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

http://ecomodder.com/forum/attachmen...1&d=1212424018

http://metrompg.com/posts/photos/met...scale-1200.jpg

Very good info!

Hello,

It is interesting to note that the plateau seems to happen roughly at the 44psi sidewall max pressure.

Nice to see that 50psi seems to be the magic number ...

Not really. Very good info would have been multiple runs at each pressure level to ensure I was getting accurate readings.

(You can tell the accuracy is questionable by the pair of identical distances at different pressures.)

It's useful info as a "snapshot", but I wouldn't draw any hard & fast conclusions from it. Maybe it'll encourage other people to try it for their particular vehicles / tires.

Cool Test :) How fast were you getting up to? I'm wondering if the air drag at the higher speeds is what is leveling off the results.

Not fast - 18.5 km/h @ 60 psi (the only time I checked).

But potentially approaching 28.5 km/h with the cross breeze (anyone want to do the vector calcs? :)), but the road was also partially sheltered by trees & buildings, so I doubt I was seeing the full wind reported at the weather station.

I figure the varying breeze may have contributed to the pairs of matching results.

Based on my aero+rr spreadsheet, at 10 mph (16 km/h), the proportion of power required to overcome rolling/mechanical vs. aero losses is: 0.44 to 0.05 hp.

At 15 mph (24 km/h) it's 0.74 hp vs. 0.16 hp for rolling/aero.

Mehbe some more people will get out there and look for some data, but that's encouraging stuff, I think.

"Not really. Very good info would have been multiple runs at each pressure level to ensure I was getting accurate readings. "
You did do well, and deserve the credit. Very-very good info would be to either measure the temperature of the tire before and after each run, OR take an air tank with you and do the test again, but INCREASE the air pressure after each run.
You might consider checking the tire pressure before you begin the run, and right after you finish, to see if there is a measureable difference.

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

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

thats what i was thinking :thumbup:

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

http://ecomodder.com/forum/attachmen...1&d=1212686195

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

wind ??? maybe a little gust

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!!!

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...

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

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?

I think it kind of interesting that the smaller tire there was a pretty big jump until about 45 psi while the bigger tire was at a lower pressure of around 35. I would of thought that it would of been the opposite.

No, I didn't. It's possible (likely) I was off laterally a little from run to run.

On the starting hill, I moved into position each time by sighting down a wall beside me that was perpendicular to the roadway, and creeping the car until my line of sight was straight down the wall. Then I stopped completely and released the brakes when the coast was clear.

I wondered that too. I suppose it's possible.

To put this into perspective for a metrompg.com post I'm working on (not up yet), here's the Firefly coasting results shown to scale:

click to zoom...

http://metrompg.com/posts/photos/met...scale-1200.jpg

Regarding Lazarus' post on the larger tire having less difference in distance per PSI level:

The larger tire has significantly stiffer sidewall area, which reduces rolling resistance. Obviously it's 40-60mm wider (manufacturer's measurements vary), but that may not win-over the loss of friction and warpage associated with tall/thin sidewalls.

Maybe the lube cooled off. There is a huge difference in coasting speeds in my truck w/ a 50F difference, and given the operating range of greases/oils I imagine there could be a pretty big increase in drag relative to the change in Crr at those speeds as stuff cools.

MetroMPG:
What pressure are you using for every day ?

Thank you.

50 ish.

distance
 

Darin,my thoughts are that if the car had been tested from some initial velocity,to a lower velocity,or stop,that you might have seen a greater spread in the data.Evidently,the starting force,which must be overcome just to get a car moving is remarkably high,compared to once its rolling.This is a reason for the traffic light synchronization,as "stops" eat a lot of energy.---------------- I think this effect is cutting into your numbers,although I realize the added complexity and variability of the coast-down.My suspicion is that your RR performance is actually much better than the numbers reflect.Your test does reflect a measurable effect so I say it's useful to all,kudos to you and !muchas gracias!,Phil.

I agree, a "normal" coastdown from some moving speed would have made for a better test, Phil. No argument there.

Thanks for posting and running the test. Has anyone had a blowout from running pressures exceeding the tirewall max?

Yes, there have been reports of blowouts, as well as other issues - groove wander, uneven wear, etc. Spend some time searching this web site and you'll find them. BTW, other web sites have also reported similar issues.

where was there a report of a blowout because of extra psi?

Ideally you would want to conduct the test with the measuring device attached to the rear bumper rather than trying to hold it while trying to drive. Each time you pull the wheel to correct off coarse you add drag to the vehicle and affect the test results. I located a yourg man on you tube screen name=(landongendur)= that did some test with his metro. He took the mirrors off tested. He took the regular tires off and used 4 spare tires test vs regular. He ran vehicle with hatch back open test vs closed. He did speed test you name it if he thought of it he posted the results on youtube.

More inflation vs. rolling resistance data for the pile.

It matches the previous tests (meaning, rolling resistance decreases significantly up to a certain pressure, then plateaus).

Tire tested: Bridgestone Potenza RE92 LRR tires in 165/65R14 size (Honda Insight OEM tire, installed on Geo Metro electric car - ForkenSwift)

http://forkenswift.com/album/23-gt-w...re92-tires.jpg


Weather: 25C, 8 km/h SW wind (test course ran NW/SE)

Methodology:

• tires were pumped up to 70 PSI, drove to test route (< .5 km), pressure adjusted
• car was reversed up a small hill (approx. 2 ft. elevation) and stopped at a marked point
• hand brake was used to stop - hydraulic brakes were not used at all during the test or on the short drive to the test area
• transmission in neutral, hand brake was released
• car rolled down short hill onto a flat run-out area
• where the car stopped, the road was marked
• pressure was adjusted (dropped 10 PSI)
• rinse & repeat

Results:

http://forkenswift.com/album/8-chart...arious-psi.gif

For kicks, see also: comparing RE92's @ 50 PSI to space saver spare tire donuts @ 70 psi - http://ecomodder.com/forum/general-e...donuts-70.html

What's the rated tyre pressure for those tyres ?
Looks like there isn't very much to be gained by going beyond 40/50 psi

44 PSI... and I agree, that's what it seems to show.

Tests were done in a straight line with start/end points marked on the pavement. No handheld/bumper mounted instruments required!

Yes, that's "Peakster" - a member here (you can search for his posts). He cut way back on driving, for the most part, so hasn't been around in a long time.

Are you going to try the reverse, 'normal' tyres on the insight ?

Nope!

I learned what I needed to learn from this test & the comparison of all the tires in the Suzukiclone fleet - RE92's at around 50 PSI are my best option for LRR.

Something confirmed again and again at Insightcentral. Thanks for adding some science to it.:thumbup: