engineer May 17 '17 at 7:25
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For maximum drag force, I recommend to have beam (long) chasis. Wheels on both farthest ends. This will maximize topple resistance, at every weight (more weight is better).

Wheels, if possible is gear wheel on linear gear track affixed to the floor. This will enable drag force exceed friction weight.

Gear ratio should be as big as possible (if no limitation, maybe 1:9999 or 1:99999 or more). You can use planetary gear for compact size.

Another tweak from me, pull point should be lower than wheel centerline. So that pull force will help to push the wheel more to the ground (I should draw this).

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  • $\begingroup$ can you please draw it? Thank you and as for the chassis, what shape would you suggest? $\endgroup$ – elf on the shelf May 15 '17 at 11:48
  • $\begingroup$ Also, if possible can you draw one of the views for this design? I'm new to robotics so I'm not the most clear on the subject. $\endgroup$ – elf on the shelf May 15 '17 at 11:53
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If this is for a robotics competition, you might keep in mind your design vs the opponent. If they put their weight on the rear of their vehicle and you can move your tie on point you can lift their rear wheels off the ground by tying off at an elevated location at the front of your vehicle. The downward force of the cable would actually add downforce to your wheels if they are close enough. You need to draw a tri-angle from the tie off point to the 2 wheels, then draw a straight line to the tie off point of the other robot. If the slope of the line is shallow enough it will add sufficient downforce to your rear wheels.

Watch some videos of tow trucks pulling vehicles out of ditches on you tube to get a better understanding of what will help.

If you are required to use the same tie off point as your opponents Ranier J above has good advice on a long chassis and weight placement to minimize forces that might tip your robot over...although, thinking outside the box I wonder if a single axle robot might have more traction on the single axle and be 'un-tippable'. Look at the recent fad of 'hover boards' that have a platform located at the centerline of the axles.

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  • $\begingroup$ We won't get to see anyone's robots before hand. but I don't quite grasp how to make the robot with the most torque and the shape of the chassis (right now mine is trapezoidal) and we can't trial with them at all. Can you please sketch the design about the single axle? $\endgroup$ – elf on the shelf May 16 '17 at 0:44
  • $\begingroup$ You can see in the linked webpage how having a high mounting point can lift the opposing drive axles off the ground, theirs would have been better if the tie off point were further forward. I am going to try to draw something on my phone for a single axle design. legoviks.blogspot.com/2008/03/tug-o-war-robot.html $\endgroup$ – Gwydionforge May 16 '17 at 13:18
  • $\begingroup$ From looking at my drawing, the single axle would only really work if you could pre-load it by pulling against the other robot a little to set it up. It would also take alot of design development to make it effective. You would need stabilizer wheels to help if you get pulled off center and it would take trial and testing. Please let me know if you do go this direction, I would be really curious if it could work. What level of competition is this? Lego / plastic robots or complex computer controlled steel and composite large frame? $\endgroup$ – Gwydionforge May 16 '17 at 13:58
  • $\begingroup$ We are not allowed to use legos :(. Right now the materials the leader gave me is an 8" x 8" wooden board to laser cut things with like a chassis and wheels. I would love to go in this direction, but I cannot see your drawing, so can you please help me? $\endgroup$ – elf on the shelf May 16 '17 at 22:55

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