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Understanding locking differentials...

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PackerEdgerton
Posts: 168

Understanding locking differentials...

Post#1 » Thu Feb 13, 2014 11:30 am

Hi guys,

I own a W460 300GD, Manual transmission, bone stock.

I've been reading up on how various 4WD mechanisms work (lockers, open differentials, Torsen differentials, Viscous couplings, front, rear and center lockers (auto, electric, hydro))

OK, am I correct in understanding that my W460 has front and read hydraulic locking differentials, but no center differential lock?

A buddy of mine who is an off-roading expert said that in his view, the most-capable off-road vehicles have THREE locking differentials, front, center, and rear.

He said that my Gwagen is very capable, but that without a center locking differential, the vehicle would not move in the case where, for example, the rear axle totally lost all traction but the front axle had traction. He said that the lack of a locking center differential would cause all torque to be sent to the rear, where both wheels (even if rear locker was on) had no traction. He said that in this situation, only a vehicle with a central differential lock would ensure that both front and rear received equal amounts of torque.

It sounds right, so my questions are:

1) Is this analysis correct? Why or why not?
2) If so, why do you suppose Mercedes didn't put a central differential lock in on the W460s? They certainly didn't spare expenses elsewhere.

Thanks for putting up with me, I'm a 4WD noob.

Thanks,

Packman

PackerEdgerton
Posts: 168

Re: Understanding locking differentials...

Post#2 » Thu Feb 13, 2014 12:45 pm

Thank you for the quick reply. Actually, that's what I thought was happening, but when I mentioned it to my friend, he said "No, you need a center differential in order to allow the front and back axles to rotate at different speeds." I said "I've been under my truck. What I see is a transmission, a transfer case, and three drive shafts. I don't see a center differential" He just said "I think you're wrong".

OK, some more Googling brought up this site: http://www.fordification.com/tech/transfercase.htm

It has some good definitions:

"Full-time vs. Part-time - A full-time 4WD system provides engine power to both the front and rear axles on all surfaces at all times. This usually requires a transfer case with a center differential, a viscous coupling, or both. Full-time 4WD improves traction and handling on paved surfaces in inclement weather, but may reduce fuel economy.
A part-time 4WD system is designed to be operated only on reduced-traction surfaces. The transfer case lacks any mechanism to allow front-to-rear axle speed differentiation. Thus, any accumulated driveline bind must be released via tire scrub. Extended use of part-time 4WD on a high-traction surface, such as dry pavement, can adversely affect handling and damage the driveline. Some, but not all, part-time 4WD systems allow the operator to shift from two- to four-wheel drive "on-the fly.""

OK, that makes sense then. My W460 has no center differential, thus when the transfer case is in 4WD and the front and rear differentials are locked, then any there is no speed differentiation between front and rear axles. Thus, even if only 1 tire has traction, you can still move.

Did I get that right?

Thanks,

Packman

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Flanker525
Posts: 1448

Re: Understanding locking differentials...

Post#3 » Thu Feb 13, 2014 4:59 pm


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diesel_jim
Posts: 974

Re: Understanding locking differentials...

Post#4 » Fri Feb 14, 2014 12:03 am

Not sure if your buddy is clued up on Land Rover products, but if so, then tell him (with the centre diff lock thing) to compare the 460 to a "series 1/2/3" and the 463 (and late 461) to the 90/110/rangerover classic/discovery1

the series 1/2/3 is only part time 4wd, that is, rear OR 4, when you engage 4 a dog clutch engages to engage the front axle. so there is a 100% lock between the front & rear axles (same as the 460)

Great in the mud/gravel, but no good on tarmac, as your buddy suggests, without the front and rear axle being able to rotate at different speeds, then transmission wind up will occur.

461's, like the 90/110/rangie etc, are full time 4wd, with a centre difflock. so on tarmac you have full 4wd AND can safely drive around bends without winding anything up, but jack one wheel off the ground (or park one wheel on a set of rollers/mud etc) then all the drive will take the easiest route and you'll be stationary (until you engage the centre difflock, where the vehicle will then perform exactly the same as the 460 with 4wd engaged

PackerEdgerton
Posts: 168

Re: Understanding locking differentials...

Post#5 » Fri Feb 14, 2014 9:02 am

Thank you guys for the swift and very detailed responses. I feel like I learned a lot.

Thanks,

Packman

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Gelande
Posts: 4113

Re: Understanding locking differentials...

Post#6 » Fri Feb 14, 2014 2:33 pm

There is never a difference in speed between front and rear axles or diffs on your W460.

And btw tell your buddy if he does have a car with awd and no locking center diff, you can trick the diff by squeezing the brake lines on the wheels with traction available and then get in the truck and hold down on the brakes to stop the wheelspin on those wheels with no traction and thus the awd center diff would be "tricked" into putting the power to the other axle that has the traction

if you need a diff locker in your axle, you can do the same....put smooth welder's vice grip or similar flat jawed clamp on brake line of wheel that is not spinning....apply brakes....brakes will send fluid only to spinning wheel...diff is tricked.... sends torque to wheel with traction

boom
now you got lockers with no lockers
old skool style.

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AlanMcR
Posts: 5543

Re: Understanding locking differentials...

Post#7 » Fri Feb 14, 2014 3:57 pm

If the car has 4-channel ABS, then one could trick the ABS pump into braking any combination of wheels. The modern version of 4matic is simply a software algorithm that watches the wheel sensors and applies the brakes on the wheel(s) spinning significantly faster than average.

Actually, one could probably build a CAN bus module that took over control of the factory ABS module and implemented a crude 4matic. Hmmm.

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Gelande
Posts: 4113

Re: Understanding locking differentials...

Post#8 » Sat Feb 15, 2014 7:37 am

"one" named Alan McReynolds!!!!

get hopping on it brother!

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4x4abc
Posts: 533
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Re: Understanding locking differentials...

Post#9 » Fri Feb 28, 2014 2:21 am

AlanMcR wrote:If the car has 4-channel ABS, then one could trick the ABS pump into braking any combination of wheels. The modern version of 4matic is simply a software algorithm that watches the wheel sensors and applies the brakes on the wheel(s) spinning significantly faster than average.

Actually, one could probably build a CAN bus module that took over control of the factory ABS module and implemented a crude 4matic. Hmmm.


Not so fast my friend - ABS decreases brake pressure to selected wheels to keep them from locking up
the pressure is produced by your foot

in order to add traction control or 4matic to ABS you will need ready to go brake pressure at all times
since you can't have your foot constantly on the brake, additional hardware is required
a pump and a small reservoir with brake fluid under pressure

when the wheel speed sensors report faster than average spinning wheels valves are opened to direct the pressurized brake fluid to the wheel(s) to slow them back down to the average speed

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4x4abc
Posts: 533
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Re: Understanding locking differentials...

Post#10 » Fri Feb 28, 2014 2:47 am

PackerEdgerton wrote:
My W460 has no center differential, thus when the transfer case is in 4WD and the front and rear differentials are locked, then any there is no speed differentiation between front and rear axles. Thus, even if only 1 tire has traction, you can still move.

Did I get that right?

Thanks,

Packman


Packman,

when your transfer case is in either 4WD Hi or 4WD Lo there is no speed differentiation between front and rear axle (both drive shafts are forced to rotate at the same speed)
left and right wheels on each axle are still abel to rotate at individual speeds

only when you lock your axle differentials, the wheels as well are forced to rotate at one speed.

In theory, with all lockers on, you could still move with traction at only one wheel.
In theory.
If the amount of traction of one wheel would be sufficient to move a car under most circumstances, we could simply produce 1WD cars.
There is a reason why we invented 4WD.
To have more traction as counter force for the torque needed to move heavy cars in uneven terrain.

In some very rare cases one wheel might indeed produce enough traction to counteract the torque needed to move the car (extreme grip plus a good amount of weight). However, in that case maximum torque generated by the truck is acting on only one axle shaft - quadrupling the force on that one axle shaft. If all 4 were on the ground, each would carry 25% of the torque.
If you are not super gentle with your gas foot, you'll break that axle shaft, or the CV, or the spiders in the diff.
The strength of the G provides some protection, but it's not unlimited.

Your 300GD produces roughly 340 lb/ft. First gear roughly 4:1, transfer case 2.14:1, axles 4.9:1 = 42:1 crawl ratio.
42 x 340 = 14,280 lb/ft in low range, first gear.
So, 14,280 lb/ft would be the force acting on one axle shaft, if you had (sufficient) traction on only one wheel.
Quite a lot. And thus better spread out over all 4 wheels (3,570 lb/ft ea).

nuff for now

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