Additional Traction - at cross roads with options.

I think we all need to remember to stay on topic - this is about options for Weeds' vehicle, not a general locker debate. H's already mentioned that ATB isn't an option, and he's already decided on locking the rear salisbury.

I recently had the pleasure of following Weeds through the Vic High Country, and my Detroit had had a very easy life. That is until we hit Blue Rag, and understandably he suggested I go first due to me having the lockers, and both of us not knowing what was ahead of us. In the end, I was very impressed with how easily (and slowly) the Detroit got us to the top, even in a heavy 130 at max GVM.

Whilst not an essential side-track, it was a fantastic experience, and to me an example of what overlanding is all about - experiencing sights. We certainly didn't need to do that track, but I'm damn glad we could and did. I imagine some may argue you could also see a lot of the world with only a 2WD, but imagine all the sights you would have to miss out on, simply because a 4WD was necessary, or maybe only even preferable. Hence, whilst it might be true that a rear locker isn't necessary, I personally believe a rear locker is certainly preferential... at least to me! I can only imagine the feeling sitting at the bottom of Blue Rag, and deciding to miss out because I didn't have the belief in my vehicle (or skills!).

I was happy to admit to Weeds that I'm unsure a Detroit is the best option. It suited me at the time of choosing - under $1000 fitted seemed like great value, and all the research I did suggested it was a sound option. I've never has any other locker, and can definitely see the pros and cons in each. Personally, if I didn't have a Detroit, I'd have an ARB. A major factor being that they are very common and widely distributed, with a pretty good reputation. As much as I love how the Detroit is always already engaged when you need it, a being able to turn a selectable locker on and off is a good thing I reckon.

Anyway, I'm enjoying the on-topic replies, so keep them coming!

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Originally Posted by weeds

I don’t quite follow....how does an ATB in the front rely/work with the locked rear?

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When in 4WD, the front and rear propshafts are forced to rotate at the same speed, so the average speed of the front and rear axles are forced to be the same. (Lets say 10kmh front and rear axle speed))

The rear being locked means that both wheels are also forced to spin at the same speed (with 10kmh axles speed, both RL and RR wheels are forced to spin at 10km/h)

The front having an ATB means that there is unlocked diff action and either the front wheels are travelling at the same speed (ie the ATB is inactive so RL/RR@10kmh, FL@10kmh and FR@10kmh ) or the front wheels are spinning at different speeds (eg RL/RR@10kmh, FL@7.5kmh, FR@12.5km when you're on a dodgy track but still moving although both rears are still spinning faster than ground speed, or more to the point RL/RR@10km/h, FL@20km, FR@0kmh when you're on a dodgy track, not moving anymore and the ATB disengages because it has no torque to bias across).

The ATB works by torque biasing. Torque bias, is the value of torque the wheel with more traction gets compared to the wheel with less traction. With a Ashcroft ATB this is 3:1, Eaton Detroit Trutrac 3.5:1 and Quaife I believe is 4:1, so if you do got this way, at least look at the Detroit Trutrac (known as "TT", the Quaife is expensive and iirc runs a funny spline halfshaft. Never the less you can see that by design, these diffs are much better suited to road and rally race cars on race tracks rather than 4WD's or in 4WD's to those with ETC, but I still don't think they are at all comparable to the positiveness of locking diffs. If you do want ATB front and rear I believe you can get the TT for Rover and Salisbury axles - edit, sorry, I think this is for the 4.7 Salisbury..

Just press the front locker button on the dash and it just works 100% when you want.

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Originally Posted by rangieman

In some down hill situations brakes are not advisable .
The way i drive is engine braking and foot brakes are a last resort and with that abs does not help in these situations [wink11]
[...]

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What situations are you thinking of? (Regards, "...some down hill situations brakes are not advisable")

In the situation I described - steep downhill, light braking intended to load the driveline a little in case a wheel lifts - the driveline is fully engaged, engine braking is still in play and overcoming the light braking forces, centre diff locked of course. The trailing braking just ensuring no upset should a wheel lift. Always a good outcome with this.

(Taking it to extremes... if one had to come to a complete stop on a steep downhill its safest to brake til engine stalls. No use of clutch, no disengaging the transmission.)

Of course if no risk of lifting a wheel - just let engine braking do its thing. (I don't have ABS in any case - so its no help to me.) :)

Certainly any action that disengages the driveline, in the scenario I describe, is outright dangerous (things like pressing the clutch pedal, as mentioned in my post)

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Originally Posted by Slunnie

Something worth considering... a ATB requires wheel spin to work [...]

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Absolutely not true! Please!

The best way to think of an ATB is:
- Its an open diff that just doesn't do the differentiation bit very well (unless one side of it is completely unloaded).
- And the more "weight" on the drive line (the more torque applied) the "worse" an ATB will be at allowing differentiation. Even before there is any difference in shaft/wheel rotational speeds across it.
- i.e. it is actually passively resisting differentiation while you drive a straight line, BEFORE you even ask it to differentiate. BEFORE any wheel loses its grip on the trail. i.e. Preemptive!

What you may have been thinking of is that, as (preemptively resisted) differentiation occurs (often a VERY good thing denied by a full locker) - the helical gears in some ATB designs like the Ashcrofts and Truetracs get driven harder into the ends of their carrier pockets and that increased friction INCREASES the torque bias ratio a little.

An ATB doesn't need any rotational difference (e.g. wheel spin as asserted in the quote above) across it to work. It needs load on the driveline (torque applied). The more torque applied, the more it will resist differentiation. Before you even ask it to differentiate, before any wheel spin occurs.

To bring this back to the OP's questions - now myths about ATB's are hopefully dispersed:
- good move going to a full air locker in the Sals rear. Even if there were an ATB option, your clarified use cases would need to have a fully locked rear axle at times and an ATB can never deliver that. (ATB combined with traction control would come close but wouldn't be my choice. Am rather glad my Malaysian spec 2013 TDCi doesn't have TC)

- An ATB may be a good choice in the front. But if the use cases you have in mind would mean a full locker is needed in the rear then I'd be thinking you wouldn't want to be farting about with left foot braking to control a loose front end either. Go full locker in the front too!

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Originally Posted by ozscott

Another advantage of ATB is where you have tight turns or side slopes. They allow tighter turns than lockers and don't tend to throw sideways. It's a fantastic point and shoot. In the extremes of say climbing tall rock steps one after another with twists then sure then full locker would remain king but for everything else the ATBs would be better in my view and always 'on'. Cheers

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Absolutely right there ozscott.

I am reminded of the early adverts for anti skid braking: Car on wet slippery road surface, an obstacle suddenly appears, panic braking, not going to stop in time in a straight line, but with ABS can be hard on the brakes and still steer a safe line around the obstacle despite the slippery surface.

ATB's are NOT a replacement for ABS by any standard (my 2013 TDCi doesn't have ABS or TC) - but they have a similar effect (being able to have effective steering inputs in difficult situations) like slippery braking and steep descents. They behave the same whether under engine braking, or engine driving (e.g. driving up slippery slopes, side slopes etc).

Its not the OP's use case - but for anyone else not needing full locked diffs at times (even it if is just the 5 minutes needed to get to the top of a serious rock step a few times a year) - ATBs present a lot of benefit for the other half million hours in a year.

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Originally Posted by weeds

I don’t quite follow....how does an ATB in the front rely/work with the locked rear?

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In short it doesn't. Some real misconceptions in some comments being offered about ATB's. There would be no issue with having ATB's in front axle, or centre diff, or both - with a full locker or detroit (un)locker in the rear. No negatives. some definite advantages.

But as just posted in a previous note here... If you are in a seriously tough climb, implying lots of wheel spin going on, wheels in the air etc - you'd be WAY better off trading-off some of the ability to steer and going for a full locker in the front as well as rear

With an ATB in the front, on a serious hard climb, you don't want to have to deal with lots of front wheel spin. (which reduces ATB effectiveness btw, its not "needed to make the ATB work" as has been asserted). Even if you have traction control to brake spinning wheels for you, the TC still allows some wheel slip (it needs it to work!) and a full locker would just make life in that circumstance so much easier.

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Originally Posted by tact

Absolutely not true! Please!

The best way to think of an ATB is:
- Its an open diff that just doesn't do the differentiation bit very well (unless one side of it is completely unloaded).
- And the more "weight" on the drive line (the more torque applied) the "worse" an ATB will be at allowing differentiation. Even before there is any difference in shaft/wheel rotational speeds across it.
- i.e. it is actually passively resisting differentiation while you drive a straight line, BEFORE you even ask it to differentiate. BEFORE any wheel loses its grip on the trail. i.e. Preemptive!

What you may have been thinking of is that, as (preemptively resisted) differentiation occurs (often a VERY good thing denied by a full locker) - the helical gears in some ATB designs like the Ashcrofts and Truetracs get driven harder into the ends of their carrier pockets and that increased friction INCREASES the torque bias ratio a little.

An ATB doesn't need any rotational difference (e.g. wheel spin as asserted in the quote above) across it to work. It needs load on the driveline (torque applied). The more torque applied, the more it will resist differentiation. Before you even ask it to differentiate, before any wheel spin occurs.

To bring this back to the OP's questions - now myths about ATB's are hopefully dispersed:
- good move going to a full air locker in the Sals rear. Even if there were an ATB option, your clarified use cases would need to have a fully locked rear axle at times and an ATB can never deliver that. (ATB combined with traction control would come close but wouldn't be my choice. Am rather glad my Malaysian spec 2013 TDCi doesn't have TC)

- An ATB may be a good choice in the front. But if the use cases you have in mind would mean a full locker is needed in the rear then I'd be thinking you wouldn't want to be farting about with left foot braking to control a loose front end either. Go full locker in the front too!

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Of course it does require wheel spin or on the road differences in wheel speed, otherwise you can't have torque differences on a differentiating diff.

Do you disagree with these quotes?

This is from Eaton, manufacturer of the Detroit TT

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The Truetrac operates as a standard or open differential under normal driving conditions, allowing one wheel to spin faster or slower as necessary. When a wheel encounters a loss of traction or the terrain changes, the gear separation forces take effect and transfer torque to the high-traction wheel. The helical-shaped gears mesh with increasing force until wheel spin is slowed or completely stopped. When the vehicle exits the low traction situation, the differential resumes normal operation.

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This is from Ashcroft transmissions

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The Limited Slip operation differs from the Locking differential in that it is never 100% locked. It operates by applying torque to the wheel with traction when the other wheel looses grip.
The ATB gear type limited slip relies on the internal friction generated by gear sets within the diff centre, these gear sets are unsupported and designed to be ineffecient. In a normal situation, i.e. on road this diff will be totally transparent, you will not know it is fitted. Provided both wheels have grip no torque biasing takes place, on a bend or corner the faster outer wheel will try to bind up the internal gears but the inner slower wheel constantly unwinds the gears, so again on a corner no bias is felt. On low or no traction surfaces where one wheel has considerably less traction than the other and thus tends to spin, the gear action within the diff causes friction which in turn applies load to the wheel with traction. The amount of torque applied depends on what is termed the 'Bias ratio'. Bias ratios can be varied by different pre-loads and different gear profiles but the most common have a bias ratio of approx 2:1.

The advantage of a Limited Slip differential is that it is totally automatic, no driver interaction and retains drive to both wheels at all times.

The disadvantage of ATB is that if one wheel is totally off the ground ,i.e. no drive then the other wheel similarly because the bias ratio is still 2:1 i.e. 2 x nothing is still nothing. Attempts at changing the preload or left foot braking can then fool the ATB into working or better still if the vehicle is fitted with ATC (automatic traction control) the ATB will enhance the action.

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and this is from Quaife

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Once installed, drivers will notice an immediate difference, as the Quaife (ATB) limited slip differential automatically biases the torque across the axle to the wheel with the most grip. Unwanted wheelspin and slip are greatly reduced, resulting in <snip>

Silent in operation (even during low-speed parking manoeuvres) the Quaife ATB limited slip unit never ‘locks,’ making it progressive, <snip>

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Quote:

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Originally Posted by tact

In short it doesn't. Some real misconceptions in some comments being offered about ATB's. There would be no issue with having ATB's in front axle, or centre diff, or both - with a full locker or detroit (un)locker in the rear. No negatives. some definite advantages.

But as just posted in a previous note here... If you are in a seriously tough climb, implying lots of wheel spin going on, wheels in the air etc - you'd be WAY better off trading-off some of the ability to steer and going for a full locker in the front as well as rear

With an ATB in the front, on a serious hard climb, you don't want to have to deal with lots of front wheel spin. (which reduces ATB effectiveness btw, its not "needed to make the ATB work" as has been asserted). Even if you have traction control to brake spinning wheels for you, the TC still allows some wheel slip (it needs it to work!) and a full locker would just make life in that circumstance so much easier.

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I've outlined pretty clearly the issues with running a rear locked diff with an ATB front in situations where both diffs are being drawn upon and the problem of differing wheel speeds

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Originally Posted by tact

Its not the OP's use case - but for anyone else not needing full locked diffs at times (even it if is just the 5 minutes needed to get to the top of a serious rock step a few times a year) - ATBs present a lot of benefit for the other half million hours in a year.

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...which is why I think ATB’s are better for long distance overlanding if you think I you need extra traction...it’s constant as so well described Tact. ...or in the OP’s case with a sals rear diff - locker rear / ATB centre.

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Originally Posted by Slunnie

When in 4WD, the front and rear propshafts are forced to rotate at the same speed, so the average speed of the front and rear axles are forced to be the same. (Lets say 10kmh front and rear axle speed))

The rear being locked means that both wheels are also forced to spin at the same speed (with 10kmh axles speed, both RL and RR wheels are forced to spin at 10km/h)

.....

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Trying to get my head around a real world situation as described here:

that is for the rears to have to rotate at 10k/h, and therefore force the unloaded front wheel to rotate at 10k/h, the implication here is that both rear wheels are spinning at 10k/h(obvious I know).
But in what situation would you find that the rears are spinning at 10k/h and the car not moving forward by any amount?

For the front ATB to stop working in such a situation would imply that both rear wheels are off the ground and spinning at 10k/h AND the car is not moving forward. ie. you're hung up on the chassis with the air under both rear wheels.

That's a fairly extreme off road situation!

As long as some momentum is maintained, and there is a small amount of load on both of the ATB driven wheels, it doesn't actually spin. As it tries too the other wheel takes up the torque immediately.

The term spin with an ATB is referring to the relative spin .. that is how much spin one wheel has relative to the other on the ATB driven axle .. it's not actually spin.

So back to the situation described above:

If the rears are in the air, and one of the front wheels needs to spin at 10k/h, that also means that this front wheel has to also be in the air for it to actually spin.

If you have three wheels in the air, CDL locked, using full lockers on both axles, and only one wheel capable of driving the vehicle, you then have all the torque of the engine driving that one front wheel.
The two scenarios most likely to occur are that the wheel with grip will most likely spin as it probably hasn't got enough friction to pull the 2T weight of the vehicle, or if it did, you most likely break an axle or CV having all the tractive effort on just that one wheel trying to pull the vehicle out.

Either way that's some pretty extreme off roading.