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Diff Lock Simplified: Center, Rear, Front Explained :)


Zed

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Found a better illustration on the side-effect / disadvantage of differential: the point that the power will always go to the least traction wheel, shown below:

If your car specs says "Limited-Slip Differential", then you're a bit better, as some power are distributed to the wheel with traction. However note it's not an "Anti-Slip" Differential, just limited, so the arrow to the left is smaller (less power / limited) than the green arrow on the right wheel (more power / slipping). To achieve 50% left : 50% right power distribution, you still need to Lock the Diff.

OpenDiff.jpg.3cc99975e1a5c3889d361908f635d84c.jpg

 

 

LSD.jpg.33e4f658ac2f31119b0f1bf3ab03ae86.jpg

 

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1 hour ago, Gaurav said:

This is an amazing effort @Zed to enlighten everyone with your recent differential enlightenment.

Just a small add-on on few points for better clarity.

  • Point 4: Center differential engagement comes standard when you switch to LO gear. Second, you can use center differential in high using HLC (Hi Lock Center diff), hence in most cars, there is no button labeled as center diff, one can use center diff either in HLC or in LLC 4X4 gearing.
     
  • Point 5: Rear diff-lock will only help to get unstuck if any one of the rear wheels still has traction. If both rear wheels are in the air or deep inside the sand then the rear diff-lock won't be able to help much.
     
  • Point 7: Jeep Rubicon has all three diff and plus sway bar disconnect option with a flick of a button, that can work wonder even when the vehicle is crested with all 4 wheels in the air. Sway bar disconnect will drop the front wheel lower by a couple of inches and increase the chance of self-recovery if cresting is not on razor-sharp blade.

 

Thanks @Gaurav bhai for adding these extra correct points. Man how could I forget Rubicon, I almost bought that car and I did look at videos of how cool sway bar disconnect is, most will show putting the tyres on a fork-lift but as Gaurav said; when you are crested press that Electronic Sway Bar Disconnect and the front tyres will drop to touch the sand like a Long-Travel Dune Buggy would, in the hope of gaining traction to self-recover 😁

 

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On 7/22/2021 at 8:43 PM, Zed said:

7. Cars with Triple Locking from Factory

In addition to your list and Rubicon,  lc70 series also have the option with part time 4wd and F&R locks,

for part time 4WDs, when 4H/4L engaged, power to F&R is locked at 50:50 already hence no central diff lock is needed

LC70 series                                                                           

 image.png.7a6bb3713c830fcb710a2b5e1e0d7dbd.png

 

 

In China, people are so obsessed with locks, they consider more locks = better offroadability,

hence there are several options in China with part time 4wd and F&R locks, the costs from 130k AED

  • Great Wall Haval H9, positioned against Prado

 Haval H9 Price in UAE - New Haval H9 Photos and Specs | YallaMotor

image.jpeg.dad0b26c3240d70c8f2ed1193c5a8660.jpeg

  • Great Wall Tank 300The 2022 Haval Tank 300 Looks Like a 2021 Ford Bronco Chinese Copycat -  autoevolution

                 image.png.0c8a4edba7f91bea1055394352b92961.png

  • BAIC BJ40, BAIC used to own Jeep's factory in China🙃

BAIC BJ40 Price in Kuwait - New BAIC BJ40 Photos and Specs | YallaMotor

image.png.bbd99800f5738864034baf42b0f85536.png

They all proper 4x4s with frame and low range gear, but only 2.0T/2.3T engines with power to weight ratio more or less around 100, which is considered as underpowered in desert driving. 

All those cars indeed beat their competitors (originals) in most of offroad scenaries just because of the locks.

 

Tank 300 is a recent product and I think the engineers probably thought about desert driving under extreme heat:

  1. Central display setting to keep engine fan rev high all the time
  2. Thermostat can be taken out without removing any parts, just like removing a fuse from fuse box

 

Havel H9 & BAIC BJ40 are also available in UAE, without lockers sadly 0-0 (I didnt see the car but neither of dealers marketed diff locks as selling point)

 

Edited by Zixuan Huang - Charlie
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Thanks @Zixuan Huang - Charlie for the additional cars, wow I believe with the price advantage, these can be considered for dune-bashing 😁  Even though the engine is 2.0, I think we can find some garages who can add turbos to add the horsepower & torque 😃  After all, now manufacturers care more about pollution & fuel-efficiency, so they put less powerful engines but add turbos. For example Toyota stopped making V8 Land Cruisers, and instead use turbos for their V6 to achieve more power 💪

How are aftermarket compatibilities? Do you think because BAIC was a Jeep manufacturer in China, most aftermarket parts like lift kit will fit the BAIC BJ40 too?

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1 hour ago, Zed said:

Even though the engine is 2.0, I think we can find some garages who can add turbos to add the horsepower & torque

The engines are with turbo already, producing around 220-230hp and 370-380 n.m torque, but I think the bigger issue is the turbo lags, I have not tried those cars but I know turbo lag is the issue for the Chinese designed turbo engines.

 

1 hour ago, Zed said:

How are aftermarket compatibilities? Do you think because BAIC was a Jeep manufacturer in China, most aftermarket parts like lift kit will fit the BAIC BJ40 too?

Not sure about this one, but definitely there are tons of aftermarket parts avaiable in China for those mentioned cars.

 

if you plan to have a look of the BJ40 in UAE, dont. BAIC ridiculously supplied the CVT to BJ40 in UAE

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3 minutes ago, Zixuan Huang - Charlie said:

The engines are with turbo already, producing around 220-230hp and 370-380 n.m torque, but I think the bigger issue is the turbo lags, I have not tried those cars but I know turbo lag is the issue for the Chinese designed turbo engines.

@Zixuan Huang - Charlie this is why for desert we prefer SuperChargers compared to Turbos, it's powered by the Crank Pulley like Clutch Fan & Timing Belt, so the power increase is instant 😁   Turbos depend on Exhaust, hence the lag 👇

 

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On 7/22/2021 at 8:43 PM, Zed said:

I had a center diff lock issue few weeks back: after self-recovery the center diff lock remained locked and would not unlock, causing heavy steering when driving home. My mechanic was trying to solve the center diff problem by isolation: whether it was mechanical (possible fix: change center diff lock actuator), or was it electrical (possible fix: clean dirty connections). While he was doing that, I was reading the materials from my Land Cruiser Repair Manuals and some other sources. Thought I'd share my "differential enlightenment" here, and try to explain it as non-technical as possible 🙂  FYI, I've arranged it so that everyone knows the context and working background of a diff before using this knowledge to our offroad driving, so read from 1-2-3-4, but if you wanna go straight to how we use diff locks, go ahead to point #4 below:

1. The Minimum 3 Diffs required for most cars

Forget if you have 3 buttons with labels: RR/Rear Lock, Center Lock, FF/Front Lock. Forget the IFS vs Solid Axle debate for front drive. Turn your 4WD car upside-down (or easier to look at the underbody diagram in your manual 😁 ), it better have 3 differentials:

Rear Diff (Red Sphere), Center Diff hidden in what's known as Transfer Case that also holds the 4LO low-gearing (Purple Sphere) and Front Diff (Blue Sphere).

pic1.gif.54e4674032e7f4d408900cc02faa4251.gif

 

What's that, you don't like this 3D illustration? Let's show a real example with an old Toyota Fake Jeep #whoops I meant FJ 😂 You can see I circled the 3 "pumpkins" casing showing the 3 differentials location 👇

FJ_3diffs.jpg.f0ecf6fd1647d4899dc054d0a908e09c.jpg

 

2. Why do we need those spheres/pumpkins/diffs ?

The picture below shows 2 tyres on an axle with NO DIFF. Do you think it will turn nicely without slipping? Do we want this kind of horse carriage in our modern 4WD systems? 😅

no_diff.jpg.422d8e930ae5958e6e6e8e5d5df33b1c.jpg

So there you go, we need Diffs to make smooth turnings without stressing the driveline. In the illustration below, when we turn the Red Curve is longer (travel more distance) than the Blue Curve (travel less distance), so how can they turn at different rates when the engine gives the same power of 1,000 rpm? The answer: the magical mechanical device called a differential 😊

turn1.gif.66a594e65f2de8b3e5f6120cc55c5701.gif


General Motors made this video long long time back, even though it's in Black & White, it explains how the concept of Differential came about (also explained how the Spider Gears work!). The video is optional, only for the technically-inclined and curious ones...

 

3. Why do we need that Center Diff?

So from point #2 above, we know Diffs are used for turning the car, so front & rear diffs make sense, but what does a center diff do? Tilt the car?

Refer to sketch drawing below:

turn96.jpg.184d8723e889bf9a70e478b39d448c69.jpg

The car is turning left. Each wheel follows a different curve path: the inside wheels don't need to turn so fast, so in this diagram the front inside wheel needs 50 rpm and the rear inside wheel needs 48 rpm. BUT in order to turn, the front wheels need to turn faster than the rear wheels: thus the outer front wheel needs 54 rpm and the inner outer front wheel needs 50 rpm. Both front wheels are rotating faster than the rear wheels. If you add all the power required, Total Front Drive Power Needed = 54 + 50 = 104 rpm and Total Front Rear Power Needed = 52 + 48 = 100 rpm. 

WITHOUT CENTER DIFF, the transmission outputs 100 rpm to both Front and Rear drive shafts.

So what? Well, that 104 rpm needed to turn wheels is greater than then 100 rpm, so extra stress is created... if continous stress is accumulated, this may lead to the image below, kablooi!

split-transfer-case.jpg.73db7f69a8e221eba19d296e5429cd4f.jpg

So the Center Diff is similar to point #2 above, but instead of allowing inside wheel & outside wheel turn at different rates, the Center Diff allows the front axle and the rear axle to get different power output. Refer to diagram below now:

turn_ft96.jpg.c90876e066879a8ad0ef04edd42a0cf9.jpg

So by having that Sphere/Pumpkin/Diff in the middle, the front drive shaft gets more power to accelerate faster than the rear drive shaft which move slower.

Don't believe this paper drawings and theories? Well let's watch how the World Rally Championship uses Center Diffs to make turnings smoother and then lock the Center Diff to distribute power in a straight-line driving:

 

4. Press that Center Diff Lock!

Finally we can use all that theories to use. Refer to below illustration:

Red Axle is the rear and Blue Axle is the front. The Front-Right wheel is slipping while the rest of 3 tyres got some traction. Why won't the car move?

one_spin.gif.3e629661d93106cc600b25b01d0a818e.gif

Here's a little secret: Diffs are great for helping to turn, but the side-effect of a diff is that the output power from transmission will go to the wheel with the least traction. In the diagram above, that means the Front-Left Blue tyre will not turn at all! 

BUT we still have the 2 Rear Red tyres with traction, don't we? Yes, but from Point #3 we know an open Center Diff allows front & back axles to turn at different rates, so in this case more power is given to the Front Blue Axle because of that spinning wheel that has the least traction... thus the rear tyres spin r.e.a.l.l.y s.l.o.w.l.y , making the driver frustrated and leaving him sweating 😂

Now we hear those famous words from the radio: "Engage 4LO and (Center) Diff Lock

We'll talk about 4LO some other time, but the effect of pressing the Center Diff Lock is now this illustration:

locked1.gif.d153a4c82d9547b3cc871f8010e29066.gif

No it's not the same image... ok yes, but watch the rotational arrows at the back. You see that Front-Right Blue slipping wheel? That one is stealing all the drive power. BUT because of Center Diff Lock, we now forced the Front & Rear axles to turn at the same rate. So if the Front-Right Blue is stealing 100 rpm slipping, then now the Rear Tyres are also spinning 100 rpm, with probably 50:50, so Rear-Right is 50 rpm and Rear-Left is 50 rpm. Because the Rear Tyres have traction (those brown balls), then the rear tyres move the car forward... or down-right depending on the angle you wanna recover.

 

5. Press that Rear Diff Lock!

So you think Center Diff Lock is enough and no need for a rope tug? Well check the illustration below:

Now we have 2 wheels spinning: one in the front (Front-Right Blue) and one in the back (Rear-Left Red).

You press Center Diff: nothing is moving. Why? Well the Center Diff allows front & back axles to turn the same, that bit is right... BUT because power is lost to the least wheel with traction, the transmission output will just spin both wheels (Front-Right Blue & Rear-Left Red). Calculatively:

Front-Right: 50 rpm spinning
Front-Left: Zero rpm not spinning (refer to disadvantage of diff in Point #4)
Rear-Right: Zero rpm not spinning (refer to disadvantage of diff in Point #4)
Rear-Left: 50 rpm spinning (same speed as Front-Right due to Center Diff locked)

two_spin.gif.033d6624d6f21da9937009149d4f654c.gif

 

Here's what's gonna happen if your 4WD is lucky enought to have a Rear Diff Lock button:

locked2.gif.e2a9ddb0d7201bc5f0e6d07a767e5d1b.gif

It's the same image again? Yessirree but look at the rotational arrows, especially at the rear red axle. So now Rear Differential is locked, splitting 50:50 drive power to the Rear-Right Red tyre and the Rear-Left Red tyre. Now instead of Zero RPM, that Rear-Left tyre will get 25rpm and might just nudge the car forward a bit... slowly but steadily, you might self-recover! Calculatively:
Front-Right: 50 rpm spinning
Front-Left: Zero rpm not spinning (refer to disadvantage of diff in Point #4)
Rear-Right: 25 rpm  (due to Rear Diff Lock)
Rear-Left: 25 rpm spinning (due to Rear Diff Lock + Center Diff Lock)

Yes you have to do Double-Locking: press Center Diff Lock first, then Rear Diff Lock.

 

6. Press that Front Diff Lock!

That situation in Point #5? Well you have a higher chance of self-recovery if your beloved 4WD also has a Front Diff Lock. Refer to illustration below:

locked3.gif.7dbc6bbf2230ee13dad598fdaf91b1d9.gif

Calculatively:
Front-Right: 25 rpm spinning
Front-Left: 25 rpm (due to Front Diff Lock)
Rear-Right: 25 rpm  (due to Rear Diff Lock)
Rear-Left: 25 rpm spinning (due to Rear Diff Lock + Center Diff Lock)

You can now do Triple Locking: get out of the car, assess the situation, then press the magic 3 buttons: Center Diff Lock On, Rear Diff Lock On, and Front Diff Lock On. 

 

7. Cars with Triple Locking from Factory

If you made it to this point, then you may want to now look for second-hand 4WDs that have triple-locking from the factory 😊  Here are 3 that I know of (please add if you know more):

7.1. Land Cruiser 80 Series: just look at the diff lock indicators on that dashboard. No custom aftermarket buttons to show if your car is triple-locked 😁

LC80TripleLock.jpg.d7f83d0b515e6cdea847e0dbd7bf7ead.jpg

 

7.2 Mercedes Benz G-Wagon. Do you know why I said the order is Center Diff Lock then Rear Diff Lock then last is Front Diff Lock? It's because Mercedes Benz labelled it in that order too!

mercedes-benz-g-class-diff-locks.jpg.6842225c8b5a0bb56a1b563793347675.jpg

 

7.3 Land Cruiser 300: well, this one you have to wait, and there's no second-hand market yet, so this is the most expensive triple-locked currently 😅

LC300_Diff.jpg.9546b94dcfc71473ac69802503e1da54.jpg

 

Sources:
The nice 3D Illustrations are from the Land Cruiser Cool Project.

Thanks Zed @Zed very informative article. You are now slowly but surely nudging into Fred's @Frederic  territory 😁. Really good one

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