Aftermarket Underdrive

J

Jim Byrne

TJ Enthusiast
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Dec 3, 2017
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Location
Kingman, AZ, United States
Has anyone bought & installed the Advance Adapters Rubicrawler gearbox?
I have a 2004 TJ with a 42rle trans. & NP231j transfercase and am considering
the underdrive. This will allow me to keep my high geared axels which gives me
hi speed & low rpm on the highway but good low gearing when needed.
I want opinions on it's quality as well as any problems on installation.
Can I pull the transfer case & the rear of the transmission and install it while in
the car, or does the transmission have to come out? If you have one of these units
are you happy with it?
 
It's not needed when you have the automatic which is the only place the Rubicrawler can be installed. Gear for the street, your 4Lo together with the torque converter which gives an infinitely high crawl ratio eliminates the need for the Rubicrawler. Your automatic's torque convernter allows you to crawl as slow as you want, like .1 or .01 mph without stalling, without the need for the Rubicrawler. The Rubicrawler would be great for those with manual transmissions who could use the extra-low gearing but it's not available for them.
 
This unit brings the overall crawl ratio down. Has similar advantages to an Atlas (think gear reduction minus the multi speed) or even a 4:1 241 case. Think doubler except it doesnt require a shorter driveshaft because it just replaces the rear tailshaft of the trans with a gear reduction unit. All the while still maintaining the stock 2.72:1 gear reduction plus having the ability to go to a 7.40:1 4WD Low gear reduction. Thus increasing your crawl ratio and putting more tourqe to each wheel.

A stock Rubi with the 42RLE Rubi Crawler will have an over crawl ratio multipled by 2.72, giving you a new 4low ratio of 10.88:1 vs the stock 4:1 thus more power and potential capability. Yet still retaining the option of keeping it in 4:1 if so choosing.

A stock TJ with the 42RLE with the RubiCrawler will have a new overall crawl ratio also multipled by 2.72 thus giving you a new crawl ratio of 7.40:1 with an overall cost of ~$1600 which is *sometimes* cheaper than an Atlas (yes has multi speed) or even *sometimes* cheaper than the common 241 Rock trac swap from a Rubicon TJ. Choosing the Rubiccrawler over the Rubicon 241OR has the benifit of keeping and adding the 2.72 factory like gear reduction in 2wd H.

Not sure what a torque converter has to do with this thread...

Jim Byrne said:
Has anyone bought & installed the Advance Adapters Rubicrawler gearbox?
I have a 2004 TJ with a 42rle trans. & NP231j transfercase and am considering
the underdrive. This will allow me to keep my high geared axels which gives me
hi speed & low rpm on the highway but good low gearing when needed.
I want opinions on it's quality as well as any problems on installation.
Can I pull the transfer case & the rear of the transmission and install it while in
the car, or does the transmission have to come out? If you have one of these units
are you happy with it?
Click to expand...
Its on my list of purchases to come for me. Although I dont have it yet. My buddy does and he loves the options the RubiCrawler has given him. Like being able to go 2.72:1 in 2WD H. As well as the much lower crawl ratio when using it in 4WD H/L which is drastically lower than the stock rubicon and some Atlas cases.
He installed it while the trans was still in the TJ. He did however remove the transfer case and lower the skid plate.
He initially assumed he might have gear noise identical to a vehicle driving in reverse but he doesn't. Fit and finish are very nice and to my knowledge he has never had an issue with his.
 
Last edited:
Jerry Bransford said:
It's not needed when you have the automatic which is the only place the Rubicrawler can be installed. Gear for the street, your 4Lo together with the torque converter which gives an infinitely high crawl ratio eliminates the need for the Rubicrawler. Your automatic's torque convernter allows you to crawl as slow as you want, like .1 or .01 mph without stalling, without the need for the Rubicrawler. The Rubicrawler would be great for those with manual transmissions who could use the extra-low gearing but it's not available for them.
Click to expand...
Is most certainly is needed if OP wants more gear reduction in 4low without addressing the axle ratios.
 
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Fouledplugs said:
This unit brings the overall crawl ratio down. Has similar advantages to an Atlas (think gear reduction minus the multi speed) or even a 4:1 241 case. Think doubler except it doesnt require a shorter driveshaft because it just replaces the rear tailshaft of the trans with a gear reduction unit. All the while still maintaining the stock 2.72:1 gear reduction plus having the ability to go to a 7.40:1 4WD Low gear reduction. Thus increasing your crawl ratio and putting more tourqe to each wheel.

A stock Rubi with the 42RLE Rubi Crawler will have an over crawl ratio multipled by 2.72, giving you a new 4low ratio of 10.88:1 vs the stock 4:1 thus more power and potential capability. Yet still retaining the option of keeping it in 4:1 if so choosing.

A stock TJ with the 42RLE with the RubiCrawler will have a new overall crawl ratio also multipled by 2.72 thus giving you a new crawl ratio of 7.40:1 with an overall cost of ~$1600 which is *sometimes* cheaper than an Atlas (yes has multi speed) or even *sometimes* cheaper than the common 241 Rock trac swap from a Rubicon TJ. Choosing the Rubiccrawler over the Rubicon 241OR has the benifit of keeping and adding the 2.72 factory like gear reduction in 2wd H.

Not sure what a torque converter has to do with this thread...


Its on my list of purchases to come for me. Although I dont have it yet. My buddy does and he loves the options the RubiCrawler has given him. Like being able to go 2.72:1 in 2WD H. As well as the much lower crawl ratio when using it in 4WD H/L which is drastically lower than the stock rubicon and some Atlas cases.
He installed it while the trans was still in the TJ. He did however remove the transfer case and lower the skid plate.
He initially assumed he might have gear noise identical to a vehicle driving in reverse but he doesn't. Fit and finish are very nice and to my knowledge he has never had an issue with his.
Click to expand...
 
What happened to post numbers 3 & 4 above? One of those asked what an automatic's torque converter has to do with my recommendations against the Rubicrawler above. It has everything to do with what I talked above. Few people understand torque converters and fewer yet manual transmission owners understand them.

To answer, an automatic's torque converter is the sole reason I said a Rubicrawler would not be all that beneficial. Having a torque converter, which a manual transmission does not have, dramatically changes how the ratios work and how well a Jeep crawls with any given axle ratio.

What most don't understand about torque converters is they eliminate having a hard-fast 1st gear (or 2nd or 3rd) ratio. This is due to the fluid coupling inside the torque converter. There is no mechanical connection until you reach cruising speed when the torque converter's lockup engages.

The torque converter's fluid coupling gives what is in effect an infinitely low 1st gear ratio... forget the ratio given in the automatic transmission's specifications. The torque converter allows you to crawl at any speed you want which is the result of it giving an infinitely low 1st gear ratio which also gives an infinitely high crawl ratio. I have a friend with a Rubicrawler and I've driven it. He was dismayed at my ability to crawl on the same trail as slowly as he could.... all courtesy of the same torque converter both of us have.

When you have a manual transmission with fixed ratios, the low ratio afforded by the Rubicrawler would indeed be beneficial but the Rubicrawler isn't available for manual transmissions. And for automatics, in reality, it really doesn't help in the way you'd think it would. Again due to how your torque converter works.

Read up on torque converters at https://en.wikipedia.org/wiki/Torque_converter

You could gain more benefit by putting that Rubicrawler budget into something to do with the suspension, gearing, etc.
 
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A 42RLE Automatic Rubicon with the 4.0:1 transfer case is going to have more (~2x’s the power and torque) torque applied to the wheels versus a 42RLE automatic TJ with the 2.72:1 geared transfer case. Thus theoretically drastically increasing the off road capabilites of the 4.0:1 versus the 2.72:1.

Installing a RubiCrawler will give the stock TJ 2x’s the power and torque being applied to wheels along with the option to still run 2.72:1 or 7.44:1 if he/she desires. And at a cheaper cost than labor and parts for axle gear changes or a rubicon transfercase. Plus, like I mentioned options when it comes to overall crawl ratio.

Having 4.0:1 gearing in the transfer case in the Rubicon gives it a “rock crawling advantage” over the standard 2.72:1 factory TJ. This was a key selling point for the Rubicon TJ. Lower gearing means more tourqe, power to the wheels, and slower more precise obstacle crossing/traversing.

This is why Advanced Adapters offers the RubiCrawler so one can bring a TJ sport to the trail with a better crawl ratio, multiplying the power and torque to each wheel by 2.72 when the RubiCrawler is engaged...A TORQUE CONVERTER IS NOT GOING TO PROVIDE THIS. I understand that you can make your Jeep crawl at the same speed as a Jeep with a RubiCrawler but what you fail to understand/mention is your friend is making 2.72 more times the power and torque to each wheel as your Jeep. He also can select to crawl at a ratio of 2.72:1 or 7.44:1, you however can not do that in your Jeep. And you cant see that with your eyes.

OP doent seem to want to go go slower or faster. He never mentions speed. He wants more gearing options and that is exactly what the RubiCrawler does. Nor does he want to change his axle ratios as stated in the original post.

Maybe try unblocking me in your profile settings :roto2nuse:
 
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Jerry Bransford said:
What happened to post numbers 3 & 4 above? One of those asked what an automatic's torque converter has to do with my recommendations against the Rubicrawler above. It has everything to do with what I talked above. Few people understand torque converters and fewer yet manual transmission owners understand them.

To answer, an automatic's torque converter is the sole reason I said a Rubicrawler would not be all that beneficial. Having a torque converter, which a manual transmission does not have, dramatically changes how the ratios work and how well a Jeep crawls with any given axle ratio.

What most don't understand about torque converters is they eliminate having a hard-fast 1st gear (or 2nd or 3rd) ratio. This is due to the fluid coupling inside the torque converter. There is no mechanical connection until you reach cruising speed when the torque converter's lockup engages.

The torque converter's fluid coupling gives what is in effect an infinitely low 1st gear ratio... forget the ratio given in the automatic transmission's specifications. The torque converter allows you to crawl at any speed you want which is the result of it giving an infinitely low 1st gear ratio which also gives an infinitely high crawl ratio. I have a friend with a Rubicrawler and I've driven it. He was dismayed at my ability to crawl on the same trail as slowly as he could.... all courtesy of the same torque converter both of us have.

When you have a manual transmission with fixed ratios, the low ratio afforded by the Rubicrawler would indeed be beneficial but the Rubicrawler isn't available for manual transmissions. And for automatics, in reality, it really doesn't help in the way you'd think it would. Again due to how your torque converter works.

Read up on torque converters at https://en.wikipedia.org/wiki/Torque_converter

You could gain more benefit by putting that Rubicrawler budget into something to do with the suspension, gearing, etc.
Click to expand...
So take two automatic (42RLE) equipped Jeep TJ’s with 4.10s in the axles. One being a Rubicon and one being a Sport...the Rubicon doesnt have 2x’s the power and torque being applied to the wheels when in 4low? It should, because the Rubicon has 4.0:1 in the transfer case versus the Sport which has 2.72:1 in the transfercase. Theoretically that means the Rubicon has ~2x’s more overal torque multiplication going to each wheel when in 4low. Does that not matter since they are both automatic transmissions? I dont see how the Rubicon doesnt have the advantage...
 
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Jerry Bransford said:
What happened to post numbers 3 & 4 above? One of those asked what an automatic's torque converter has to do with my recommendations against the Rubicrawler above. It has everything to do with what I talked above. Few people understand torque converters and fewer yet manual transmission owners understand them.

To answer, an automatic's torque converter is the sole reason I said a Rubicrawler would not be all that beneficial. Having a torque converter, which a manual transmission does not have, dramatically changes how the ratios work and how well a Jeep crawls with any given axle ratio.

What most don't understand about torque converters is they eliminate having a hard-fast 1st gear (or 2nd or 3rd) ratio. This is due to the fluid coupling inside the torque converter. There is no mechanical connection until you reach cruising speed when the torque converter's lockup engages.

The torque converter's fluid coupling gives what is in effect an infinitely low 1st gear ratio... forget the ratio given in the automatic transmission's specifications. The torque converter allows you to crawl at any speed you want which is the result of it giving an infinitely low 1st gear ratio which also gives an infinitely high crawl ratio. I have a friend with a Rubicrawler and I've driven it. He was dismayed at my ability to crawl on the same trail as slowly as he could.... all courtesy of the same torque converter both of us have.

When you have a manual transmission with fixed ratios, the low ratio afforded by the Rubicrawler would indeed be beneficial but the Rubicrawler isn't available for manual transmissions. And for automatics, in reality, it really doesn't help in the way you'd think it would. Again due to how your torque converter works.

Read up on torque converters at https://en.wikipedia.org/wiki/Torque_converter

You could gain more benefit by putting that Rubicrawler budget into something to do with the suspension, gearing, etc.
Click to expand...
Jim Byrne said:
I would like to talk to your buddy that has installed the Rubicrawler, if he wouldn't mind. He can contact me at [email protected]
Also I would like to hear from anyone else that has installed or driven with one of these gearboxes.
Thanks for the input.
Click to expand...

For future references and for folks via searching...

Please read Post #65 in @Chris 's build thread as well as and Post #3,541
 
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