Experiment: coast down distances (rolling resistance) @ various tire pressures

[COMP]

i may try it on my truck by using a scale to see what it takes to move my truck

[MetroMPG]

Comp - that's not a bad idea. I guess a fish scale could work...

[COMP]

Originally Posted by MetroMPG

Comp - that's not a bad idea. I guess a fish scale could work...

thats what i was thinking

[MetroMPG]

Here's more data. Same road, same conditions (except zero wind - it's blocked a ridge & buildings to the NW).

This time I did 2 runs per PSI, at a 10 PSI interval

Car: 1999 Toyota Camry sedan V6

Tire info: 205/60/16 BF Goodrich Traction T/A, rated 44 PSI max sidewall.

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: 20C / 68F, 4 kts N wind (NW sheltered 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 10 PSI)
• rinse & repeat
• NOTE: two runs per PSI (blue), average (green)

I used a bicycle wheel to measure the rolling distances - counted revolutions from the starting point, then converted circumference to total feet.

Raw results for PSI/feet travelled (1)/ feet travelled (2) / average

20 / 568 / 570 / 569
30 / 662 / 672 / 667
40 / 677 / 679 / 678
50 / 667 / 679 / 673
60 / 692 / 681 / 686

[MetroMPG]

What I find so interesting is the level of variation between runs, even with such a simple test.

[COMP]

Originally Posted by MetroMPG

What I find so interesting is the level of variation between runs, even with such a simple test.

wind ??? maybe a little gust

[mattW]

Yeah the wobble in the graph for 50psi on an otherwise upward trend is weird, did anything unusual happen on the 667,50psi run?

Thanks for your commitment to real world testing... Always so interesting!!!

[MetroMPG]

It wasn't windy today - just a 4 knot, light breeze, and the road is very sheltered in that direction. I would have been more inclined to blame wind during the Metro runs the other day.

Nothing unusual happened that I noticed...

[COMP]

for your next test,,,,,,,tire's at 50psi and alignment at 0 toe 0 chamber

[dcb]

Thanks for doing this Metro.
re: variance. Did you mark your starting position in two dimensions? Were there any variances in steering input maybe?

Is there any reason to think that the brakes might not release evenly?