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  1. #1
    Reputation: cycling's Avatar
    Join Date
    Jul 2004

    700x20 tire question

    I need new tires. Will i be able to put 700x20cc tires on my alex 330's? Will i get less rolling resistance?

  2. #2
    RoadBikeReview Member
    Join Date
    Aug 2001
    I'm not sufficiently familiar with those rims to know how wide they are. You'll have lots of pinch flat issues if you attempt to install a too narrow tire on a too wide rim. If you check Sheldon Brown's website, I think that he has some rim and tire width recommendations.

    The relationship between tire width and rolling resistance seems to mystify a lot of riders. Take a bare tire and try to bend it front to back. The energy that it takes to bend the tire isn't pushing you down the road. That's what causes most of the rolling resistance. A tire with thin supple sidewalls will be easier to bend and have less rolling resistance than one with thick stiff sidewalls.

    Now think about what happens where the rubber meets the road. Your tire flattens out a little, front to back, at the bottom. As you ride down the road, your tire is constantly bending and unbending as you chase that flat spot around the tire with every revolution. The more it flattens, the more the tire is being bent and the more energy you are losing to rolling resistance.

    There are two common ways to reduce the amount of flattening that occurs.

    One way is to increase the tire's internal air pressure. That will reduce the rolling resistance, but it makes the bike less pleasant to ride and it means that your tire is going to bounce off of, rather than absorbe, minor road variations. Bouncing means the tire isn't in contact with the pavement so your handling suffers. Some people think that this "dancing around" makes a rock hard tire feel faster at the same speed than a more supple one.

    Another way to reduce the front to back flattening that occurs is to use a slightly wider tire. A wider tire at the same air pressure will have same size contact patch, but it will be wider. That makes it unnecessary for the tire to flatten quite so much front-to-back so it consumes less energy flexing the tire and you have less rolling resistance. Surprise! a wider tire has less rolling resistance than an equivlent narrow one.

    Rolling resistance of bike tires is probably less important than you think. Once you pass about 15 mph, most of your energy is being consumed to overcome air resistance. The faster you go, the more important aero drag becomes. Compared to the energy required to push your torso through the air, everything else combined is small potatoes. If you want to go faster, work on maintaining a good aerodynamic body position on your bike.

  3. #3
    Reputation: cycling's Avatar
    Join Date
    Jul 2004
    But wont incresing pressure increase risk of popping
    Im running mich pro races @116psi

  4. #4
    RoadBikeReview Member
    Join Date
    Feb 2002

    Blow-off risk

    Quote Originally Posted by cycling
    But wont incresing pressure increase risk of popping
    By popping do you mean blowing the tire off the rim? Higher pressure increases this risk, but only very slightly unless you go to extreme pressures. I believe that Michelin recommends a max pressure of 110-115 psi. If you need higher pressures than that to prevent pinch flats, then you should be riding on wider tires. Also, don't kid yourself that your pressure gauge is accurate to 1 psi (115 psi is the same as 116). 20 mm tires are only good for light people (say 130 lb or less) and for front tires on time trial bikes to reduce a bit of aero drag. You get less rolling resistance by increasing tire pressure up to around 110-120 psi. After that, on real road surfaces, the harder tire bouncing off the rough surface prevents any further reductions in resistance.

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