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  1. #26
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    Quote Originally Posted by bholwell View Post
    By increasing the tire volume, you might actually be increasing the likelihood of pinchflats. Air is compressible, and since casing height is not increased much, the tire still has nearly the same distance to travel before contacting the rim, but since there is more air volume (like in a high-volume shock), compressing the air is more linear, i.e. easier to bottom out.
    I think that the percentage of the total total tire volume that is displaced is so small, that the actual pressure is going to change very little in a typical compression of the tire resulting in a pinch flat or rim strike. But even if it does change a relevant amount, I think it would change pretty much the same in tires of different volumes (of identical heights), because in the case of a larger volume tire, more volume needs to be displaced in order to hit the rim.

    So it is like having a higher volume shock (more linear), but being able to push the piston in farther without bottoming.

    I think the moral of the story is to choose a rim that is wide enough to laterally support your tires- there's probably not much you can do against pinch flats regarding rim selection, except for avoiding those rims with sharp edges. If you're still pinch flatting, either choose a higher volume tire or run more air pressure.
    This is true. I understand that this thread is largely theoretical, and hair splitting.
    15mm is a second-best solution to a problem that was already solved.

  2. #27
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    I wonder if the point of diminishing returns for tire height as you increase rim width is a more-or-less fixed ratio of rim width:casing width.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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  3. #28
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    Quote Originally Posted by kapusta View Post
    This is true. I understand that this thread is largely theoretical, and hair splitting.
    I've read here a few times that wider rims increase tire diameter and reduce pinch flats so I think the information serves a purpose. Yes, the differences are small...exactly!

    I'm just trying to show that this "benefit" is really an illusion. Not trying to say wider rims aren't good, just separating fact from fiction.

  4. #29
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    Quote Originally Posted by meltingfeather View Post
    I wonder if the point of diminishing returns for tire height as you increase rim width is a more-or-less fixed ratio of rim width:casing width.
    I believe it is, and I believe the peak occurs at an inner rim width of b2b casing width divided by Pi (provided you determine that width correctly).

    P.S. Handy rule of thumb would be 80% of rated tire width.
    Last edited by craigsj; 12-15-2011 at 08:02 AM.

  5. #30
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    Quote Originally Posted by bholwell View Post
    By increasing the tire volume, you might actually be increasing the likelihood of pinchflats. Air is compressible, and since casing height is not increased much, the tire still has nearly the same distance to travel before contacting the rim, but since there is more air volume (like in a high-volume shock), compressing the air is more linear, i.e. easier to bottom out.

    I think the moral of the story is to choose a rim that is wide enough to laterally support your tires- there's probably not much you can do against pinch flats regarding rim selection, except for avoiding those rims with sharp edges. If you're still pinch flatting, either choose a higher volume tire or run more air pressure.
    IME a narrowish tire on a wide rim is more prone to pinch flats than on a narrowish rim, for whatever reasons.
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  6. #31
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    This thread is very interesting. Thanks for all the information.

    Quote Originally Posted by craigsj View Post
    I've read here a few times that wider rims increase tire diameter and reduce pinch flats so I think the information serves a purpose. Yes, the differences are small...exactly!

    I'm just trying to show that this "benefit" is really an illusion. Not trying to say wider rims aren't good, just separating fact from fiction.
    I always thought that wider rims diminished the chances of pinch flat by preventing the tire from folding over itself and therefore pinching the tube... As bholwell said, "sidewall buclking" certainly plays a role... I'm not convinced it is an illusion.
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  7. #32
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    Quote Originally Posted by PissedOffCil View Post
    As bholwell said, "sidewall buclking" certainly plays a role... I'm not convinced it is an illusion.
    Yes, I agree. I was referring to the rim making the tire taller. I believe other factors could exist.

  8. #33
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    Quote Originally Posted by craigsj View Post
    I believe it is, and I believe the peak occurs at an inner rim width of b2b casing width divided by Pi (provided you determine that width correctly).

    P.S. Handy rule of thumb would be 80% of rated tire width.
    ha... that was kind of a dull thought, wasn't it?
    that was covered (more-or-less) in the OP... I was just looking at bholwell's figure and thinking sort of abstractly.
    bholwell... next time you mess around in that file, put some dang units on your x-axis!
    kidding (sorta)... also nice work.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

  9. #34
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    Quote Originally Posted by meltingfeather View Post
    bholwell... next time you mess around in that file, put some dang units on your x-axis!
    Whoops! Fixed, now.

    Why don't you go ahead and take that sabbatical? I've got lots of projects you can help me on, and then I wouldn't be so rushed on extracurricular activities like this.
    Tire Design & Development Engineer. The opinions expressed in this forum are solely my own.

  10. #35
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    I derived some equations for tire width and volume and used the tire height formula I post earlier to graph the details for a tire 157mm wide bead-to-bead using rims from 0-100mm wide. 157mm is conveniently 50mm * Pi so a 50mm produces the tallest tire. It would be rated somewhere between 60-65mm I'd imagine, but remember these dimensions don't include any tire thickness which would add to width and height. Volume is internal volume ignoring rim volume. Here are the results.
    Attached Thumbnails Attached Thumbnails Tire height vs. rim width-tire_dims.jpg  

    Last edited by craigsj; 12-16-2011 at 05:05 AM.

  11. #36
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    Although not related to the OP, this article is interesting for putting things in perspective :

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  12. #37
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    Quote Originally Posted by PissedOffCil View Post
    Although not related to the OP, this article is interesting for putting things in perspective :

    Tech Tuesday
    I thought pslide's quote was apt / accurate:

    Quote Originally Posted by pslide
    This article is psuedo-science...

    I wonder if RC contacted any actual tire engineers when he wrote this? Because certainly they would talk about things like bead seating, bead compression, bead shape vs. rim shape, etc.

    Can't argue with his conclusions though. Wider is better, but there is a lot more to the story when you get into the actual engineering involved.
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  13. #38
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    Quote Originally Posted by bholwell View Post
    I thought pslide's quote was apt / accurate:
    Yeah... it's pretty hard to take anything Richard Cunningham writes seriously.

    (I recognize and own the irony of this comment in the context of the Bike Magazine fork testing discussion, btw... my apologies for that)
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

  14. #39
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    Quote Originally Posted by bholwell View Post
    I thought pslide's quote was apt / accurate:
    I don't think it's meant to be mathematical science, rather a vulgarisation people can easily understand. Basically the principles involved are right but they let out some elements to keep it simple... nothing wrong with that
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  15. #40
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    Quote Originally Posted by PissedOffCil View Post
    I don't think it's meant to be mathematical science, rather a vulgarisation people can easily understand.
    I get that. But some of the things stated are just plain incorrect. The article made me
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  16. #41
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    Since craig linked to this thread, I have a question.

    The initial calculation assume that the tire cross section is circular, but is that really true for bike tires? Car tires, for example, have radial(?) belts that make them keep a square cross section.

  17. #42
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    Quote Originally Posted by beanbag View Post
    Since craig linked to this thread, I have a question.

    The initial calculation assume that the tire cross section is circular, but is that really true for bike tires? Car tires, for example, have radial(?) belts that make them keep a square cross section.
    bhowell is probably better to answer that question but I'll take a shot.

    The internal pressure of a tire will force the casing to be round absent anything that prevents it from doing so. Bicycle casings may have a distribution of materials that could distort the roundness but those materials will be quite flexible compared to the construction of car tires in your example. The more interesting question is, then, in what way could tire construction invalidate the basic conclusions above? How could a bicycle tire be made that has a fundamentally different, and beneficial, reaction to rim width?

    What I tried to show in the analysis is that tire height doesn't vary as much as some people think and therefore minimum tire pressures may also not vary so much assuming that pinch flats are the limitation (which I realize is not always the case). Widths do vary a fair amount but it's not clear why anyone cares about that at all. The tire still has the same tread, that doesn't change, it's contact shape with the ground will be the same and the unloaded curvature of the tire, as little as it changes, won't affect that. It's commonly accepted that wider rims are better and it's often said that's it's because of the increased volume. I think the volume increase isn't the reason at all.

  18. #43
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    Quote Originally Posted by beanbag View Post
    Since craig linked to this thread, I have a question.

    The initial calculation assume that the tire cross section is circular, but is that really true for bike tires? Car tires, for example, have radial(?) belts that make them keep a square cross section.
    A tire's carcass is completely circular in cross section. There is nothing like a belt to alter the profile in any way. Some tires have breakers, but this affects the profile very little, if at all.

    Even motorcycle tires, which have a much more substantial construction, are pretty circular in cross section.

    With bicycle tires, the tread depth is usually varied to alter the tire's profile.
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  19. #44
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    Quote Originally Posted by craigsj View Post
    bhowell is probably better to answer that question but I'll take a shot.

    The internal pressure of a tire will force the casing to be round absent anything that prevents it from doing so. Bicycle casings may have a distribution of materials that could distort the roundness but those materials will be quite flexible compared to the construction of car tires in your example. The more interesting question is, then, in what way could tire construction invalidate the basic conclusions above? How could a bicycle tire be made that has a fundamentally different, and beneficial, reaction to rim width?

    What I tried to show in the analysis is that tire height doesn't vary as much as some people think and therefore minimum tire pressures may also not vary so much assuming that pinch flats are the limitation (which I realize is not always the case). Widths do vary a fair amount but it's not clear why anyone cares about that at all. The tire still has the same tread, that doesn't change, it's contact shape with the ground will be the same and the unloaded curvature of the tire, as little as it changes, won't affect that. It's commonly accepted that wider rims are better and it's often said that's it's because of the increased volume. I think the volume increase isn't the reason at all.
    Pretty spot on.

    An increase in volume should theoretically improve the comfort of the tire. But the real benefit (imo) is that when the distance between the two beads is increased, the lateral stability of the tire is improved, allowing less tire 'roll' when cornering hard. I think this improves up to a point, and then the improvement in lateral stiffness starts to level out. Going too wide, however, will slow the steering response and will expose the sidewalls to trail damage more.

    I think your next project should be measuring lateral deflection for a given tire at a given load, using several different rims of varying widths.
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  20. #45
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    removed - I "luv" the random thread a post goes to when quotes are not used.
    Last edited by derby; 03-02-2013 at 01:32 PM. Reason: dumb software

  21. #46
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    A wider rim changes a tire's profile, but height at the center of the tire does not change.

    The original drawings posted are true for a single line or end view of a square sheet, but not true for a belted tire. The tire height doesn't change with rim width, because the tire circumference remains constant. ...well the circumference does stretch a couple millimeters as it ages from tire pressure.

  22. #47
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    Quote Originally Posted by derby View Post
    A wider rim changes a tire's profile, but height at the center of the tire does not change.

    The original drawings posted are true for a single line or end view of a square sheet, but not true for a belted tire. The tire height doesn't change with rim width, because the tire circumference remains constant. ...well the circumference does stretch a couple millimeters as it ages from tire pressure.
    Hmmm, good point.
    15mm is a second-best solution to a problem that was already solved.

  23. #48
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    "...but not true for a belted tire."

    Not true for a belted tire where the belt cannot stretch. Such a tire would have a flat tread even when unloaded. How many bicycle tires look like that?

    Bicycle tire circumference does not remain constant and that is trivially verifiable. Try measuring it, derby.

  24. #49
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    Quote Originally Posted by derby View Post
    The original drawings posted are true for a single line or end view of a square sheet, but not true for a belted tire. The tire height doesn't change with rim width, because the tire circumference remains constant. ...well the circumference does stretch a couple millimeters as it ages from tire pressure.
    You are seeming to suggest that a bicycle tire is belted like an automotive tire... that's not the case and the height of bicycle tires definitely changes with rim width.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
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  25. #50
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    Quote Originally Posted by meltingfeather View Post
    You are seeming to suggest that a bicycle tire is belted like an automotive tire... that's not the case and the height of bicycle tires definitely changes with rim width.
    The center strip of the tire is a certain length. Once inflated, the diameter (height) of the tire is that length / pi. Yes, tire pressure could stretch that length slightly, but that is a separate issue from rim width.

    If two identical tires at identical pressure on rims of different width were to have different diameters (height), that would mean that some force is stretching the tire material along the center strip more in the taller one. I can't think of anything that would explain that, which is why I agree with what Derby is saying.

    I think that the problem with the diagrams above is that they assume the cross section of the tire retains a perfect arc shape as you change the rim width. this would be true for a straight tube, but not for one in a ring like a tire.
    15mm is a second-best solution to a problem that was already solved.

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