Thread: 6WD vs 8WD
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Unread 06-01-2011, 11:49
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Re: 6WD vs 8WD

Quote:
Originally Posted by KrazyCarl92 View Post
The equation is actually Ff <= μN, which brings us back to the question of traction limitted drive trains. Because 67 used traction wheels that would be driven and in contact with the ground at all times, I highly doubt that they would be fraction limited. Let's assume that the coefficient of friction of their wheels is .8 (conservative estimate), then their force of friction would be .8 x 90 <= 72 lbs. That means that they would have up to 72 pounds of force moving them forward. This lead to the acceleration of a 90lbs robot up to:
mass = 40.82 kg
force = 320.27 N
acelleration = 320.27N / 40.82 kg = 7.846 m/s2 = 25.74 ft/s2

Given that no robots can get up to 25.74 ft/s in a single second (or at all for that matter), the acceleration is clearly limited by gearing/motors, not the traction of the wheels. So in this case 67 would in fact accelerate about at about 4/3 the rate of the 120 robots that they compete against.
You raise valid points about what is limitting acceleration (and, as you and JamesCH95 alluded to, it's often elements of the motor/drivetrain itself), but you have a pretty significant flaw in your argument. 25.74 ft/s2 isn't equal to 25.74 ft/s. Just because you're accelerating at rate x, doesn't mean your maximum speed is x.

Another point is that radial wheel [drivetrain] acceleration and linear robot acceleration are two different matters, though often linked together in FRC scenarios.