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Any thoughts on what may be happening here? From the data he collected, it appears the rolling distance goes down when psi goes from 60 psi to 70 psi.; 227.4' vs. 226.3'. :confused:

http://ecomodder.com/forum/showthread.php/experiment-coast-down-distances-rolling-resistance-various-tire-2721-2.html#post265379

Change in gravitational pull due to a change inthe the earth's rotational axis.:D

My guess would be wind change or if he didn't drive in exactly the same track on the road. You know how some roads are sticky in the main path.

Sounds like a question for CapriRacer.

Change in gravitational pull due to a change inthe the earth's rotational axis.:D

My guess would be wind change or if he didn't drive in exactly the same track on the road. You know how some roads are sticky in the main path.

Sounds like a question for CapriRacer.

I kind of wondered about that but, the way he did the tests it doesn't seem like a different track would be a factor as it looks to have been done in a parking lot (see link below). Yes, it would be interesting to hear CR's thoughts on this.

http://ecomodder.com/forum/showthread.php/testing-rolling-resistance-various-tires-suzukiclone-fleet-19126.html

Looks like Not much point in going much over 45 PSI-nothing new there

Looks like Not much point in going much over 45 PSI-nothing new there
As I see his results, it looks like the optimal pressure is actually about 60 psi. What's most intriguing is the decrease in rolling distance over 60 psi. :confused:

Probably just a measurement problem-hard to have exactly the same conditions.
Simple way to see if it actually does decrease-keep pumping and measuring
Pump it up to 100 psi or so-big decrease??-then you have something

Rough road surface may be the cause of this scenario? H

MetroMPG's test methods are some of the best around. I trust his results to be accurate. Confusing in this case, but accurate.

Extend it to 100 psi-see if you-he gets the same result-

I suppose you could speculate-increase pressure decreases size and changes the shape of contact patch-somehow the patch has "opened up" and now "grabs" the road "harder".

However I would still bet it is just a measuring blip 1/228 not much- certainly within what you would expect of this sort of real world test.

Just pump it up to 80 90 100 110-
If there is really something there- it will be obvious.
Charlie

MetroMPG's test methods are some of the best around. I trust his results to be accurate. Confusing in this case, but accurate.

MetroMPG responded in the ecomodder thread that the decrease at 70 vs 60 was too small to be statistically significant in his test.

True. The average of the 60 and 70 readings are less than 1/2% different.

MetroMPG's test methods are some of the best around. I trust his results to be accurate. Confusing in this case, but accurate.
Agreed. He is quite thorough and conscientious in his methods.

MetroMPG responded in the ecomodder thread that the decrease at 70 vs 60 was too small to be statistically significant in his test.

Extend it to 100 psi-see if you-he gets the same result-

I suppose you could speculate-increase pressure decreases size and changes the shape of contact patch-somehow the patch has "opened up" and now "grabs" the road "harder".

However I would still bet it is just a measuring blip 1/228 not much- certainly within what you would expect of this sort of real world test.

Just pump it up to 80 90 100 110-
If there is really something there- it will be obvious.
Charlie

Sean and I, through observation, have found that closer to 80 on the Insight is about the optimum pressure.

There's no significant difference between the 50, 60, and 70 psi coasting distances. 2 feet over a 225 foot distance isn't even 1%. Oops. I see you've discussed this while I was composing my reply.

......... Yes, it would be interesting to hear CR's thoughts on this....

Thanks for the vote of confidence.

Around here it seems to be heresy to point out that the tread actually bulges out more as inflation pressure increases. I think this test is manifesting one of the effects this has.

In order to carry the load, a tire deflects. The formula is: Load = Spring Rate (of the tire) X deflection.

Since the spring rate is largely a function of the inflation pressure, more pressure = less deflection.

As the tire gets stiffer and stiffer, and the tread bulges out further and further, there is a point where the tread shoulders have minimal ground contact, and the bulged out tread (in the center) is doing most of the deflection. Go beyond that point and the deflection is pretty much all taking place in the center of the tread.

- BUT - the spring rate of the center of the tread is a fraction of what it is if the entire tread face is involved.

The net effect is that it takes MORE deflection to generate the amount of resistance to carry the load - hence the RR starts to INCREASE.

And those are my thoughts.

Capri
Yes that is plausible-crossed my mind
Just keep increasing the pressure and testing-see what happens
At what pressure do you start to worry about the tire blowing off the rim- or just blowing up in some fashion??
At what pressure do you have safety concerns?
Sure looks like anything over 45-50 psi or so is pointless.
Charlie

Charlie, what I found confusing about MetroMPG's results is that Sean and I have both seen an increase in results at pressures significantly higher than in Metro's test...and we have the same tires.:confused: As for what pressures would cause the tire to come off the rims, hard to say. The stunt driver Joey Chitwood inflated the tires on his cars to a little over 100 psi for his famous driving on two wheels stunts. He found that less than that and the tires would pop off of the beads. Granted, driving on two wheels has safety concerns anyway but in this case, safety depended on the higher pressures. In daily driving, I have 65K miles on the Prius (never under 60 psi), about 15K miles on the Insight(70-90 psi), and several 10's of thousands of miles on other cars at pressures above placard and sidewall max, without any safety related incidents or uneven wear. If ever there was a case of YMMV it is with the subject of tire inflation. While there is "official" evidence showing concerns with higher pressures, there seems to be nearly as much evidence to the contrary. I wonder if the evidence may not be skewed towards the "official" results since most just take the pressure recommendations at face value and just don't test the higher pressures as much. I remember when tire experts used to say that you shouldn't reverse the direction tires rotate because it causes blowouts. That seems to have been disproved as with used tires, you generally don't know what side of the vehicle they came off of; tire makers and stores like to sell tires and fear can be a useful tool. Enough said. Again, YMMV.

In daily driving, I have 65K miles on the Prius (never under 60 psi), about 15K miles on the Insight(70-90 psi)

How do you decide what pressure to use?

How do you decide what pressure to use?

The Prius was the first car I went that high (60 psi) on. I had went to about 50 psi on a regular basis on previous cars. Actually I may have briefly experimented with that on other cars but, the Prius was the first one I used that as a minimum. Fear from all the naysayers of higher pressures probably had the biggest influence prior to the Prius. As for going higher, I have landmarks that I use on my normal routes that I start my FAS coming up to stop signs and noticed that with higher pressures I could either start the FAS earlier or, begin at the same point with a lower speed. I try to check my pressures on a regular basis but it quite often is a shorter than expected FAS distance that alerts me to check on the tire pressures. I've pretty much settled on the 80-90 range as I don't notice much more increase in FAS distances as I was approaching that range. That said, my methods may not be as precise as MetroMPG's which are as good as any around, over a long enough time, I can expect a certain result with various combinations of weather conditions and pressures.

DiamondLarry-
Right- my guess is the odd result is just a measuring blip-hard in the real world to have conditions exactly the same- little bit of wind-slightly different actual path-1" or 2' this way or that.

Usually when I have to come up with an exotic (but plausible) explanation for something (change in shape of contact patch causing more RR)- the somewhat exotic explanation turns out to not hold up.

Easy enough to find out- just retest-which you have sorta done, right- same tires same cars-
Yeah just normal variation you find in real world testing.

I've been slowly inching my tire pressure up. The Prius I used to keep about 38 psi-then 42 psi- but lately 50 psi.I finally got my compressor working-well in position to use easily-so I'm not having to herniate myself with the bicycle pump!
The Suburban-which I drive- does seem to respond very well to 50 psi vs 38psi.
At normal-door post #s- 32 ft 41 rear- it will glide about 3 seconds per 1 mph in the 37>28mph range(motor on in D). With 50 psi it is more like 4 seconds per mph.This is crudely-me just counting to myself- but I'm a good time estimator.

I get 16.5 mpg average on my 5.5 miles average trips with cold motor-leave 6:40-arrive 6:55-park- home 2:42-2:58. Dead cold motor 5.7 5200 lbs 218,000 mile dated guzzler.The 12psi seems to be worth about 1 mpg-shouldn't be that much- but seems to be.

EVERYONE WATCHING MY SAINTS BEAT Carolina like a drum!!!
Ha,Ha
Looks like 2009 from here.
Can't believe NE- a pitiful team- gets home field and a bye and much better NFC teams-us- have to go on the road!! The AFC is pitiful!!
Can't say I'm looking forward to Atlanta again-they hate us-and they are unfortunately for us-pretty good. I would greatly prefer almost any other team.

Oh well

DiamondLarry-
Right- my guess is the odd result is just a measuring blip-hard in the real world to have conditions exactly the same- little bit of wind-slightly different actual path-1" or 2' this way or that.

Usually when I have to come up with an exotic (but plausible) explanation for something (change in shape of contact patch causing more RR)- the somewhat exotic explanation turns out to not hold up.

Easy enough to find out- just retest-which you have sorta done, right- same tires same cars-
Yeah just normal variation you find in real world testing.

I've been slowly inching my tire pressure up. The Prius I used to keep about 38 psi-then 42 psi- but lately 50 psi.I finally got my compressor working-well in position to use easily-so I'm not having to herniate myself with the bicycle pump!
The Suburban-which I drive- does seem to respond very well to 50 psi vs 38psi.
At normal-door post #s- 32 ft 41 rear- it will glide about 3 seconds per 1 mph in the 37>28mph range(motor on in D). With 50 psi it is more like 4 seconds per mph.This is crudely-me just counting to myself- but I'm a good time estimator.

I get 16.5 mpg average on my 5.5 miles average trips with cold motor-leave 6:40-arrive 6:55-park- home 2:42-2:58. Dead cold motor 5.7 5200 lbs 218,000 mile dated guzzler.The 12psi seems to be worth about 1 mpg-shouldn't be that much- but seems to be.



I don't know if I would call it just an exotic explanation since I'm really not familiar with the actual science of tires and lab testing methods. This could just be yet another case of realworld results just not quite syncing up with on paper/lab results. As I mentioned earlier, I use landmarks and know I have to be a certain speed at that landmark to make it to the stop. If my tires aren't inflated to their usual level, I won't make it to my stop without an extra pulse when all other conditions (wind, temperature, road surface wet or dry, etc.) are similar.

Capri
Yes that is plausible-crossed my mind
Just keep increasing the pressure and testing-see what happens
At what pressure do you start to worry about the tire blowing off the rim- or just blowing up in some fashion??
At what pressure do you have safety concerns?
Sure looks like anything over 45-50 psi or so is pointless.
Charlie

I start off with the idea that tires are designed to operate within certain conditions: Load, inflation pressure, road surfaces, weather, etc. I think many discussions of this type generally don't start with that in mind.

I also think in terms of "Risk", rather than "Safety". "Risk" is the inverse of "Safety", and I think better expresses that this is a shade of gray kind of thing - not a black and white sort of thing.

Having said that, I know that passenger car tires are designed to operate in the 26 to 35 psi range with maximum loadings in the 85% range (or so). Needless to say, moving out of that range increases the risk - and I get proportionally more nervous the further away from that the situation is.

But I would like to point out that "blowing off the rim" is not the only possible thing that can happen as a result of increased inflation pressure. Obviously ride quality goes down, but from a "Risk" point of view, increased inflation pressure decreases the resistance to impact failure, decreases traction, etc. I think "blowing off the rim" is not in the top 10 of important affects when it comes to inflation pressure.

There's so many variables - the track you take on the road, temperature, etc. - I'd chalk it up to my statistician in chief - Marginof Erra :D

My experience suggest that optimum fuel efficiency is achieved
when overinflation of the tire pressure is matched with top speed of the vehicle.

As the top speed decreases, we can increase tire pressures to increase a vehicle's MPG.

Tire over inflation decreases it's rolling resistance but
its how overinflation affects a tire road surface adhesion/traction performance
which changes its near-stead-state top speed of when
rolling resistance(inertia-momentum)/traction(acceleration) = aerodynamic drag
and hence the vehicle's overall optimum speed for gliding and for the best fuel efficiency.

Given
a smooth dry asphalt flat road (with the understanding that no road is completely smooth)
no wind resistance, no precipation(no rain, sleet, snow), warm weather ( 70F degrees)
a warmed up Prius ( gen 3) with no cargo except the driver (uniform curb weight)
using stock oem Yokohama Avid S33d tires ( RVMTP = 35 psi front, 33 psi rear, max sidewall =44psi)

I hypothesize that that the Prius would behave somewhat as follows

The recommended vehicle manufacturer tire pressure (RVMTP) is designed for 75 mph
so if your top speed is +75 mph
your best fuel efficiency is acheived for +75mph when your tire pressure is set to RVMTP
as your top speed decreases to 65 mph
your best fuel efficiency is acheived for 65mph when your tire pressure is set to RVMTP+2.5psi
as your top speed decreases to 55 mph
your best fuel efficiency is acheived for 55mph when your tire pressure is set to RVMTP+5psi
as your top speed decreases to 45 mph
your best fuel efficiency is acheived for 45mph when your tire pressure is set to RVMTP+10psi
as your top speed decreases to 35 mph
your best fuel efficiency is acheived for 35mph when your tire pressure is set to RVMTP+15psi
as your top speed decreases to 25 mph
your best fuel efficiency is achieve for 25mph when your tire pressure is set to RVMTP+20psi

I've been driving a 2010 Prius with my
Yokohama Avid S33D set at 50psi/48psi (RVMTP+15psi ) for over a year which is 6psi over the max sidewall
I'm getting over 60 mpg overall but that's because I'm mainly hypermiling at speeds lower than 45 mph
On short superhighway trip at 60 mph at these higher tire pressure settings I've notice that when using cc
my MPG = 53 mpg drops which was lower than the 58 mpg I achieve with my tire pressure setting
40psi front /38psi rear on my road trips to michigan at the same speed of 60mph using cc.

I plan on buying an air compressor and do further testing of tire pressure vs speed vs fuel efficiency.