Which transfer case is best?

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    steffen707

    steffen707 Well-Known Member

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    I have earned 0 out of 5 stars in off-roading, so i'll probably end up with a Rubicon after a short lease. I like the "most stuff is already done" for the build.
    As always, thanks for the help @Malarkey21 and @homerun
     
  2. 5JeepsAz

    5JeepsAz Well-Known Member

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    Gone Resto update... I'm considering adding overdrive to my 64 Gladiator. It would add one shift lever and offers 6 gears because it adds a high low to 1,2,&3. Not sure what I'm going to actually do; fun to think about. Anyway found cool pics!

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  3. Ole Cowboy

    Ole Cowboy Well-Known Member

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    Rock Crawling Index:

    The Jeep Rubicon 6 sp trans the ideal ratio would be about 66:1. This becomes our point of reference based upon the Rubcon with std trans.

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

    As you head towards a Crawl Ratio 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 effect changes will make. This gives a relative 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.

    Throw on a set of BFG KM2's in 37x12:50 and life will be a LOT better with a 4:88 axle ratio, everything else staying the same.
     
    SleepyJeep and steffen707 like this.
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