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Understanding 4WD


Cravey Dockett

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Can I get a read on my understanding of drive systems? It's my observation that the typical open rear 2WD is actually 1WD, because if one rear wheel can spin uncontrollably, it will. Thus:

2WD open = 1WD (if 1 wheel can spin, it will)
4WD open = 2WD (1 driven wheel per axle)
2WD posi = 2WD (each rear wheel gets some portion of the available power)
4WD posi = 3WD (rear wheels each get some power, plus one front wheel)

With 4WD posi, in 4LO, will each wheel be driven without uncontrollable slipping?

Thanks, I'm new to all this!
 


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Well, you are mostly sorta correct. On either diff if one wheel is given a situation where is has little or no friction between the ground and the tire, while the other has a lot, you will get one wheel spinning and one sitting still. I do still take issue with your math. The reason being is that with an open diff if one wheel has enough traction to grab, and you are moving, then both do, so it is 2wd.

The advantage that 4x4 gives you in bad, or off road, conditions, is that the rear wheels may be in a position where one or both can't get a grip on the ground, while the front wheels are both perfectly capable of grabbing. 4x4 is now an advantage because you get get power up to those front wheels that have traction, and they can pull you out of your stuckness.
 

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Thing to remember:
An open diff always sends the same amount of TORQUE to both wheels.
If one tire has less traction than the other, it will be the first tire to slip (spin). That does not mean the other wheel is not trying to propel the truck forward (it is), it's just that the torque going to that wheel is limited to that of the other wheel that is slipping (and is why you may be stuck). If one tire is hanging in the air for example (takes virtually zero torque to spin it), the torque available to the wheel still on the ground will also be nearly zero.

A locking differential or limited slip on the other hand has the ability to provide ADDITIONAL torque to the wheel with more traction, so it is able to get more power to the wheel with more grip (and get you going again).

Hope that helps
 

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An OPEN differential allows each axle to spin at a different rpm
This allows a vehicle to go around a corner without dragging the outside tire, which it would do with solid or locked differential.

So OPEN is 2WD it just allows cornering :) which is quite handy in street driving
And yes the axle with less resistance, highest RPM gets all the power
So if one wheel is on ice and spins easiest it will get all the power, it must because differential considers it the "outside wheel" on a corner, higher RPM wheel/axle

Limited slip(posi-traction/Trac-Loc) balances RPM differences, if one axle starts to spin "to fast" in relation to the other axle then power is transferred to that other axle

The "limited" part allows you to go around corners without dragging a tire, most limited slip differentials require a Friction Modifier so they don't grab to soon, without it they will make tires chirp when cornering.

Locking differential does just that, locks both axles to the same RPM, so no cornering except in slippery conditions, outside wheel will be dragged.

Most front wheel drive(FWD) cars are OPEN
And most 4WD have OPEN front axles
But with the weight of the engine on the front axle either wheel will have better traction than rear wheels, especially in pickup trucks.

The weight on front axle is one of the reasons FWD cars do better in snow and ice, but other is physics of PULL vs PUSH
It is easier to pull a vehicle thru snow(or mud) that to push it thru the snow(or mud), so FWD is better, and also why 4WD in a pickup does better, if you have even one wheel with weight on it PULLING you, you have better chance of not getting stuck or getting unstuck.

OPEN is not 1WD, a motor cycle is 1WD :)
And two OPEN axles is not 2WD, it is 4WD

Reason being is either wheel on each axle CAN drive the vehicle, so ALL 4 can be the drive wheel at any given time, hence 4WD
 
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pjtoledo

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and to confuse you even further....

a posi, (also known by other names, trak-lok whatever) is a limited slip that utilizes a set of clutch packs on each side of the axle. a posi works exactly like an open diff but the friction from the clutch packs imparts resistance to the axles rotating at different speeds. once the limit of that friction is reached the clutches slip and the tires can indeed spin at different speeds. that's exactly what happens going around curves or parking on hi traction surfaces. I'm thinking that friction relates to about 300 or so lb-ft of torque, the "bias". that means if you parked a posi axle with one wheel on ice or in the air and the other on good pavement you will only get the 300 lb-ft of torque on the good wheel then the clutches will slip and let the bad wheel spin. granted, 300 lb-ft usually gets you going but it does happen that the limits of a posi can be overcome.

that 300 lb-ft that ties the 2 wheels together is what makes the posi a poor choice for a steering axle. it puts a lot of binding on the front axle while turning.
 

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That 300ft-lbs is also just the preload. When power (torque) from the engine is put through the axle, the clutches then receive additional pressure from the spider gears pressing against the side gears (the clutches are located behind the side gears). This increases the amount of torque bias tremendously. But indeed there comes a point that it can only do so much... If that spinning wheel loses too much of it's grip, the pressure against the clutch stacks decreases (along with the torque bias), and ends up leaving you doing a one-wheel-peel just the same.


and to confuse you even further....
Hopefully we haven't already overwhelmed the OP with too much info. lol

This page was posted awhile back which should help explain some things as well.
http://www.therangerstation.com/Magazine/Feb 06/tech.htm
 

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That 300ft-lbs is also just the preload. When power (torque) from the engine is put through the axle, the clutches then receive additional pressure from the spider gears pressing against the side gears (the clutches are located behind the side gears). This increases the amount of torque bias tremendously. But indeed there comes a point that it can only do so much... If that spinning wheel loses too much of it's grip, the pressure against the clutch stacks decreases (along with the torque bias), and ends up leaving you doing a one-wheel-peel just the same.




Hopefully we haven't already overwhelmed the OP with too much info. lol

This page was posted awhile back which should help explain some things as well.
http://www.therangerstation.com/Magazine/Feb 06/tech.htm
so we don't want to explain how the driveline torque is attempting to rotate the axle housing, thus decreasing traction to the right rear wheel?

and while doing a burn out in reverse the left wheel gets to smoke?
 

don4331

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You guys missed the 2nd half of the question - what if difference between 4H and 4L. ;)

In 4H, a collar in the transfer case links the front axle to the rear, powering the front axle in addition. In the Ranger, there is a clutch on the collar, so this can be done at any "reasonable" speed.

In 4L, in addition to the collar connecting the front axle; another collar disengages the transmission from the rear wheels and inserts a set of planetary gears. The planetary gears slow down the truck by about 2-1/2:1 (2.48 if you want to be real technical).

So, instead of going 25 in a given gear/rpm combination; you will be going just under 10. Which is useful in off-roading (and a few other situations) when you want to go slow and in control. It also provides 2-1/2 the torque if you are going up something real steep.

But, in order to drive the wheels, 2-1/2 times slower, the "planets" in the planetary are going 2-1/2 times faster. So, it should only be used at slow speeds. Running at freeway speeds in 4L would result in planets needing to turn >10k and they weren't designed for that. When failure happens to something turning that fast, the results are destructive.

So, 4L has no impact on number of wheels powered or amount of "power" to each wheel.

To OP; hopefully, we are helping you understand.

To my expert colleagues, I'm testing you: :)

And I'd like an explanation on how spider gears increase the pressure.

Also, how the right rear (going forward) has less traction; I thought it had same traction, just less load and same force on less load results in it being more prone to overcoming static friction and spin.

The reduction in grip once the threshold is of the preload is surpassed is due to dynamic (sliding) friction being less than static. (Which is why getting off the gas and letting the wheel stop spinning is advantageous most of the time).

Doesn't the SHO have a limited slip in the front axle? My Focus SVT did. And it was real advantage even though it was on the steering axle.

And we probably don't want to get into that it will be the left front wheel that spins due to decreased load...
 

adsm08

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And I'd like an explanation on how spider gears increase the pressure.
Well, technically they don't. If you want the real technical details of it, it is either the S spring or belleville spring that creates the pressure. It depends on the axle you are working on.

An S spring is easy, it just slips in the middle between the side gears and pushes out, creating pressure on the clutches.

If you are unlucky enough to be working on a 9.75, or a FW 8.8 made after 09 a belleville spring placed under the clutch pack is used. Then you need the gear retracting tool and carrier holder to pull the side gears back in against the springs. If you don't pull them in far enough there isn't enough room for the spiders to get in, if you pull them in too far you engage the clutches and can't turn the gears, and if you get is perfect half the time one gear or the other is half a tooth off and doesn't line up enough for the king pin to go in.
 

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And I'd like an explanation on how spider gears increase the pressure.
The spider gears press against the side gear when power is applied (the gears would push away from each other if not contained by the carrier). This forces the side gear harder against the inside of the carrier. With the clutch stack behind there, the pressure against the clutch stack is increased at the same time, giving it additional torque transfer capability.

Also, how the right rear (going forward) has less traction; I thought it had same traction, just less load and same force on less load results in it being more prone to overcoming static friction and spin.
Less load = less friction = less traction.
Like pjtoledo said, power transfer from the driveshaft puts a rotational moment on the whole axle housing, which tends to reduce weight on the right tire (and increases it on the left tire). It's not a huge amount of difference (maxes out around couple hundred lbs maybe), but with an open diff that is just enough to allow the right-side tire to slip first (when going forward).
 

pjtoledo

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You guys missed the 2nd half of the question - what if difference between 4H and 4L. ;)

In 4H, a collar in the transfer case links the front axle to the rear, powering the front axle in addition. In the Ranger, there is a clutch on the collar, so this can be done at any "reasonable" speed.

In 4L, in addition to the collar connecting the front axle; another collar disengages the transmission from the rear wheels and inserts a set of planetary gears. The planetary gears slow down the truck by about 2-1/2:1 (2.48 if you want to be real technical).

So, instead of going 25 in a given gear/rpm combination; you will be going just under 10. Which is useful in off-roading (and a few other situations) when you want to go slow and in control. It also provides 2-1/2 the torque if you are going up something real steep.

But, in order to drive the wheels, 2-1/2 times slower, the "planets" in the planetary are going 2-1/2 times faster. So, it should only be used at slow speeds. Running at freeway speeds in 4L would result in planets needing to turn >10k and they weren't designed for that.

you will never get to freeway speeds, 45 mph was about all I could get out of my 2000 in low range. the brakes work real good then too!!

When failure happens to something turning that fast, the results are destructive.

So, 4L has no impact on number of wheels powered or amount of "power" to each wheel.

To OP; hopefully, we are helping you understand.

To my expert colleagues, I'm testing you: :)

And I'd like an explanation on how spider gears increase the pressure.



simple physics. the faces of the teeth are at an angle. not only do they cause rotation to the opposing gear, they also push it away. think inclined plain.

Also, how the right rear (going forward) has less traction; I thought it had same traction, just less load and same force on less load results in it being more prone to overcoming static friction and spin.

my definition of traction (from high school physics in 1970) is how much "sticky" and pressure something has against something else.
we are mixing terms here. I'm guilty. oh yeah, "sticky" is co-efficient of friction. so sticky multiplied by pressure equals traction/total friction.
anyway, sitting still, or in docile motion each rear wheel has approximately the same traction. when you romp on the gas the drive shaft tries to rotate the pinion. because of traction the pinion fights back and doesn't want to rotate faster. that results in the entire axle assembly attempting to rotate about the axis of the pinion. net result is less force/traction on one side, more on the other. yes, I know the pinion also wants to crawl up the ring gear which lifts the front of the car. kinda complicated back there.

The reduction in grip once the threshold is of the preload is surpassed is due to dynamic (sliding) friction being less than static. (Which is why getting off the gas and letting the wheel stop spinning is advantageous most of the time).

Doesn't the SHO have a limited slip in the front axle? My Focus SVT did. And it was real advantage even though it was on the steering axle.

what SHO years are you referring to?

I'll stick to 89-99 for this discussion.
as far as I know, there were no limited slip in the 93-99 automatics. possibly a custom setup someplace, but nothing common.

the 89-95 manuals came open from the factory. aftermarket soon offered a Quaife. its a worm gear torque sensing unit. then along came the Wave Trac torque sensing unit. same worm gears as the Quaife but it also had ramps that increased the bias under load.

And we probably don't want to get into that it will be the left front wheel that spins due to decreased load...

torque arm. basically the difference from the center of the differential to each wheel. something about vectors and resulting forces. think 2 lines, one each from the center of the differential to where each tire touches the ground. because the drivers side is closer to the diff, the lines/vectors have different angles which result in different torque values.


enough bull for the evening?
 

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