Before you over inflate watch this.

Discussion in 'Fuel Economy' started by ALS, Jul 2, 2008.

  1. kwj

    kwj I hypermiled this

    This is what we are all about, going where no man/woman has gone before. Think about it, where else have you been enticed to such a deep discussion of the benefits of Max Sidewall pressures?

    We are in a grey area. No one normally operates here - we do. We are building up that body of experience.

    Needless to say, our good friends at the AAA would leap for joy if all of our membership suddenly started sliding off the roads and into other cars - but it just ain't happenin'. That bothers them and our other individual naysayers, who like nothing more than to bring up something they barely understand, as some extreme danger that we are foisting on an unwary public.

    I run my tires at pressures well above Max Sidewall. With my long driving experience in a variety of types of cars, I'm very comfortable where I'm at. It handles great, rain or dry, hot or cold. It is rough. I'm conducting my own experiment. In 15,000 miles, my tires are not wearing on the center. I'll keep you posted of any success or failures I might experience. But as to safe? I'm one of the safest around. Safety was drummed into me in my career. I can function in the most hazardous of environments, as long as I know the danger and the techniques to protect myself.
  2. 2way

    2way Electromagnetic Wave [:-h

    Above placard pressure can improve normal cornering stability. However, it may not necessarily improve "grip". Especially, if you go too high. By reducing the rolling resistance and the contact patch, you have also lowered the break free limit. Not that it would probably matter to most hypermilers. It just means that you have lowered the max speed with which you can take a particular corner without losing it.

    Still, I view advising max sidewall as just too arbitrary and doesn't take into account several factors (like load & tire construction). On a light vehicle, with only one occupant, I would think it could be dangerous if you were to corner @ speed with overinflation. I tend to hit offramps at speeds probably much higher than most hypermilers would and use them for glides:D I did have to adapt to the loss of cornering ability/confidence going from sticky summer tires to performance all-seasons. So, I can also understand the point about adjusting your driving to accomodate such changes.
  3. kwj

    kwj I hypermiled this

    With my tires at over 100% higher than placard and 50% above Max Sidewall, I had the opportunity presented to me, to stand on my brakes when some NASCAR van (yep, had a NASCAR bumper sticker), abruptly came from the third lane, across 2.5 lanes, trying to make it to an exit, when he saw that there were other cars in the exit lane. He trapped me between him and them. I had to hit the skids and steer into the upcoming Vee where the exit lane broke away to the right. Even though there was sand on this part of the road, I did not enter a skid, and controlled my car the way I was always taught - DON'T LOSE STEERING. My tires never locked up, and once again I avoided an accident because of my defensive driving skills (always have a safe out).

    Now, please tell me how small my tire's contact patch must be before I experience this "break free limit"? Obviously, I haven't reached that point yet, and must pump my tires up still higher to achieve what the nay sayers are saying. I'd rather not pump them up any higher cause it's getting kinda rough.
  4. Tochatihu

    Tochatihu Well-Known Member

    kwj, does your Aveo have anti-lock brakes? Those are really handy to help maintain steering when you are standing on pedal #2.

    I operated 2001 Prius 3 times in "ABS territory", with excellent results. For the purpose of this discussion, all were at the tire sidewall max psi or slightly below.

  5. kwj

    kwj I hypermiled this

    No, didn't get ABS, although I would feel a bit better if I had. This is a small car, but surprizingly heavy for it's size. I suppose that's how they achieved a 5 star front collision rating.
  6. Jimmy

    Jimmy Well-Known Member

    The AAA, whether you agree with them or not, and whether they are correct or not, would not "leap for joy" if its members "started sliding off the roads and into other cars".

    Let's at least keep these discussions on a level above that of hate mongering. Polarization solves nothing.

    You said "Needless to say, our good friends at the AAA would leap for joy if all of our membership suddenly started sliding off the roads and into other cars - but it just ain't happenin".
  7. kwj

    kwj I hypermiled this

    Thanks Jimmy, go tell that to the AAA.
  8. 2way

    2way Electromagnetic Wave [:-h

    Even @ hypermiler speeds, if you stood on your brakes, you should've locked up w/o ABS. Something there just doesn't add up.
  9. jamesqf

    jamesqf Well-Known Member

    Had a similar opportunity the other night, with an apparently suicidal deer. Jumped out in front of me, I braked & turned to avoid him. So he changed direction so as to get in front of me again, so more brake & turn. And wouldn't you know it, he changes direction too. And so on, me making S-turns on the road until I'm at a dead stop with Bambi about a foot from the headlights. All this with Bridgestone RE92s at 50 PSI.

    Which brings me to a point that seems to be missing from this discussion. Americans have apparently been willing to sacrifice handling, braking, & stability in order to drive oversized, jacked-up "drives like a waterbed" SUVs and pickups, and the AAA and such don't seem to have any problem with that. So why get worked up about possibly trading a small amount of that for increased fuel economy?
  10. kwj

    kwj I hypermiled this

    Hi 2-way, I was doing 55 in the right lane, at the intersection of 695 north (Baltimore Beltway) and I-95 south. There's a lot of construction as they are redesigning all the interchange. Consequently, there is some dirt and sand in the portions of the road not travelled. I was worried that I was going to lock them up and then lose control, so I was able to get very close, and may have dragged a bit, but left no skid marks. Fortunately, the van was not able to stop faster than I, and that allowed me to clear his rear bumper before being hit and forced into the turn lane traffic.

    The worst thing about this incident was that the van driver realized that he didn't want that exit anyway, and he pulled back out into traffic (maybe worried that I was going to get out of my car).

    I've not been on my brakes like that in years. I had to sit there and regain composure and stop shaking prior to pulling back out. And again, never experienced breakaway. I've got 15,000 on the front tires, and 60,000 on the rear tires. They are all in very good shape, with lots of tread.
  11. vtec-e

    vtec-e Celtic MPG Warrior

    On my Kia, I have been running my michelin energy 205/55R16 tires at sidewall (50psi) since new and 15,000km later it appears there is about 1.5 to 2mm less tread depth at the centre of the tire than at the edge/shoulder. Could this be due to the centre of the tire bulging from sidewall pressure or due to the design of the tire in that the tread depth was less in the centre from new. I did read a guys report over at and he said his tires wore more at the centre when at sidewall. It would appear that some tires dont like sidewall pressure. Is it a brand thing or a size thing, or a combination, (yay!)? The tires(185/65R14) on my civic have been at sidewall for 100,000km with no uneven wear so it would seem narrower tires cope better with sidewall than wider tires. This makes sense to me but i'm open to suggestions otherwise. I've since lowered:eek: the kia's tires to 45psi to see if the wear evens out. Mpg wise, i don't think i've suffered much of a loss but i'll monitor progress. I'm still trying to overcome other issues like running in a newish diesel engine and grille blocking while not affecting the intercooler, oh and the weather changing so much! Plus i dont drive the kia much.(too much love for the honda!!) :p

  12. vtec-e

    vtec-e Celtic MPG Warrior

    I love it when real world experience flies in the face of the "conventional wisdom"
    Actually, i might refer any future naysayers to this very page! If you don't mind of course!

  13. vtec-e

    vtec-e Celtic MPG Warrior

    Yeah, tires defy the laws of physics! Sorry, capriracer, you know tons about tires but you cant defy the laws of physics.

  14. kwj

    kwj I hypermiled this

    vtec-e, thanks for the information on your tires. If in fact they are wearing in the center, that will be inportant information for all who use the same tire, and will remind owners of similar tires, to keep a very close watch.

    For the benefit of all who run their tires higher than placard, how did you determine the 1.5 to 2 mm of difference across the tread? How often would you say a driver should check this?

    I check pressure about every two weeks, perhaps at the same time, I should check tread depth. As we gain more experience, this could be useful for other hypermilers.
  15. -mr. bill

    -mr. bill Senior Member

    You might want to read this book. If you don't want to spend the money, I'd understand. At least "SEARCH INSIDE" and read the excerpt. I'll quote one line:

    "Unfortunately, many introductory physics textbooks do not emphasize to the reader that the laws of metallic friction are for metal on metal only and have no scientific applicability to other engineering materials, including rubber."

    My money is on CapriRacer, but I'm just a naysayer.

    -mr. bill
    Last edited: Jul 14, 2008
  16. -mr. bill

    -mr. bill Senior Member

    Here's both pairs:


    BOTH ARE AS IS, faithful reproductions of the original jpeg posted. No manipulation. The top and bottom are not "nearly identical" as claimed last week.

    -mr. bill
    Last edited: Jul 14, 2008
  17. -mr. bill

    -mr. bill Senior Member

    Yes, you should inspect your tires if you are operating in the "grey area." And you can't measure these with your eyes. You should purchase some tools.

    I can recommend this one, with 1/128 inch gradations. This might allow people to catch toe-in and camber problems early, rather than blaming underinflation tens of thousands of miles later.

    -mr. bill
  18. kwj

    kwj I hypermiled this

    Thanks for the tool reference. We need to do this to track our progress (or lack thereof) as to center wear on some tires.

    As to the tire pictures, I still see where the scale for the one on the top left is larger than the one on the bottom left. I think it is easy to see. If you could bring the top one to the same scale as the bottom, you would be able to make a better comparison.
  19. -mr. bill

    -mr. bill Senior Member

    You are welcome about the tool.

    But you are asking me to photomanipulate the data?

    I will not. Do that yourself, or find someone else to do that.

    -mr. bill
  20. CapriRacer

    CapriRacer Well-Known Member

    I thought we settled the footprint size issue.

    Not only does it NOT make sense that the higher pressure has the same size footprint, but if you count tread segments, you get different answers.

    Photo manipulation or not, higher pressure yields a smaller footprint - just like the laws of Physics say.

    Oh, and one additional thought about the laws of Physics - you have to know when to apply them.

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