Mechanics and Construction > Mechanics and Construction

Wheel width for differential drive robot

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Somchaya:
I'm thinking of building a differential drive robot, with the front 2 wheels powered by servos. I was wondering if the width of the wheels will make a big impact on the effectiveness of the turning.

Essentially, the wheels I'm considering are 6" in diameter and 1.5" in width. Will these wheels be too "fat"? I haven't calculated the final weight of the robot, but I estimate it to be around 3-4kg. Will standard servos (4.1kg/cm at 6V) provide enough torque to do the job well? (Edit: I just looked at the dynamics tutorial page, I'll do some of those calculations later, but if anyone can give a rough idea of yes/no, that'd be nice  ;D)

Also, I'm wondering about the back wheels. I'm thinking between using a single caster wheel (maybe with an omniwheel as the caster), or maybe 2 omniwheels (so the robot looks kind of like a 4 wheeled car but with 2 omniwheels at the back) for added stability. Which would be better?

--- Quote ---I was wondering if the width of the wheels will make a big impact on the effectiveness of the turning
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What material are the wheels made out of? What terrain will the robot run on? How wide is your robot (wider robots have lower turning friction)? The highest friction scenario I have done is .75" width foam wheels on concrete, and didnt have any noticeable issues.

As for the caster, one is better - less friction, less to buy, less to connect, etc. Your robot will be perfectly balanced as long as mass is equal on both sides of the castor. One additional note, make sure you locate your heavy stuff on top of the castor - if not, your robot will do pop-a-wheelies when it stop-starts (I learned this the hard way :P).

Somchaya:
The wheels are made of foam, and the robot should be around 9-10" wide. I'm thinking of building a base to place my laptop on for mobot, so the terrain will be concrete with cracks :D I also want to possibly use the base for other things in the future too, but the terrain then will most likely be flat, or low carpet.

With such a wide base, I'm wondering if a single castor will be stable enough. The weight should be basically evenly distributed, but it may still be unstable?

Im gonna say it straight . . .
Ive never seen a mobot with a laptop make it more than 10ft past the starting gate :P

--- Quote ---With such a wide base, I'm wondering if a single castor will be stable enough. The weight should be basically evenly distributed, but it may still be unstable?
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One will work fine. Just keep your weight really low and balanced. The only way your bot can tip is if its top heavy and making sharp/fast turns.

The best way to test friction is to jerry-rig up something quick with the wheels you want, motors you plan to use, and 'simulation weights.' With just 2 or 3 hours of work you can determine if your ideas are valid or not. I did something similar my first year doing mobot - it involved lots of tape . . .

I will see you at mobot :)

annoyin_kid:
what is a mobot and why do you want a laptop on a robot?