LSD vs Traction Control

[apensity]

Reading on my Sport build sheet, I have open differentials. However, reading about the SelecTrac Traction Control, it basically describes it like having a computer controlled LSD. So would adding a LSD prove any benefit? Or does the Traction Control do it all already? (4xe specific)

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[Tethmes]

Traction control reduces engine output to stop wheel spin, whereas the lsd transfers some power from the spinning wheel to the one on the other side thats not spinning. TC is good for low traction situations on road where you might otherwise lose control of the vehicle, but it will get you stuck offroad. They're two different systems with two different purposes.

If you've only ever had open diff vehicles off road or in snow, the lsd is a game changer.

 

[Jeep Wick]

It depends on how good the traction control is. If it can brake each wheel appropriately, it could be just as effective. I watched Matt's off-road drive a Sport over some pretty tough obstacles.
If you need one, I'd look at the TruTrac helical limited slip.

 

[oldcjguy]

The traction control system can apply the brakes to individual wheels. So, if one wheel is spinning TC can apply the brakes to that wheel which helps transfer the torque to the other wheel that has more traction. LSD is also used to transfer torque to both wheels, but it does it through clutches and dynamic loading. Depending on the kind of wheeling you do, just having TC use the brakes will wear out your brakes faster. Ask guys that drive on the beach or heavy sand a lot. Rear brake pads wear quickly. That's a simplified explanation, but you get the point.

Personally I'd prefer to have both. Having TC apply the brakes actually helps the LSD work better as it increases the dynamic load on the clutches when needed. It acts like a helper.

 

[Tncdrew]

It depends on how good the traction control is. If it can brake each wheel appropriately, it could be just as effective. I watched Matt's off-road drive a Sport over some pretty tough obstacles.
If you need one, I'd look at the TruTrac helical limited slip.

Or as an alternative to the TruTrac, Nitro Helix... (same basic design).
I bought one to replace the clutch type LSD in my Willys rear M220 last year when they were running an incredible sale.
Love it!! ?

 

[Tethmes]

TRACTION CONTROL is different from Brake Lock Differential

This is the most important point of the thread imo. People often conflate the two systems which leads to a lot of misunderstanding of how TC works in general. They are separate systems with separate functions working toward separate end goals, but can and do work in tandem when the situation arises that those goals are similar. Wrangler's TC reduces power, period. BLD (and LSD) transfer power to where it's needed. But one isn't a substitute for the other.

 

[STW]

Very useful run-down. Thanks Jim.
Can you (or someone) add to this a description of why sometimes a locker on ice/snow can put you into the snowbank? Mostly an issue with non-switchable front lockers that can surprise you on snow/ice when you're in 4wd because of the snow.

If I understand it right, the reason an LSD can be better than a locker on snow/ice is that an LSD balances the torque between a low-traction spinning wheel and sends torque to the other wheel which might have some traction. And LSD is dynamically balancing that torque per wheel based on how much traction.

But a locked axle sends the same constant torque to each wheel. So if you have one locked axle wheel spinning on ice the other side continues to have full traction and, if circumstances permit, the wheel with full traction can pivot you right around the spinning wheel and put you in the snowbank. Anyway, I think that's how it works.

Full-time front locker does just great crawling rocks though, with few drawbacks if you don't live around snow and ice. But if you do, you're better off with selectable lockers so you can have 4wd with an open diff, or better, an LSD.

BLD works extremely well in conjunction with any type of limited slip differential.
BLD is always on. You can't turn it off. It uses sensors to detect wheel spin and apply brake only to the spinning wheel. It does NOT control the throttle. Traction control also controls the throttle.

An LSD works great unless you have ZERO traction and a spinning wheel.
BLD will apply braking force if a wheel is spinning faster than the other on the same axle, and it is smart enough to know if you are turning or not.

OPEN DIFF EXAMPLE
1 wheel in the air or on ice, zero traction. This wheel spins with essentially zero torque. An open diff forces the same torque to each wheel, so the other wheel also has essentially zero torque.
You are stuck because the wheel with good traction can only get the same amount of torque - zero - as the spinning wheel.

without an LSD, the BLD applies brake force to the slipping wheel. Let's say It takes 10 ft lbs of torque to overcome this brake force. The other wheel gets some 10 ftlbs of engine torque = the brake force. And if 10 ft lbs is enough to get you going you move. If not, you don't.

LSD EXAMPLE
1 wheel in the air or on ice, zero traction. This wheel spins with essentially zero torque.
The LSD has a bias factor, let's say 3;1. So the other wheel gets 3x0 or still zero torque.
Which is why people tell you that an LSD isn't any good when you have a wheel in the air.
But wait, there's more.
BLD notices the wheel is spinning and applies brake force - assume the same 10 ft lbs.
Because you have an LSD, the bias will apply 3x = 30 ft lbs to the other wheel.
AKA, 3 times as much torque as you got from BLD alone - which is usually enough to get you moving.

BOTTOM LINE
LSD and BSD work very well together.

 

[stumblinhorse]

Very useful run-down. Thanks Jim.
Can you (or someone) add to this a description of why sometimes a locker on ice/snow can put you into the snowbank? Mostly an issue with non-switchable front lockers that can surprise you on snow/ice when you're in 4wd because of the snow.

If I understand it right, the reason an LSD can be better than a locker on snow/ice is that an LSD balances the torque between a low-traction spinning wheel and sends torque to the other wheel which might have some traction. And LSD is dynamically balancing that torque per wheel based on how much traction.

But a locked axle sends the same constant torque to each wheel. So if you have one locked axle wheel spinning on ice the other side continues to have full traction and, if circumstances permit, the wheel with full traction can pivot you right around the spinning wheel and put you in the snowbank. Anyway, I think that's how it works.

Full-time front locker does just great crawling rocks though, with few drawbacks if you don't live around snow and ice. But if you do, you're better off with selectable lockers so you can have 4wd with an open diff, or better, an LSD.

@jimcoffey62 and I have discussed this before. That is a good explanation that was provided.

To answer your question a locker will not allow the wheels to turn at different RPMs. What happens then, you go straight, even if you don’t want to. Then snow bank…. So pretty much no lockers in snow ever. If you are completely stuck in deep snow and need to pull forward a locker might help, but if you leave it on you will be stuck again at the next turn.

Where I differ from many is that I prefer an open diff on snow and ice Over an LSD. An LSD will slide you sideways with more torque being applied to one wheel versus the other. An open diff you will stay straight, your wheel(s) might spin more but you stay straight. I drive 6 months of the year in snow and ice. In terrible conditions I always drive my open diff vehicles. It is just way more predictable.

 

[KevinBoston]

Limited Slip Differentials, LSDs should be on all Wrangler models not equipped with lockers. It's inexpensive and makes the wrangler much more capable. BLD is great if you do not have LSD or lockers. I have found traction control is nice and kicks in during some situations. For me, traction control engages a couple times a year during snow or heavy rain.
My opinion

 

[John Benoit]

Traction control reduces engine output to stop wheel spin, whereas the lsd transfers some power from the spinning wheel to the one on the other side thats not spinning. TC is good for low traction situations on road where you might otherwise lose control of the vehicle, but it will get you stuck offroad. They're two different systems with two different purposes.

If you've only ever had open diff vehicles off road or in snow, the lsd is a game changer.

The two systems work well together. My 2017 Rubicon had true trac in both differentials and it went wherever I asked it to go rain snow mud and everything else.

 

[stumblinhorse]

Limited Slip Differentials, LSDs should be on all Wrangler models not equipped with lockers. It's inexpensive and makes the wrangler much more capable. BLD is great if you do not have LSD or lockers. I have found traction control is nice and kicks in during some situations. For me, traction control engages a couple times a year during snow or heavy rain.
My opinion

I am glad it is an option on all 4WD vehicles. I never want it regardless of lockers. As well my opinion only.

 

[Bzinsky]

Hate my lockers in my rubicon. I got the rubi because I always wanted them just because I understood how all the drivetrains mechanically function and on paper lockers are awesome. In the real world they are worthless unless you are rock crawling. I much prefer lsd’s for mud/snow/rain.

The only nice thing about lockers for mud situations is that you can just try anything without them activated. Then when you get stuck, you activate them for a slight bump in traction to back out of it.

Lsd’s front and rear would be ideal for anything that did not involve crawling over obstacles.

I guess thats why raptors have an lsd in the front and the wrangler doesn’t, because the wrangler is all about getting over obstacles.

 

[Pape]

I did enjoy the factory LSD on my JK and was sad when he eared out and left me with BLD. Got a working one again on my JL and certainly enjoy it

 

[jeepoch]

This is the most important point of the thread imo. People often conflate the two systems which leads to a lot of misunderstanding of how TC works in general. They are separate systems with separate functions working toward separate end goals, but can and do work in tandem when the situation arises that those goals are similar. Wrangler's TC reduces power, period. BLD (and LSD) transfer power to where it's needed. But one isn't a substitute for the other.

Dylan, et., al.,

Yes there is certainly a lot of confusion about the different systems and modes. Each have their specific use case(s). Let's see if I can do a good job at describing them:

First, the glossary:
ABS - Automatic Braking System
BLD - Brake Limited (or Lock) Differential
ERM - Electronic Roll Mitigation
ESC - Electronic Stability Control
FLA - Fully Locked Axle
HDC - Hill Decent Control
LSD - Limited Slip Differential
RAB - Ready Alert Breaking
RBS - Rain Brake Support
TCS - Traction Control System

All of the above have very specific and intended purposes but many overlap in functionality and hence causes some confusion. They all however address some type of traction loss condition, they just differ in what they're trying to accomplish depending on the driving context.

The ABS is simply the overall system that supplies nearly all of the automated braking functionality. It is the main brake control computer.

Within the ABS environment the following subsystems are managed:

BLD - This subsystem monitors just the two wheels on each axle. If one wheel 'slips', rotates at a faster rate than the other wheel on that particular axle, then braking is applied to the slipping wheel with enough force so that both wheel's rotational speeds are matched. This ensures that with an open differential, energy is applied evenly to both wheels thus allowing the non-slipping wheel to receive some amount of torque. And something is far better than nothing.

It is worth noting that BLD works independently on each axle and is always enabled independent of any other traction system or transfer case setting. However, when lockers are enabled providing a fully locked axle, BLD will naturally not engage since both wheels will always rotate at the same rate. No braking action whatsoever will automatically be applied while that axle is locked.

ERM - This subsystem attempts to mitigate vehicle roll based upon steering wheel position and vehicle speed. Both braking action and reduced output torque are automatically applied when wheel lift is anticipated. This system is always enabled in both 2WD and 4HI modes unless ESC is fully disabled via cockpit control. It is also disabled, along with all other traction control functions when in 4LO.

ESC - This subsystem provides overall vehicle stability control by correcting for either over or under steering conditions. It works by detecting intended path (steering wheel position) relative to actual path based on wheel slip. Both braking action and reduced output torque are automatically applied to bring the vehicle back to intended path.

There are three ESC modes. Fully on, partially on and fully off. Fully on is the normal 2WD and 4HI operation. Partially on, controlled by cockpit pushbutton will give the driver a more aggressive 'feel' and can be used to get unstuck in potentially slippery conditions. Fully off can also be selected via cockpit pushbutton (longer button press or toggle) or when in 4LO.

I've personally found that 'partially on' works much better in deep snow. I've gotten unstuck when in 4LO by simply moving up to 4HI. And found even better traction by partially enabling ESC. Reduced automatic braking during wheel slip seems to do wonders (at least for me) in extreme snow conditions. I rely on it a lot.

HDC - I swear by it. It is so cool to maintain 0.6 to 5.0mph on the steepest of descents without riding the brake pedal. This subsystem controls both output torque and braking action to provide a downhill cruise control at very slow speeds. I would never leave home without it especially on any steep terrain adventure.

It uses the gear shift lever to set the speed in 0.6 mph increments. The AutoStick (+/-) lever is used. If you don't know how to use this subsystem, learn it. It will literally knock your socks off, it's that good. Way (way) cool.

RAB - This subsystem helps the driver in extreme, emergency braking conditions. It helps maintain a straight path and greatly assists in achieving the smallest possible stopping distance in any very heavy foot pedal application.

RBS - Rain Brake Support very gently applies very minimal braking action to dry the rotors when the windshield wipers are in either a low or high (non-intermittent) setting. This helps ensure good stopping action in wet conditions.

TCS - This is effectively similar to BLD but on steroids. It senses all four wheel speed sensors and automatically applies braking action on any wheel and overall output torque to keep the vehicle traveling straight. It only differs from ESC in that it's more concerned with straight line travel. This subsystem is always on in 2WD and 4HI but disabled when ESC is fully off or when in 4LO.

Non-ABS systems:

FLA - The fully locked axle is everyone's go to mode in any bit of trouble. With both axles fully locked just one gripping wheel will typically provide enough traction for motive effort.

However too many Jeepers tend to over muscle their way into even more trouble with lockers engaged. Good throttle discipline is always required in order to minimize wheel slip. Locked axles are magic when used properly and problematic when misused. A pretty obvious negative side-effect with locked axles is that they can bind the drivetrain even with the slightest cornering. A single locked rear axle with BLD in the front is generally the proper choice for most difficult situations.

LSD - This provides for limited slip. The axle becomes partially or almost fully locked depending on wheel slip conditions. It does this automatically with no driver intervention. Many will say they prefer this to a fully locked axle any day of the week and twice on Sundays. It gives almost all the same benefit of a fully locked axle but completely automatically. It's only downside is the internal clutch packs tend to require more periodic maintenance.

I hope this helps with the most basic of traction control concepts coupled with the 2 or 4WD operating modes. The good thing is that the Jeep engineers have done a pretty remarkably great job in implementing all these features seemlessly. Almost to the point you don't even know that they're there. However, it's always to your advantage to understand how to manipulate their functionality or know how to make full use of their magic during your travels. Especially in off-road or slippery conditions.

I hope I've just given you enough background info to more fully investigate these systems on your own. You own one of the best all-terrain vehicles on the planet. Take advantage of that. Your JL is even that more capable when you know exactly the right modes to set when you find yourself in a tight spot. Your Jeep will do everything it can to help you get to your destination. No matter what obstacles and terrain is in your path.

Jay

 

[Wbino]

LSD = 4WD Auto correct?

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