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Re: 6WD vs 8WD
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However, consider that the driving force (i.e. output torque of the motors) decreases with speed. At some speed the motors will lack the torque required to spin the wheels, barring extreme cases. At that point, the situation will become the F=ma situation we all know and love and the lighter robot will be able to accelerate faster. Another interesting tidbit is that the wheels' coefficient of friction may not be constant, but rather vary with contact pressure. Assuming that the coefficient of friction is constant the heavy and light robots will accelerate at the same rate in a friction-limited case. However, if the tread/playing surface is sensitive to contact pressure then the lighter robot will have the advantage, all else being equal. Also note that a drive-train does not require power to break traction, but rather requires torque, specifically torque on the drive wheels. |
Re: 6WD vs 8WD
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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. |
Re: 6WD vs 8WD
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Re: 6WD vs 8WD
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Another point is that radial wheel [drivetrain] acceleration and linear robot acceleration are two different matters, though often linked together in FRC scenarios. |
Re: 6WD vs 8WD
Sorry I used the equation V = at without mentioning it, using 1 second as t for reference because I figured that we would all agree that no robots would reach a velocity of 25.74 ft/s at one second at that acceleration, indicating that it is other factors, not the friction that limits the acceleration of the robot.
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Re: 6WD vs 8WD
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-Brando |
Re: 6WD vs 8WD
Which was the point I was making, the gearing, not the friction is what limits the force of acceleration and thus the acceleration of the robot. We can come to this conclusion because the acceleration of 25.74 ft/s2 is so unreasonable that it is obviously not the limiting factor. So if it were geared as such, a lighter robot could in theory accelerate faster than other heavier robots.
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Re: 6WD vs 8WD
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The ride height was controlled by the claw, and is automated (more drop when turning, less when not). The driver must request a bump-cross, but when not bump-crossing, it handles ride height on its own. From Danial Ernst's pictures from the State Championship: ![]() The rear chassis (the part we attach the bot-bottoms to) is mobile, pivoting around the rear axle (omni wheels), and containing the drive motors (4 CIM + AM shifters), and most of the control system. The gearbox is chained directly to the middle and rear wheels, and the rear wheels are chained to the front wheels via two jack shafts per side (over the top above where the chassis floats). In software, the normal claw movement (including the bump-crossing and flat centered positions) is handled with the claw axis pot, and center dropping is handled by a string pot in the chassis which measures drop. |
Re: 6WD vs 8WD
So, I was very inspired by some of the physics guys up above, and have a challenge:
Assume 16m dash (approx. 54 feet- robot length), and COF of 1.0. Also assume a 4 cim power-train at 300 Watts/CIM. What weight is your bot so that you are just barely traction limited the moment you strike the other wall? Is this less than 4 cims & a battery? |
Re: 6WD vs 8WD
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He implied an acceleration of 25.74 ft/s^2 over a time interval of one second would result in a velocity of 25.74 ft/s. He then commented based on experience/anecdotal evidence that teams are not doing this, and therefore are not traction limited. |
Re: 6WD vs 8WD
Can someone tell me what a "West coast drive" is?
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