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Thread: Wood choice for crutches?

  1. #1
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    Wood choice for crutches?

    Just curious. I have a distant in law with a spinal thing he was born with. We hit off, he and I, decades ago. He uses I think half crutches, the kind with a regular handle, but a leather strap about mid forearm rather than a padded knob in his armpit. When he has some width to work with, he can motate. Like on a driveway or sidewalk where he can swing the tips of the crutches wide as a pair, and move his legs as one appendage, his stride length is like 48-60 inches instead of the 28-35 inches most of us have on two legs. Indoors in a regular hallway he moves about as fast as anyone else, but he can outrun most of us outdoors on short courses.

    He is hard on his equipment. He snapped another crutch a couple days ago, again, at the bolt hole for the handle, again, but this time the leather straps I made for the top bands (29 years ago) gave way too. He has a few bruises, didn't go to the ER, and I already have a new pair of leather bands ready to ship tomorrow. He strongly prefers wooden to aluminum crutches, we have shared several pints on that discussion over the decades. Typical off the shelf crutch for him is a glue lam of some unknown white wood.

    I am wondering about upgrading his woodwork though. It makes sense that any species suitable for stick furniture (hickory, red oak) should be suitable for crutches, but weight is an issue. And they usually break (for him) at the bolt hole for the handle for his hands.

    To make one pair of half crutches I am going to need four (riven) pieces steam bent into a gentle S curve, so air dried stock maybe nominal 1x2 by 60 inches long for starters. It seems to me walnut would be a possibility, walnut is about as strong as the oaks for impact, but weighs half as much; therefore gunstocks. I have never seen straight grained air dried walnut for sale.

    Doug Fir could be done, but DF does not, at least at my house, stay strong around through holes without some volume and mass. I could get all the usual KD suspects at my local purveyor and do a glue lam with West system epoxy, likely 105 resin with negotiable hardener. I am moderately adept at using heat to increase depth of epoxy penetration in face to face joints.

    Hardware is 3/16 inch. I am thinking about drilling the wood at 1/4 inch and using bushings. I could go a couple inches wide at the top ends of the S curved pieces where the through holes are, and then shape more narrow going lower.

    What I want to do is get him setup for life in the next few months while I have the mental acuity to solve the problem, and the dexterity and strength required to execute the solution.

    Local I can purchase, rive and air dry...birch. I am not sure which design parameters I need to look at regarding resistance to friction at metal fasteners and strength around drilled holes for through bolts.

    Appreciate any and all input. If weight was not an issue, I got plenty of hickory in the shop. I don't even like hickory for axe handles because of the weight. Great axe haft, wears me out before lunch. He is on these crutches anytime he is standing- so looking for light, strong and tough. What I don't want to do is build something that might work and have my well admired inlaw in the ER with a broken bone if I am wrong.

  2. #2
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    No offense, but I think you would open yourself up for a personal injury lawsuit by making crutches, should they fail and a serious injury result.

    I believe the problem is the crutches are being used in a way they were never intended to work. They are intended for straight down load support. Having them splayed out at a significant angle would reduced the strength significantly.
    Last edited by Lee Schierer; 05-12-2022 at 9:56 AM.
    Lee Schierer
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  3. #3
    If you really think this not a dangerous idea, I would look at ash. Straight grained ash. Itís light and strong. Thatís why itís used for guitar necks and baseball bats.

    However, if you really want to do your relative a favor, buy him aluminum or fiberglass or carbon fiber crutches or whatever non wood material is being used these days. They will be lighter and stronger.

  4. #4
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    I also thought of carbon fiber, but am wincing at the thought/risk to both of you of doing this.
    - ďItís not that Iím so smart, itís just that I stay with problems longer.Ē Ė Albert Einstein
    - Welcome to Florida. Where the old folks visit their parents

  5. #5
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    Ash is what came to mind, but you do want to make sure it wonít break. Donít do what failed.

    You already know the bolt hole makes a weak spot at high stress, so bent lamination might allow extra wood there while allowing fibers to pass continuously on both sides. You could even laminate in some fiberglass.

    Maybe veneering a set of aluminum crutches would be an option.

  6. #6
    I'm not sure of the shape. I googled "wooden half crutches" and got a slghtly curved fairly long stick design with a post for the hand at right angles to the long stick and a half cuff for the arm above the hand hold. If that is the design then the hand hold being supported at only the one end is a definite weak point. If you could work in even a thin support that goes from the main piece out over the otherwise unsupported end it would add significant strength. I don't see where it really needs a bolt there if you use a bent lamination to attach the handle at both ends.

    Bent laminations are used in structural applications in houses in lieu of bigger solid timber due to their strength. I like the idea of ash but I would caution that a single piece, regardless of species, is going to be weaker than a laminated piece. If he is breaking the laminated ones.... I like the idea of combining metal and wood with wood for it's appearance and the metal to make it strong.

  7. #7
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    Straight grain hickory with brass bushings for the bolts would be the way I would go, BUT as others say, you are putting yourself at risk.

  8. #8
    Outside the box here but maybe laminated carbon fiber? The lateral force is a bit much around those bolt holes for wood (imho).

  9. #9
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    I would do a bent lamination of VG DF or similar grained Ash with Kevlar or carbon fiber between the laminations, and wrapped around the outside at the critical hardware, with epoxy. I think it would be a fun and interesting project. for the short time i used crutches, The aluminum ones are very unforgiving and I can imagine a wood one would be much more comfortable to use.

    I am thinking DF vs oak for its bending properties and lightness. to me, oak and ash are much more brittle and have wilder grain compared. maybe some really straight QS maple or Long leaf pine? I think i would be more concerned with grain structure than species.

    I would look at things like skateboard decks, canoes and paddles and recurve bows for inspiration. It sounds like you have a good relationship with the user and understand the problems, so I would not be as fearful of the project as some here are. You will likely learn some things and build something way over built compared the the commercial unit being used now.

    I think of strip built and skin kayaks and the abuse they take with just a thin, but well designed wood frame, or wood sailboat parts.

    having a bit of boat building, commercial composites manufacturing and wood working knowledge leads me to these thoughts.

    edit. inspiration?


    https://www.etsy.com/listing/7152780...ery-1-39&sts=1


    bamboo strips would also be fantastic from a strength/weight perspective, but I have no idea where/how to source or mill them for something like this. bamboo does hold epoxy and laminate very well and is stable though. you would have to get strips like plyboo is made of.
    Last edited by Adam Herman; 05-12-2022 at 11:40 AM.

  10. #10
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    White ash would be my choice, too. Steam bend to whatever shape is needed, then spiral wrap it with epoxy and fiberglass or carbon fiber, with an extra wrap around the area where the hardware goes. Sort of like making skis. 3/16" seems pretty light duty for the hardware. I'd upsize that to 1/4, or maybe even 5/16", and use bushings as you mentioned, and maybe add a metal plate under the fiberglass on both sides where the bushing goes.

    John

  11. #11
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    Is it possible to redesign the handle so it clamps around the stick with no holes? Motorcycle handlebar clamp stuff is one inch diameter. I am thinking something similar to a hand held drill auxiliary handle.
    Bill D

  12. #12
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    All of you respondents - all y'all - are exactly right. The thing is he is a dude, just like most of us. He is still tempting gravity, and he is still pushing himself. "Forearm crutch" is a search term that brings up the correct product in aluminum and one vertical post. He likes two wooden posts with one bolt through both vertical posts and the handle.

    He never actually coached T-ball, but he got pretty close until someone saw him bat with his left crutch. He had his right crutch 'too far' down the first base line (his feet were inside the batter's box), and was hitting grounders towards third base super super slow - but knocking the bouncing ball into play was just a thing that happened on the way to getting his left crutch onto the first base line, and then he was off like a shot. If the parent's had let him coach there would be a bunch more switch hitters out there today.

    I am not worried about him bringing a lawsuit. If I build him something stronger he is going to test it until something breaks. I clearly remember the day we met, our common relative says it was June 1993. The pair of them showed up in my shop while I was building a leather cuirass for one of the RenFaire people out of 16 oz leather. The leather bands at the top of his half crutches or forearm crutches was about 4 oz shoe leather, sad and saggy. FWIW 16 oz leather is about 1/4 inch thick, 6.4 mm. The bands he was on were shoe leather, 4 oz leather is 1/16 thick, 1.6mm. I just told him to sit down, made him a new pair of leather bands in 45 minutes or so, and then the three of us went out for pizza.

    If you are going to hook a horse to a chain that is already hooked to a log on the ground, you want 16 oz leather. Since I upgraded his leather, he has been breaking the wooden uprights on his 'sticks' at the bolt holes. I have not yet built him a stick with a spring loaded knife blade in it, but it comes up again every time ultrasounds of his wife show another daughter on the way. The 3/16 diameter through bolts (through both wooden uprights and on center through the handles) have never yet failed on him. They look like they shouldn't be "enough" but they seem to be, even when he is moving out for first base while the third base man is sprinting into the infield to scoop up his grounder. The family did chip in on a pair of crutches with about a 5" diameter disc on the base so he could walk over the beach sand and actually get in the water. He is a good dude, we are glad to have him in the family.

    "Oh I am handicapped, I need to move slow" is not a thing for him.

    After I post this I shall go start a WTB (wanted to buy ) thread in the for sale section. I will be looking for 8/4 Ash, air dried, roughly 8" width by uncertain length, probably 48 inches. It MUST be air dried for later steam bending. I am looking for zero grain runout. If your piece has runout it will need to be wider so I can rip/rive on the grain lines. Waiting for a text response from back east regarding the length of the four uprights I would need to make one pair. 9PM here, already 1AM on the east coast, maybe tomorrow.

  13. #13
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    Quote Originally Posted by Bill Dufour View Post
    Is it possible to redesign the handle so it clamps around the stick with no holes? Motorcycle handlebar clamp stuff is one inch diameter. I am thinking something similar to a hand held drill auxiliary handle.
    Bill D
    This is a distinct possibility. As my inlaw is fully grown, he doesn't need a dozen bolt holes to adjust the handle height, the handle needs to go at one specific height. I have a return PM already from one of our more creative engineers, suggesting a Sam Maloof technique where I could cut a dado around the uprights, and a matching tongue on some handle stock, and them steam bend the handle through 360 degrees into the dados on the upright pair. I have steam bent both KD hickory and KD ash. To do this Sam's way absolutely requires air dried stock. I do have to go read up on "Maloof Joint," it is the only name I have.

    Up next, not attached, is a cellphone pic of the most recent break, they have looked the same for 20+ years. Seems like the pic is in my "internal storage/ download" folder in my phone but not in my gallery folder. I shall go find an eight year old to help me work my telephone. BRB hopefully...

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