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Discussion Starter #2
Thanks. My rig is so inefficient I really need to look closely at this
and give it a try. The first question I have is if the relation ship is
linear enough to use rolling to a stop as a point. Maybe slowing to
5,10,15,20 mph would give a better indication of the rolling
resistance?. It seems when I push a car that it takes more force to get
it rolling that to keep it rolling. like static and dynamic coefficients
of friction.

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Discussion Starter #3
If nonlinear rolling resistance or static vs dynamic effects are really a
problem you could do as you mention,
e.g. time the coast from 10km/h to 5km/h
and also the coast from 5km/h to 0, they should come out the same.
(If you have a timer with 'lap'
it will finally come in handy)
I havent seen many speedometers that actually measure decently on the low end
so you could try another way using distance and time instead of velocity:
Put 3 markers evenly spaced (e.g. at 0, 20meters, and 40meters).
Hit the timer as soon as you coast past the 1st marker.
Hit lap as soon as you coast past the 2nd marker
Hit stop as soon as you coast past the 3rd marker.
What you want is the time from 1st to 2nd marker and time from 1st (not 2nd) to
3rd marker.
If you manage to get those times you can use
a=Crr g = 2 (x1 t2 - x2 t1)/(t1 t2 (t2-t1))
where x1 is distance between 1st and 2nd marker
x2 is distance between 1st and 3rd marker
t1 is time from 1st to 2nd marker
t2 is time from 1st to 3rd marker
thats your value of Crr g, (rolling resistance coeff x gravity)


Death to All Spammers where are you?? You asked for this analysis about half a
year ago,I blew a Sunday on it so stop killing spammers and start measuring!

On Sun, 12 Aug 2007 13:29:40 Jeff Shanab wrote:
>Thanks. My rig is so inefficient I really need to look closely at this
>and give it a try. The first question I have is if the relation ship is
>linear enough to use rolling to a stop as a point. Maybe slowing to
>5,10,15,20 mph would give a better indication of the rolling
>resistance?. It seems when I push a car that it takes more force to get
>it rolling that to keep it rolling. like static and dynamic coefficients
>of friction.



Jeremy Rutman
Technion Physics Dep't.
Haifa 32000 Israel
phone 972 4 8293669
fax 972 4 8295755

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Discussion Starter #4
The test I outlined will work no matter what kind of setup you have.
If you want to see the changes due to your mods, test once before mods
and once after mods and see how the coefficients change.
again, the full test description is at
http://physics.technion.ac.il/~rutman/car/Roll-down%20test.pdf

or as was mentioned you can just run a set route a bunch of times and forget
the
quantitative stuff. On the other hand the White Zombie dude can surely tell you
theres nothing like hard numbers (e.g. quarter mile time) to cut the wheat from
the chaff, and those coeficients are directly comparable to other rigs.

On Sun, 12 Aug 2007 "FRED JEANETTE MERTENS" wrote:

>how do you figure co-efficient of drag after I put fender skirts' front and
>rear smooth skinned the bottom for aerodynamics ' and found a way to reduce
the
>drag from the vacuum in the rear between the tail lights ?

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Discussion Starter #5
What I am most interested in in this test is to isolate the majority of
my losses to aero vs rolling I drive the same route to work every day
and it is a pretty constant amount.

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