Tire pressures

Discussion in 'General Bike Discussion' started by Xela, Jul 1, 2009.

  1. Xela

    Xela New Member Tavern Member

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    There are probably many articles out there written by "experts" regarding this, but I would like to get a feel from the community here about your common practice.

    What tire pressures do you run in your road tires? I'm about 6' and 175 lbs. I usually run around 115 to 120 psi. I was told by a mechanic at a huge shop once that that is too high. However, another mechanic at another shop was praising the Vittorias because you could run pressures up to 140 psi +. So, there's one reason for my question.

    The other is that my (read sarcastically) wonderful county road department is deciding to totally screw up perfectly good roads with thick, large aggregate chip-seal. :mad: Very frustrating as a lot of the roads they are doing this to appeared to not need any work at all, yet some of the worst roads around here are not being touched. Because of the new gravel, the ride is very rough, and I was thinking about dropping my pressure.

    What pressures have you all found to be the best compromise between rolling resistance, smooth ride, and cornering ability? Also, if you were going to start experimenting with lower pressures, what increments would you drop the pressure in (i.e. 5 psi, 2 psi, etc)?

    Thanks,

    Alex
     
  2. camilo

    camilo New Member

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    Alex, just like motor vehicle tires, bike tire pressure depends on the tire, the size of the tire and the load. Many, (most?) bike tire manufacturers have pressure charts with the variables generally being rider weight and tire size. There are also generic formulas. If you do a search on other cycling websites (bikeforums.net and roadbikereview.com for example), you'll see references and pointers.

    I think it's pretty well accepted now that unless one is riding on a perfectly smooth surface, the old saw about "higher pressure = lower rolling resistance" is false. On imperfectly smooth surfaces, and especially on rough surfaces, too-high pressure can slow you down because (a) it's uncomfortable and (b) the tire tends to bounce over irregularities rather than rolling over them. Bouncing causes energy loss because it's, in essence, lifting the bike up over the irregularities rather than rolling over them.

    Of course, there's a point where a too-soft tire will be inefficient too, but the idea is that there's an optimal pressure. That's where manufacturer's pressure charts come into play, and look on line for your tire. I know Mich. has such a chart, and others do too.

    A chart that is pretty well circulated which I have used successfully for years is:

    Calculate Rear tire pressure:
    Tire Width=20: Pressure(psi) = (0.33 * Rider Weight in lbs) + 63.33
    Tire Width=23: Pressure(psi) = (0.33 * Rider Weight in lbs) + 53.33
    Tire Width=25: Pressure(psi) = (0.33 * Rider Weight in lbs) + 43.33
    Tire Width=28: Pressure(psi) = (0.33 * Rider Weight in lbs) + 33.33

    Tire Width=32: Pressure(psi) = (0.17 * Rider Weight in lbs) + 41.67
    Tire Width=37: Pressure(psi) = (0.17 * Rider Weight in lbs) + 26.67

    Front Pressure = .9*Rear Pressure
     

  3. Xela

    Xela New Member Tavern Member

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    Camilo, thanks for the great info! I searched Continentals website yesterday, but couldn't find any information on my tires. So, I went with the recommended pressures from your table. It was only about an 8 psi change, but it made a world of difference on the smoothness of the ride. It was like a completely different bike. Only minor degradation, as would be expected, was the cornering on smooth roads.
     
  4. Pickle

    Pickle Guest

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    The best way is to leave the pressure caps off, remove a little air at a time from each tire, test it.....

    How does this effect getting flats? Is more air or less better?