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so few sailing - anybody thinking ?

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  • cjecje1
    I had Capital 215/70-15 tires on my Tacoma pickup. For various reasons I decided that I would prefer to have Michelin 235/75-15 tires on it instead. It s a
    Message 1 of 72 , Jan 9, 2014
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      I had Capital 215/70-15 tires on my Tacoma pickup. For various reasons I decided that I would prefer to have Michelin 235/75-15 tires on it instead. It's a better tire and will stand up to my occasionally abusive weight and high speeds better. The 235's are 2" taller so I speculated that the resulting higher final drive gear ratio might improve my gas mpg as well.

      I mentioned that concept to someone who told me that no; the bigger tires would kill my gas mileage - because of all the added weight. So I checked the weight difference and found it to be 9 lbs per tire / 36 lbs. total. Which is about the weight of two cases of beer. Two cases of beer seems to me like it would make a gas mileage difference too small to even measure.

      That was my return comment to him and his retort was: No; it's unsprung weight and rotating mass - so it's effect is equal to seven times it actual weight. So it's as if you had added over 250 lbs of weight - and that will really reduce your gas mileage.

      I thought about that, concluded that it made no sense to me, and asked for details from him.

      His response:

      "Because it's rotating mass and unsprung weight. If you're having a hard time understanding it stated that way then an in-depth explenation isn't going to make you feel any better because it gets into a LOT of physics and more serious topics. Just know that a 1 lb added past your suspension is the same as 7lbs elsewhere. You add 10lbs to each wheel (or tire, or brake rotor) and that's the same as 70lbs per wheel elsewhere in the vehicle. At four corners you're talking 280lbs sitting next to you. Weight savings on wheels & tires is critical and this is why race teams focused on the wheels & tires for so many years because ounces they shaved there equaled pounds shaved elsewhere. No need to get super technical just know that unsprung weight is roughly equal to 7lbs of sprung weight and there ain't no arguing with physics."


      OK smart people - does any of that make any sense to you? And if so; can you tell me how rotating a balanced weight makes it seem to weigh more? <g>

      stephen
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    • cchl74
      You are not wrong. There is a point at the leading edge called the stagnation point where the air flow divides to go either over or under the foil so the flow
      Message 72 of 72 , Jan 14, 2014
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        You are not wrong. There is a point at the leading edge called the stagnation point where the air flow divides to go either over or under the foil so the flow is dividing ahead of the leading edge, but like I said, for air under sailing conditions this is not very far out. The reason I cautioned you is because I and others have been criticized by some on this list when using terms that imply that the air has a crystal ball and knows about the approaching sail. Fluid flow is quite complicated but I think that most texts oversimplify, some to the point of being wrong. I've got sailing books that show air flow at the trailing edge leaving the two different sides of the sail at different velocities. The classic Bernoulli diagram does not explain why planes can fly upside down, how the old balsa F-86 models flew with dead flat wings, and where the big wind shadow behind a sailboat comes from. A good discussion, Marty.


               Bruce K
               Challenger # 74, "Ouroboros"
               Los Lunas, NM 




        Yeah, that is why I put "see" in quotes.  It is just a shorthand for
        describing the timing and sequence of good old cause and effect. A
        common device for the popular communication of hard to visualize stuff.

        As for testing based on false premises, you nailed it.  Even scaled
        model tests in "realistic" settings aren't realistic at all because not
        all forces and effects scale the same etc.

        Marty

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