Tires to 52 PSI...Let the experiment begin

[evilgeniustodd]

DISCLAIMER: In posting this experiment and it's results I am in no way suggesting you should do this with your vehicle. I am simply reporting my results. Running a higher tire pressure will almost certainly negatively affect certain performance characteristics (like stopping distance and accident avoidance). If you are a conservative driver, higher inflation pressures may be for you. If you are an aggressive driver, they almost certainly ARE NOT for you. Though we shall all find out.

Personal note: I'm not interested in hearing or rehashing the same old wives tales about ballooning tires, exploding beads, and other similar failure modes. We've all heard them dozens, if not hundreds, of times. No need to repeat them again and again. A great many of the opinions surrounding this issue are vestigial assumptions from bygone eras. Unless you have some long term/relevant experience with this issue please refrain from speculating or concern trolling. Your humble opinion isn't necessary or invited, your facts are.

For those particularly concerned with catastrophic failures from running over max side wall on your Z. Wayne Gerdes, Patron Saint of hypermiling(cleanmpg.com), drove his Z for nearly 1000 miles. During the entire test he was running all 4 corners at 60PSI. Most of the test was completed with the trunk area packed full of gear (probably something like 60 pounds of kit) and Waynes not insignificant self. His testing included plenty of highway driving and I'm sure plenty of pot holes. Despite all of this he didn't have any problems. So relax, running a single pound over max side was isn't uncharted territory.

This test run will be running 13% lower than that. Again, I am exceeding side wall max by 1 psi, so less than 2% over max recommended sidewall. Sidewall max pressure is intended for use under maximum load conditions(source NHTSA & Consumer reports). IE with the tire supporting its maximum side wall weight the user should set the tire at the maximum side wall pressure.

Michigan, and specifically the area I live in near Detroit, is home to some of the worst roads in the country outside of Louisiana. So if anyone is going to have pothole related issues it's going to be me.

Motivations and Methodology: Tires affect vehicle fuel economy mainly through rolling resistance. As a tire rolls under the vehicle’s weight, its shape changes repeatedly as it experiences recurring cycles of deformation and recovery. In the process, mechanical energy otherwise available to turn the wheels is converted into heat and dissipated from the tire. More fuel must be expended to replace this lost energy.

For a tire inflated to pressures between 24 and 36 psi, each drop of 1 psi leads to a 1.4 percent increase in its rolling resistance. I'm unable to find empirical data pertaining to the reverse scenario. However, I am willing to assert that at least to a certain extent the opposite is likely true. IE over-inflation will lead to reduced tire deformation and a reduction in the RRC. Though I doubt to the same extreme degree as underinflation, and most likely with a diminishing/vanishing return past a certain pressure.

According to the Transportation Research Board (TRB) for highway driving characterized by higher and more consistent travel speeds, a 10 percent reduction in rolling resistance coefficient (RRC) increased fuel economy by 0.9 to 2.1 percent.
I’d certainly appreciate some input on how exactly to test the RRC of a specific tire and pressure. That is currently over my head.


During this test I hope to test as many performance parameters as is technological and economically feasible. Including:
• Tread wear
• Tire pressure as a function of temperature
• Stopping distance
• Ride quality
• Mileage
• (any other suggestions on what and how to test are more than welcome)


Watch this space for updates.

Starting on 8/14/2012 at 17K miles the tires had previously been at 35-37. I've now set them to 52 psi or 1 pound over max recommended side wall pressure. It was positively terrifying to get on the highway(fear of the unknown). But everything worked out just fine so far. I'll probably head over to autozone to get a depth gauge for monitoring wear and what not. The goal of this action is to increase overall MPG.

8/19/2012
Double checked all 4 corners pressure today along with taking tread depth measurements with a digital micrometer.
These measurements are presented for future reference. The depths are presented from the outside in starting with the first 2 valleys ending with the inner most. All measurements are in millimeters:

Driver side rear 4.8 5.5 5
Driver side front 4.3 4.9 4.1
Passenger side front 4.2 4.6 4
Passenger side rear 4.7 5.1 4.9

I also decided to do some temperature testing tonight. Took the Z out on to the highway to see what kind of tread temps and pressure changes occur at highway speeds. I drove roughly 20 miles around the highway ring that surrounds Ann Arbor. During the test my speed varied between 68 and 73 MPH. I'm using a no touch IR gun for temps.
Baseline: The ambient temperature for the test was 70F according to the Z's on board sensors. The tires started the test with both sidewall and tread temperatures of 73F.
Results: Rear sidewall temperature remained unchanged. Rear tread temp rose to 92F. Front sidewall temp up to 82F. Front tread temp 95F

20 miles at 70F is far from a proper test but it's what I had time for this evening

[wkqkqkfk]

why

[OneFitZ]

Lets see how quickly you blow a tire or break the bead.

[ITEM9]

I wonder how many people die in the name of fuel economy...

Just keep in mind the drawbacks of over-inflating; mainly reduced traction. This is very important for safety, especially when it comes to braking and you never know when someone is going to pull a bonehead maneuver in front of you. Compromising safety just isn't worth the savings or being able to brag about your fuel economy.

[Jayemtee]

He didn't really say "fuel economy". Shall we assume that and run with it.?

It should be especially interesting in the wet/freezing weather.

Might as well take 'em to 90 as long as you have a death wish.

[wkqkqkfk]

Syria blames blast on tyre blowout - Middle East - Al Jazeera English

[OneFitZ]

Originally Posted by Jayemtee

He didn't really say "fuel economy". Shall we assume that and run with it.?

It should be especially interesting in the wet/freezing weather.

Might as well take 'em to 90 as long as you have a death wish.

Why else would you run super high PSI in your tires?

[TytaniumK]

Originally Posted by ITEM9

I wonder how many people die in the name of fuel economy...

Just keep in mind the drawbacks of over-inflating; mainly reduced traction. This is very important for safety, especially when it comes to braking and you never know when someone is going to pull a bonehead maneuver in front of you. Compromising safety just isn't worth the savings or being able to brag about your fuel economy.

VERY well-said!

[cyeung]

I would not run them that high. My understanding is that the pressure will increase slightly from normal use. I would probably go up to 75% of sidewall pressure if I wanted low rolling resistance. I value my safety more than my MPGs.

[Jwarian]

I've been running 51 psi for 10thousand miles

Any questions I'm the one to ask