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

[3-Wheeler]

Originally Posted by diamondlarry

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

Yes, the smaller contact patch of the rubber on the road!

This smaller contact patch in turn, creates more sidewall flex and localized stress. This effect is more pronounced in pedal bikes with their curved carcasses.

It's been found that, at least for rolling resistance, a larger tire has less carcass deflection and because of this, less rolling resistance. Air resistance is a whole other matter however.

Although to be fair, the Tour deFrance guys pressure their tires to 150psi or so on those extremely narrow tires.

Jim.

[MetroMPG]

Nice to add more data to the pile.

Originally Posted by jimepting

I think that one potential problem the OP had was that his method did not adequately warm up the engine

You're quite right there! The car used doesn't have an internal combustion engine. I used the electric ForkenSwift.

and transmission, so that viscous drag changed somewhat during the tests.

I would say that's possible, though more likely the transmission was at ambient temperature throughout. The parking lot used is only a few hundred meters from where the car was parked (cold soak). I bet it wasn't driven fast/far enough to significantly change the temperature of the drivetrain components.

When I get a nice day, I intend to drive my long test course at 70&80 psi and see if the fuel economy differs. After all, that is the real test.

It may be "real", but it's less of a "test" (less validity). I'm not sure why you would want to do this, given your understanding that "controlled as possible" testing resulted in nearly insignificant changes in rolling performance at the highest pressures. Actively driving a test route will be vastly less controlled than a coast down test, wouldn't you say?

[3-Wheeler]

Originally Posted by MetroMPG

.....Actively driving a test route will be vastly less controlled than a coast down test, wouldn't you say?....

Hi Metro,

I would agree 100%!

All of the coast down tests I did showed that it's not hard at all to get very inconsistent results, even on the same stretch of road.

I really appreciate your testing, along with Jim's and take it for what it is.

Even the coast down test inside the factory does not consider tire warm up, and I am not privy to what affect that would have on the overall outcome of their testing.

It *might* be possible, at least in the summer time, to get an idea of just how much heat is put into the tire carcass during normal driving, and that would be, to take very careful tire pressure measurements of tires, both before driving the car and after driving on a controlled route.

I know that motorcycle race teams routinely check tire pressure buildup to determine the proper cold tire pressures before a race and keep notes of this throughout the season.

Jim.

[jime57]

Originally Posted by 3-Wheeler

I know that motorcycle race teams routinely check tire pressure buildup to determine the proper cold tire pressures before a race and keep notes of this throughout the season.

Jim.

I think all teams do this on racing cars and motocycles. Routinely, the tire temp is checked immediately on leaving the track. It is only the hot pressure which matters to a race driver. The recommendation to set pressures cold is only for street vehicles.

[jime57]

Originally Posted by MetroMPG

It may be "real", but it's less of a "test" (less validity). I'm not sure why you would want to do this, given your understanding that "controlled as possible" testing resulted in nearly insignificant changes in rolling performance at the highest pressures. Actively driving a test route will be vastly less controlled than a coast down test, wouldn't you say?

I'm not so sure about that, like much of what we amateur engineers do. I test fuel economy by the long course method because it has worked for me personally.

Because my test course is long, fairly flat, constant averge speed, constant driving style, is a two way course, and I start with the car thoroughly warmed, I can stay in lean burn about 95% of the time, and I get very high consistency of results. It's just a theory, but my therory is that by using a long two way course, many of the smaller errors, which would affect a short test are averaged out and down into the noise. I said it was a theory.

The negatives are that it takes time, puts miles on the car, and wastes fuel.

Here again are my mpg numbers for a recent test of some aero effects:

Baseline: 93mpg, 93.7mpg, 93.6mpg

Test results: 98.3mpg, 99.4mpg, 99.8mpg, and 99.6mpg.

I won't mention the test, I think you might remember it.

[MetroMPG]

Sounds like a pretty ideal test "course". I'm lucky in that I've got one around here too.

[jime57]

Originally Posted by MetroMPG

Sounds like a pretty ideal test "course". I'm lucky in that I've got one around here too.

It's decent. I'm still looking for a better one.

Richmond is not quite at the edge of tidewater Virginia, where the really flat coastal area sets in. I would like to find a course with no hills where I can stay in lean burn 100% of the time. I would also like to find a course closer to my garage which is also a bit shorter. I think 10 miles and flat is the perfect course. I've seen some like that in Louisana, but not on the east coast.

We have only been here a couple of years, so I haven't scouted everything.

[CigaR007]

Originally Posted by jimepting

When I get a nice day, I intend to drive my long test course at 70&80 psi and see if the fuel economy differs. After all, that is the real test. JMO

Yikes ! 70-80 psi in standard car tire ?! That is asking for trouble imo.

[redpoint5]

Originally Posted by CigaR007

Yikes ! 70-80 psi in standard car tire ?! That is asking for trouble imo.

I've read that the bursting pressure of a car tire is several hundred PSI, of course you can experience trouble at lower pressures. At 45psi my car suspension has to work harder to absorb bumps, road noise is significantly louder, and snow/loose ground traction is worse.

[MetroMPG]

Not to mention, a puncture is more likely from debris with a large enough item on the road (or a pothole with a sharp enough edge), vs. lower inflation where more tire deflection would absorb the impact.

That said, apparently there are more than a few people running around with 70+ PSI in their tires on a regular basis (not me!).

...tidewater Virginia, where the really flat coastal area sets in

My run is also near a body of water - running alongside a quite wide section of the St. Lawrence River.