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  1. #76
    Senior Member OldsCOOL's Avatar
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    Quote Originally Posted by HawkOwl View Post
    Regular curb. Not a ramp.
    Oh man, that would knock a Schwinn Varsity rim out of true. That kind of curb you just get off and walk it over.
    Having a flat tire as part of the total cycling experience is highly overrated. Knowing how to fix one quickly is not.

    '85 Trek 460 road racer

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  2. #77
    Senior Member
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    How far the center of gravity (CG) of the bike/rider must travel to get from "A" to "B" determines how "fast" a tire is.

    If the tire is too 'hard' for the surface conditions, it will raise the CG higher when hitting a bump than it would if it were 'softer'. The consequence of this is that the bike/rider must travel farther to get where they want to be. Thus, ‘too much pressure’ (for the conditions) requires that the assembly (bike + rider) must travel farther to ‘get there’. Correct pressure, therefore, is a rather complicated matter. Almost everybody gets it wrong.

    Joe
    Living a fact-based life.

  3. #78
    Senior Member mapeiboy's Avatar
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    5'8" . 127lbs , on Conti 4000s 23mm , 120psi . Since everyone is moving away from this tire size I have no problem finding it on sale all year along .

  4. #79
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by Joe Minton View Post
    How far the center of gravity (CG) of the bike/rider must travel to get from "A" to "B" determines how "fast" a tire is.

    If the tire is too 'hard' for the surface conditions, it will raise the CG higher when hitting a bump than it would if it were 'softer'. The consequence of this is that the bike/rider must travel farther to get where they want to be. Thus, ‘too much pressure’ (for the conditions) requires that the assembly (bike + rider) must travel farther to ‘get there’. Correct pressure, therefore, is a rather complicated matter. Almost everybody gets it wrong.

    Joe
    That's an interesting way of putting it. But almost everybody wants to avoid too high pressure on rough surfaces realizing that the bumps slow them down so I don't think they're really getting it wrong.

  5. #80
    Email for new group DnvrFox's Avatar
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    When I got my Lemond BA in 1999 (yes, that's 1999) the wise owner looked at me and said, "Always run your tires at 120/120." Who am I to dispute this wisdom from someone so experienced? I've followed that advice ever since, with 19's, 20's, 23's and 25's. And, if I let the pressure drop to - say 105 - because I was lazy and did not top off the tires that day, I can tell absolutely no difference in ride quality. I am no lightweight, currently at 206.

    Of course, the Lemond is 853 steel with a carbon fork. To me, that makes the difference in ride quality, not 10 lbs of psi. Still ride that bike almost daily. Still keep it at 120. Still love it.
    Almost gone from the 50+ forum. - Email me at dnvrfox@aol.com for another fun new group of 50+ folks

  6. #81
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by Carbonfiberboy View Post
    I got the bolded "mistake" from Jobst Brandt. If there is in fact a lack of a forward vector, then how is the rough roller data to be explained? Why isn't it the vertical movement, since there is no return on the energy required for that acceleration? ...
    Well, gravity is a conservative force so maybe that's what Brandt meant.

  7. #82
    aka Phil Jungels Wanderer's Avatar
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    I just got new tires for my car, and it rides MUCH SMOOTHERRRRRR, even at higher pressures. Quiet too! Forget all that other stuff, it's all in the tires!

    p.s. they really are noticeably much smoother

    "Retirement is the best job I ever had!" Me, 2009


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  8. #83
    What??? Only 2 wheels? jimmuller's Avatar
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    Quote Originally Posted by wphamilton View Post
    Well, gravity is a conservative force so maybe that's what Brandt meant.
    And the argument that a tire skips over a bump applies only if it contacts the back side of the bump for a significantly shorter time period than it contacts the front, or if it leaves the ground entirely. This is analogous to the downhill ski racer, where speed is lost whenever the skis leave the snow. Over normal non-bumpy road surfaces, especially chipseal-or-smaller-embedded gravel, this doesn't happen.

    It should also be pointed out that when the tire hits a bump the center of mass of bike and rider is not what needs to move vertically. The frame deflects like a spring while the tire is deflecting, and unless the rider is riding heavily on the bike his body will not move as much as the mass of the frame. Much like the sprung/un-sprung weight ratio of a car wheel holds the road better in a bump, a lighter wheel and tire combination does so on a bike.

    Finally, the fact that "everybody is doing it" doesn't prove it better or worse, especially in cycling! I don't race but a few years ago I asked a friend who did race long ago (the same guy who gave me my Masi frame) about this very question, skinny tire/wide tire, high pressure/low pressure. His response was that if skinny high-pressure tires weren't the right thing to do, racers wouldn't have doe it for more than 50 years. Back then "everybody" was doing it. Now "everybody" is doing it different. So I'm generally cautious about accepting anything except what I experience or what I can "intuit" (always dangerous) by applying my own (fairly extensive) understanding of the principles.

    YMMV.
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    jimmuller

  9. #84
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by jimmuller View Post
    And the argument that a tire skips over a bump applies only if it contacts the back side of the bump for a significantly shorter time period than it contacts the front, or if it leaves the ground entirely. This is analogous to the downhill ski racer, where speed is lost whenever the skis leave the snow. Over normal non-bumpy road surfaces, especially chipseal-or-smaller-embedded gravel, this doesn't happen.
    ....
    Except that the tire isn't perfectly elastic, and also it may have already started reforming its shape before getting over the hump, so it isn't all returned. Picking nits, I know.

    So the main point is that if the high pressure small tire doesn't leave the ground, it isn't costing much (any) speed.

  10. #85
    Senior Member
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    Some years ago I saw a film that involved two bicycles coasting down a long grassy embankment. One had suspension and the other was rigid. They were precisely similar otherwise. Same rider on both and he stayed on the seat.

    There were four tests: First, each bike/rider assembly coasted down the slope on the smooth grass and reached the bottom with the same elapsed time. Then 2x2" boards were placed at intervals across the path and the tests were repeated. The rigid bike was significantly slower: its CG (Center of Gravity) had to travel farther to get to the finish line.

    Near the end of the film, a comparison of the CG paths was demonstrated with a line that followed the CG of each bike as it traversed the slope over the boards. The reason for the superior performance of the suspended bike over the boarded path was made very clear and obvious -- it did not have to travel as far to reach the bottom.

    This was not actually a test; it was a demonstration. What happened was obvious and predictable.


    There are a number factors that influence what is the best tire size, construction and pressure for any given set of conditions or goals. Track tires traverse a completely smooth surface; they can use 18mm tires and 140# pressure (the CG has a straight path). However, don't try a downhill mountain bike trail with such a combination; you'd spend way too much time in the air and likely smash your head against a tree because you simply cannot steer while airborne.


    BTW: Where I live and ride, the roads are clean, smooth and dry. The tires I use reflect that. If I were to live in some other place (rough pavement, potholes, gravel, etc.), my tire selection would reflect the differences. This is not a simple subject and I'm happy that I am not a tire engineer trying to please everyone.


    Joe
    Living a fact-based life.

  11. #86
    What??? Only 2 wheels? jimmuller's Avatar
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    Quote Originally Posted by Joe Minton View Post
    Some years ago I saw a film that involved two bicycles coasting down a long grassy embankment. One had suspension and the other was rigid...
    I'd say you should be careful what you conclude from that. I see quite a few reasons why it doesn't apply to any tire pressure discussion.

    First, I'd guess (but could be wrong) that those were mtb's, while (in my own mind at least) any discussion of values, say, 80psi or higher are for road bikes. (I had thought we were talking about road bikes.)

    Second, the road surface irregularities between pavement and grass (of all things) would be very different. You didn't say how large the boards were, but only really bad pavement would have repeated bumps/dips of 1" or more, especially not with repeated sharp-edged bumps.

    Third and probably most important, the pitch characteristics of those two bikes could be very different. The bike with suspension would be more compliant in the front, far more than any difference in tire pressure would produce. It would also be heavier in the front, so the front's sprung/unsprung weight ratio would higher. That would keep the tire pushed downward faster and of course make it accelerate better on the back side of each board. It is quite possible that the no-suspension bike's front wheel was leaving the ground while the suspension bike's was not. In addition, on a car the pitch behavior would be determined by the oscillation frequency of front and rear but on a bike the rider can ride very reactively. Since the rider is so much heavier than the bike that that could dominate the behavior. So the two bikes would not necessarily be comparable to each other, let alone provide any lesson you could apply to real-world riding.

    I'm not saying that video was tricked. (Such things have been done in the past - remember the Ford commercials with a pickup truck running on RR track ties? That worked because the speed was chosen to match a null in the oscillation frequency of the truck body. Run it 1/3 faster or slower and the result would have been very different.) I'm just saying the result though impressive don't mean as much as they would appear. What it does do is demonstrate the value of a compliant bike, i.e. one which keep the wheels pressed down. Depending on the time constant of a bump compared to the wheels, a lighter wheel combination on a more compliant frame could have the same benefit.
    Last edited by jimmuller; 07-31-14 at 12:40 PM.
    Real cyclists use toe clips.
    jimmuller

  12. #87
    coprolite fietsbob's Avatar
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    406-47 , with thorn resistant tubes, in the tires, I top them up every 3 months.. maybe..

  13. #88
    Senior Member
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    The principle is simple:
    The farther the CG has to travel to get from one point to another, the more power input required to maintain a given speed, or, in a coast-down test, the longer it will take to get to the second point. This applies to bicycles, motorcycles, cars, trucks, tricycles, wheelchairs, etcetera.

    If the coast-down test is on a level surface and from a particular speed until the vehicle comes to rest, it will come to a stop sooner and not go as far if the surface is 'rough'. Rough being defined as being uneven enough so that the CG is jounced and does not follow a straight path.

    Yes, there are many other variables but likely none as fundamental as CG travel distance.
    One instance: bicycle riders are soft and giggly and outweigh their rigid bikes several times over. When the bike/rider assembly hits a bump, both are elevated to some degree or other; they acquire vertical energy taken from the kinetic energy of their forward movement. Some of that acquired energy is turned into heat; tire friction, internal friction of the giggly rider and so on. That energy is lost to the enterprise of getting down the road; the bike slows down. Most of us have experienced this when we ride from a smooth portion of road to a rough one -- the bike slows down, sometimes rather quickly.

    If tire pressures were lowered a bit, the CG might not raise as far, less energy would lost and the bike wouldn't slow as much. This would also be true if one were to fit more compliant tires such as going from Conti Touring Plus to Gatorskins.



    Joe
    Last edited by Joe Minton; 08-03-14 at 12:52 PM. Reason: cleaning the meaning ;o)
    Living a fact-based life.

  14. #89
    rugged individualist wphamilton's Avatar
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    Quote Originally Posted by Joe Minton View Post
    The principle is simple:
    The farther the CG has to travel to get from one point to another, the more power input required to maintain a given speed, or, in a coast-down test, the longer it will take to get to the second point. ...
    Joe
    Keep in mind that gravity is a conservative force, and the energy required is the same regardless of the path taken.

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