JKlad

Ok, so I'm fairly mechanically minded and believe I have a sound working knowledge of the mechanical principals of engines, transmissions and drivelines. However I was asked a question the other day by a friend's inquisitive 12yo daughter recently when a few of the families went camping that though I thought I knew the answer to eventually left me stumped.

So she was asking my about the manually operated part-time 4WD system my JK has; she wanted to know what that smaller lever was next to the gear shift as her dad's Nissan AWD didn't have one so I began an explanation of the different 4WD systems commonly used (she tops her class in science apparantly so I was able to get reasonably technical in my explanation). So I got to the bit about the JKs part-time system only being suitable in situations of low traction (and not used for highway driving unless snow or ice is present) as due to there not being a centre diff like her dads car, both driveshafts are locked together by the transfer case which means the wheels all spin at the same time and thus must be able to slip on the driving surface otherwise strain on the drivetrain will result.

All good so far, but then she tripped me up be earnestly asking: "...but then why would you need (manually operated) diff lockers then to stop one wheel spinning when it loses traction off-road, if as you said the driveshafts are locked together making all the wheels turn at the same rate? If you need lockers to force both drive axels to turn at the same rate and give equal drive to both wheels on the axel, then why can't you drive onroad in 4WD and just turn the lockers off then?".

So I realised that I didn't have an exact explanation as to the difference between having differentials locked or open offroad and having driveshafts locked by a transfer case with regards to why the wheels spin at the same rate in 4WD but not when one looses traction offroad.

I know it seems like a dumb question, but when a few of us adults discussed it the finer points seemed to have us all disagreeing. Anyone who could spell it out here would be much appreciated as it sucks looking stupid to a smart-ass 12 year old girl!

ronjenx

When the JK is in 4wd, both drive shafts HAVE to turn at the same speed.
However, the axles can still spin a wheel.
So, when in 4wd, if a tire on each axle loses traction, you will sit there, spinning two tires (one on each axle). That's where lockers come in handy.

Also, when rounding a curve, the two axles have to turn at different rates.
They cant do that if the drive shafts are locked together.

genesbro

With automatic 4wd and fulltime 4wd the xfer case has clutches in it to allow slippage. With part time like we have the road has to have a loose or uneven surface which serves the slippage part because when engaged the front and rear driveshafts are locked together.
Since the front and rear axles take a turn along different arcs they will travel different distances and that causes the driveline to bind up if something can't slip.
Just like if you have a locker in the rear axle and you are in 2wd the inside tire will want to chirp if you are on the gas hard so the locker can't let the rachet system slip and let the diff work like it is open. The main reason arb or electric lockers is better.
Pretty basic description, hope it helps.

CerOf

locker in a differential either locks the drive shafts front to rear or locks the axle side to side.


Jeep JK 4WD system when in 4WD or 4LO locks the drive shafts front to rear.


There is no lock side to side (right to left)

A locker for your front or rear axle will lock side to side.

On other forums, mainly Range Rover types, you'll see people talk about a front, center, and rear differential.

We typically call them a front and rear differential and talk about the center differential as the transfer case or short hand "t-case."


As mentioned above, there are many types of center differentials or transfer cases.

Some are two-speed (meaning they have a low range) others are single speed and over no reduction in gearing. These are commonly called "AWD" or all-wheel drive systems, though Honda will call theirs 4wd because they have the capability of locking the front and rear driveshafts together.

Jeep ZJ Grand cherokee uses a viscous fluid that was EXTREMELY sensitive to temparature change and when slippage occurred, it would cause the fluid to expand. This would force the fluid through a valve body to engage a set of clutch packs front to rear.

The WJ grand cherokee used a gerotor pump to force the fluid to engage a set of clutch packs front to rear. When slippage occurs, it spins an impellar of sorts that forces fluid to exert pressure and engage the clutch packs.

Most all systems use some type/form/variation of this for their "all time" or "full time" 4WD systems.

part time systems like ours have a direct mechanical lock front to rear. There is no "slippage" front to rear.

How stuff works website has great explanations on this.

ninjajmoney

It is amazing what kids know and can understand now a days.

animated_robot

After reading all this my brain hurts. Now I see what my kids go through in school.