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-   -   Help me Understand the 3.21 vs 3.73 vs 4.10 Axle Ratio on JKs (http://www.jeepforum.com/forum/f96/help-me-understand-3-21-vs-3-73-vs-4-10-axle-ratio-jks-1417335/)

TrailRated2011 09-15-2012 04:29 AM

Help me Understand the 3.21 vs 3.73 vs 4.10 Axle Ratio on JKs
 
I am having difficulty in understanding the concept of axle ratio as it relates to adjusting / improving my driving skills and competencies on difficult off-road trails.

Need help...please.

My 2011 JK 2dr Sport model w/manual transmission has an axle ratio of 3.21.

How does its handling and performance off-road differ from a JK with 3.73 or 4.10?

What's the difference, if any, between the expression "crawl ratio" and "axle ratio"?

When I am observing other drivers on the trail ahead of me (with 3.73 or 4.10---eg. Unlimited / Rubicon) approach and overcome an obstacle in 4L, how does my 3.21 gear ratio figure into my strategy when I take on the same obstacle? What considerations, limitations, adjustments do I have to consider in my approach on the same obstacle?

To-date I've found my 3.21 doesn't "crawl" as slowly as I'd like---if I'm in a precarious situation---when I want to creep in 4L it seems that as I slowly let out the clutch, my JK still jack-rabbits forward.

Also, when in 4L when going down a really steep slope...by the time I get to the bottom my speed is still fast enough to bang the front end of my JK----if the slope levels out abruptly.

RockyClymer 09-15-2012 10:05 AM

I don't post too often, but thought I would try and give some help here.

Axle ratio is simply that, the number of times the drive shaft turns to make one turn of the tire (or axle shaft if you prefer). So your rigs drive shaft will have to turn 3.21 times for one tire rotation. So an axle ratio of 4.10 would mean the drive shaft has to turn 4.1 times for a complete revolution of the tire....that is almost one more turn for the same tire rotation! Quite a bit of difference when you think of it, the result is that your engine will run faster for the same speed, or, by running the engine slower, you will get a much slower wheel rotation and hence be able to "crawl" slower for the same speed than when using a 3.21...

Crawl ratio is simply the combination of the lowest transmission ratio in low range, the ratio of the transfer case in low range and the axle ratio discussed above. Simply stated, the lower the combination (obtained by the multiplication of all three) the "higher" the crawl ratio...as above, this means that for a given engine speed (rpm) the vehicle will go or crawl slower. An example (for my JK) first gear in low range is 4.46, I have installed a RockTrac 4.0 transfer case and my axle ratio is 4.88 - so, multiplying these, 4.46 x 4 x 4.88 = 87, my crawl ratio is 87. On your vehicle, and making some assumptions, your first low gear is 4.46 (same as mine on the 6 spd) your transfer case is 2.7 (std JK) and your axle ratio is 3.21. 4.46 x 2.7 x 3.21 = 38, so your crawl ratio is 38 compared to my 87 or about 43% of 87....this roughly means that for the same engine rpm, my vehicle will go a little less than half the speed yours will....this is a significant difference when rock crawling.

Third point, what does it mean? Could mean a lot of things, but a few basics. Going slower when rock crawling gives a lot more control over wheel placement and and tire speed control...that is not "jack rabbiting" as you explained it. You gain a lot of tire spin control with the higher crawl ratio just through throttle control. Going slower can allow your tires to "dig" in or "bite" at the rock or obstacle instead of just spinning. Going downhill the higher gear ratio will allow the engine compression to provide much more braking effect...again, more control using the throttle.

This is a real short story, so hope it helps. For more info, discussion you may PM me.

222Doc 09-15-2012 10:28 AM

Well said. Rocky

with 3:21 you have about street gears. If you use a bigger tire than stock everthing you are having issue with will get worse as well. Most the issue you are having in "rocks" as it gets harder, is where a Rubi transfer box comes to play.

Cheepest fix is get the the highest gears you can live with on road.

Not the cheep way but get a Atlas, have a few extra loons. get a 2speed atlas then you can have 1low and 2 low.

spyder6 09-15-2012 04:13 PM

you must also recognize that there is a difference in ratios in the transfercases as well. as stated above, your Jeep has the NP241 with a 2.73:1 Ratio for low range. Rubicons come with the NP241OR with a 4:1 ratio low range.

there are aftermarket transfercases like the Atlas 2 that can be geared for a 10:1 reduction.

also what was not mentioned is that the lower gears (higher numeriacally) also have a multipication effect on torque output. you start throwing mathamatics into the mix and suddenly you have a lot of theoretical lbs-ft of torque.

perryhooah 09-15-2012 06:35 PM

Rockyclimer (?) said it well.. but basically, with 3.21 gears, you have a higher top speed on the highway, and not a lot of torque for pulling. That being said, to reiterate previous post, at low engine speed, you will still move faster with less pulling power and more (potential for) wheelspin.

I have a 6spd and 3.73. Mine will not go as fast as yours on the highway, but has a little more torque at the wheels, meaning more low-end power and less wheelspin on the trail.

the 4.10 is the next step (and from the factory, the final) to lower top speed and more torque.

There is also the math part about transfer case and transmission ratios, to calculate engine rpms and final output per engine rev, etc. but this is quick down and dirty ratio basics. Mostly just trying to simplify prev post into newbie terms--NO OFFENSE ROCKYCLYMER!!! IMHO, before you drop in new transfer case, while a good option, would look into lower gears such as the 4.10 or lower. depending on the shop, changing those varies (based on forum info) by location from $800 to $2600 for front and rear axles, regardless of ratio-most of that is labor. I am not sure what the Atlas costs, but i am pretty sure that the case and labor would be dropping a fair amount of money on it.

My Jeep is 08 JKU, with 6 sp and 3.73, and i would like to go to 4.56s at some point, for 35s... but that may be a while yet.

Hope this and prev posts help!

TrailRated2011 09-16-2012 05:10 AM

Thank you all, sincerely appreciate your efforts.

Given my existing axle / crawl ratio --- What off-road driving tips, tricks, and techniques do you suggest that I can use when I'm off-roading on difficult trails (rock clusters, tight turns between obstacles, deep mud, steep slopes).

I'm assuming that I won't have the same vehicle-assisted crawl control as the Rubis and Unlimited that I'm following through obtacles---therefore different driving techniques and strategies.

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perryhooah 09-16-2012 11:46 AM

you should have the hill assist (which i think you are referring to as "crawl control") as it is a pretty standard option, although, with your 3.21s, i am not sure. the 3.21 is capable, once you learn to use it, but is meant for the "soccer-mom" set or for the commuter who wants mileage and highway speed, rather than off-roadability. NOT saying it can't, just not the ideal setup. only advice is to get grippy/high-traction tires (to reduce slippage) if you can afford it. Also, just go slow, and be gentle goosing the throttle over obstacles... if you ARE into offroading a good bit, then you may want to consider regearing... again, kinda pricey, and you lose some mpg/top end... otherwise, just experiment with throttle control yourself and see what works for you!

RockyClymer 09-16-2012 12:15 PM

I would not try myself to address all the circumstances you list, but I will offer my experience when it comes to throttle control with a JK on a steep decent. Remember the JK is a drive by wire system, that is, there is no direct linkage between the gas pedal and the throttle body on the engine. The foot pedal just sends a signal to the computer which in turn interprets that, along with other collected information, as a setting for the throttle. My experience, and I have taught this to other JK drivers which report an increased level of throttle control when making a steep decent.
Simply put, how to "talk" to the vehicle computer! With no keyboard or mouse, the only thing you need is the brake and gas pedals believe it or not. The computer monitors these two inputs and reacts accordingly in a steep decent scenario...so all you have to know is how to
using these two inputs to convey what you want to do to the computer.

When starting a steep decent, if you stay off the gas the throttle will stay very closed and the engine will tend to run slow. Now with the gearing you have, the engine may speed up, so this may not totally apply. In fact, the engine may run too slow, esp with low gearing, and you might want to speed the engine up a bit. So, you tap the gas pedal sending a request to the computer to speed up and the engine speeds up, in fact, when you take your foot off the gas pedal, the engine will stay at the higher rpm and will not return to the slower speed you were at! What you did was "tell" the computer you wanted a higher rpm and the computer answered your request....but, it thinks you want to stay at the higher rpm and will not return to the slower speed when you take your foot off the gas pedal....so, how do you "tell" the computer you want to slow down again? Simple, just tap the brake pedal and send that request to the computer...and it will now know that you want to go slower again and the throttle setting will return to a lower level. Sounds funny, but it works every time for me and others that I have gone through this scenario under real driving conditions...

The actual way the system "works" may be totally different than I describe, but I am describing the way it "behaves." Enough BS for now.

PS - this is for a manual tranny only - no experience with an auto.

DoctorTim 09-16-2012 12:26 PM

this guy deserves a thumbs up from OP

perryhooah 09-16-2012 12:46 PM

that is the best description of hill assist i have seen!!!

JIMBOX 09-16-2012 01:27 PM

:cool: Withj an auto--it'll all depend on your diff. ratios and xfer case gearing-

With extreme gearing you can ascend/descend a lotta hills/rocks that would normally NEED 4lo app's

:tea: JIMBO

HappyTrails 09-16-2012 10:02 PM

5.38's and 4LO (Rubicon t-case) = who needs brakes while descending steep slopes? You might even need to give it gas to keep it rolling. :laugh:

cdnroadwarrior 09-16-2012 10:28 PM

Quote:

Originally Posted by TrailRated2011 (Post 14172907)
Thank you all, sincerely appreciate your efforts.

Given my existing axle / crawl ratio --- What off-road driving tips, tricks, and techniques do you suggest that I can use when I'm off-roading on difficult trails (rock clusters, tight turns between obstacles, deep mud, steep slopes).

I'm assuming that I won't have the same vehicle-assisted crawl control as the Rubis and Unlimited that I'm following through obtacles---therefore different driving techniques and strategies.

Sent from my iPhone using JeepForum

I've got a 3.21 running on 32's. Like the others have stated above, your crawl speed will be faster than most others on the trail. I spend more time then my friends in 4L, but that's fine. When you come up to an obstacle, take the time to analyze it and get out for a look if in doubt. Plan the best approach and remember, momentum is your friend.

TrailRated2011 09-16-2012 11:31 PM

Wow! Thank you all.
This entire thread has helped big time --- surpassed my expectations. All the best to you all. Mark

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TrailRated2011 06-16-2013 08:38 AM

My 2011 JK 2dr Sport model w/manual transmission has an axle ratio of 3.21. Would simply buying and installing a Terra Low 4:1 system be a 100% remedy to my current crawl being way too fast? Pros/Cons? Would doing this just create another issue somewhere else?

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