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Thread: Wheelchair physics -- need an explanation

  1. #11
    Have you recently become wider (pushing the side guards out) via extra clothing, cushion change, or weight gain?
    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>

  2. #12
    It would be informative if others examined the distance between the tops of their tires relative to some stationary marker on the frame of their chair (e.g., side guards) and see if they, too, observe this phenomenon (regardless if the tires are making contact with anything). I suspect this is a near-universal phenomenon.

    Just the same, I'll give my chair a more studied look-over tomorrow with fresher eyes.

  3. #13
    Quote Originally Posted by chasmengr View Post
    Have you checked the toe?
    On my list for tomorrow.

    Quote Originally Posted by chasmengr View Post
    Have you recently become wider (pushing the side guards out) via extra clothing or weight gain?
    I beg your pardon, sir! Even if I put on weight, which I suspect is highly unlikely, it doesn't go to my hips.

  4. #14
    Quote Originally Posted by stephen212 View Post
    I beg your pardon, sir! Even if I put on weight, which I suspect is highly unlikely, it doesn't go to my hips.
    Just brainstorming - long distance dx is tricky.
    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>

  5. #15
    Senior Member tooley's Avatar
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    Quote Originally Posted by stephen212 View Post
    On my list for tomorrow.
    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.

  6. #16

  7. #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.

  8. #18
    Quote 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?

  9. #19
    Quote 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.

  10. #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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