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Thread: Tire size, opinions:)

  1. #1
    Junior Member Shelly1's Avatar
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    Talking Tire size, opinions:)

    I'm new to the site...have 2010 Wrangler Unlimited. Awaiting my 4" suspension lift. My opinion request for tire size. 35" or 37"? I want big but not obnoxious, it's my daily driver.
    Thanx!
    Shelly

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    F it! It ain't my Jeep. Ricky Booby's Avatar
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    I would say 35s. Especially if it isn't a Rubicon, and has a Dana 30 in the front. Just my .02

    Bobby
    www.pnwjeep.com

    Quote Originally Posted by Walter View Post
    Alcohol, Tobacco and Firearms should be a convienent store, not a government agency.

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    Devil Doc Navydoc8778's Avatar
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    I would agree with him. That axle tends to do funny things once you get past a 35 and do much if any offroading.

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    Senior Member SavageSun4x4's Avatar
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    Shell,
    Tire size is driven by a series of factors: Axle strength, gear ratio and transmission (auto vs std) and TORQUE curve of the engine.

    Tell you straight up, if you don't have a Rubicon 37's are out and if you do have a Rubicon + an auto trans, forget them. 37's will overload your axle capacity and the auto trans and axle ratio will slow you down to a crawl both in town and on the highway. In fact so bad that it becomes a safety factor in driving your Jeep.

    35's: Outer limits of the OEM axle configuration of a non Rubicon rig.

    33's: Detriment to the overall performance of your non Rubi rig but sufferable, loss of mpg.

    This topic comes up a lot. SO if you want to see the effect tires (height) will have on your rig turn to the RCI (Rock Crawling Index) and its very accurate in projecting performance results on most rigs.

    To see the charts on engine torque curves then go to here: http://www.savagesun4x4.com/enter/te...x_rci_dis.html

    RCI:

    The Jeep Rubicon with OEM 31 in tires and using the 4.0L Jeep engine and 5 sp trans the ideal RCI ratio would be about 66:1.

    NOW you have to factor: tire size (diameter in inches) & RPM width of engine & HP curve & torque curve & weight of rig. This is where it gets dicey.

    As you head towards a Crawl Ratio (CR) of 100:1 and beyond you are going to start needing some deep engine rpms, but you can mitigate that with taller tires, but then where is the torque/hp curve?
    1) From a drive-train viewpoint, ideally you want each set of gears (transmission, transfer case and rear axle) to within .5 +/- .5 of each other. Example: Jeep Rubicon, 4:1 1st gear, 4:1 transfer case, 4.1: rear axle, this is EXCELLENT! Why is this? Strength and equal distribution – multiplication of torque in the driveline! Each component is of equal relative strength and torque multiplication is equal thru the drive train (4 x 4 x 4.1 = 66). Is it acceptable to exceed these values? Certainly, just remember added stress on components will require increased strengths and may lead to premature failure of certain components.

    2) Why does the Rubicon do so well off road and especially in the rocks? Crawl ratio is balanced with the ability of the Jeep engine (RPM width, torque curve, hp curve) and the Jeeps weight and it all comes together quite nicely.

    You have to consider all the factors. Pull the 4.0L out of the Rubicon and put in a good engine, say a high winding Chevy that does not get torquey until 2500+ RPM and its a whole different ball game.

    The 4.0L delivers about 75% of its torque just past idle. Start going much beyond 3200 RPM and the engine is out of breath.
    I like to use the RCI (Rock Crawling Index) to see what affect changes will make. This gives a realitive score based upon a known performance index and it factors in tire size.

    TM = TransMission (driven gear ratio, 1, 2, 3, etc)
    TC = Transfer Case
    AR = Axle Ratio
    CR = Crawl Ratio
    TD = Tire Diameter in inches

    RI = Rubicon Index (2.12) Note this is a not fixed number as it is based upon OEM specs for the Rubicon w/NV3550 transmission. You could use anything you wanted, but the Rubicon is well known and this provides an index of known performance on which you can compare.


    RCI = Rock Crawling Index
    TM x TC x AR = CR
    CR divided by TD = RI
    RCI = RI / 2.12
    TM (transmission ratio) = 4.01 < enter data
    TC (transfercase ratio) = 4.00 < enter data
    AR (axle ratio) = 4.10 < enter data
    CR (crawl ratio) = 65.76 TM x TC x AR
    TD (tire diameter, inches) = 31.00 < enter data


    CR 65.76 / TD (31) = 2.12 RI (Rubicon Index)


    4:1 TM x 4:1 TC x 4.1 AR = 66 CR/31 TD = 2.12 = OEM Jeep Rubicon



    RCI = 2.12 (rock crawling index FOR the Rubicon), now enter your data from your rig and compare with the Rubicon or ???? what ever you chose.

    NOTE: as you go up in tire size you need to increase the index slightly to compensate for the added mass the larger tires bring. Start adding extra weight, wider tires (increased CoF), trail tools etc etc and the OEM Rubicon 2.12 will not cut it so the RCI has to continually drift upward.

    Last edited by SavageSun4x4; 01-10-2011 at 11:51.
    Don

    '15 Jeep Unlimited Rubicon Hard Rock...OLD SCHOOL 6 Spd Manual Trans
    17 Oaks Ranch Companies LLC

    www.savagesun4x4.com

  5. #5
    High Speed, Low Drag XtrmTJ's Avatar
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    ^^^ Yea, ... What he said ! ^^^
    Most people go through life wondering if they have made a difference, ... United States Marines, do not have that problem. Pres. R. Reagan 1985

    I am a Sheep Dog, ... Wolves Beware!

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    Member JimhatTJ's Avatar
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    he said 2010 so wouldnt have the 4.0.

    i thought the JK rubicons came stock with 32's?

    with a 4" lift on a JK and 35 is going to look a bit strange. just lifted a friends with 3.5" and 35's and really needs 37 to fill out the wheel wells.

    but if you have a non-rubicon model then i would not go over 35's
    03 Rubicon, 6.0L Chevy, 700R4, 241, ORI struts, custom F&R dual traiangulated longarms, 39.5 Irok, frame backhalf, 35 Spline Ford 60's, Full Hydro steering, 110"wb
    05 TJ (salvage), 6.0L aluminum head Chevy with LS1 parts (@400hp) , 4L60E, home made longarm on 14" SAW coilovers, 35 spline Dana 60's, 100"wb

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    Wheelin requires wrenchin Doppler's Avatar
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    Good info!

  8. #8
    2010 JK BigD76's Avatar
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    Quote Originally Posted by Shelly1 View Post
    I'm new to the site...have 2010 Wrangler Unlimited. Awaiting my 4" suspension lift. My opinion request for tire size. 35" or 37"? I want big but not obnoxious, it's my daily driver.
    Thanx!
    Shelly
    I have a new JK with 2.5" lift running 35's and I love it.
    Big D
    2010 Wrangler JK Rubicon
    HHB, FA SQDN, 2 CR
    Vilseck, Germany



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    Go 35's..

    you won't regret it...it will look good, fill out your fenders ok. it's a good compromise..
    37's would look better, but for a daily driver, would be too much without a regear

  10. #10
    Senior Member SavageSun4x4's Avatar
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    Quote Originally Posted by JimhatTJ View Post
    he said 2010 so wouldnt have the 4.0.
    I use the 4.0L for the example as there are far more of them out there. That said, the 3.8 V6 chart is included and the results for that engine are dismal. Why? See the chart(s)154_0706_03_z2007_jeep_wran.jpg'07 Jeep 3.8L vs '06 4.0L engines HP/Torque



    40_torquecurve-5.jpg


    The 4.0L engine is the best engine that has ever resided under the hood of a Jeep since day 1.

    The Jeep Rubicon IMO is the best and most well balanced OR rig available world wide. Sadly Chrysler Jeep has to move forward and the Jeep engine is truly a engine with 50's technology. Jeep had to grab something off the shelf and the best they could do given corporate parameters was the 3.8. The JK Rubicon is a FAIL in the driveline analysis but has made up for it at least partialy with the suspension which is a vast improvement over the TJ.

    Don't get me wrong, I don't pine for the 4.0 but would not trade it for the 3.8, but rather looking forward to the Pentastar that is due in '12.

    The V6 is dismal in its weak as tea torque curve. The 4.0L torque curve is as flat as my first girlfriend without her wonder bra.

    Whether one chooses 32, 33, 35 or 37 can be a matter of appearance or from an off roading standpoint or anywhere in between. The point here or the 'take away' is to provide an understanding of the ramifications of your choice.

    Point: Its your $, your Jeep, your pride of ride chose as you see fit and let no one cast stone. BUT chose wisely and do so by arming yourself with facts and balancing that off with your desired results.


    Our poster is located in Newport News, a city I know well, having spent 9 years on station at Ft Monroe. That said I will stand by my suggestion on 33's. Its my guess that trips down to Nags Head and other jaunts into the outer lands with 35's and beyond will drag you down to the point of unhappiness in spite of the better look.

    Few realize the effect of spinning 35/37's on a Jeep with an already anemic engine. My MTR 37's mounted on Walker Evans BL's with ORF rock crawling domes bounced off the scales at right at 132 lbs EACH x 4. The centrifugal weight of these spinning at city or highway speeds is staggering. The effect upon your ability to brake, steer or accelerate clearly puts you in the danger zone of travel with 37 and you are pushing the safety limits with 35's not counting the damage its doing to your rig. FACT, it ain't built for it and forget extended warranty coverage on the driveline. + you will consume ball joints, control arm bushings and shocks like candy; along with that goes increasing bouts of death wobble.

    Get on a kids playground small carousel or take a spin on a set of skates. Note the effect as your bring your hands into your body vs extending them. Wanna try something, then do it with a 10 lb weight in each hand...note the disastrous effect as your arms are ripped out of your shoulder sockets like those of a rag doll being drug behind Jeep.

    For those of your making the recommendations to go 35 and beyond, perhaps you can offer some engineering or facts behind your suggestion. Maybe some STOPPING distance charts or some acceleration tests.

    NOTE: I ran 37's on my rig for right at 7 years and 75 k miles, bolted to a full set of Dynatrac ProRocks with upgraded 1/2 ton brakes, Rancho shocks and HD ball joints. I know well how a Jeep performs and this is not the terrain for the novice or faint at heart.

    You now have the facts now make your choice based upon your wants and needs.
    Last edited by SavageSun4x4; 01-10-2011 at 18:45.
    Don

    '15 Jeep Unlimited Rubicon Hard Rock...OLD SCHOOL 6 Spd Manual Trans
    17 Oaks Ranch Companies LLC

    www.savagesun4x4.com

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