Base Model Rear Locking Differential

[Austin26]

The short answer for those that don't want to read everything: in the front, up to about 2.5:1 TBR. The rear, anywhere from maybe 2:1 to 4.5:1, depending on how the vehicle will really be used. More detail:

For background, on the most basic level, the term "torque bias ratio" is what we use to characterize a differential's performance level, ie "locking effect". Torque bias ratio relates how much friction the differential generates in order to provide resistance to wheel spin, and transmit torque between outputs. So, the higher the TBR, the "stiffer" or more aggressive the differential is tuned to be. For reference of scale, an open diff is roughly 1:1 TBR; a locker is infinite:1. So while we're definitely playing on the low end of the scale when dealing with even very aggressive LSDs, that scale is somewhat logarithmic.

That's important, because it has direct bearing what is most appropriate for a given application, based on how the vehicle is to be used. The right choice is, as all things in engineering, whatever compromise works best for your needs. For starters, its important to understand that the more aggressive the differential is, in terms of either TBR or preload, the more it will also resist allowing the vehicle rotate, change direction, turn, etc.

This is especially important if the driven axle in question is also a steering axle. The more aggressive the differential in a steering axle, not only will the vehicle be less inclined to change direction, but it will also be more difficult to steer - the steering will be heavier, awkward, possibly having a gyroscopic feeling. It can also accentuate torque steer. Anyone that has tried to steer a 4x4 with a locked front diff will attest to impact it can have. So, for that reason, I would set a practical cap of about 2.5:1 TBR on a front axle.

There are varying schools of thought, of course, and the design of the suspension and steering systems have bearing on this too. But in my experience, something around 2.5:1 is still benign enough to be liveable in a typical SLA front suspension. It will still have a slightly heavier steer feeling than an open, but you get used to that quickly. I ran one like that in my Mountaineer for years. But if the suspension is torque steer prone, something more like 1.7 or 1.8:1 would possibly be a better choice.

In the rear axle, TBR can be more flexible (bearing in mind the comments of the first paragraph). It really does depend on what you're doing. The higher the TBR is, the less you'll spin a tire. The more torque you'll be able to send to the high traction side. The more readily and effectively it will respond to brake modulation (or traction control intervention), should it come to that. But also the more quickly you'll be able to saturate the traction limits of both tires in quick succession. This later point is more applicable driving in bad weather, snow, etc. A high TBR diff will "snap" on the driver fast, with less warning, possibly leading lateral instability / tail slides / oversteer skids.

So for that reason, and this may seem counter-intuitive, but high TBR levels are more beneficial with higher traction surfaces. You can take better advantage of higher TBR when the overall traction limits are raised. For a daily driver in all weather conditions, the 2-2.5:1 is still a good pick. For more aggressive driving in sportier cars, closer to 3:1 seems to be a good starting point. For all-out track cars, 4:1 might be desirable, but it depends on the chassis set up and track you're driving. Higher TBR can work off-road as well, where it can be beneficial when there are wide splits in traction on varied surfaces (grass/dirt/mud/gravel/etc). I think in general off-road (non rock crawling) situations, 4ish:1 is probably the practical limit if combining with a daily driving duties. I'm talking on fire roads, trails across open fields, going to the beach, prerunning, or even rallying, etc. But you need to be aware if the TBR is high, it will come with the aforementioned challenges in winter use.

Of course, I say all that, but the V8 Mountaineer I mentioned above also had a 5:1 TBR, high preload diff in the rear end. Combined with the vLSD transfer case, the 2.5:1 front diff and studded snow tires, it was extremely capable in places I had no business being. But that also had direct bearing on the skill level needed to stay on top of what the vehicle is doing. I often come back to the question of "what would I want my mother to drive?". And that would be back to something like 2.5:1 TBR...

It's difficult to understand how it will feel to steer an equipped vehicles and what it feels like when it kicks in as I've not driven one before, only a Ford t-lok. When you say the steering is heavy do you know why that may be? If it isn't being engaged I don't understand why it would feel any different than an open. While driving with normal amount of traction the worm wheels and gears on each side do their own thing as I understand it, just like an open which is obviously useful for turning. Further, does heavy steering imply a less comfortable daily driving experience?

Without having driven a helical equipped car I'm also not sure how it would feel when it kicks in. I don't want a violent over correction while driving on the highway in the rain, but I'm pretty sure they wouldn't be popular if they weren't safe for a daily driver.

If one drove an "aggressive" TBR helical, could you describe what that may feel like / what happens to the vehicle? Like I said, I had a t-lok on my Mustang so that's about all I know.

I'm guessing a mellow TBR would just feel like it isn't working much as an LSD but please correct me if I'm wrong.

For a fair number of us on the site I'd say that an LSD instead of lockers would be more beneficial the more I learn about all of the differences. Less expensive, practical, a bit safer if designed appropriately. I'm surprised and disappointed Ford didn't offer an option for it. And yes I did read your post or maybe it was someone else's who talked about why they're being phased out and how lockers are more marketable. Still unfortunate. I am feeling more comfortable with going with a base and regear and install LSDs.

Is the reason why a helical won't lock when one tire is hanging up in the air is because zero times whatever the TBR is is still zero force applied to the other wheel? Pretty sure that's why but wanted to check.

For anyone interested, here are the helpful videos I've watched and articles I've read about helicals. EDIT: I can't find the articles in my history right now but when I'm at my computer I'll try to find them. Some are dry, well most are, but one has video of an SUV with differential comparisons on the same terrain. Things are clicking better for me so maybe it'll help y'all.







Rocketeer Rick, does this video seem accurate to you? Seeing it in action is selling me pretty well on the idea.

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[Rocketeer Rick]

OK, let me start with this: I think the producers of the last video used the term "Torsen" in the general sense; I'm not aware of Torsen-branded products being made for Lada models (though Torsen Europe has some oddball things over the years). The first "How it works" video gets right the part that Torsen is a trademark of JTEKT. After that, it repeats the old fallacy about worm gears and wheels, which is probably what led you to previously believe that they were self-locking when a tire is off the ground. But this is an old inaccuracy that has been tough to kill.

The Engineering Explained video gets it pretty close to the mark, and it sounds like he read and drew from the SAE white paper that used to be on the Torsen site. He seems to have gotten past the worm wheel myth (though his earlier videos have touched on that). He's right that's all about the friction, and that's true of any LSD. The practical differences between types are how they generate and manage friction. This is what I've been trying to get at.

Where EE misses the mark, somewhat, is that the crossed-axis design he is explaining (as well as the first video) is obsolete tech. Very few Torsens are still produced with the old Type-1/Type-A crossed axis configuration today, and those that are are old holdover applications from the '80s. Any newer design Torsen are of parallel axis configurations of varying sorts, depending on application. To be fair, his video is pretty accurate (though he sort of skipped/glossed over the gear to gear mesh contribution), just no longer applicable.

Parallel axis Torsen models still work by generating friction as a result of forces on the gearing that are derived from torque application. They're just applied a little differently than on crossed-axis gearing. But the fundamental principle is the same, which is what I've been describing here and there - friction from gearing forces provides resistance to spin. By using friction to resist spin, you can support torque transfer without slippage.

As far as how it feels in a steering axle, by heavier steering, I meant just what that implies. There is more weight to the steering effort. The differential adds resistance to slip, which in turn means more resistance to direction change. Also, by allowing more torque to go to one side than the other (by allowing a greater than 50/50 torque split), you'll have one side - the one that probably is more loaded by weight transfer and has better traction - that has more torque translating into a drive force at the tire patch. This is where steer geometry comes into play. The tire contact patch to the road is offset from the pivot point (this is steering castor), and the offset distance multiplied by the drive force at the contact patch creates a torque about the steering pivot.

Since a biasing diff allows this steering torque to be greater on one side than the other, the left steering torque and the right steering torque are less balanced. With an open diff, the 50/50 torque split would mean they'd mostly cancel each other out. When you bias more torque to one side, you need more steering effort to overcome the inbalance than with an open diff. That increases the perceived feel of the force the driver has to apply, which is what gives the steering a heavier weighted feel. It is also what can lead to an increase in torque-steer feel if the traction is real unbalanced between the sides.

This somewhat heavier steering feel would only be omni-present in a full-time AWD/4WD (or FWD) scenario. When a part-time transfer case is used, then the differential won't have any real influence on steering feel, because there's no torque across it. In someways, the full-time model is better because the steering feel is consistent at all times. When I drove that in my full-time Mountaineer, you get used to it, it because part of the vehicle's baseline.

Whereas the difference would be more apparent when it only is used sometimes in a part-time situation. Though when I've driven that in a Ranger (which has the same suspension/steer geometry as the Mountaineer), it would also coincide with the front being locked to the rear since it had a manual t-case. So that transfer case lock will add enough steer resistance that it might mask the diff contribution to the issue, so the driver would feel like its all lumped together in 4x4 mode.

But with that said, you won't feel the diff "kick-in" per se, it isn't a Gov-Lock that abruptly engages. Generally, other than the slight increase in steer weight, the existence of the biasing diff would be quite transparent to the driver unless it was a pretty high TBR (but as described before, the operation would be progressive). If you stuck with 2-2.5:1 in the front end, most people wouldn't know its there. In fact, that's the best thing to happen from the engineer's point of view - the driver doesn't know its there because its isn't doing anything objectionable. When driven through mixed traction situations, it responds instantaneously to conditions, so you still don't know its there because it prevented anything objectionable from occurring. It basically does what its supposed to do so the driver doesn't have to think about it.

Regarding a wheel in the air, you're on the mark that Zero x any TBR = Zero. The EE video had a pretty good description of TBR affect. But more fundamentally, the differential's function is contingent on creating friction as a result of supporting a torque load. I.E., the input torque is key to creating the friction forces. So to support the torque load, it has to have something to react against on both outputs (comes back to Newton's 3rd law). If one output has no resistance, then the gearing can free wheel and there's nothing for the torque to load against, so no torque is supported. No torque supported means no friction forces. Thus Zero X TBR = Zero.

Is this making sense?

 

[kodiakisland]

I rarely know when my True Trac is engaged. I just know it goes where I point it.

 

[Rocketeer Rick]

I rarely know when my True Trac is engaged. I just know it goes where I point it.

And that is exactly the point!

 

[grimmjeeper]

I rarely know when my True Trac is engaged. I just know it goes where I point it.

Same here. I have a 2001 Cherokee with TrueTracs front and rear. It also has the NP242 transfer case that has both part time 4HI (tcase locked front to back) and full time 4HI (AWD). On the road it's an absolute beast in mixed snow/ice that we get in the winter. It's also very pleasant to drive on dry pavement. Beyond that it does reasonably well on moderate to difficult trails in the mountains. At least until I lift a tire or two.

 

[Austin26]

@Rocketeer Rick Yes, it is making sense. I just told my wife actually that I have to read your posts, digest them, look things up that I don't understand and then re-read the next day before it clicks, ha. Otherwise I'd be quite lost.

That's a good point about the steering feeling heavier or torque steer only happening in 4x4 setting. It could be another factor but Humvees also are harder to steer in 4LO rather than in normal 4HI. I'd also read briefly that they had helicals front and rear as well. Interesting. Maybe I have more experience with them than I think.

The "kick-in" or lack thereof makes more sense to me now. Basically, just because one wheel is slipping a lot won't mean that a bunch of torque will get immediately dumped on the other wheel. No matter the throttle, the torque sent to the gripping wheel is maxed out at your TBR, and that's only multiplied from the little amount of traction you're getting on the slipping wheel. Either way, not probably a substantial amount. For some reason I was visualizing in the correct proportions but a larger torque value to start with on the slipping wheel. I doubt all of this makes sense. I got it, basically.

The part where you mentioned that the point of engineering is to have the driver not even know it's working / any difference from before is indeed what I'm after as well. That, and obviously I don't want to make a brand new vehicle less safe or predictable than before.

As far as diagonal terrain where one wheel is in the air in the front and the same for one wheel in the rear, at this point one would be at the mercy of traction control, correct? There would be nothing else with a helical setup that could help with that besides getting into the suspension and articulation, tires, etc.

I will have questions about the T2, T2R, and T3 / C. I haven't done enough reading yet to know what I need help on, except that we were grouping all LSDs that were Torsen/TrueTrac/other brand names into the group "helical". Well, with the development of the T2 and beyond, do you categorize them all the same? Which type (under Torsen, Eaton, or any other brand) is likely the best for a daily driver Bronco owner such as myself?

Is this picture the T1? I'd have guessed it's the parallel type which is T2.

https://www.flashoffroad.com/features/Torsen/Torsen_white_paper.pdf

 

[Rocketeer Rick]

Your pictured image is, indeed, a Type-1/Type-A design. Apologies for the confusing nomenclature; the original naming convention was numeric (well, with the introduction of Type-2, prior to that it was just "Torsen"). But that evolved over time and is now primarily letter based. To the point of your question - the pairs of smaller gears that connect the left and the right together are turned 90 degrees to the axis of axle rotation. This is thus referred to as "crossed-axis". A Type-B parallel-axis model would have them oriented on a axis parallel to the side gears and axle shafts:

This is the form most other helical diff manufacturers essentially follow. As mentioned before, crossed-axis designs are out of favor today.

Re: HMMVWs using helical diffs, they have had Type-A diffs in both axles from the very beginning and still do to this day with very few exceptions. It is worth noting that the vehicle's transfer case has a full-time "AWD" mode when not locked. So, it would stand to reason that steering would be more difficult between 4LO (with center locked) and 4HI (in unlocked mode), for the reasons I previously described.

I would classify all Torsens (Types A, B [and T-2R variants], C & D) as helical gear types, and helical gear designs more generally as a sub-category of LSD. They individually have their own purposes and uses, and some of them (Type-C in particular) are only marketed as OEM products and not sold aftermarket.

But as far as which design is best for the Daily Bronco, well, that's simpler than you might think - whichever A) you feel suits your needs & B) is available. That might sound like it takes it too lightly or glibly, but what I mean is that there will only be certain choices available for you to pick from.

A TrueTrac offering will be pretty similar to what they already offer for Dana 30 & 44 axles, I suspect. Neither AAM or GKN are likely to enter the aftermarket with their products for this application. Quaife and Wavetrac might, if they see the market worthwhile, and their products will, again, be similar to what they offer now. A potential Torsen offering will be whichever Type that is deemed most appropriate for the specific axle versions. They won't offer Type A, B & D models for it, but rather one or the other.

So, once we see what actually hits the market for these specific axle versions, then you'll have to choose what seems most suitable to you. In the end, it may not matter too much, as the actual driving performance probably won't vary too much. I know that skirts your question, but right now, we don't know what will be offered or what their specific performance characteristics will be.

Oh, also, I apologize if I wasn't as clear in my explanations as I could've been. When you do this long enough, sometimes one forgets that folks that don't live and breath it might not make the same assumptions or leaps of logic that I might default to...

 

[Austin26]

Ah, a post I can understand easily. You've lost your touch (Sarcasm).

I didn't realize there may few offerings and likely one configuration per company. I don't know much about Danas and had assumed that if it was a "Dana XX" that that meant it'll have plenty of interchange parts and aftermarket offerings. But like you said, that'll make a decision easier ha. It's good to hear that there isn't that much difference in the types as whatever is offered will be spec'd for the Bronco. Side note.. When an engineer basically tells me I'm overthinking things.... Maybe I am.

Don't apologize. I'm just happy to learn from someone that sounds like they know what they're doing. Since the words you choose are deliberate I can look up things to fill in the blanks.

Another side note. Do you feel the 4A "advanced 4x4" mode to be comparable to full-time AWD?

 

[Rocketeer Rick]

4A is intended to supplant a true full-time system (ie something that has an actual center diff), to fill the same role. A fundamental advantage of this sort of thing is that it operates without the driver having to monkey with it, but still allow part of the drivetrain to stop rotating so that drag is minimized. Because it operates without driver intervention, it is essentially full time. It just does not operate continuously. This basically the same function as most CUV and AWD sedans that are based on FWD, but its flipped around 180 degrees on the chassis. Again, the intent is drive the secondary axle seemlessly (so its transparent to the driver) while still minimizing drivetrain drag.

I will withhold judgement about how effective the advanced 4x4 is until I've gotten to drive it. I have owed 7 vehicles with a "proper" full-time system, and am now driving a truck that traditional part-time 4WD. I had to sort of relearn how I'd drive it on the snow compared to what I've been used to. So I am anxious to see how the 4A behaves on a short wheelbase (since I'm planning on a BL 2-door). I also just got my first FWD car in 15 years, so I'm relearning that as well...

FWIW, this 4A system has also been in use in the Expedition, Navigator, and fancier F-150s. So, that should be some indication as to how well it'll work on Bronco.

 

[dgorsett]

4A is intended to supplant a true full-time system (ie something that has an actual center diff), to fill the same role. A fundamental advantage of this sort of thing is that it operates without the driver having to monkey with it, but still allow part of the drivetrain to stop rotating so that drag is minimized. Because it operates without driver intervention, it is essentially full time. It just does not operate continuously. This basically the same function as most CUV and AWD sedans that are based on FWD, but its flipped around 180 degrees on the chassis. Again, the intent is drive the secondary axle seemlessly (so its transparent to the driver) while still minimizing drivetrain drag.

I will withhold judgement about how effective the advanced 4x4 is until I've gotten to drive it. I have owed 7 vehicles with a "proper" full-time system, and am now driving a truck that traditional part-time 4WD. I had to sort of relearn how I'd drive it on the snow compared to what I've been used to. So I am anxious to see how the 4A behaves on a short wheelbase (since I'm planning on a BL 2-door). I also just got my first FWD car in 15 years, so I'm relearning that as well...

FWIW, this 4A system has also been in use in the Expedition, Navigator, and fancier F-150s. So, that should be some indication as to how well it'll work on Bronco.

Rocketeer Rick: I wanted to pick your brain. Your posts have been most informative, I've understood about half and retained half of that. So i have a question you've probably already answered but...

Since the cost of the Sasquatch pkg jumped in the B&P a Base Base becomes more interesting again. You have said you believe the ring gear is welded making locker installation more challenging. How about "Lunchtime" or "Lunch Box" lockers like the old LA Lock right? Been a long time since I studied them, but installation should be possible, maybe even easy with the welded ring? Are these still a thing? How is their performance from a function and invisibility standpoint? And do they hold up?

Sorry if you've already addressed...retention issues ya know.

 

[Rocketeer Rick]

The so-called "lunchbox" lockers are, essentially a conversion kit to update an open differential into a Detroit Locker -style positive locker. This type of mechanism is basically a ratchet. That means that if you're applying power, its locked. If you lift, it can freewheel. And it sounds like a ratchet (click click click click) when you turn. The way you get one to differentiate around a corner is to coast. If you give it any throttle at all, it'll lock up and the truck will understeer. And scrub tires.

OTOH, the factory eLocker only locks when you tell it to, so it is a much better compromise for a daily driver. The flip side of that is the lunchbox is a way around the whole welded ring gear thing, assuming one is made for these specific axles. It is a relatively simple install (comparatively) and fairly inexpensive. But it comes with less tangible costs.

Ratchet lockers are usually quite stout, they typically are comprised of massive dog clutches, so they're hard to break. But that's more true of purpose built designs - like the Detroit Locker. The "lunchbox" conversions have to compromise that somewhat to fit into an existing carrier, so are likely less stout. But probably still sufficiently strong.

 

[grimmjeeper]

One thing to add. The lunchbox lockers (all automatic lockers actually) can confuse the traction control systems when they engage. It can cause the system to disable itself and go into some sort of limp mode. I know the Wrangler will just turn off the traction control but otherwise run fine. I don't know how Ford designed the new Bronco's system.

Lunchbox lockers are a cheap way to get traction but they do have downsides.

 

[Ecobeast]

Reviving ye olde base locker thread to see if we have any new info on this conundrum. The specifics on changing over the open diff to a locker are well covered here in the event that the Bronco does use the same welded ring gear as the non-locked ranger. The real question though is is this actually confirmed or still just speculation?

It’s going to suck for the folks that upgrade to a higher trim if it turns out that it’s a bolted ring gear and the switch to the e-locker is simple and cheap. I know I would be 100% going base if that was the case. Instead it pushes me all the way up to BD since the price difference between that and BB with a locker is negligible.

 

[Halfsmoke00]

Reviving ye olde base locker thread to see if we have any new info on this conundrum. The specifics on changing over the open diff to a locker are well covered here in the event that the Bronco does use the same welded ring gear as the non-locked ranger. The real question though is is this actually confirmed or still just speculation?

It’s going to suck for the folks that upgrade to a higher trim if it turns out that it’s a bolted ring gear and the switch to the e-locker is simple and cheap. I know I would be 100% going base if that was the case. Instead it pushes me all the way up to BD since the price difference between that and BB with a locker is negligible.

I was hoping for some DIO accessory list options for this, but it doesn't look like it's going to happen.

 

[Ecobeast]

I was hoping for some DIO accessory list options for this, but it doesn't look like it's going to happen.

I wish we could just get a straight answer out of Ford on this. Since I’m going 2.3 7mt the base would be the best possible starting point. The only other question remaining is how Ford would handle unrelated warranty issues. For instance if the truck eats a turbo it wouldn’t be related to the locker but would they screw you?

I’m still having visions of a perfect base model with aftermarket wheels and 33s and a locking diff. It’s literally all I need.

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