How does a low range box work?: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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April 9, 2010 12:15 p.m. daytonaer Reader

I've had a jeep for half a year now and love the freedom of not worrying about bad roads and crashing into stuff. The community for 4wd's is also very grassroots and I love it.

I have a jeep with a NP231 transfer case which has a low range and 2 open differentials.

I understand numerically how the low range works but do not understand how that transfers to driving and exactly when I should use it.

As an example I drove it through a muddy field in low range in 3rd gear and it seemed sure footed. On the way back I was in high range 4wd 1st gear and it was hard to get power down. Maybe I was just imagining things?

Is the advantage of lower ratios that it will change the torque output to the wheels which prevents excessive wheel spin? (ie 3rd in low is pretty equal in forward speed and rpm of 1st in high)

Or is it only that it allows you to drive slower?

When is an appropriate time to use 4lo?

for fun, picture after 3 hours of digging it out. Had it buried to the floorboards, used plywood and a jack to lift it out of the ruts and fill them with rocks.
April 9, 2010 12:23 p.m. joepaluch Reader

low range just a chage in driveshaft speed.

So the engine turns which drivers the tranny. The tranny has an output shaft that normall turns the drive shaft. With alow range transfer case the output is direct 1:1 in high range or can be changed to 2.71:1 for XJ low range.

Evey MFG low range is different, but 2.x is pretty commong. Rock crawler type will go 4:1 for Jeep low ranges.

Now the benefit.

Simple.... More torque to the rear wheels for less engine speed. This very handly on rocky terrain as you don't need to spin the motor as fast. If you hvae an autobox it also means more time with tranny lock up vs slipping the torque conveter. If you have manual it means less clutch slipping when going 10 mph over rocks.

I used to have an 98 XJ like that. It was daily driver off-roader. I put 4" lift and 31's on it and in low range it would crawl over anything pretty much.
April 9, 2010 12:39 p.m. Woody SuperDork

Low range is really just a torque multiplier (or divider, depending on how you look at it). The only change is the engine rpm required to turn the wheels (or driveshaft, as mentioned above) any given rpm.

This isn't a direct answer to you question, but you may be interested in this post from Jerry Bransford on the Jeep Forum (about half way down, he explains Part-Time and Full-Time 4x4 systems):

http://www.jeepforum.com/forum/f9/4x4-torque-answers-628225/
April 9, 2010 2:04 p.m. Trans_Maro Dork

some XJ's have a differential in the transfer case to allow for full time 4wd.

The bad thing is, you actually only have one-wheel drive with this system when things get really slippery.

There should be a "lock" setting for the transfer case diff if yours has this system.

Shawn
April 9, 2010 2:12 p.m. DILYSI Dave SuperDork

The NP231 has a (I beleive) 2.72:1 low range. That means that when you're in 4 Lo you are spinning the tires 2.72X slower, but with 2.72X more torque. That said, 4 Hi in 1st gear and 4 Lo in 3rd gear are nearly identical final drive ratios. So in this case, it shouldn't make a difference.

As for when to use low range, I like it pretty much any time I'm off road. I'd rather cruise around in 4 Lo in 4th or 5th than 4 Hi in 1st or 2nd, because if I need more pulling power, it's a simple matter of downshifting versus putting the transfer case into low range. I typically don't go fast enough off road to even get to the upper gears while in 4 Lo.
April 9, 2010 2:43 p.m. joepaluch Reader

Another off roader gave some interesting advice about lockers. "Sure lockers are great in that you don't get stuck as easily, but when you do get stuck you are just deeper in it."

My XJ was going to get ARB airlockers one day, but I never saved up the money for it. I was also going to regear from 3.55 to 4.10 to counter the impact of my 31's.

the NP231 uses a chain to drive the front wheels. So no differentials in there. When I did my lift I also changed drive shafts and got rid of slip yoke. So in the process I split the transfer case and swapped in output shaft. Interesting the stuff in there.
April 9, 2010 4:48 p.m. Trans_Maro Dork

joepaluch wrote:

the NP231 uses a chain to drive the front wheels. So no differentials in there. When I did my lift I also changed drive shafts and got rid of slip yoke. So in the process I split the transfer case and swapped in output shaft. Interesting the stuff in there.

Chain drive doesn't mean no diff.

The Quadratrac case uses a chain and a diff.

Shawn
April 9, 2010 6:28 p.m. Woody SuperDork

joepaluch wrote:

the NP231 uses a chain to drive the front wheels. So no differentials in there. When I did my lift I also changed drive shafts and got rid of slip yoke. So in the process I split the transfer case and swapped in output shaft. Interesting the stuff in there.

They look a lot like this inside:
April 9, 2010 7:49 p.m. 914Driver SuperDork

My Toyota 4-Runner had 4 lo & 4 hi. Four wheel peel is good. 4-low is NFG above 30 mph, it's for climbing telephone poles and such. High speeds will chew things up.

Think about lockers. If you have one wheel peel in a car, one tire bites, three just follow. 4WD is one wheel peel on both axles. In the woods you can get hung up, two wheels in the air with the wrong two on the ground.

The kit I bought for my X-Fer case lowered the 4-lo ratio by 185% but left highway 2WD stock.

Seriously, with the lower transfer case gears, I beat the crap outta Jeep guys. But to answer your question, watch your tach. If you're revving high, go to 4-hi. If you're chugging or burning clutch, 4-lo.

Dan