Does A 4WD Truck Need Limited Slip

[mdc1990zr1]

Semantics, what does "4wd" actually mean?

A 2wd truck with open diffs is actually a 1wd truck; the second one wheel in the rear loses traction, you won't go any further. Both wheels do get the same amount of torque when traveling in a straight line and when both wheels have traction, but if either wheel loses traction it's game over, all the torque goes to the spinning wheel and the wheel with traction won't turn or help you out. So this is the same effect as if you actually had a single drive wheel which then lost traction, you're not moving.

A 2wd truck with a locker/limited slip, is a true 2wd truck. When the locker is engaged, you have 2 wheels which can provide movement as long as they have traction.

A 4wd truck with open diffs is actually a 2wd truck; if your left side of the truck hits a ditch and you lose traction on that side, you won't go any further; both the front left and rear left spin, the torque is "lost" to those spinning wheels and you won't go anywhere.

A 4wd truck with a center diff lock and with a locking/limited slip in the rear, is a 3wd truck; now when you hit the ditch on your left side, your rear wheels are locked so the right wheel will still get torque and provide you with movement as long as it has traction.

A 4wd truck with a center diff lock and front/rear lockers is a true 4wd; all diffs are locked, all 4 wheels turn at exactly the same speed, you only need traction at any single corner to get you moving again.

I agree with everything you say but the 2WD scenario. The 2WD truck with the locker will lock both wheels when slippage is detected and you will have both rear wheels turning. The LSD rear of a 2WD vehicle will only transfer the power of the drive wheel to the other wheel via clutch packs when slippage is detected but both wheels will not turn forward at the same time. You still have only one drive wheel in the rear.

 

[kdoublep]

I'm no engineer but I saw "My Cousin Vinny"

 

[Mike Wenrich]

To answer the question of why would you not? The 2024 trucks are quite a bit more money than 23's and if I could find everything I want but it did not have L/S I could save a bunch and maybe it would not matter. For my situation. I'm not going to be rock crawling and I don't plan to intentionally drive in ice/snow/horrible weather. Maybe sand. I still have my 4WD with L/S 2004.5 Ram truck and would drive that in known bad weather. It's the what if's that made me ask the question and my lack of knowledge on what works and what does not. I already know what does not work on a one legger. Twice.

 

[PaulTGarrett]

My '75 Bronco had a posi rear end and a limiited slip front. I can only remember ONE time I got stuck out in the woods hunting and that was when I found some ruts that were deeper than my tires. Bottomed out the axles... Winch time!

My '79 Bronco had a posi rear end and an ARB locker on the front. I don't remember ever getting it stuck in any enviroment.

OP... Get the limited slip rear... You are dropping a lot on coin on your truck, get it right from the get go...

 

[kdoublep]

Great info here. I learned a lot.

 

[TestPilot57]

Long story short, nothing needs anything. All depends on expectations.

Personally I choose LSD since 99+% of my travel is on road. I don't want to have to anticipate additional traction needs and lock a diff. LSD is "always on". Is it as strong as a locker? No. Does it need to be? Not for me. If you are doing heavy off-road travel you should seriously consider a locker.

And for those saying that without LSD or locker a 4WD is really only 2WD, go back to school. Just because you can get stuck with only 2 wheels are spinning doesn't mean it's 2WD. Seriously, look it up. I'm not going to school you - there are plenty of resources free online. But if you really want to know, you can spend the $200k and get an engineering degree that will tell you the same thing.

 

[TestPilot57]

Semantics, what does "4wd" actually mean?

A 2wd truck with open diffs is actually a 1wd truck; the second one wheel in the rear loses traction, you won't go any further. Both wheels do get the same amount of torque when traveling in a straight line and when both wheels have traction, but if either wheel loses traction it's game over, all the torque goes to the spinning wheel and the wheel with traction won't turn or help you out.

This is where you're wrong. With an open diff both wheels get the same torque REGARDLESS of the amount of traction available.

The problem is that if one wheel only gets 10 LB/FT of traction, both of them get that, and it's not enough to move the vehicle.

 

[Wild one]

This is where you're wrong. With an open diff both wheels get the same torque REGARDLESS of the amount of traction available.

The problem is that if one wheel only gets 10 LB/FT of traction, both of them get that, and it's not enough to move the vehicle.

How do you explain a block long one wheel blackie on the pavement then,otherwise known as the one wheel peel,the vehicle is definitely moving to.I've had cars that'd roast one tire at 50 mph pretty easily,curious on how you explain that scenerio if both wheels are getting the same amount of applied torque to them.I'll admit they have the same applied torque till one wheel is spinning,then i think the spinning tire is getting the majority of usable engine torque

 

[TestPilot57]

Rather than re-write what has already been written, from https://www.rrtransmissions.com/technical/open-locked-differentials

Before describing how useful limited-slip differentials can be, and how they differ from open differentials, we should start by describing how open differentials work and their limitations. Open differentials are fitted to most road cars and allow a difference in wheel speed when cornering. They do this by splitting the torque evenly between both wheels at all times. In a straight line this means both wheels have even amounts of power applied to them.

 

[Wild one]

Rather than re-write what has already been written, from https://www.rrtransmissions.com/technical/open-locked-differentials

Whoever wrote that isn't really what i'd call up on how things work with comments like this.Power does not stay the same at the wheels with a transmission ahead of the diff.You always dyno a vehicle in it's direct drive transmission gear,otherwise you get skewed results.Lets say you could use 2 dyno hubs to check torque output at the tires (virtually impossible) with an open diff,but say you could,and 1 dyno hub,had the ability to lock one wheel,and the other didn't,do you think you'd have the same torque output at both wheels,no you wouldn't,the 1 spinning tire/dyno hub would be the only 1 giving you an output. About all i agree with in that article is the fact Torque X RPM divided by 5252 is how Horsepower is calculated,and they forgot that there's also a time factor included in that equation if you wanted to get really technical.If that's the article you're using for your comments,i'd disregard alot of it.

• Gearing is used to increase the amount of torque available at the wheels. Power at the wheels remains the same regardless of the selected gear.

 

[Bike_Pilot]

It may be that the trucks you are looking at have it as a standard feature. A locking rear diff was standard on my '18 3500 SRW diesel. I added an ARB air locker to the front so I can be fully locked with all four turning if I want.

As to whether it's necessary or not it really depends on your uses. The more you find yourself in situations with very limited traction or a wheel of the ground the more important it is. These trucks are really heavy and have stuff suspension, so on steep, uneven terrain it doesn't take much to unweight a wheel and lose traction on that wheel.

You can add lockers as aftermarket. Figure on 1100 for parts per diff.

 

[TestPilot57]

Okedoke, I already commented on this at least once if not more...

Bottom line? Here are the three answers that apply to ALMOST any question.

Yes.
No.
Maybe So.

The "Maybe So" answer is where it can get complicated, and in this question (as in most/all) that answer depends on what your expectations are.

 

[fireflymedic]

My thoughts on this is your buying 4 wheel drive to not get stuck and paying slot for that feature up front and forever with poorer fuel milage. Limited slip also helps you not get stuck. It is cheaper and does not drop your fuel milage. So yes get it. You know without it you could get stuck with 2 tires on pavement. Shameful if this was to happen your only half off the road yet stuck.

 

[TestPilot57]

Whoever wrote that isn't really what i'd call up on how things work with comments like this....

Can you point me to your commercially supported website with a better technical explanation of how it works? Or, even better, an article in terms a redneck could understand?

Signed,
Redneck Dev

 

[Travelin Ram]

My thoughts on this is your buying 4 wheel drive to not get stuck and paying slot for that feature up front and forever with poorer fuel milage. Limited slip also helps you not get stuck. It is cheaper and does not drop your fuel milage. So yes get it. You know without it you could get stuck with 2 tires on pavement. Shameful if this was to happen your only half off the road yet stuck.

That is never going to happen in any modern 4x4 with traction control.

 

[ramffml]

That is never going to happen in any modern 4x4 with traction control.

Exactly. Excluding offroading, the amount of situations one can get into in a modern 4x4 truck that absolutely requires a LS to get out of, has to be close to zero. Might take a tiny bit more effort without, but really, "need"? No. Nice to have? Yes.

 

[Wild one]

Can you point me to your commercially supported website with a better technical explanation of how it works? Or, even better, an article in terms a redneck could understand?

Signed,
Redneck Dev

I did explain it,in laymens terms,but i can't help it if you don't understand how an open diff works

 

[ramffml]

Can you point me to your commercially supported website with a better technical explanation of how it works? Or, even better, an article in terms a redneck could understand?

Signed,
Redneck Dev

Take a 2wd drive truck with open diff and jack one wheel fully in the air and turn of all traction controls. Put it in drive and rev it up, does the truck move at all? My guess is it won't move an inch. I've seen this demonstrated a few times (instead of jacks they used rollers, same idea).

We can talk about torque until the cows come home. Bottom line is, without traction control, an open diff will stop your truck from moving the second one of your two wheels loses traction. However traction control is so good these days that if its enabled it will brake the spinning wheel, causing the torque to flow to the wheel with traction. In the past before TC we just used to apply a bit of brakes manually while giving it some gas to get the same effect.

 

[Travelin Ram]

How do you explain a block long one wheel blackie on the pavement then,otherwise known as the one wheel peel,the vehicle is definitely moving to.I've had cars that'd roast one tire at 50 mph pretty easily,curious on how you explain that scenerio if both wheels are getting the same amount of applied torque to them.I'll admit they have the same applied torque till one wheel is spinning,then i think the spinning tire is getting the majority of usable engine torque

Where your example breaks down is in the conflation between torque and HP. Yes, an open diff guarantees equally applied torque. Say, just for the sake of example it takes 1,000 lb-ft to maintain slippage at the roasting tire. So the same 1,000 is applied to the gripping tire on the other side.

Now HP on the other hand, is the product of torque and RPM. So, if the spinning tire is turning 3x the revs, it is consuming 3x the power. In which case 25% of the net power is on the slow tire, and 75% on the fast tire. Ergo the smoke.

 

[Wild one]

Where your example breaks down is in the conflation between torque and HP. Yes, an open diff guarantees equally applied torque. Say, just for the sake of example it takes 1,000 lb-ft to maintain slippage at the roasting tire. So the same 1,000 is applied to the gripping tire on the other side.

Now HP on the other hand, is the product of torque and RPM. So, if the spinning tire is turning 3x the revs, it is consuming 3x the power. In which case 25% of the net power is on the slow tire, and 75% on the fast tire. Ergo the smoke.

I refer back to my example of using 2 hub dyno's to measure torque output at the wheels.The only dyno that's going to give you a torque reading is the 1 hooked to the spinning tire,the locked stationary dyno isn't going to give you a torque output reading.Dyno's read torque then compute that number to a horsepower reading