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| View Poll Results: Rear wheel drive vs. Front wheel drive vs. All wheel drive | |||
| All wheel drive |
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110 | 88.71% |
| Rear wheel drive |
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7 | 5.65% |
| Front wheel drive |
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7 | 5.65% |
| Voters: 124. You may not vote on this poll | |||
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#1
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4 wheel drive vs. 2 wheel drive
we have been debating whether all wheel drive or front wheel drive or rear wheel drive was better.
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#2
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Re: 4 wheel drive vs. 2 wheel drive
General rule of thumb is that you want any wheel touching the ground to be powered. If not, you're cutting into your team's available pushing force.
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#3
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Re: 4 wheel drive vs. 2 wheel drive
Indeed. If you are imparting robot weight on a wheel, make sure it is powered.
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#4
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Re: 4 wheel drive vs. 2 wheel drive
all wheel drive will be the best
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#5
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Re: 4 wheel drive vs. 2 wheel drive
That being said, you really want your weight as far back as possible so that your pivot point is as close to the hitch pivot. That reduces the moment that the trailer imparts on you robot, which it needs to counteract to turn, and will allow you to turn better.
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#6
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Re: 4 wheel drive vs. 2 wheel drive
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Edit: All wheel drive might help but might be wasting power, motors, and battery life... (i have done some tests and this is what i get, but its up to you) Last edited by Betty_Krocker : 06-01-2009 at 08:10. Reason: last minute thought |
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#7
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Re: 4 wheel drive vs. 2 wheel drive
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The physics reason for all wheel drive is this: 1) Let's say your robot has 4 wheels 2) Each of these wheels supports some fraction fi of your robot's weight. The sum of all Fi = 1. 3) Each wheel will thus have a normal force of mRobot*fi*g = 9.81*mRobot*fi 4) Each wheel will thus be able to generate (if powered) a frictional force of Ff = u*Fn Ff = 0.06 * (9.81*mRobot*fi) Ff = 0.5886*mRobot*fi 5) Your robot's total potential frictional force is the sum of the frictional forces of all of its driven wheels. 6) For an all wheel drive robot, FfRobot = 0.5886*mRobot (because Fi sums to 1) 7) For a 2WD robot with its weight evenly distributed, we know all the Fis are 0.25 (due to equal weight), but only two of them are driven. So FfRobot = 0.2943*mRobot, or half of the AWD's robot full force. So based on 5), 6), and 7), we know that by choosing not to drive some of your wheels, you are throwing away potential friction. Wheels that are touching the floor while the robot is driving and are not powered are completely wasting potential friction. A robot with its weight evenly distributed over 4 wheels that only drives two wheels will be two times slower than an identical all wheel drive robot. The two wheel robot could potentially employ a very complicated suspension so that its weight shifts onto its driven wheels when acceleration or decelerating, but it would be a waste of time when you could just drive all four wheels. Last edited by Bongle : 06-01-2009 at 08:30. |
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#8
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Re: 4 wheel drive vs. 2 wheel drive
if you are going to attempt to prove me wrong, at least understand my argument. I never said an even weight distribution, in fact i did say WEIGHT OVER THE FRONT DRIVE WHEELS, plus my system cuts down on weight thus allowing you to allocate that precious weight to other areas of the robot like the heavy control system, or ballast weight that you can add where needed...
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#9
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Re: 4 wheel drive vs. 2 wheel drive
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I understand your idea (though I did gloss that over in my original post, sorry) and it is definitely the most optimal way to implement a 2WD system, I just think that _any_ 2WD system is far from optimal this year. |
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#10
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Re: 4 wheel drive vs. 2 wheel drive
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#11
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Re: 4 wheel drive vs. 2 wheel drive
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And by the way, the whole "pushing is better than pulling" thing is more or less a myth. RWD makes more efficient use of it's traction than FWD, no matter what surface they're on. The reason why FWD is better in the winter is that when they're varying levels of traction on different parts of it road, it's easier to control, and understeer is safer than oversteer. Manufacturers don't like selling cars that oversteer, is because the majority of people don't understand vehicle dynamics, and don't have the experience to correct oversteer instinctively. If a RWD car starts to slide out, most people are going to slam on the brakes. This transfers weight to the front of the car, increasing the grip on the front wheels, and decreasing the grip in the rear, causing the car to pivot further. To correct a slide in a RWD car, you need to use steering input and throttle modulation, not panic braking. In the context of Lunacy, where the traction on the regolith is entirely uniform, and it doesn't matter if you slide into other robots, RWD would be far superior. Just because traction is reduced this year doesn't mean that the laws of physics went out the window. FWD still sucks. In fact, it sucks even more this year, because the robots are going to be driving at the limit of their grip all the time. Last edited by SWIM : 06-01-2009 at 11:18. |
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#12
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Re: 4 wheel drive vs. 2 wheel drive
Agreed. My comments were was based upon the assumption of a standard kit style frame with no steering and the choice being between 2 or 4 wheel drive. Most steered mechanisms will be better than that, but if you are limited to a standard kit drive system, it is my experience with the kit base and attached trailer that the more rearward your pivot is, the better able you are to turn.
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#13
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Re: 4 wheel drive vs. 2 wheel drive
Quote:
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#14
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Re: 4 wheel drive vs. 2 wheel drive
right but since there are no sway bars present, the tongue weight and force it exerts will be concentrated at the hitch, if the hitch is in the center of the two steering wheels, the force will be distributed evenly between them giving the needed down force to turn the bot...
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#15
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Re: 4 wheel drive vs. 2 wheel drive
To everyone considering not using all wheel drive: DON'T DO IT!!!
I don't care if you are steering, not steering, using 2 wheels or 18. With the limited amount of usable force available you want every wheel touching the ground to contribute to converting normal force into tractive force. Every non powered wheel you have touching the ground will reduce your ability to accelerate and push. |
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