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Old 06-02-2008, 12:27 PM   #1 (permalink)
MetroMPG
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Blackfly - '98 Geo Metro
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Experiment: coast down distances (rolling resistance) @ various tire pressures

The purpose of the test was to get an idea of the difference in coasting distances in my '98 Metro/Firefly over a range of tire pressures, 20-60 psi.

EDIT Jan 23/09: Posted a summary of this thread at: http://metrompg.com/posts/tire-press...resistance.htm

Tire tested: 155/80/R13 Goodyear Invictas, rated 44 PSI max sidewall.

EDIT - Also tested Oct 13, 2011...
Bridgestone RE92 tires, multiple runs at pressures from 20-70 PSI. See this post: http://ecomodder.com/forum/showthrea...tml#post265379

Placard info: vehicle placard recommends 32 PSI front/rear @ max load.

Pressure Gauge: readings were taken with an Accutire digital gauge, 5-99 PSI rated, with a manufacturer accuracy claim of +/- 1% + 0.5

Weather: 19C / 66F, 10 km/h SSW wind (roadway ran SW/NE)

Methodology:
  • tires were pumped up to 60 PSI, drove to test route (< 5km), pressure adjusted
  • car was driven up a small hill (approx. 6 ft. elevation, 8:1 slope), turned around and stopped at a marked point
  • engine off, transmission in neutral, brakes were released
  • car rolled down short hill onto a flat run-out road
  • where the car stopped, the road was marked
  • pressure was adjusted (dropped 5 PSI)
  • rinse & repeat
  • NOTE: only one run per PSI
I used a bicycle wheel to measure the rolling distances - counted revolutions from the starting point, then converted circumference to total feet.

Ideally, I would have simply measured the coastdown distance from a constant speed at the same point on a level road, but my cruise control isn't working, and I didn't want to deal with the possibility of driver error (varying speeds). Even more ideally, I should have done multiple runs per pressure level.

Since the car accelerates from rest and coasts to a stop, the differences in pressure are amplified compared to a simple coastdown test.

Raw results for PSI/feet travelled

20 / 479.3
25 / 524.8
30 / 621.0
35 / 621.0
40 / 639.6
45 / 687.5
50 / 702.0
55 / 699.3
60 / 702.0




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