Tire Pressure

Discussion in 'Cycling Equipment' started by CatSpin, Mar 1, 2005.

  1. CatSpin

    CatSpin New Member

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    My Zipp rep just gave me an earful about riding my clincher 404's at 140lbs pressure with a 130 rate tire. He stated that one should only ride a tire at rating, not above and a harder tire (over-inflated) causes more problems wiht rolling resistance and traction than good.

    Why are some clincher tires rated so low 120 psi and others 140psi. Why would a racer choose a higher pressure than a lower one? Does racing dicipline, weight and one's personal cycling stength play a role in tire choice?

    Help,

    CatSpin
     
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  2. artmichalek

    artmichalek New Member

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    The theory that higher tire pressure reduces rolling resistance only works on a perfectly smooth surface. On a typical road, resistance stops going down at about 110. Basically, you're riding up and down a bunch of little hills. A lower pressure allows the tire to conform around them and not waste as much energy. If you're on a track however, the surface is a lot smoother, and you can go as high as 200psi before performance drops off.
     
  3. RC2

    RC2 New Member

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    The skinny on Zipps and pressure...
    http://www.velonews.com/tech/report/articles/4177.0.html

    I don't think there's a concern at 140 if the tire is rated to handle it, but keep in mind that a long descent with contant braking will heat the rims... and with a ~1 psi increase per 10 degrees of heat, you're no longer at anything close to 140. And if you're not on a track, even the smoothest blacktop, you probably want to be in the 100-115ish range anyway for best wear and rolling resistance. There is another Zinn article in www.velownews.com tech section that talks about that in depth.
     
  4. RC2

    RC2 New Member

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  5. dhk

    dhk New Member

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    Agree with you and RC2. I've read the same theory several places now, and my own experience with lower pressures confirms it. I used to run the max sidewall rating too, but began to experiment when I got Michelin Pro Race tires, since their sidewall specs a range (87 psi min to 116 psi max). Found then that 100f/105r works much better for ride, handling, grip and tire life than the 120 psi I had been using, with no increase in rolling resistance on real roads. For reference, my weight is 175 lbs.

    People still using very high pressures also need to be aware of the rim limits. Believe most MAVIC rims are only rated for up to 130 psi....you can check their website for the specs on your rim.
     
  6. garagedog

    garagedog New Member

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    I've wastold by a LBS guy back in the early 90's that the max rating was 1/2 of the blowout pressure. Example...If a tire sidewall blowsout at 240lbs of air pressure, then the max raiting is set at 120.
     
  7. Jaguar27

    Jaguar27 New Member

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    Talk about blow-out pressures.....I keep our Bikes in a spare Bedroom...one night, I was changing my wife's MTB Tires out to those specialized Fatboy slicks...I pumped to around 80psi and KERRRRRBOOOOOOMMMMM!!!

    I can tell you from experience that a Tube bursting in an enclosed Room makes your ears ring for over an hour....

    WHAT???!!!!

     
  8. serenaslu

    serenaslu New Member

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    LOL Reminds me of way back in college when the same type of thing happpened to me with a road tube. I had a single dorm room; 8.5'x11', cinder block walls, concrete ceiling and floor. Talc cloud. Would love to have had a picture of the look on my face.:eek:
     
  9. LockinUp

    LockinUp Banned

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    WORD!
    www.kingzoftheroad.com
     
  10. artmichalek

    artmichalek New Member

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    Sort of. A tire rated for 120psi will probably blow out around 240psi or so depending on what rim it's on. More than blowout pressure goes into the manufacturers rating though. The laminated structure of a tire is designed around a specific set of load and deformation parameters. If you consistently over or under inflate a tire, it's going to delaminate and fall apart long before the rubber wears out.

    Also, in addition to increasing drag an overinflated tire can be pretty dangerous for cornering. When a 23mm tire inflated to 110psi (good fun with mixed units...) rolls over a piece of debris on the order of 1mm in diameter, the tire deforms around it and still maintains contact with the road. (Actual performance varies based on things like thread count and tread thickness.) Pump that same tire up to 140psi and instead of deforming it rides up on top of the debris. Kind of like riding around a corner on a road covered with ball bearings.
     
  11. Wurm

    Wurm New Member

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    Here's the real poop guys:

    [/size]

    [size=-1]http://www.velonews.com/tech/report/articles/7507.0.html
    [/size]
     
  12. domaindomain

    domaindomain New Member

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    The well known, large tyre brands spend considerable R&D dollar$$ on designing and developing their tyres.

    To go totally against their recommendations on pressures etc. doesn't seem to be the best idea. That said, some small variation around the recommendations might make the ride more suitable to your own style.

    Cheers,

    bicyclerubber.com

    All the rubber your bikes need !
     
  13. dhk

    dhk New Member

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    Yes, but in most cases they don't give recommendations; only the maximum rated pressure is put on the sidewall. Michelin Pro Race is the only exception I know; they show 6-8 Bar (87-116 psi) as the recommended range. Believe this is a good working range for any 23 mm tire.

    This is different than cars, which are required to show the recommended pressure ratings on a placard where it can be easily read (at least in the US). My car has four different sets of pressures, depending on load and speed range. The max pressure on the tire sidewall is 44 psi, much higher than any pressure recommended for the car. Running the tires at max sidewall pressure would give very poor roadholding and ride.
     
  14. artmichalek

    artmichalek New Member

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    As far as I know all of the Michelins have both a minimum and maximum on the sidewall. At least the Axial BiSport and HiLite Prestige do.
     
  15. Fat Hack

    Fat Hack New Member

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  16. Wurm

    Wurm New Member

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    "What is the optimum tire pressure?"

    THE answer is: experiment between known reasonable limits for your specific tire casing/size, total bike/rider weight, conditions, and personal preferences. When you find what works best for you, stick with it and ignore the debates.

    Example: for me @ about 225 lbs. total bike/rider weight, I get good results from the Vred Fortezza 25c @ around 115 front/128 rear.

    Otherwise, I really don't care if Your Mileage May Vary! You're not on MY bike.
     
  17. el Ingles

    el Ingles New Member

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    Maximum rated pressure is usually the result of an argument betwwen the marketing people , who want it as high as possible , and the lawyers , who want it as low as possible - if there´s a reccomended pressure on the sidewall or website then that´s the technicians imput : go from that figure and see how it feels but don´t forget that too much pressure causes as many problems as too little .
     
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