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Look at the Tail Wheel on this Bot — Parallax Forums

Look at the Tail Wheel on this Bot

Martin_HMartin_H Posts: 4,051
edited 2012-03-02 12:28 in Robotics
My recent Ebay suggestions came up with this item:

http://www.ebay.com/itm/Smart-car-chassis-car-body-tracking-tracing-robot-car-/390348701968?_trksid=p5197.m185&_trkparms=algo%3DSIC.NPJS%26itu%3DI%252BUA%26otn%3D12%26pmod%3D170778881346%26po%3DLVI%26ps%3D63%26clkid%33D6682998385877695355

That tail wheel looks suspiciously similar to the one on this bot:

attachment.php?attachmentid=89399

I guess hardware store ball bearing casters are popular.

Comments

  • TtailspinTtailspin Posts: 1,326
    edited 2012-02-29 18:12
    Thats not a caster... that is a "universal wheel"....

    Have they no shame? casters... Bleh.:sick:

    -Tommy
  • photomankcphotomankc Posts: 943
    edited 2012-03-01 07:06
    Well they are inexpensive and easy to find, but my experiance with one thus far is not positive. It isn't grippy enough to get back into alignment on smooth floors but is just grippy enough to try and push the butt-end in whatever direction it's pointing. I keep seeing my bot dragging it around nearly sideways on the tile floor.
  • ercoerco Posts: 20,255
    edited 2012-03-01 09:46
    +1

    Stick with rubber wheel casters, they run true and track better. Counterintuitive, but true.
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2012-03-01 10:10
    It's all about weight and how it's distributed. The ideal caster has little or no influence at all, and is just there to keep the base from tipping over.

    Nothing beats 4WD where the wheels are close-coupled, like on a Bobcat, and this (sadly no longer available) base:

    http://www.budgetrobotics.com/item/Rigel---4WD-Robot-Base-233

    At $30, from China no less, that eBay base is way overpriced. Those are DAGU wheels and motors that cost under $15 in the US for the pair, coupled with a $1.50 caster. The extruded acrylic (ack!) base is about $5, tops, if you were to have it custom-cut. And they want $10 more for shipping.

    -- Gordon
  • Martin_HMartin_H Posts: 4,051
    edited 2012-03-01 10:29
    GordonMcComb wrote: »
    Nothing beats 4WD where the wheels are close-coupled, like on a Bobcat, and this (sadly no longer available) base:

    http://www.budgetrobotics.com/item/Rigel---4WD-Robot-Base-233

    I've seen a few Rigel owner pages and it looked like a pretty sweet robot.
    GordonMcComb wrote: »
    At $30, from China no less, that eBay base is way overpriced. Those are DAGU wheels and motors that cost under $15 in the US for the pair, coupled with a $1.50 caster. The extruded acrylic (ack!) base is about $5, tops, if you were to have it custom-cut. And they want $10 more for shipping.

    No kidding.
  • ercoerco Posts: 20,255
    edited 2012-03-01 10:55
    GordonMcComb wrote: »
    The ideal caster has little or no influence at all...

    Wow. That makes me an ideal caster!
  • Martin_HMartin_H Posts: 4,051
    edited 2012-03-01 17:47
    In case anyone wants to see a video of Gordon's Rigel in operation:

    [video]www.youtube.com/watch?v=VmtA_SomdVI[/video]

    Gordon, couldn't the Tamiya spiked tires (http://www.pololu.com/catalog/product/1687) work for this bot to replace your now defuncted source?


    Update: I think this chassis kit wins as the worst value:

    http://www.engagerc.com/servlet/the-2608/EngageRC%2C-2WD-Light-Weight/Detail
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2012-03-02 11:58
    Hadn't seen that video before. Looks like it has trouble with the height of some of those obstacles, though the purpose of their demo is to detect the proverbial "stuck bot on an incline" problem. My favorite obstacle testing involves old socks and lumps of tee-shirts scattered on the floor.

    If you look more closely at RIT's construction choice, you'll see they aimed the servos away from each other, giving the wheels a wider coupling. That wasn't the intention of the base, though it would still work over many kinds of surfaces. The wheels were meant to be as closely coupled as possible, with the servo shafts on each side closest to one another.

    I'm not sure if Tamiya's wheels will work, but I know anything with any amount of stiction is a no-go. The "nubs" on the Rigel wheels is what made it work. Those were terrific wheels when I could get 'em. Since those problems with paint lead in toys it's been harder for guys like me to be able to buy decent wholesale toys from China. About 2/3rds of the wholesalers in the LA area stopped importing anything but the cheapest toys for the dollar stores. No more tanks, friction cars, nothin'. And not working at a toy maker, like some of us here, I have no access to anything better.

    Erco, irrelevancy is the common refrain of the married male! Swiveling in the wind is about as productive as the other ____ in the wind activities.

    -- Gordon
  • ercoerco Posts: 20,255
    edited 2012-03-02 12:28
    Martin_H wrote: »
    Update: I think this chassis kit wins as the worst value:

    http://www.engagerc.com/servlet/the-2608/EngageRC%2C-2WD-Light-Weight/Detail

    But Martin_H: It is

    2. Light weight and beautiful !!!
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