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Wheelchair physics -- need an explanation

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    #16
    http://www.out-front.com/twinstar_overview.php

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      #17
      I'd think you'd be able to shake your happy ass side to side with some force while in your chair and see if it's spoke flex or what. That's what I did when my tires were rubbing my pants. lol
      DIGG.

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        #18
        Originally posted by tooley View Post
        don't waste your time, it's not that.

        Okay, I'm not a mechanical engineer like chas. I was (and still think like) an auto mechanic. So bear with me while I attempt to put this into words.

        Think about the force put on the tire at the contact patch while turning. It pushes the wheel out thereby causing the opposite effect at the top of the wheel where your sideguards are. How noticeable this is will depend on type of wheel and wheel spacing. I noticed from day 1 on my Oracing because of the incredibly tight wheel spacing to my solid sideguard tubing, magically it never touches but does get really close.

        To better amplify the force I am trying to explain turn on some carpet, you will notice as you turn how much the wheel flexes and then when you come out of the turn (moving straight) the lateral force is no longer present and the wheel spacing increases.
        I think I followed that. What kind of wheels are you running on your Oracing?
        stephen@bike-on.com

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          #19
          Originally posted by DIGGER View Post
          I'd think you'd be able to shake your happy ass side to side with some force while in your chair and see if it's spoke flex or what. That's what I did when my tires were rubbing my pants. lol
          While stationary and pushing my wheels in toward my body I am not able to get the top of the wheels as close to my side guards as when occurs when I'm turning.

          Another related matter is that no matter how finely adjusted I get the quick release axles, no matter how absent the amount of play at the hub, there's always some slop at the perimeter of the wheel. This has been the case for every wheelchair I've owned. It's a function of the coupling of the wheel-axle-camber tube.

          I think Schmicking designed a more robust wheel axle design that may minimize this but that's not in the cards for either of my chairs. Here's an old photo from my archives.
          stephen@bike-on.com

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            #20
            The only way this can happen is if your chair has camber, so I'm guessing you probably have some.

            When you turn, the distance between the wheels at the bottom gets larger, because wheelchair tires are treaded to go straight. When you turn, the tire "wants" to go straight while the tires are actually turning, which causes the wheels to move apart, which effectively increases your camber by a few degrees - until you negate it by going straight again.

            It is for the same reason that wheelchair wheels tend to squeak on hard surfaces while not squeaking when going straight.

            Your tires will probably do this more on carpet (as the tires can't just slide about) and less on hard surfaces.

            I don't know if I explained that in a way that makes any sense, so... you'll let me know I am sure. All the wheelchairs I've ever had have done this.

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              #21
              That is a perfect explanation, Chrissy. I can understand it, and Stephen is much smarter than me - his IQ, after all, is 212!
              Don - Grad Student Emeritus
              T3 ASIA A 27 years post injury

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                #22
                I have fold-down aluminum side guards, 4-degrees camber, and <0.75" rear-wheel spacing at the backrest post. When my chair was brand new, the tires frequently rubbed the side guards. To remedy it, I shimmed the side-guard hinges with brass sheets so the guards tipped in just slightly. They have never rubbed since.



                When you turn, not only could your wheels be flexing, but so could your side guards (from your hip's pressure) causing them to rub.
                Last edited by chasmengr; 28 Aug 2012, 1:56 AM.
                Chas
                TiLite TR3
                Dual-Axle TR3 with RioMobility DragonFly
                I am a person with mild/moderate hexaparesis (impaired movement in 4 limbs, head, & torso) caused by RRMS w/TM C7&T7 incomplete.

                "I know you think you understand what you thought I said, but what I don't think you realize is that what you heard is not what I meant."
                <
                UNKNOWN AUTHOR>

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                  #23
                  Originally posted by stephen212 View Post
                  I think I followed that. What kind of wheels are you running on your Oracing?
                  I'm a SpinergyLX guy, 2 sets - the set with treaded tires do it more than the set with SpeedRuns. I love Dino's but just can't justify more wheels.

                  Those Schmicking axles you posted are used on their rugby chairs and are indestructible. The SpinergySLX's with the 5/8" axles have experience catastrophic failures (very rare) but that Schmicking design is bullet-proof.

                  As for Chrissy's comment about cambered wheels only doing it I would gamble otherwise. The user weight plays a big factor, as well as the torsional strength of the rim and spoke tension - my chrome hubbed/white spoked LXs (not a factory option) do it far more because whoever built them when they came on my Lasher barely tightened the spokes at all.

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                    #24
                    I think we have a winner!



                    Sounds good to me!

                    And accounts for my direct experience, as well.

                    Thanks!


                    Originally posted by ChrissyOT View Post
                    The only way this can happen is if your chair has camber, so I'm guessing you probably have some.

                    When you turn, the distance between the wheels at the bottom gets larger, because wheelchair tires are treaded to go straight. When you turn, the tire "wants" to go straight while the tires are actually turning, which causes the wheels to move apart, which effectively increases your camber by a few degrees - until you negate it by going straight again.

                    It is for the same reason that wheelchair wheels tend to squeak on hard surfaces while not squeaking when going straight.

                    Your tires will probably do this more on carpet (as the tires can't just slide about) and less on hard surfaces.

                    I don't know if I explained that in a way that makes any sense, so... you'll let me know I am sure. All the wheelchairs I've ever had have done this.
                    "I have great faith in fools; self-confidence my friends call it." - Edgar Allen Poe

                    "If you only know your side of an issue, you know nothing." -John Stuart Mill, On Liberty

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                      #25
                      Originally posted by ChrissyOT View Post
                      When you turn, the distance between the wheels at the bottom gets larger, because wheelchair tires are treaded to go straight. When you turn, the tire "wants" to go straight while the tires are actually turning, which causes the wheels to move apart, which effectively increases your camber by a few degrees - until you negate it by going straight again.

                      Isn't that what I tried to say?

                      I knew I should have added an OT to my screen name when I joined.

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                        #26
                        Originally posted by tooley View Post
                        Isn't that what I tried to say?

                        I knew I should have added an OT to my screen name when I joined.

                        Yeah, you said it, but I didn't read it.

                        My bad.
                        "I have great faith in fools; self-confidence my friends call it." - Edgar Allen Poe

                        "If you only know your side of an issue, you know nothing." -John Stuart Mill, On Liberty

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                          #27
                          heh, I'm just breaking balls...

                          good question/discussion though. Made me think about why this happens (especially for those times when I get my thumb caught in between there.

                          We really do need to start wiki'ing some of these things... chas?

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                            #28
                            Originally posted by tooley View Post
                            We really do need to start wiki'ing some of these things... chas?
                            I am only partially tech literate (I'm retired you know). What is wiki'ing?
                            Chas
                            TiLite TR3
                            Dual-Axle TR3 with RioMobility DragonFly
                            I am a person with mild/moderate hexaparesis (impaired movement in 4 limbs, head, & torso) caused by RRMS w/TM C7&T7 incomplete.

                            "I know you think you understand what you thought I said, but what I don't think you realize is that what you heard is not what I meant."
                            <
                            UNKNOWN AUTHOR>

                            Comment


                              #29
                              I thought wise had something setup so as to create CC's own version of Wikipedia. I haven't really looked into it, just noticed one day looking at other user profiles - "Wiki Contributions: x"

                              Seems like a great way to spend time, creating your very own Encyclopedia Chastanica.

                              Comment


                                #30
                                Originally posted by tooley View Post
                                I thought wise had something setup so as to create CC's own version of Wikipedia. I haven't really looked into it, just noticed one day looking at other user profiles - "Wiki Contributions: x"

                                Seems like a great way to spend time, creating your very own Encyclopedia Chastanica.
                                On the bottom of the page:

                                'CC Wiki' powered by VaultWiki 2.5.0
                                "I have great faith in fools; self-confidence my friends call it." - Edgar Allen Poe

                                "If you only know your side of an issue, you know nothing." -John Stuart Mill, On Liberty

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