Subject:
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Re: Newbie needs Help (diff sensor)
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 7 Jun 2006 17:20:33 GMT
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Original-From:
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steve <sjbaker1@airmailAVOIDSPAM.net>
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Viewed:
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4744 times
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Brian Davis wrote:
> > To measure the dynamic friction you can use the same locked-up
> > wheel rig and just pull it along flat ground with a spring
> > balance. Drag it along at a reasonably constant speed where it
> > isn't jerking along.
>
> That's not very easy in practice, and you need to make sure the rope is
> pulled parallel to the floor. I've always wanted to use the RCX and a sensor to
> do this automaticly, but the results "by hand" have always been so poor I've
> never bothered.
Hmmm - that's unfortunate...but if the 'right' strategy is to
go with a non-slipping robot, the measurement of sliption is
really a lot less important.
> > Furthermore, realise that the high school physics equations for
> > sliption say that the frictional force is proportional to the
> > weight - but NOT related to the contact area. People find that
> > counter-intuitive - but it's true.
>
> It's wonderfully true in the world of high-school physics, but the real world
> can be significantly different. Since the behavior of the rubber depends on the
> pressure exerted, as well as the dynamic physics of what happens as the tire
> repeatedly sticks and slips (both are still occuring), this is another one of
> those places where I suggest you do some empirical tests, and not worry too much
> about what the physicist say (and for the record, I *am* one of them... I have
> great fun in my college level classes showing the students where the textbook
> solution fails miserably in the realy world).
Yeah - so many of those basic mechanics equations are only
approximations - yet they are taught as if they were laws.
I enjoyed Richard Feyneman's commentary on this subject.
However, whilst it's not 100% true - it's not such a terrible
approximation. Several people here have noted that adding more
wheels doesn't help.
> > Tank tracks are useless for the same reason - they increase
> > the contact area
>
> The problem is, the LEGO tank treads really don't - even if you include a
> bunch of idler wheels like on a real tank, take a look at how much of the tread
> actually contacts the surface and you'll see what I mean.
Yeah - the ribbing on the track looks to be the problem. A smoother
track would work better. (That's back to having nice knobbly off-road
tyres versus racing slicks).
> > Stiffer rubber is better...
>
> All other things being equal, perhaps... but how "hard" or "soft" the rubber
> is plays a major role in the coefficient of friction. "Stiff" tires often have
> lower coefficients of friction (both static and dynamic) on certain surfaces...
> which brings up another factor that hasn't been mentioned (too much),
Having soft rubber on car tyres eats energy at higher speeds because
energy is absorbed in repeatedly flexing the side-walls of the tyre.
At lower speeds, compressing the sidewalls as the tyre approaches
the ground is kinda like driving uphill...it's generally undesirable
for all kinds of drive - but if your wheels are spinning, that's a
MUCH bigger effect than when they are moving quite slowly.
> > and you'll need to be aware that stictional forces ARE
> > proportional to the contact area...
>
> OK, I'm confused. Why would one of these forces be proportional to the
> surface area, and the other not?
Dunno - I'm happy to defer to your expert knowledge!
> In both cases, the frictional force is just the
> normal force (usually the weight) times the coeffecient of friction. Period.
> Surface area (in "textbook" physics) just doesn't enter into it.
That's certainly not true in reality. Cars with wider tyres grip
better - that's absolutely for sure - using dead smooth racing slicks
gets you better grip (in dry conditions) than tyres with grooves cut
into them...just see the effect on a Formula I race car when the team
guess wrong about the weather and put on the wrong kind of tyre! Both
are made of soft compound rubber - both have about the same contact
patch - but one has a bunch of places where its not touching the ground.
The car weighs the same in both cases...the only difference is the area.
On the other hand, I remember the high school physics class where
we took a heavy wooden block and dragged it along a nice level surface -
noting that it didn't matter whether we put the block on it's big flat
side, it's narrower side or on one end - the frictional force was
the same...so the contact area didn't matter.
I presumed that was because wheels that aren't slipping are reliant
on static friction and that the experiment we did in school was
dynamic friction. Perhaps I came away with the wrong impression
here and the dependence on area depends more on the nature of the
materials involved (rubber is weird stuff).
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Message is in Reply To:
 | | Re: Newbie needs Help (diff sensor)
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| (...) As you note in your mini-cooper example, *if* you can keep the wheels from "spinning out", but instead are always in rolling contact with the ground, static friction is what's important... and generally, the static coefficient of friction is (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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