Howdy! I recently bought an '02 F150 Supercrew FX4. This is my first 4WD truck (and first truck at that). Being an engineering student, I have taken an interest in exploring the complex mechanisms which provide us with offroad capability! Anyway, I have a question about the 4WD gearing, and gearing in general. I'm pretty sure I've got 3.55 gears (part of the FX4 package), but correct me if I'm wrong. I know the basics of how the transfer case works and all that stuff, but how does driving in 4 High compare with driving in 4 Low? What are the differences? Also, how does the tranmission gear selection (drive, OD, 2nd, or 1st) affect driving in 4WD (either High or Low). Am I supposed to drive in 1st gear when in 4 Low? etc... I know I probably should know this stuff, but I'm new to 4WD trucks. Thanks in advance for any advice given!

 

The transfer case serves 2 functions: first is to switch from 2wd to 4wd by engaging and disengaging the front axle. Second function is to switch between a 1:1 gearing out of the two output shafts, and 2:1 (low range) in which the output from the trans (whichever gear it is in) is TWICE that of the output from the Transfer case. This in effect doubles the effective gear ratio in each gear.

There is no hard and fast rule for what transmission gear selection you use. This will depend on the traction, speed, surface etc. You will learn what works by watching fellow 4wheelers, and through good old experience.

What you want to avoid is spinning your tires in a low traction situation. A harsh upshift can cause a disaster in a dicey low traction situation! That is when you would want to set the trans in 1 gear and not let it shift!

The other factor in 4WD is the front axle. In the old days, you had to manually engage a set of dog gears on each front wheel to engage the wheels. Now it is done automatically. The other factor with 4WD is open vs. limited slip axles. Limited slip, or lockers, or other traction differentials, will keep power to the wheel in proportion to their traction on the ground. On the other hand, open diffs will bias power to the wheel with the LEAST traction. What this can amount to is 2WD: 1up front slipping and 1 out back slipping! Limited slip, which is what the factory provides as a solution, will bias power to the wheel with the least traction.

I hope this gives you a groundwork to understand 4WD!

Experience (with caution) will be a great teacher.

 

So wait a minute... I'm assuming that since my rear axle has 3.55 gears, the front axle has 3.55s also. Now I have some more questions:
1) So when I engage the transfer case (4 Low), it effective reduces my front gear drive ratio to nearly 7.10:1 ???

2) What about the back? Don't the final drive ratios for the front and back axles have to be the same since the tires are the same size?

3) I still don't understand how my tranny gear selection (D, OD, 2, 1) works with my 4WD gear selection ( high or low). I'm assuming that I'll get the most speed out of Overdrive while in 4 High. Should I even be in overdrive while in 4WD? Also, I'm assuming that I should get the most pulling power in 1st gear while in 4 Low. Is this correct?

 

Wait, I think I got it. Okay, the transfer case is the link between the front and the rear driveshafts/axles. When the transfer case is engaged in 4 Low, it offers an additional gear reduction to BOTH the front AND the rear driveshafts/axles. When in 4 High, it simply links the front and rear driveshafts/axles without any gear reduction other than that from the transmission. Is this correct? I was thinking that the rear driveshaft wasn't even connected to the transfer case, only to the tranny.

 

correct
you get an "a" in transfercase 101
(i've never used 4 low and i plow LOTS of snow)

...zap!

 

Howdy, nice to see a fellow engineering student with a truck. None around here at widener. hahaha. You got it on the how it works for the transfer case deal.

Now how to use it. I am assuming the supercrews all have electronic shift on the fly and not the manual shift. To use 4H, just turn the **** while stopped or moving at a slow speed. It can be done moving fast, but that is a lot of stress to put on the front end. To use 4L, you NEED to be stopped and the tranny in Neutral (I'm assuming an automatic) and your foot on the brake. The same is true to get out of 4L.

Adrianspeeder

 

Awesome, I got it. I read the Owner's Manual to find out how to shift to 4WD. Damn this thing has some ***** in 4 Low!! Anyway, thanks to everyone in helping me figure this stuff out. For some reason, I just like to know what's going on underneath me...

 

You ain't kidding! I had to use my 4 Low to pull my dad's flatbed trailer up a steep and washed-out hill. It just hossed that trailer right on outta there. The 5.4L engine coupled with a 4X4 Low system can be a true workhorse. But like these guys have said in your post, treat it with respect. And NEVER use it on dry pavement if you can help it.

 

Yes, avoid 4H or 4L on dry pavement if you can cpaggie. The reason being that under normal driving while turning the front and rear wheels turn at different speeds. While in 4x4 they are locked together and it puts a lot of undue stress on the driveshafts, ujoints, transfer case, ect. While on dirt, stone, rain, or snowy roads, there is enough "slip" for one of the tires "give" taking the stress of the driveline while turning.

What school do you go to and what engineering?

Adrianspeeder

 

Wow, I was searching for my transfer case ratios and I found my old thread! Oh, to answer your question adrianspeeder: I'm studying Aerospace Engineering at Texas A&M.

 

Try this link.

http://auto.howstuffworks.com/four-wheel-drive.htm

JMC