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Thread: Wheelchair physics -- need an explanation

  1. #41
    Senior Member tooley's Avatar
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    Quote Originally Posted by stephen212 View Post
    OTooley?
    with the O being short for Obnoxious.

    Quote Originally Posted by -scott- View Post
    sounds Irish.
    lol. you better believe it, look out when I've been drinking.


    It really makes me wonder why people are still throwing out shots in the dark at the OP's post when I firmly believe we've explained it away. Might not be as evident for some with high spoke tension on their wheels, but is obviously present for folks using wheels with low spoke count.

  2. #42
    There's no good way to get around the physics of the wheel "scrub." When turning in place, it generates forces that create deflection. Essentially, the only way to eliminate the aforementioned scrub would be to add a LOT of camber to the wheels. When wheels are straight up and down, they like to go in the direction in which they are pointed. Think about being on carpet and turning in place while using a basketball or tennis chair. How would it compare to making the same turn in your everyday? The cambered wheels on the ball chairs are sort of always in a "turn." So, you'll feel a lot less scrub when you're turning in place.

    Of course, with a lot of camber, you can't get through doorways. Plus, if the camber isn't dialed in just right, it can induce rolling scrub (from toe-in or toe-out.) The best way I've found to get rid of sideguard rubbing is with a solid axle system. Wheel rigidity and integrity also plays into it, but a solid axle setup probably makes the biggest difference. The Schmicking axle system that was pictured is fantastic. I would love to have that on an everyday chair. My everyday rigid chair is set up with threaded axles (same as one might find on a racing chair or handcycle.) I rarely take the wheels off, and I wanted to see how much difference it makes. The difference between fixed and standard QR axles is considerable. My wheels don't deflect nearly as much, and the chair tracks a straight line MUCH better.

    Unfortunately, crappy 1/2" quick-release axles are still the industry standard. At the Schmicking factory, I did see a set of QR axles that had an interesting mechanism that would sort of "expand" the end of the axle to make it more rigid. It seemed to work pretty well, but it wasn't anywhere near as nice as their 25mm setup. I've found that a solid axle system also makes pushing more efficient. There isn't as much energy lost to rolling wheel scrub, and there's a lot less deflection when pushing on the handrims.

  3. #43
    Welcome "wheel scrub" to my wheelchair lexicon! Never heard the word scrub used this way before.

    TWL, what's involved in converting a standard axle into a threaded axle setup? Are there axle receivers that can be easily swapped out into, say, a TiLite camber tube?

    And of course you would like the photo of the Schmicking setup -- I got it from you!

  4. #44
    Senior Member lazierdog's Avatar
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    Quote Originally Posted by ChrissyOT View Post

    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.
    Your explanation was excellent, but I have to comment that most of my tire squeal is the result of underinflated or damp/wet tires. Although tire squeal is more likely to occur with more camber.

  5. #45
    Senior Member Skogy's Avatar
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    Great discussion!

    I learned a good one this weekend. I decided to make my chair a little narrower by switching from 2° to 0° so I might fit in a travel trailer my in-laws bought.

    Got done and tried out my chair with 0° camber and was amazed how much harder it pushed AND how much harder it was to turn the chair. I went 2 days to see if I would get used to it, I didn't.

    I switched back to 2° and could not believe how much nicer it turned and pushed. Wish I could try 4° but 1 door in my house is too narrow.

  6. #46
    Quote Originally Posted by stephen212 View Post
    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.

    .
    Spoke flex.

    I also have my skirt guards attached to the tube to keep them in place.

    Straight & no rub then turning & rubbing with no bearing play or lees mounting hardware = spoke flex.

  7. #47
    First I read this thread and informative.
    The "wheel scrub" is actually the result of lack of lateral stiffness. In a quest for lighter wheels one has to deal with torsional forces, vertical forces, and Lateral forces. The goal is to get the best benefit from all without sacrificing too much.

    Turning the chair deflects the rim. Factors making the rim stiffer include...
    spoke lacing. (Radial lacing pattern usually provides the best lateral stiffness.) Also benefits from the lightest design.
    Use of a stiff and deep rim which will come with shorter spokes.
    Larger spokes.
    Finally increase the bracing angle of the spokes. (Hubs with flanges.)
    Finally High tensioned spokes which will delay the moment when spokes on the opposite side of the load lose tension.

    All of this comes with some sacrifice. I would also consider bearings going bad and axles that are flexible can add to the lateral movements.

    I have noticed on my chair when turning on surfaces that have a lot of traction in relationship to the rubber and the road I can hear the spokes make noise. Also when the tires are lower in pressure the rubber can take up some of those lateral forces.

  8. #48
    Quote Originally Posted by The_Wheel_Life View Post
    There's no good way to get around the physics of the wheel "scrub." When turning in place, it generates forces that create deflection. Essentially, the only way to eliminate the aforementioned scrub would be to add a LOT of camber to the wheels. When wheels are straight up and down, they like to go in the direction in which they are pointed. Think about being on carpet and turning in place while using a basketball or tennis chair. How would it compare to making the same turn in your everyday? The cambered wheels on the ball chairs are sort of always in a "turn." So, you'll feel a lot less scrub when you're turning in place.
    You are describing what is known in bike terms as trail & flop, here is a good tool/description:

    http://yojimg.net/bike/web_tools/trailcalc.php

    We also use caster angle (kind of analogous to a bike's head tube angle) and I'm not sure if we use Ackerman in chairs due to the casters……….but the rear must, as a multi-wheeled vehicle excites a turn the insides of the wheels transcribe a tighter circle then the outsides, check out this video………but I still think your spokes are just loose……...

    https://www.youtube.com/watch?v=W8RZS85u6Sg

  9. #49
    I think outside the box. At least on hard turns your body twists. Maybe your butt is pushing the side guards out against the wheel if you have a chose fit. I kept my manual chairs as narrow as possible and this happened to me with plastic side guards. I solved the problem by going to a metal shop and getting a pair of side guards made out of 3/16" thick aluminum. Solved the problem. The guys in the shop were proud of doing a good deed for a crip and only charged me for the metal.
    You will find a guide to preserving shoulder function @
    http://www.rstce.pitt.edu/RSTCE_Reso...imb_Injury.pdf

    See my personal webpage @
    http://cccforum55.freehostia.com/

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