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Author Topic: Pulk Design  (Read 4882 times)

Offline K.

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Re: Pulk Design
« Reply #15 on: March 30, 2018, 01:09:14 pm »
There's certainly no end of angles to the design question , but one I haven't seen addressed yet is getting rid of ice encumberment, from slush and so on. An advantage of the ski sled is the there is less surface area to scrape, while the toboggans and pulks would have the full surface area to deal with. However, being somewhat flexible, the toboggan could possibly be a little easier to de-ice. I can't see there being any innate advantage in any of the designs in terms of friction though, friction is proportional to force not pressure. All that actually matters there is weight (ie force) and material (ie coeffient of friction as in uhmw vs hd), and all three sled,pulk,toboggan can be optimized for weight and built from any material.

Offline kinguq

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Re: Pulk Design
« Reply #16 on: March 30, 2018, 04:17:19 pm »
"I can't see there being any innate advantage in any of the designs in terms of friction though, friction is proportional to force not pressure. All that actually matters there is weight (ie force) and material (ie coeffient of friction as in uhmw vs hd), and all three sled,pulk,toboggan can be optimized for weight and built from any material."

Actually I don't quite agree. Friction on snow is quite a dynamic process that depends on temperature and loading among other things. Also there is compressive friction, essentially the force needed to push through the snow, or move it out of the way. I wrote about these things here, although the photos have disappeared.

https://www.wintertrekking.com/community/index.php?topic=3917.msg35277#msg35277

I found a pulk with runners slides more easily than a toboggan under almost all conditions. A sled with runners has "enough" flotation under most conditions, because it is almost always following at least one snowshoe or ski track, and the loading on your skis and snowshoes is inevitably greater than that on your sled.

Kinguq.

Offline rabbit

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Re: Pulk Design
« Reply #17 on: March 30, 2018, 05:54:29 pm »
"friction on snow is a dynamic process" I am chuckling to myself as I type this. Several years ago a friend said to me and I quote " Did you know that Eskimos (Inuit) have 35 words to describe snow but don't have a word for digital watch?" Snow conditions are definitely dynamic. Ask a competitive cross country skier. Enjoy the snow.

                                                                                  Les

Offline Dave Hadfield

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Re: Pulk Design
« Reply #18 on: March 30, 2018, 08:31:29 pm »
I thought my HDPE toboggan, an idea I stole, was the finest modification in the history of the universe, until I invented the ski-sled. (It was the '80s. This kind of plastic was new.) Then, I found that the new design, based on old downhill skis, pulled easier.

And was a lot cheaper and easier to source. (I "borrowed" an old set from a friend's parent's garage.)

Most people build ski-sleds too heavy. You have to build them like a canoe, or an airplane -- always fighting weight gain.

If it never breaks, you screwed up -- you made it too heavy.

Offline AunNordDuNord

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Re: Pulk Design
« Reply #19 on: March 30, 2018, 11:03:10 pm »
HDPE is one thing, UHMW is a totally different animal, I built a few out of both material and I can tell you that the later is way easier to pull and will pull easier for a longer time!

Offline Kaifus

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Re: Pulk Design
« Reply #20 on: April 02, 2018, 10:33:16 pm »
Here's some pictures of when I was building my latest sled:

I sandwiched the UHMW between metal and used a propane torch for bending the sides where I work...





Then took it home and build it in my living room...



I used 3/16 aluminum rivets for everything, using wide-flange rivets for the bow curve and making washers out of the wide-flange rivets for the back side. Four inches between the cuts...





The stern was a bit different and I spaced the notches at about 1-3/4, 3, and 3 inches...



With the military sled style lashing installed...




Offline AunNordDuNord

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Re: Pulk Design
« Reply #21 on: April 02, 2018, 11:06:47 pm »
Kaifus that looks fantastic! Great work!!

Offline Inabell

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Re: Pulk Design
« Reply #22 on: April 03, 2018, 08:24:49 am »
Do you think a construction method like this would work with HDPE? 
« Last Edit: April 03, 2018, 08:57:00 am by Inabell »

Offline Hutchy

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Re: Pulk Design
« Reply #23 on: April 03, 2018, 09:16:01 am »
Kaifus, what did you use for the metal edges? is that some type of stainless extrusion?

This is the exact type sled I am making currently. I am going to get it bent soon. I was told that cold bending uhmw was best, but it looks like the heating didnt hurt a thing.
Used to be the man made the gear, now it seems the gear makes the man...

Offline AB_Winter

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Re: Pulk Design
« Reply #24 on: April 03, 2018, 10:43:32 am »
This is great work Kaifus. Thanks for sharing.

Offline AunNordDuNord

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Re: Pulk Design
« Reply #25 on: April 03, 2018, 11:05:47 am »
Do you think a construction method like this would work with HDPE?
It does work I made a few for the school with HDPE but I would spend the extra bucks and get UHMW, you will never regret it!

Offline Kaifus

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Re: Pulk Design
« Reply #26 on: April 03, 2018, 07:36:19 pm »
Kaifus, what did you use for the metal edges? is that some type of stainless extrusion?

This is the exact type sled I am making currently. I am going to get it bent soon. I was told that cold bending uhmw was best, but it looks like the heating didnt hurt a thing.

It's an aluminum extrusion and it wraps up and over the top edge and turned out really nice. I had first used 1/16 x 3/4" aluminum bar stock but with the 1/16" UHMW the top edge of the sled was too wavy and not so nice looking (on my sled out of 1/8" UHMW the top edge is nice and straight with only another strip riveted to it). With the extrusion it's basically the same weight but provided the perfect degree of rigidity. It was originally meant for use on a greenhouse and I had to cut off part of the extrusion. I don't think it would work with 1/8" thick material like it did with the 1/16".

I think the heat definitely helped bend it but I really kept the torch moving and tried my best to heat the entire bend evenly.


Offline Hutchy

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Re: Pulk Design
« Reply #27 on: April 03, 2018, 09:18:06 pm »
So I guess not needed on 1/8" eh.

What made you go to 1/16 from 1/8? Juet weight savings or...?

Anything in particular you didnt like about the 1/8?
Used to be the man made the gear, now it seems the gear makes the man...

Offline Kaifus

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Re: Pulk Design
« Reply #28 on: April 03, 2018, 10:23:46 pm »
So I guess not needed on 1/8" eh.

What made you go to 1/16 from 1/8? Juet weight savings or...?

Anything in particular you didnt like about the 1/8?

There was nothing I didn't like about the 1/8 inch, it was strictly weight savings but based off how durable the 1/8 seamed and figuring I could save some weight by going to 1/16 inch. You might still want a strip of something at the top edge of an 1/8 inch sled to keep it nice and straight.

Offline Hutchy

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Re: Pulk Design
« Reply #29 on: April 04, 2018, 07:03:51 am »
Awesome. Thanks. I figure based on a density of 0.033 lbs/cubic inch thst my pulk will be a shade over 12 lbs. Yours will be just over six And a regular toboggan will likely be around twenty two with all crossers etc in place. Ill see how 1/8 goes but based on those numbers it would indeed make sense to go to 1/16.
Used to be the man made the gear, now it seems the gear makes the man...