In recent years, most top teams have been using either 6 or 8 wheels in their drivetrain, but what are the advantages of this besides the ability to do center traction? Many teams run 6 or 8 wheels without any traction wheels. Wouldn’t advantages like the robot not sinking into the tiles as much be outweighed by the fact that more wheels adds weight and rotational inertia?
Additionally you will have more contact points on the ground which means you can get better traction.
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I thought of this, but more surface area in theory doesn’t increase friction. The formula for friction is the coefficient of friction times the normal force, and adding more contact area doesn’t change either of them. Am I missing something?
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You are correct. More wheels touching the floor means less force downwards per wheel. Therefore traction is primarily based upon the material of the wheel surface, the material of the foam tiles, and the weight of the robot.
Basically:
Heavier robot => more traction
More wheels => Less chance of bottoming out while over a bump
More wheels => More weight and spin-up inertia (which slightly increases traction with the added weight)
More wheels => Slower acceleration
Stickier rubber on wheels => more traction
The quickest way to add traction to your robot is to change the material of the wheels to whichever has the highest friction coefficient (like the rubber of flex wheels).
The second quickest way is to bite the bullet and add more weight to your robot, accepting that you may have lower acceleration.
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Having more wheels distributes the normal force across the wheels more evenly. Meaning that each wheel has less individual force on it but the same total force which means each wheel is less likely to slip causing you to have better traction.
Additionally, having a larger surface area contacting the ground increases the probability of molecular interactions between the floor and the wheel, effectively causing an increased coefficient of friction. (Technically the coefficient of friction is really only supposed to be used for ideal situations with perfectly uniform molecular structures, which vex parts are not, but using it this way effectively communicates the applicable concept in an easy to understand way for people who already have a basic understanding of physics. Kind of like the ideal gas law which perfectly explains ideal gases, and is a really good estimator for real world gases)
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I don’t understand this. If the total force is the same, shouldn’t traction also remain the same?
All of the advantages and disadvantages of adding more wheels seem so minute. If it does increase traction, it only does by a tiny bit. It also only increases weight and rotational inertia by a tiny bit. I’m just struggling to understand why almost every high level team goes through the effort of adding more wheels if it barely affects anything.
The number of wheels can also increase stability.
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Adding more wheels isn’t hard. I’m taking a 5% benefit for 5 minutes more of build time.
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If you are struggling to see the effects and feel like the changes are miniscule, I would recommend building 2 drivetrains so that you can see the difference in pushing power.
It’s not too difficult to build, will provide you with a good notebook entry and will help you understand the mechanics a little bit better.
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Thank you for the suggestion! I will try to do this when I get back to school.
Sorry, I should’ve been more clear. What I meant was that if adding more wheels has clear advantages that outweighed the disadvantages, it is obviously worth it to add more wheels. However, the advantages that more wheels come with also have disadvantages, such as increased weight. Both the advantages and disadvantages seemed very small to me, so I didn’t understand why it was worth it. It also confuses me that many of the teams that add more wheels to their drivetrain are the same teams that cut flex wheels in half to save weight even though it might reduce grip on game objects.
To be clear, I’m not claiming that fewer wheels in a drivetrain is better, and I have tremendous respect for all the high level teams out there who have much more experience than me. I’m sure there are good reasons for why people tend to use more wheels, and I asked this question so that I could better understand them myself.
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