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Rolling Radius With A 33” Tire

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

  1. Aug 5, 2020
    zinertia2

    zinertia2 "and which part are you keeping?" 2020 Sponsor

    San Diego
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    Anyone measured the actual Rolling radius of their 33 inch tire. ( if you use one)
    Half tank gas, top, or other weight related items during measurement?
    Tires aired for freeway, etc?

    working through gear ratios and rpm, formula works best if there is an accurate/actual measured radius instead of using tire man’f stated diameter

    rpm= (mph* gear ratio*336)/tire diameter
    Half inch difference due to weight at 60 mph with a 33” tire means a 16” radius = 78rpm difference.
    Not a lot but if its an inch due to weight.......

    ignoring the more complex issues as a tire spins.

    thanks
    Dennis
     
  2. Aug 6, 2020
    boopiejones

    boopiejones Member

    California east bay
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    Probably easiest to measure the rolling circumference and then use math to back into the radius.

     
  3. Aug 6, 2020
    Jw60

    Jw60 Chim chiminey Chim chim cher-ee!

    Sedalia MO.
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    It's gonna be far less accurate for someone else to measure there tire unless It is the same exact make, model, and size.

    I could have 4ply and if you are looking at 6ply or kevlar reinforced, bias vs radial. I would run with the mfg numbers and drink another beer to feel better about it.
     
    Walt Couch and bigbendhiker like this.
  4. Aug 6, 2020
    zinertia2

    zinertia2 "and which part are you keeping?" 2020 Sponsor

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    Good point!
    :beer:
     
  5. Aug 6, 2020
    ITLKSEZ

    ITLKSEZ Volvophilic

    Post Falls, ID
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    So what’s your make/model/size? And requested air pressure?
     
  6. Aug 6, 2020
    zinertia2

    zinertia2 "and which part are you keeping?" 2020 Sponsor

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    Don't have a requested air pressure - thinking someone running a set in this size would have worked out optimum for road (assuming a road and off-road vehicle). I don’t have tires yet in my build, thats why asking for some info if avail.
    I put 37” toyos on my J2000 that I put a 4BTA w NV4500/NP205 in a few years ago and had to experiment w pressure till I got an even pattern when driving on the street.

    As noted by jw60 would ask for load range “C” tires to normalize the rigidity of the tire (or the squash factor :))

    I am working from these kinds of info to see if it makes sense to buy a 33” tire or find a 33.5” size etc,

    so the question might be better stated:

    Q- if you are running load range C 33” tires and they are aired for street driving in your early CJ5 what is the rolling radius?

    getting several folks answering with data could help me with this - even w the other variables

    Dennis
     
    Last edited: Aug 6, 2020
  7. Aug 6, 2020
    ITLKSEZ

    ITLKSEZ Volvophilic

    Post Falls, ID
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    I have a set of 33x12.50-15 Goodyear Wrangler MT (2nd gen) on my 3b I could check on pavement if that would help.
     
  8. Aug 6, 2020
    zinertia2

    zinertia2 "and which part are you keeping?" 2020 Sponsor

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    That would be perfect, thank you!
     
    ITLKSEZ likes this.
  9. Aug 6, 2020
    ITLKSEZ

    ITLKSEZ Volvophilic

    Post Falls, ID
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    I got 98 1/2” at 20psi. Tread is 98% (only driven on pavement about 5 miles). The standing height of these tires on my 3b is 31” from the ground to the top of the tread.
     
  10. Aug 6, 2020
    timgr

    timgr Eppur si muove. 2020 Sponsor 2019 Sponsor

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    The circumference is not going to change with weight, inflation, etc.

    If you look at the tire specs at TireRack.com, it will tell you the actual diameter. Typically it's a little less than the "advertised" size. For example, a 33x10.5R15 KM3 on an 8.5" rim width (their nominal size) is listed as 32.5" in diameter.
     
  11. Aug 6, 2020
    ITLKSEZ

    ITLKSEZ Volvophilic

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    In theory, yes. But it really does change. There’s a bit of scrub that happens at lower pressures that loses a little bit of distance in each revolution.
     
  12. Aug 6, 2020
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member

    Tantallon, Nova...
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    We've been down this hole before :)

    Rolling radius doesn't affect how many times the tire revolves for a given distance, it's the tire's circumference which *will not* be 6.28 times the loaded tire radius because 6.28 only holds for a perfect circle, which a loaded tire isn't.

    There's a neat video I can't find at the moment where some guy proves this with some masking rape & chalk.

    Edit- Found it!


     
    Last edited: Aug 6, 2020
  13. Aug 6, 2020
    Mcruff

    Mcruff Earlycj5 Machinist

    Albertville, AL
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    Unless they have the exact brand of tire your looking for anything they measure could be off by as much as a 100 rpm. Not all listed 33” tires are 33” in diameter, different ply numbers, different load ratings, different tread depth. If your getting picky over 75-100rpm you need the exact same tire and loading on the vehicle.
     
  14. Aug 6, 2020
    zinertia2

    zinertia2 "and which part are you keeping?" 2020 Sponsor

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    Howard
    Good point the circumference is constant fir a given tire!

    Mcruff
    True, but gives me an “idea” if someone provides their specific tire data (like ITLKSEZ did)

    ran Armstrong Tru Trac tires on my first M38A1 in ‘72, noted then when I got their contact patch reasonable on the road by dropping from the tire store inflated pressure to a “good” pressure that for the same speed (measured mile vs that day mechanical spedo) that RPM increased slightly - a memory anyway. Was trying to see if with field info and math that I could roughly determine any difference- or faulty memory :)

    Dennis
     
  15. Aug 6, 2020
    nickmil

    nickmil In mothballs.

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    Can you explain to me how indirect TPMS (Tire Pressure Monitoring Systems) work?
     
  16. Aug 6, 2020
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member

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    Nope not at all.
     
  17. Aug 6, 2020
    termin8ed

    termin8ed I didn't do it Staff Member

    Mason, MI
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    :pickme:
     
  18. Aug 6, 2020
    timgr

    timgr Eppur si muove. 2020 Sponsor 2019 Sponsor

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    Go for it.
     
  19. Aug 6, 2020
    Mcruff

    Mcruff Earlycj5 Machinist

    Albertville, AL
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    "Indirect Tire Pressure Monitoring Systems (TPMS) are the systems that do not have air pressure sensors inside the tires. Rather, they detect a low tire by comparing relative wheel speeds via the Anti-lock Brake System (ABS) wheel speed sensors. When a tire loses air, its diameter decreases slightly."
     
  20. Aug 6, 2020
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member

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    Ok lets settle this like Gentlemen- by having flunkies sort it out :D

    Would someone, anyone, with a mounted steel belted radial sitting around please measure the tire circumference at 0, 10, 20, 30 & 40 PSI?
     

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