wider tire, lower rolling resistance?
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  1. #1
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    wider tire, lower rolling resistance?

    I have read in the threads time and again that a 25mm tire offers lower rolling resistance compared to 23mm tires, assuming both tires are properly inflated according to a rider's weight. What is the science behind it?

  2. #2
    wim
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    Bad science because it's a false statement. The claim is only true if the 25 mm and the 23 mm tires are inflated to identical (not "proper") pressures.

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    Best way to explain it is starting with identical pressures (which pretty much nulls the whole thing).Looking at the deflection and contact patches of the 23mm and 25mm tyres: You'd see that the 23mm would have a narrow, but long contact patch whereas a 25mm tyre would have a wider, but substantially shorter contact patch - like a dot,

    Until 25mm's are built to handle higher pressures, this fact isn't applicable even if it is true. But even more so, the ride quality of a stiff 25 would be unfavorable.

    Beyond the tyre design, wheel manufacturers like HED have designed thier rims to be wider; HED's rims are 23mm wide. They pretty much say the same thing occurs:

    "With a better rim to start from, the Jet wheels are faster. Jets are made for 23mm tires. Larger tires have lower rolling resistance than smaller versions. They put more rubber on the road for better cornering traction. They require less psi to support the same load as a 19mm tire. 23mm tires are superior – now that we have the proper rim for them. With the C2 rim, tire width matches rim width, so the wheel is more aero. The tire sidewalls are part of the aero shape of the entire wheel, they don’t bulge over the brake track like they do on a 19mm rim. The same straightened sidewalls support the tire better in hard cornering and out of the saddle efforts, so the complete tire is more rigid and less squirmy. Stiffer is more efficient than squirmy, and more efficient is faster."

    http://www.hedcycling.com/wheels/jet9.asp


    My setup coincidentally has a 25mm rear tyre on a 22mm rim. I think there is a noticableble effect if you pump the tyre to a pressure beyond reason,
    Last edited by Ventruck; 05-26-2009 at 04:15 PM.

  4. #4
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    Quote Originally Posted by orbeamike
    I have read in the threads time and again that a 25mm tire offers lower rolling resistance compared to 23mm tires, assuming both tires are properly inflated according to a rider's weight. What is the science behind it?
    It's geometric. As mentioned above, a narrower tire will have a longer contact patch than a wider one. Therefore, the force component in the direction of travel will be a larger fraction of the total force for the narrower tire. So if the total deflection force for the wide and narrow tires are equal, the wider one will produce lower rolling resistance. That force depends on many factors including tire construction, rubber compound and inflation pressure. There's a good explanation of it here, http://www.amazon.com/Bicycling-Scie...3366979&sr=8-1

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    Quote Originally Posted by asgelle
    It's geometric. As mentioned above, a narrower tire will have a longer contact patch than a wider one. Therefore, the force component in the direction of travel will be a larger fraction of the total force for the narrower tire. So if the total deflection force for the wide and narrow tires are equal, the wider one will produce lower rolling resistance. That force depends on many factors including tire construction, rubber compound and inflation pressure. There's a good explanation of it here, http://www.amazon.com/Bicycling-Scie...3366979&sr=8-1
    So let me get this straight.......with the same tires (Pro Race 3 for example), one with 23mm pumped to 110 psi and one with 25mm pumped to 100 psi (tuned to my weight), the 25mm will still have smaller but wider contact patch?
    Or does this only occur with identical 110 psi in both tire?

  6. #6
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    It's not precisely correct but close enough, to say that any two tires with the same pressure will have the same size (but not shape) contact patch. And generally, a short, wide patch deflects less, creating less loss from hysteresis than a long, narrow one.

    Now, 'same pressure' does matter, but not in the way that has been assumed in some of the posts above. Highest pressure does not equal lowest rolling resistance, because at too high a pressure, you force the bicycle over road imperfections rather than allowing it the tire to conform around them, and lifting the bicycle requires more energy than bending the tire. The opposite end of the barbell is that with too low a pressure, too much energy is lost to sidewall flexion (hysteresis.) What's more, there's a necessary relationship between pressure and weight, as a narrow, high-pressure tire can't support a certain load without risking pinch flats as well as a wider tire at a similar (or even lower) pressure can.

    So...

    If you weigh more than 160 lbs or so, you'll be able to more easily get the ideal pressure in a wider tire than in a narrower one. If you only weigh 110 lbs or less, riding 21's might be more efficient than 23's. Please note that these weights listed are examples only, and not meant to have any mathmatical strength behind them. It would also vary by surface ridden, type and construction of tyre, tread design (if present, but let's hope not) and so on.

    Bottom line, just saying so is a gross oversimplification, but for the large majority of riders reading this, 25's are likely to be a more appropriate choice.
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    Not straight

    Quote Originally Posted by orbeamike
    So let me get this straight.......with the same tires (Pro Race 3 for example), one with 23mm pumped to 110 psi and one with 25mm pumped to 100 psi (tuned to my weight), the 25mm will still have smaller but wider contact patch?
    Or does this only occur with identical 110 psi in both tire?
    The size of the contact patch is the load on the tire divided by the tire pressure. For simplicity's sake, if you have 100 lb of load on the tire and 100 psi in the tire, you will have a 1 square inch contact patch (simply cancel the units when dividing lb by lb per square inch). To obtain that contact patch, a larger tire will have to deform less, and therefore the energy losses will be less as the casing deforms. In real life, narrower tires are typically inflated to a higher pressure, and so the rolling resistance argument cannot be made one way or the other since the comparisons are not direct.

    The main take away should be that larger tires do not necessarily mean higher rolling resistance, despite what many peoples' "intuition" tells them. Further, higher pressures do not necessarily reduce rolling resistance because the tire is less able to deform over road roughness, and so energy is wasted as the tire "kicks back" when hitting roughness.

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    I also think that some riders who pump up their tires to higher pressures get the "feel" of speed because they feel more of the road buzz with the higher pressures and the lower pressures feel more dampened and "feel" slower.

  9. #9
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    being a heavier rider I always keep my 25's inflated to the max or slightly more and compared to the 23's I used to ride the 25's have a smoother ride.
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    Just to add fuel to the debate. I read in a recent magazine article about "cycling myths" that higher pressures do not lead to less rolling resistance.
    I rann one bike with 23s at 120 and other has 25s running at 100 over a 10 mile course in as near as I could tell identical conditions. The speeds/effort are actually the same as far as my legs and Garmin could tell - about 15seconds in favour of the 25s. Statistically insignificant.

    PS Another Myth (not cycling) - That we lose most heat through our heads.

  11. #11
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    DIRT BOY

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  12. #12
    wim
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    Quote Originally Posted by DIRT BOY
    Here is some good info:
    Good info indeed—one of the few articles that comes clean about the identical pressure stipulation. Which, as Ventruck said above, pretty much nulls the whole thing.

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    wow, you guys are way over my head. How about adding one more to this discussion? Theoretically, a 25 will definitely weigh more than a 23, and thus causes additional rolling weight, and causes more energy lose when accelerating?

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    Quote Originally Posted by FlyingYeti
    wow, you guys are way over my head. How about adding one more to this discussion? Theoretically, a 25 will definitely weigh more than a 23, and thus causes additional rolling weight, and causes more energy lose when accelerating?
    How much? And is that with or without the valve cap

  15. #15
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    Quote Originally Posted by FlyingYeti
    wow, you guys are way over my head. How about adding one more to this discussion? Theoretically, a 25 will definitely weigh more than a 23, and thus causes additional rolling weight, and causes more energy lose when accelerating?
    Meh, it's not like 25s are pigs or anything... for example, Conti GP 4000s weigh 215 grams in 25C size, hardly porkers.

    And you can throw other x-factors into the mix, such as, how much more does a rider get beat up by road shock on skinny tires run at ultra-high pressures, rather than wider ones run at lower pressure? And how much does that start to degrade his or her performance on long rides?

    It's just not a simple, easy comparison. For myself, I bailed on skinny tires a long time ago, due to comfort issues and pinch flats, but I am a heavy rider.
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  16. #16
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    Now add that 25mm tires are less aero to all this. It all depends on your needs on what will be faster.
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    Rotating weight

    Quote Originally Posted by FlyingYeti
    wow, you guys are way over my head. How about adding one more to this discussion? Theoretically, a 25 will definitely weigh more than a 23, and thus causes additional rolling weight, and causes more energy lose when accelerating?
    Yes, it will take more energy to spin up a 25 than a 23 due to the higher weight of the bigger tire. For example, an extra 15 grams would require 0.0002 watts to accelerate from 30 to 40 km/hr. That same 15 grams in your water bottle would require 0.0001 watts to acclerate. You might not be able to see this difference on your power meter, or despite the claims of some, even feel it in your legs.

  18. #18
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    Here's my thoughts and experience:
    The rolling resistance difference between the 2 is insignificant. I've ridden 23's for a while at both max psi and lower - lower lead to pinch flats galore at my then weight of 200lbs. High psi led to many puncture flats. On 25's, high psi yielded punctures, whereas 85 to 90psi has led to lots of enjoyable ride time without flats.
    You can to too far in either direction. I've found what works best for me by experience. I hope you can too. Don't sweat the small stuff about rolling resistance.
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    I'll add to the argument that Continental GP 4000s come in both 23 and 25c sizes. The PSI ratings are 90-120 psi for the 25s and 110 (or maybe 105) - 120 psi for the 23s. So same pressure at the top end.

    I have the 25c version and ride them at around 105 psi, just from feel. But how do you calculate the ideal pressure based on load? I weigh about 180 pounds.

  20. #20
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    Quote Originally Posted by iamnotfilip
    I'll add to the argument that Continental GP 4000s come in both 23 and 25c sizes. The PSI ratings are 90-120 psi for the 25s and 110 (or maybe 105) - 120 psi for the 23s. So same pressure at the top end.

    I have the 25c version and ride them at around 105 psi, just from feel. But how do you calculate the ideal pressure based on load? I weigh about 180 pounds.
    http://www.vintagebicyclepress.com/images/TireDrop.pdf

    Read carefully, and remember, the chart is per wheel, your bike and equipment weigh something too, and the back wheel takes on more than 50% of the weight.
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  21. #21
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    Quote Originally Posted by iamnotfilip
    I have the 25c version and ride them at around 105 psi, just from feel. But how do you calculate the ideal pressure based on load? I weigh about 180 pounds.
    There's no such 'calculation' possible, as we can't come up with figures for how smooth or rough your road surface, whether it's wet or dry, whether you ride 'heavy' or 'light', and so on.

    Peanya's notes about balancing pinch flats and puncture flats is one way to look at it, as is ride comfort vs perceived tire deflection.

    Here's Michelin's take on the question, though I personally run about 10 psi short of these pressures:
    https://www.michelinbicycletire.com/...rpressure.view
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  22. #22
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    Quote Originally Posted by SystemShock
    http://www.vintagebicyclepress.com/images/TireDrop.pdf

    Read carefully, and remember, the chart is per wheel, your bike and equipment weigh something too, and the back wheel takes on more than 50% of the weight.
    .
    Thanks a lot, that article makes a lot of sense to me. I realized that I am very close on my rear tire, but could go quite a bit lower on the front. I'll give that a try, see how it feels.

    When going for a long ride, sometimes I over-inflate by a few PSI, the logic being that the tire gradually looses pressure, and I don't want it to be too low at the end of the ride. Is this nonesense? How much PSI can you loose during a 4 hour, 100 km ride?

    Another thing I consider is the temperature outside. The reasoning being that with the hot air as well as sun and black tires, the air inside the tires is going to get warmer during the ride and actually have higher pressure than the cold air I inflate in the tire. So I underinflate a bit when it is very hot and sunny. For example if it is 30 degrees Celsius outside and sunny, I'll go about 8 PSI lower than my desired pressure.

    I never really read anything on the issue, but just something that has crept up from discussions with a friend who does auto racing. Am I silly to even consider this?

  23. #23
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    I have the 25c version and ride them at around 105 psi, just from feel. But how do you calculate the ideal pressure based on load? I weigh about 180 pounds.
    I'd say you're about a good spot. I'm right around 190 and run 90 to 100psi. Lighter weight can add a little. (yes it seems backwards from the chart, but I'm not selling tires)
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  24. #24
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    Quote Originally Posted by iamnotfilip
    When going for a long ride, sometimes I over-inflate by a few PSI, the logic being that the tire gradually looses pressure, and I don't want it to be too low at the end of the ride. Is this nonesense? How much PSI can you loose during a 4 hour, 100 km ride?

    Another thing I consider is the temperature outside. The reasoning being that with the hot air as well as sun and black tires, the air inside the tires is going to get warmer during the ride and actually have higher pressure than the cold air I inflate in the tire. So I underinflate a bit when it is very hot and sunny. For example if it is 30 degrees Celsius outside and sunny, I'll go about 8 PSI lower than my desired pressure.

    I never really read anything on the issue, but just something that has crept up from discussions with a friend who does auto racing. Am I silly to even consider this?
    Yah, you might be overthinking it just a tad.

    You might want to run a little lower pressure in the rain... but beyond that, once you have the right psi for your weight and comfort, I think worrying about every little factor may be a bit much. But, that's me.
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  25. #25
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    Quote Originally Posted by danl1
    There's no such 'calculation' possible, as we can't come up with figures for how smooth or rough your road surface, whether it's wet or dry, whether you ride 'heavy' or 'light', and so on.

    Peanya's notes about balancing pinch flats and puncture flats is one way to look at it, as is ride comfort vs perceived tire deflection.

    Here's Michelin's take on the question, though I personally run about 10 psi short of these pressures:
    https://www.michelinbicycletire.com/...rpressure.view
    Kinda crazy that Michelein does not distinguish front/rear tire pressures, non? I mean, their chart acknowledges that load relates to ideal tire pressure (the entire point of the chart). And not even randonneurs (who are not their target audiences in 700c X 25c, X 23c, etc.) ride 50%/50% weight distribution front/rear.

    When I rode Krylions (formerly Carbons), I routinely pushed the rear tire pressure to 120lbs (even 125lbs) with no problem whatsoever, so I find Michelein's max pressure recommendations to be extremely conservative (vs. Maxxis, which is perhaps too generous).

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