Subject:
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Re: Newbie needs Help
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 7 Jun 2006 03:20:20 GMT
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Original-From:
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steve <sjbaker1@SPAMLESSairmail.net>
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Viewed:
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4522 times
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Dave Curtis wrote:
> There is a torque of L times the distance from drawbar to ground that wants to
> lift the front end. All I'm saying is that raising the drawbar increases that
> torque.
Yeah - bad for a tractor with front-wheel steering - and a need to steer
- not so bad for a robot with either no steering or rear-wheel skid
steering.
This robot really doesn't care if it's front end gets light - but for a
tractor it's a major disaster.
But the other major difference is that we're pulling the other robot
via a flexible rope - the only forces are along the line of the rope.
With a tractor and trailer, the trailer pushes down vertically onto
the tow bar as well as being a rearward drag on forward progress.
That also drives the tractor design in directions that aren't
necessarily appropriate for a tug-o-war robot.
> The farm tractor solution is: 1) low drawbar, 2) long chassis, 3) as much weight
> forward as possible. Without going through the analysis, my guess is that
> getting the weight forward will be a bigger contributor to traction than you
> could get by using your opponent's weight to pull your own back end down.
I disagree - friction (for static and dynamic friction) is proportional
to the downforce on the wheels. More friction with the ground is more
traction.
Why waste our limited weight budget weighing down wheels that aren't
generating any traction? (Presuming we aren't concerned about steering
by turning the idle wheels).
Better still is if we can redirect some of the backwards force generated
by your opponent into pushing the wheels harder against the ground by
moving the rope attachment point.
You don't do that on tractors because roll-over accidents on sloping
ground are very dangerous - and raising the tow bar above the center
of gravity would make that lethally worse.
> By now, the original poster is probably sorry he asked :-) I'm certainly not
> sorry he asked, this has been fun to think about.
Of course!
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Message has 1 Reply:
 | | Re: Newbie needs Help
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| (...) Thought provoking question. It led me to the conclusion that when the bot is generating it's maximum possible drawbar pull (as limited by the motor and drive train), the weight on the undriven front wheels (I'm assuming a rear wheel drive (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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Message is in Reply To:
| | Re: Newbie needs Help
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| (...) Sure, I understand the argument. But this old farm boy has both a few hours in the driver's seat of a farm tractor, as well as a couple of engineering degrees, both of which prompted my cautionary statements. Let me start by saying that I (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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