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Rock Crawling Index (RCI)


'07 Jeep 3.8L vs '06 4.0L engines HP/Torque

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

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.

 As you head towards a 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.

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