In Search of a Bobsled

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Viewing 8 posts - 61 through 68 (of 68 total)
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  • #71924
    Ed Thayer
    Participant

    Preety cool history there.

    #71936
    Andy Carson
    Moderator

    Getting back to the physics of the sled runner and the placement of the bunk… I have had difficulty modelling the drag from the sled runner with different runner shapes and with different bunk placements when the sled is on dirt. I pulled a “phone a friend” to my friend who is a mechanical engineer. After much discussion, I found that this modeling is indeed very hard for a lot of reasons. I am still thinking about what this all means, or how this knowledge can be applied, but I wanted to share none-the-less. I find it interesting at least.

    So, the first thing I leanred is that modelling the pressure underneath the runners is complicated because one surface (the steel runner) is non-flexible, and one (the soil) is complient (flexible compressible). Because the soil is flexible, the vast majority of the pressure along the runner is going to be carried by the leading edge and the trailing edge. Kinda like if you put a log in a hammock: almost no pressure is exerted by the center of the log. This explains the wear on the rear edge, which has been a particular fascination of mine.

    The other thing that I found very interesting is that the friction coefficient of steel on dirt is not constant, but varies with the pressure placed on the soil. This is due, at least in part, to the ability of loose soil in direct contact with the steel runner to roll and slide over the soil particles beneath as well as slide and roll on the steel runner. So, the soil particles have two surfaces that they can interact with a several options as to the nature of these interactions. Obviously, the interaction with the least friction will be preffered. This ability of soil to act as a semi-fluid occurs only at low pressures and greatly reduces the relative draft. When soil is compressed over a runner, the particles aren’t free to slide against each other or roll and there is an increase in friction and drag. I think this a good explaination for the reduced draft on the stoneboat vs the sled in Tim’s work. Interestingly, the stoneboat advantage goes away on the Hayfield, which could be because the soil is fixed in place by sod in this setting and not able to act as a semi-fluid. Also, I learned (and this is a gut reaction on the part of my friend) that this kind of semi-fluid movement would only occur on pretty large surface areas with pretty low pressures. The exact numbers and pressures that form this threshold would vary with soil conditions, but this probably won’t happen with “standard” runners. One would probably need something like a stoneboat to see this. So, simply widening the sled runner a few inches is not going to make much of a difference… You would need a plate (or similar) which seems less practical…

    Again, I am going to have to digest this a little to see if this might be just food for thought or if there might be any jewels that might lead to improvements in sled design. Just thought I would share…

    #71909
    Mark Cowdrey
    Participant

    Thanks Andy, fascinating as usual. Makes me step outside my box to think of “dry” soil as a fluid.
    Mark

    #71942
    mitchmaine
    Participant

    @Michael Low 32642 wrote:

    That would have been Francis Foster. Lived in Walden, and was one of the first to import Brabants into the country. That’s the same Foster who owned the mill shown in the Ben Thresher video. He supported a family, 12 kids plus 10 foster kids, on horse logging and eventually his sawmill. Quite a charactor who unfortunately died just a few years ago.
    Michael Low

    i think ben thresher sold his mill to the rockefellers who wanted to move it to the billings museum, but the locals put up such a fuss, rockefeller sold it back to ben and ben sold it to a local named hiram hill or something like that who set up the nonprofit and so on. is this true? in that case, francis foster would have had to have it first and sell it to thresher. can you fill me in on the particulars? lots of fosters in maine, including my mom.

    #71921
    Does’ Leap
    Participant

    Mitch, Francis Foster didn’t own Ben’s mill but the mill from which he purchased his ash runners for the sled (a brief scene in the movie).

    George

    #71943
    mitchmaine
    Participant

    thanks george, i remember now. i misunderstood, but it makes sense now, thanks for clearing that up. mitch

    #71922
    Does’ Leap
    Participant

    Here a some pictures of the new bobsled. I ended up printing this thread and sending it to Dwayne Langmaid – the Walden, VT sled-maker. He was impressed by the thought and reasoning and agreed to make a sled with the same basic principals as Carl’s bobsled. I headed up the “mountain” yesterday and took down a load of firewood which is a good half mile skid. Before purchasing the bobsled I envisioned logging this part of my property by twitching to my main skid road and hiring a fowarder because of the steep terrain and long skid. Having learned about the benefits of the bobsled, I figured it would be a good alternative to this plan.

    My first load went surprisingly well. I figured the bobsled would be harder to load than the scoot. However, I found it much easier to find small depressions to park the sled because it is so much smaller than the scoot. The first tier of logs went up easily and I got them chained down based on Carl’s directions. The second tier of logs was a little more challanging as my skids tended to roll back toward the back of the load and my log would slip back a bit. I compensated for this by pulling my logs further forward. I really liked being able to load 20 footers on the sled – very difficult to do with the scoot.

    The trip down went great. I thought I would have to put on the bridle chains, but the horses seemed to be holding back the load o.k and I left them off. The sled pulled well on the steeps and flats with the load pivoting beautifully on the fixed bunk. I have a good hill leading up to the barnyard where I cut my firewood and had to get after the horses to make it up, they did. More pics of the sled can be seen here: https://picasaweb.google.com/102626207397974443261/Bobsled?authkey=Gv1sRgCMnM5-3ep9qVDg#5717283283848248674

    Thanks again to all for the input on this.

    George
    [IMG]https://lh4.googleusercontent.com/-WzgWKPr46-M/T1fe4ScP8bI/AAAAAAAABSQ/2a6f7I7tlsk/s800/P1030707.JPG[/IMG][IMG]https://lh3.googleusercontent.com/-ZZWi8VOixjQ/T1fe2u4NvWI/AAAAAAAABR8/KtqN4mFPtc0/s800/P1030706.JPG[/IMG][IMG]https://lh4.googleusercontent.com/-YTouc0hurTU/T1fenppmqeI/AAAAAAAABQ0/KgcdAa4sRO4/s800/P1030697.JPG[/IMG]

    #71907
    Carl Russell
    Moderator

    @Does’ Leap 33176 wrote:

    ….. as my skids tended to roll back toward the back of the load and my log would slip back a bit…..

    Try cutting the bottom of your skid flat with the saw to keep them from rolling…..

    Very nice sled and operation George.

    Have fun, Carl…

    So now you’re ready for this poem I wrote a few years back in memory of the old-timer who showed me how to do this..

    Bobsledding Logs

    It’s 4:30, quarter to five,
    on a crisp mid-winter afternoon.
    The sun has gone down behind the hill,
    and you can see dusk forming in the shadows.

    Dry powder snow covers the ground.
    The temperature never got above fifteen degrees today,
    but working in the sun, you warmed up easily.
    Now a slight chill seeps into the dampness of sweat on your shoulders.

    The team is ready,
    hitched in front of a nice jag of fine straight logs.
    Put your coat back on, and check for mislaid tools.
    Climb up on the load, gather lines,
    brace your feet, and calmly call on your animals.

    As the load lurches forward,
    puffs of steamy breath float from the horses’ nostrils.
    You can feel the weight of the load through your legs,
    as the bobsled travels over humps and through hollows.

    Hooves thump and squeak,
    as chips of ice fly from sharpened caulks.
    From below the load comes the jingle of bridle chains,
    and as the logs rasp over the frozen snow
    they put forth a whine, like from a bow on a fiddle.

    Feel that?… That’s the spirit in your soul dancing the bob-sledder’s jig.
    ” Now we’re logging”.

    1998; For Walt Bryan; Farmer horse logger, family man.

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