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post #1 of 7 Old 05-16-2011, 04:24 PM Thread Starter
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gearing question

I was getting frustrated in the rocky terrain at RC this weekend! I have stock 3.73 gears and 34" tires. I was spending so much time on the clutch trying to slow it down that I started smelling clutch! Instead of clutching I tried just braking without clutch and it worked sometimes but the majority of the time it would kill it. My buddies with Rubicons with stock 4.10's had no problems whatsoever. So I pulled the trigger and set an appointment for 4.88 gears and a rear ARB locker. My question is the transfer case. Will the diff gears get me to the point where I will stop killing it all the time or am I going to need to find a rubicon t-case to get the real low crawling power as well? How do I calculate my final gear ratio?

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post #2 of 7 Old 05-17-2011, 12:24 AM
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You'll be much better off with the gears that you are installing. Yes the 4.1 Rubi xfer case is an advantage but not as much of a big deal as axle gears imo. And don't forget that part of it is experience and skill. I'm still work in progress as far as that wheeling skill thing goes

If you decide that you have to have the lower range xfer case you can find pulled rubi cases at a deal from the guys who go Atlas.

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Last edited by TEXASKEV; 05-17-2011 at 12:35 AM.
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post #3 of 7 Old 05-17-2011, 02:07 AM
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The new lower R&P will help some when it comes to crawling but not much. R&P in the JK are done for hiway livability. With the new gears it will perform on road much better.

Sounds like you are trying to crawl. The issue is your 2.73:1 low range in your transfer case. This is the reason for a 4:1 in a rubi; it is for crawling. The 2.73 is much better for other higher speed applications. It is all according to the offroading that you do.

Gear ratios multiply together. The ratio means so many turns of the input shaft to this many turns of the output shaft. You have a first gear of 4.46:1 in your transmission; the tranfer case has a 1:1 high range and a 2.73:1 low range. You have stated you have a 3.73 R&P in your axle differential so....

4.46 x 2.73 x 3.73 = ~45.41 to one This means your motor crankshaft (RPMs) has to turn 45.41 times to turn your axles (or tires) once in your lowerst gear.

With the new R&P you get 4.46 x 2.73 x 4.88 = ~59.42:1

A rubi factory gets you 4.46 x 4 x 4.1 = ~73.14:1

My first suggestion would be to get a rubi transfer case take-off. Second would be an Atlas with a gearing close to the 4:1. Third is an Atlas 4 speed (a lot more money) this is the planetary gear of a rubicrawler made onto an Atlas 2 speed. For example, you can have 1:1, 2.73:1, 3.8:1, and 10.x:1 gear ranges with the 4 speed.

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Originally Posted by virtual_mage View Post
.....a dark forum, were people would sometimes be rude but yet be helpful.
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post #4 of 7 Old 05-17-2011, 05:12 AM Thread Starter
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Quote:
Originally Posted by toymaster View Post
The new lower R&P will help some when it comes to crawling but not much. R&P in the JK are done for hiway livability. With the new gears it will perform on road much better.

Sounds like you are trying to crawl. The issue is your 2.73:1 low range in your transfer case. This is the reason for a 4:1 in a rubi; it is for crawling. The 2.73 is much better for other higher speed applications. It is all according to the offroading that you do.

Gear ratios multiply together. The ratio means so many turns of the input shaft to this many turns of the output shaft. You have a first gear of 4.46:1 in your transmission; the tranfer case has a 1:1 high range and a 2.73:1 low range. You have stated you have a 3.73 R&P in your axle differential so....

4.46 x 2.73 x 3.73 = ~45.41 to one This means your motor crankshaft (RPMs) has to turn 45.41 times to turn your axles (or tires) once in your lowerst gear.

With the new R&P you get 4.46 x 2.73 x 4.88 = ~59.42:1

A rubi factory gets you 4.46 x 4 x 4.1 = ~73.14:1

My first suggestion would be to get a rubi transfer case take-off. Second would be an Atlas with a gearing close to the 4:1. Third is an Atlas 4 speed (a lot more money) this is the planetary gear of a rubicrawler made onto an Atlas 2 speed. For example, you can have 1:1, 2.73:1, 3.8:1, and 10.x:1 gear ranges with the 4 speed.
Thats what I was looking for! Perfect explanation. Thanks

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post #5 of 7 Old 05-17-2011, 06:04 AM
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I agree that rubi or atlas transfer case is a great way to get awesome crawl ratio but dont discount the benefit for regearing off road. I went from 3.21s to 4.88s with 34s and it was a HUGE difference off road. stuff that I had a hard time going slow enough got much easier to crawl. Perfect combo would be both but gears are going to be cheaper and get you highway performance to.

While we are on this topic, how much does a brand new rubi transfer case cost from dealer ready to install? The chance of find of a take-off transfer case is pretty slim where I live.
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post #6 of 7 Old 05-17-2011, 06:54 AM
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Quote:
Originally Posted by Olean NY Jeeper View Post
While we are on this topic, how much does a brand new rubi transfer case cost from dealer ready to install? The chance of find of a take-off transfer case is pretty slim where I live.
About as much as an Atlas...

OP, IMO go get your gears swap done. This will help out your driving off road, and on, and then when the time comes you can upgrade the tcase and you'll be that much better of a driver. Just my opinion.

You can take what toymaster said a step further.

your setup now (3.73*2.73*4.46= 45.41) with a final crawl ratio of 45.41:1

So say your trying to get over something just idling along at 1000 RPMs

1000/45.41= 22 tire RPM/ 60 = .36 tire rotations per second.
You have 34" tires so
34*3.14=106"

106*.36=38.5
38.5/12=3.2

Your speed now at 1000 rpms is about 3.2 feet per second. If you followed that then
Now with your new gears at 4.88

4.88*2.73*4.46=59.41
1000/59.41= 16.8 tire RPM/60 = .28 tire rotations per second
106"*.28=29.7
29.7/12=2.5

Your new speed at 1000 rpms would be about 2.5 fps. It doesn't look like much on paper, but its a significant differance when you actually jump back in your ride.

No doubt, in the math the place to get the super low crawl ratio is in the Tcase, but for starting out, you'll be happy, and impressed with the drivability of a new gearset in your axles. And like Olean said, they're cheaper than a new tcase.

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Thats a special kind of stupid right there.......
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post #7 of 7 Old 05-17-2011, 11:15 AM
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You don't need a new transfer case. I just did the exact same setup as you. Installed 4.88 gears and put in an ARB in the rear. (I also have 35's) I went wheeling on a pretty difficult trail here in Colorado on Sunday for the first time with the new setup. It was a very steep trail with lots of big rocks/small boulders and I just crawled right over everything. I didn't even have to give it much gas. I found myself getting out in front of the rest of the group and had to go back and help the TJ's that were with me. They struggled on stuff that I simply rolled over.

I had the same experience as you did before the gears/locker. I was always stalling on trails. Even on easy stuff. It's not only annoying, but it can be scary losing your power steering and brakes. Now with my gears and locker I didn't stall out once.

Regearing and putting in the locker will make all the difference in the world. You won't need the Rubi Transfer case unless you're doing some crazy hardcore stuff.
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