Driving Under Pressure (full article)

Discussion in 'Articles' started by lamebums, Jun 1, 2008.

  1. lamebums

    lamebums Member

    Proper tire pressure could save your life!

    [xfloat=left]http://www.cleanmpg.com/photos/data/500/1133915712312_Bobby-Ore-Pic.jpg[/xfloat]Sgt. Dave Storton - Officer.com - December 21, 2005

    Proudly debunking the PSI = POP myth. Posted because of its frequent citing by CleanMPG members. -- Ed.

    How many officers check the tire pressure on their patrol car on a regular basis? We all seem to be great at checking that the lights and siren work, because the time to find out they don’t work is not when you get a Code 3 call. Likewise, the time to find out your tire pressure is too low is not when you are in a pursuit and trying to take a corner at high speed.

    What is proper pressure?

    The proper tire pressure for the Police Crown Victoria is 44 psi. If you look on the sidewall of the tire, you will see that it lists 44 psi max pressure. Regardless of what vehicle you have, use the maximum pressure listed on the sidewall. Higher pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed. [bolded for emphasis] This number on the sidewall lists “the maximum amount of pressure you should ever put in the tire under normal driving conditions.” Pursuits and Code 3 responses are not “normal driving conditions.” Many agencies maintain tire pressure at 35 psi since this is what is listed in the owner’s manual and on the door placard. The reason the owner’s manual lists 35 psi is because we get the same manual as the civilian version of the Crown Victoria. The police version, however, is fully loaded with communications equipment, a cage, and your gear. You are not looking for a soft and cushy ride, you want performance.

    Myths about pressure

    Let’s put to rest some common misconceptions. The tires will not balloon out creating a peak in the center portion of the tread when tire pressure is above 35 psi. There is a steel belt that prevents this from happening. Also, you are not overstressing the tire with higher pressure, and the tire will not be forced off the rim with higher pressure. The picture above is Bobby Ore of Bobby Ore Motorsports driving a Ford Ranger on two wheels. The tires on the left side have 100 psi in them, and they happen to be tires and rims from a 1999 Crown Victoria! This is a dramatic example of how pressure holds the tire in shape, and how much stress a tire can handle.

    Performance

    If you were able to watch a tire as it travels across the ground at high speed, you would see that it deflects to one side during cornering. The faster you are going through a corner, the more tire deflection you get. As the tire deflects over onto the sidewall, you get less traction and more of a tendency to understeer or oversteer. This could spell disaster when negotiating a corner at high speed during a pursuit or a Code 3 run. Higher pressure keeps the tire from deflecting onto the sidewall as much, which keeps more of the treaded portion on the road.

    A good demonstration for EVOC instructors is to have students drive a high-speed course in a vehicle with 32 to 35 psi. Then have them run the same course with 44 to 50 psi in the tires. The student will experience a marked difference in performance. Having officers experience this difference in vehicle performance is much more effective than just telling them to check their tire pressure.

    Hydroplaning

    When a tire rolls across a road covered with water, the tire tread channels water away so the rubber remains in contact with the road. The factors that affect hydroplaning are speed, and water depth. Conventional wisdom says that vehicles will hydroplane in as little as 1/16th of an inch of water. Not so coincidentally, legal tread depth is 1/16th of an inch.

    Tire manufactures and the Association of Law Enforcement Emergency Response Trainers International (ALERT) have shown that tires have more of a tendency to hydroplane when pressure is low. This happens because the tire footprint (the portion of the tire actually in contact with the road) is larger. For those of you who water ski, think of which is easier to get up on: a fat ski or a skinny ski. More tire surface in contact with the water makes it easier to hydroplane, just as it is easier to water ski on a fat ski. Also, a soft tire can be pushed in more by the pressure of the water on the center portion of the tread. This results in less rubber in contact with the road.

    Tire wear

    Much better tire wear results from maintaining proper pressure. Tires with lower pressure will wear off the outside of the tread faster from the deflection of the tire during cornering, and the tires will heat up more from increased road friction. This is one of the factors that caused the failure of a certain brand of tires on Ford Explorers some years ago. In 1999 the San Jose Police Department realized a significant cost savings by increasing the pressure in the training fleet to 50 psi. They soon followed up by increasing the pressure in the patrol fleet to 44 psi. For liability reasons, most agencies are reluctant to exceed the maximum pressure listed on the tire for actual patrol vehicles, but they reap the cost saving when going to 50 psi on training vehicles.

    Next time you inspect your vehicle, make sure you check your tire pressure since your ability to performance drive is significantly affected by it. You are not driving to the store to get a loaf of bread! You may be called upon to chase a dangerous criminal or respond to assist another officer in trouble. You don’t wonder whether or not your gun is loaded before you hit the street; don’t’ wonder whether your tire pressure is correct once the pursuit starts. Check your tires routinely, just as you do with all other critical equipment... [rm]http://www.officer.com/web/online/Editorial-and-Features/Driving-Under-Pressure/19$27281[/rm]
     
    Last edited by a moderator: Jan 25, 2009
  2. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Austin:

    ___It appears as if somebody put some pressure of their own on Officer.com and the article was updated with much of it magically missing? You did good :)

    ___Good Luck

    ___Wayne
     
    Last edited: Aug 2, 2008
  3. jdhog

    jdhog Hyper Smiler

    This is interesting. I bet it would be way more difficult or impossible to drive that truck on two wheels if the tire pressures were at door placard. Also regarding performance and safety, it is easy to see how running at max sidewall would probably decrease road traffic accidents by giving drivers better chance at correcting a mistake, or having a much wider margin of error before getting into a sticky situation. Prevention being better than cure kind of thing....So I wonder why don't car manufacturers recommend max sidewall then?
     
  4. voodoo22

    voodoo22 Cheaper than the bus

    As one of my best professors said, it's about money, everything is about money and the sooner you figure that out, the better off you'll be!

    Hehe.

    IMO lower pressure is recommended because:

    it gives people a more comfortable ride and thus the perception of more quality
    tires wear out faster, so tire companies make more money
    you use more gas, so oil companies make more money
    maybe even it increases your probability of getting into an accident which makes more money for the bodyshop
     
  5. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    Two words: Ride Quality. A soft ride sells cars.
     
  6. jdhog

    jdhog Hyper Smiler

    Yeah I see. Money makes the World go around.

    I wonder then which side of the fence car safety governing bodies would sit, Door placard or Max sidewall? surely if there was enough evidence of max sidewall being safer then it would be required by car manufactures to post that instead of cushy feeling psi that sell more cars and so on.....oh well. Maybe that's why new cars are coming mandatory with TPMS, but along with TPMS they should also make the public aware that max sidewall is safer than placard.
     
  7. WoodyWoodchuck

    WoodyWoodchuck Sophomore Hypermiler

    Thank you lamebums. I’m printing a copy to share with my dealer when I go in for my 5,000 mile oil change and they deflate my tires for me.
     
  8. voodoo22

    voodoo22 Cheaper than the bus

    I insist that they put on my service sheet not to touch the PSI and they usually listen. Ask them to not check your tire pressure.
     
  9. abcdpeterson

    abcdpeterson Well-Known Member

    My experience with higher Tire PSI so far has been surprising, at least to me.

    I always believed the tire PSI listed by the auto maker was for ride comfort. But I also was of the belief that High PSI would result in a decrease in traction.

    I was right and wrong.
    Right in: With higher pressure ride has suffered, as I thought, in fact more than I thought it would.

    Wrong in: Traction has NOT decreased, in fact it’s better. With lower (auto manufacture listed) PSI my car would under steer. Front tires losing traction before the rear.

    Next question for me is: will snow and ice traction still be better with the higher PSI?
    Cold pavement so far has not yet been an issue.
    I had a deer jumped out in front of me, on a Cold night. I jammed on the breaks and turned just a bit in an attempt to avoid the collision. Missing the deer by less then 12inches. Traction and control was GREAT, better than I would have expected.
     
  10. lamebums

    lamebums Member

    The Echo's handling at door placard - for lack of a better word - sucks. It understeers easily and will chew up the outside of tires pretty quickly.

    They have to be at sidewall or better at all times.

    When I took my car into Tire Discounter's I told them to press up the new tire to sidewall. They did (and also let the other three down to sidewall).
     
  11. JasonA

    JasonA New Member

    Hey guys. First post here. Question on this topic:

    I've heard, and this may be unsubstantiated, that increased tire pressure increases drag in the struts by forcing a strut movement on a small road impact where a softer pressure in the tire wouldn't have been able to force that movement (and the impact taken up by the sidewall of the tire). I know from experience that increasing tire pressure improves rolling resistance, but the thrust of what read was saying that there's a point of diminishing returns as well...that once you get high enough, you're not doing any more good for rolling resistance, and you may be bringing on unintended negative consequences as well.

    Any truth to that?
     
  12. abcdpeterson

    abcdpeterson Well-Known Member

    I feel there is some truth to it. BUT!

    I think you would be hitting some dang big bumps before it would more of a negative impact then the lower rolling resistance of the higher PSI.
    I would recommend keeping the PSI up before worrying about it.



    Now along that line. Finding the smoothest line down the road with the lease amount of car directional adjustment also improves FE.

    There has been a time or 2 when in a glide and catching up to slower traffic instead of using the break I will look for a rougher track in the road (like the crack at the edge) and ride that a sec or 2, the rougher surface will give you a gentile slowdown.
     
  13. run500mph

    run500mph Well-Known Member

    i could not agree more about tire pressure. my experience has been the same. better handling, taction, and no hydroplaning. and of course much higher fe and looong tire life. at 75-85psi its been very nice .
     
  14. Wheeee!

    Wheeee! New Member

    Hi All,

    I am in Ireland and motoring is a bit different here! Most cars are small/medium sized with manual transmission and there are very few hybrids.

    I have been thinking about how driving style affects fuel consumption ever since my father used to coast down hills and have come up with a few conflicting principles.

    I also ride a motorbike and this has raised some issues for me regarding furl consumption. My bike (F650GS) is a single with fuel injection and it normally does around 55mpg (imperial) around town. On the highway cruising at around 70mph, my consumption increases to over 70mpg. This raise two issues with me:

    1. Bikes are not very aerodynamic and I would expect the consumption to worsen with increased speed. I understood the resistance increases exponentially with speed. So why does the consumption increase?

    2. Cars here should return around 30-40mpg. A car is considerably heavier than a bike but returns the same mpg as a mid-size or large bike.

    Weight and aerodynamics are the two big factors influencing consumption but there seems to be some conflicts in what actually happens. Any explanations?

    The other area that occupies my mind concerns engine efficiency. My bike provides maximum power/torque at about 4000rpm. Is this the figure that returns the highest mpg? In my car I always thought that the lowest rpm would give the best mpg.
    I figure now that this depends on whether you want constant speed (in which case lowest rpm), or if you will be changing speed a lot such as in traffic or perhaps going uphill (in which case max torque rpm).

    I hear often that you should reduce speed to improve consumption. For my bike 70 mph seems good -so I should actually speed up!!! However, for my car I am not sure by how much. On the principle that resistance increases with speed, then I should drive really slow. Obviously 10mph wont result in good consumption as it would take a lot of time to cover the journey. So just what is a good efficient speed? 30mph? 40mph? 50mph? I know this depends on the car the conditions etc, but if they can generalise that 60 is too high then there must be a general speed figure to aim for. What is it?

    Anyway it is good to see that there are other people out there who are concerned with MPG instead of BHP.

    Cheers
     
  15. Shiba3420

    Shiba3420 Well-Known Member

    As I like to simplify;
    Holding the accelerator on the floor wastes a lot of gas, but idling wastes even more as your not going anywhere.

    Usually your highest/longest gear will give the best economy. At what speed do you get the best? As you say, it would depend heavily on the car. For my HiHy, between 45 & 50 gives me the best mileage I can get on flat ground with no wind while holding a steady speed. With a tail wind, my best economy may be as high as 55. With a head wind, maybe as low as 40. Downhill...speed doesn't really matter as I can just coast pretty much for free at any speed the car wishes to reach. For a typical uphill, I think around 40/45 is best, but since there are few places where I can play with my speed while maintaining the same climb, its pretty much a guess.

    The best way to find out its a monitoring device that can show your instant mpg. Set you speed and watch the gauge for a minutes. Bump the speed up or down and see if the mpg improves or gets worse. However, as you see all over the site, there are techniques that offer far better economy than just setting a best speed and holding it.
     
  16. Tomjones76

    Tomjones76 Well-Known Member

    Has anyone tried asking the publisher or author to post the entire article again?
     
  17. CapriRacer

    CapriRacer Well-Known Member

    The problem is how you are measuring the consumption.

    Stop and go traffic results in idle time - and as was pointed out, this only consumes fuel without generating distance.

    If you really want to compare what speed does for consumption - without the effect of idling - then you need to drive the same road at different speeds.
     
  18. Seraph

    Seraph Well-Known Member

    Thanks for this article, really cleared a few things up
     
  19. infitom

    infitom New Member

    Just curious,

    What is the effect of alternates to air in tires, and their impact on mileage. Is this bunk?

    Some claim a 4-10% difference (Something I have a hard time believing):

    Has anyone tried this, and did it make ANY difference?
     
    Last edited by a moderator: Dec 30, 2009
  20. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi infitom:

    ___N2 will not help and when it heats up, its pressure increase is less which will give less FE over air. Low moisture content is good but air is usually free... Or close to it and has been working fine for well over one-hundred years ;)

    ___Good Luck

    ___Wayne
     

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