1. Registration trouble? Please use the "Contact Us" link at the bottom right corner of the page and your issue will be resolved.
    Dismiss Notice

Rolling Radius With A 33” Tire

Discussion in 'Early CJ5 and CJ6 Tech' started by zinertia2, Aug 5, 2020.

  1. Aug 6, 2020
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
    Joined:
    Jul 18, 2013
    Messages:
    5,593
    I am honestly asking as I do not know or claim to know these details in anyway...does the wheel speed change because the tire diameter changes, or is it because rolling friction of the low air pressure tire causes it to do something different that the wheel speed sensor picks up?
     
  2. Aug 6, 2020
    Mcruff

    Mcruff Earlycj5 Machinist

    Albertville, AL
    Joined:
    Sep 22, 2002
    Messages:
    5,349
    Yes the wheel speed changes just enough from lower pressure for the speed sensor to pick it up. If you have traction control on the newer cars they sometimes go nuts when going around a corner with a little to much power and set off the traction control from speed sensor input varying from side to side.
     
  3. Aug 6, 2020
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member

    Bozeman, MT
    Joined:
    Jul 18, 2013
    Messages:
    5,593
    so back to the OP/Howard's discussion - does the wheel speed change because of the radius or because of the friction?
     
  4. Aug 6, 2020
    Mcruff

    Mcruff Earlycj5 Machinist

    Albertville, AL
    Joined:
    Sep 22, 2002
    Messages:
    5,349
    Probably a combination of both. As the diameter changes the friction will increase as well as the axle slowing down from the rolling radius changing.
     
  5. Aug 6, 2020
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
    Joined:
    Aug 3, 2003
    Messages:
    9,754
    I believe this is correct.
     
  6. Aug 6, 2020
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
    Joined:
    Aug 13, 2015
    Messages:
    6,656
    :pickme:

    I’ll have 30 minutes tomorrow morning while waiting for seam sealer to set up.
     
    Fireball likes this.
  7. Aug 7, 2020
    Buildflycrash

    Buildflycrash More or Less in Line. 2024 Sponsor 2022 Sponsor

    Gulf Breeze FL...
    Joined:
    Jun 5, 2007
    Messages:
    3,559
    You guys and your math!
    I used a GPS app on my phone. 40= 42
    That’s all I need to know.
     
  8. Aug 7, 2020
    Warloch

    Warloch Did you say Flattie??? Staff Member

    Falcon, CO
    Joined:
    Mar 4, 2003
    Messages:
    5,466
    I used a programmable speedo - Drive one mile and push buttons. Got me within .2 MPH at 50, so call that good.
     
  9. Aug 7, 2020
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
    Joined:
    Aug 13, 2015
    Messages:
    6,656
    If nobody gets to the math, I’ll do it later... I’ve got a build to get back to. :sneak:

    This is the 33x12.50 tire(s) mentioned earlier.
    I started with this baseline. If I backed up and it didn’t line up perfectly, I jacked it up and repositioned the tire.

    [​IMG]

    Results. I did three tire rotations so I’d get definitive results. Divide the distances by three to get numbers for a single rotation.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
    FinoCJ likes this.
  10. Aug 7, 2020
    Glenn

    Glenn Kinda grumpy old man Staff Member

    Apopka, Fl
    Joined:
    Sep 21, 2002
    Messages:
    12,367
    What is your intended vehicle usage? What type of vehicle are you putting the tires on? 33 inch tires aren't all the same diameter. Tread design creates different rolling resistance. How much same speed driving will you do? As your tires wear the diameter decreases. Rim width can make a difference also. Just throwing this out there since you are apparently focused on a 33 inch diameter tire.
     
  11. Aug 7, 2020
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
    Joined:
    Nov 22, 2003
    Messages:
    8,102
    OK I guess I've been Told. :)

    Please ignore everything I've ever posted.:(

    But returning to the OP's original post I still don't think a , say 10%, reduction in loaded rolling radius represents a 10% reduction in tire circumference for purposes of speedometer calibration. :schooled:
     
  12. Aug 7, 2020
    CJ Joe

    CJ Joe Truckhaven Tough!

    Pinon Hills, CA
    Joined:
    Nov 9, 2002
    Messages:
    390
    You guys are funny. I just let some air out of the tires and go wheeling.
     
  13. Aug 7, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
    Joined:
    Aug 10, 2003
    Messages:
    23,596
    Interesting result. Did you measure the overall height of the tire at each pressure? That's where the math comes in.

    I think the main point here is not that you can detect a change, but what people attribute that change to. The change in footprint due to lower pressure brings the axle closer to the ground, but I would contend that's not what's changing. Instead, the tread is lengthening because the entire tire is blowing up like a balloon.

    A 33" tire has a nominal circumference of ca 104". A one inch change in 3 revolutions is a change in circumference of 1/3 of an inch, or about 0.32%. If the height of the tire changes by an inch that's a reduction in rolling length by about 3", and over 3 revolutions you'd see a difference of 9" or so, nearly an order of magnitude different.
     
  14. Aug 7, 2020
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

    Liberty Lake, WA
    Joined:
    Aug 13, 2015
    Messages:
    6,656
    I didn’t. I was trying to hurry because my jeep was leaking oil all over the pavement. :D

    While I was doing this, I kept thinking about a tank. If you took tank tracks and expanded them into a circle, what would the test results show?
     
  15. Aug 7, 2020
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
    Joined:
    Aug 10, 2003
    Messages:
    23,596
    Bien sur, mon ami! (Been talking to a French colleague... puts my mind in that frame.)
     
  16. Aug 7, 2020
    Howard Eisenhauer

    Howard Eisenhauer Administrator Staff Member

    Tantallon, Nova...
    Joined:
    Nov 22, 2003
    Messages:
    8,102
    I guess it depends on what pressure they're inflated to :twist:
     
    timgr likes this.
  17. Aug 7, 2020
    zinertia2

    zinertia2 "and which part are you keeping?" 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Greencastle , IN
    Joined:
    Feb 15, 2018
    Messages:
    184
    Glen and Howard
    The vehicle I’m working has some details as noted in my “signature”.

    Unfortunately its about 3 hours to Glamis, or about the same to the dunes at Devil’s Playground or Joshua Tree, about 2 hours to Ocotillo Wells area.

    Around here - sand, dirt roads, sandy roads and in rare moments possibly some snow. Highway tire pressure and higher rpm for hours to get anywhere!

    I will be spending significant time on highways here in So Cal. :cry:
    Just trying to obtain enough info to decide (brand x right now) if it is to be a 33” or 33.5” or 33.8” tire to start to hit my optimum numbers.
    Will likely guess in the end.....note my vehicle is probably heavier w the frame boxing, full cage, dana 44 front and dana 60 rear than most( too much left of my SCORE racing in the 70s I guess)

    Dennis
     
  18. Aug 7, 2020
    Glenn

    Glenn Kinda grumpy old man Staff Member

    Apopka, Fl
    Joined:
    Sep 21, 2002
    Messages:
    12,367
    It would show someone obviously has too much time on their hands.
     
    timgr and ITLKSEZ like this.
  19. Aug 7, 2020
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
    Joined:
    Aug 3, 2003
    Messages:
    9,754
    Moi aussi.

    But our jeeps are not products of the digital age with nano-particle binary yes/no answers.

    Close is Good Enough, IMHO. Better, even...if you understand the concept of 'Anti-Fragility' and its value in these times of upheaval.
     
    gromit and Ns0mniac like this.
  20. Aug 7, 2020
    Glenn

    Glenn Kinda grumpy old man Staff Member

    Apopka, Fl
    Joined:
    Sep 21, 2002
    Messages:
    12,367
    It seems this narrows things down to an all terrain radial then.
     
New Posts