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

[diamondlarry]

Originally Posted by MetroMPG

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)




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:



For kicks, see also: comparing RE92's @ 50 PSI to space saver spare tire donuts @ 70 psi - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Poll Results

Very interesting results. Anyone have any ideas on what may be causing the distance to decrease with the higher pressures?

[dcb]

Less deflection of the tires, and resulting heat creation? There is still hysteresis in the deformation within the tread/compound, but less hysteresis as the whole tire deforms less.

[Frank Lee]

How does less deflection lead to more heat?

[MetroMPG]

Originally Posted by diamondlarry

Very interesting results. Anyone have any ideas on what may be causing the distance to decrease with the higher pressures?

I don't think I would conclude that from the raw figures.

The difference between the averages of those last 2 sets (1.1 ft, from 60 to 70 psi) is smaller than the standard deviation within each set (2.1 for 60 psi; 1.7 for 70).

It's been years since I had a stats class, but I don't think that "result" is statistically significant. It's within the margin of error.

If you wanted to tease some valid conclusions from very high pressure testing for these tires, you'd have to run another/different test.

[jime57]

Originally Posted by silverinsight2

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

I doubt that this is actually accurate. Most of the hypermiling champs over there use 80 psi.

[jime57]

There was possibly a bit of wind at the end. It would take a mere breeze to effect the results. Also, the difference at the top is so small that decrease/increase is probably within the margin of error. 3wheeler, a very smart engineering type, has had problems with coast down repeatability.

It would also be interesting to know the span of time over which the data was gathered. Looks like it would have taken several hours, and I suspect tire temperatures would have changed within that timeframe. There is another subtile question in the data. One has a tendancy to read the data top to bottom, BUT, the data was taken the reverse order with the high pressure data taken first. I kinda wonder if the tires actually warmed up a bit during the test cycle due to rubber hysteresis?

I'm going to repeat the test on an Insight when we get a bit warmer spring weather, but I'm going to do it by driving my long test course. That would be more representative than the cold stuff we are having right now. Looks like MetroMPG gathered his data on a warm day.

In any case we known that the improvement with increasing pressure is a diminishing effect. At some point the improvement has to get too small to measure, though I have some doubt that it actually goes negative.

[MetroMPG]

Originally Posted by jimepting

I doubt that this is actually accurate. Most of the hypermiling champs over there use 80 psi.

And lots of championship winning sports players also have superstitious habits they wouldn't dare miss doing before a game.

Note I'm not arguing 80 psi isn't significantly better than, say, 60 - I don't know the answer.

But I wouldn't conclude there's a significant difference solely from the MPG results of a competitive hypermiler. Too many other variables at play.

[jime57]

Yeah, you are correct on that one

You notice I hope that I couched much of my detailed response in lots of "soft" questions. I don't think we actually know how far up the pressure scale the improvement works, but for sure the effect diminishes considerably to the point that it becomes unmeasurable.

[MetroMPG]

I noticed! And I think you're right that the effect above >50 or >60 psi becomes so small that it's more or less unmeasurable on the car.

[redpoint5]

Almost everything in life is a diminishing return and necessitates compromise. Heck, even improving MPGs is a diminishing return. If you could double your MPGs from 10 to 20, the actual fuel savings is much greater than doubling again from 20 to 40.

1000 miles driven @ 10mpg = 100 gallons consumed
1000 miles driven @ 20mpg = 50 gallons consumed
doubling the efficiency saves 50 gallons

1000 miles driven @ 40mpg = 25 gallons consumed
doubling the efficiency again saves only 25 gallons.

I really appreciate the time you spent putting this data together for us Metro.