Limited slip differentials in sand?
 

Hey,

Has anyone experience with Truetrac Limited slip axle differentials in sand? Would they work against me?
I don't think they will bother to much. Maybe when the added 'stress' may cause you to break trough the top layer of soft sand when turning.

Any ideas?

Thanks,

Rob

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[This message has been edited by Robbert (edited 06 January 2003).]

I have Truetrac fitted to my Land Rover. I rate it pretty highly, but it's always difficult to tell. You do need to come off the accelerator very quickly as you get stuck, as you can get down to the axles in an instant.

I am getting a second vehicle, and will fit same again...

Sam.

Open diffs are only a problem if there is a significant traction difference between the two wheels. Or, to put it another way, locking a diff (or limiting the slip) will have zero effect if there is the same resistance under each wheel.

Usually in sand (but not always), there is pretty uniform traction under the wheels, and a locker or an lsd does not confer much advantage. In fact, in sand, traction is seldom an issue - flotation is the crux of the problem.

A locker or lsd is also very unlikely to be to your disadvantage either, though.

One thing to keep in mind, though, is that where one wheel *has* got very little resistance, and it is *freely* spinning, then locking the axle diff (or limiting its slip) will put much more strain on the other wheel - the one that is doubtless buried to the hub. Be gentle with the clutch and throttle! http://www.horizonsunlimited.com/ubb/smile.gif

Regards,

Michael...

As mentioned above, flotation is the key. Install as wide as practical tires with a not too aggressive tread (i.e.; mud tires are a bad idea) and air them down.

To answer your question, the Truetrac limited slip is a very robust and reliable unit. It uses springs instead of clutches so clutch wear and replacement is a thing of the past. That also means you don’t need any limited slip oil additives which are basically unheard off in most parts of the world.

Since the Truetrac is a replacement differential carrier you’ll need to remove your old open carrier, take the gear ring off and install it on the new Truetrac carrier. Therefore the new carrier has to be adjusted from scratch to your old ring, pinion and axle housing. Adjustment of the new carrier is crucial and includes things like: backlash, run out, pinion depth, gear pattern, etc. This somewhat expensive procedure has to be done by a very qualified mechanic with the right tools.

IMHO Limited slip differentials are of great benefit when driving in sand especially when crossing dunes, extracting another vehicle or starting to move from a stop in soft sand. With an open differential, the instant a tire looses traction it will start to spin and dig itself in, as the open differential will transfer all the engine torque to the spinning tire. That’s what it’s designed to do; send all the torque to the tire that rotates faster. A limited slip will make sure that some of that torque goes to the other wheel, which might still have some traction. A locker does even better since it splits the torque right smack in the middle between the 2 tires.


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A.B.

OasisPhoto.com – Images from the Magical Sahara.
ShortWheelbase.com – Jeep preparations.

In addition to my last, mine is a locking diff (rather than limited slip). Sorry, didn't read the initial post correctly!

Sam.

I would agree with SandyM's reasoning - my impression has been LS or locking diffs make little difference in soft sand compared to flat tyres. My TLC difs locks actually seized up through lack of use over the years. They were handy tho for really burying a car to try out various recovery tricks for the D. Driving vid.

btw, off the point but I can highly recommend stretchy ropes for quick sand or crest recoveries - i'd always though these were rather dangerous but the strain in sand appears minimal: they are by far the least effortful way of yanking a car out with another car as long as you mounts are solid.

CS

Thanks all,

I was not considering the TT's to improve sand driving. Just wondering about possibly negative impact. Since it doesn't seem to bother to much I think I'm going ahead with it. And also because I can pick them up in the US at about half the price.

I'm definitely a fan of the kinetic ropes. Well used you can have a soft recovery out of a rather sticky situation. Only used them in the mud though.

Rob


[This message has been edited by Robbert (edited 08 January 2003).]

Hi A.B.,

I've read many of your posts, and given your excellent grasp (and experience) of things 4x4, this is a rare opportunity to, ahem, correct you on a fairly crucial factual point. :-)

If an open diff did what you describe above, I would agree that a diff lock (or lsd) would make a significant diffrence to traction in sand.

However, an open diff doesn't send all the torque to one side. In fact, it always maintains equal torque on each side. It allows one side to rotate faster than the other, but the torque (and hence the motive force) on each half-shaft will be equal. The torque on the shaft is a product of the driving force from the tranmission, meeting a resistive force from the ground. If one wheel is on (say) ice, the resistive force on that shaft will be very low (not high), and the wheel will spin. An open diff is incapable of supplying more torque to one shaft than the other, the torque on the other wheel will be limited too, even though it is on firm ground with a high tractive resistance. The torque on the "good" side will fall to the same level as the wheel slippery side, and it may stop turning.

Even though it may be stationary, that wheel is still contributing the same amount of forward motive force as the spinning wheel. (Just as a person pushing a vehicle may be contributing even if he is not actually succeeding in moving).

Hence, with an open diff, both wheels always contribute the same motive force.

If you lock the diff, then the only limit to the torque on a sideshaft is the resistance from the wheel (assuming you have plenty of power from the engine and gearbox). Each shaft has a different torque. The one on the ice is still low, but the other one is no longer limited to the same low figure. It will rise until either the vehicle starts to move, or that ground also gives way, and both wheels are spinning.

The important part of the above is that a diff lock only adds torque to one of the wheels, and it often only adds a very small increment.

Two examples to illustrate the extremes (using a 2WD with a diff lock, for simplicity):

1) Assume the vehicle needs a total torque of 1000Nm to start to move up the slope.

* Left wheel on wet ice, maximum of 100Nm of torque supported before the wheel starts spinning.

* Right wheel on good rough concrete surface, maximum of 2000Nm of torque supported before the wheel starts spinning.

Open (unlocked) diff: torque on left wheel 100Nm, torque on right wheel, 100Nm, Total 200Nm, vehicle stuck.

Locked diff: Torque on left wheel 100Nm, Torque on right wheel 900Nm (then the vehicle starts to move, so no increase in torque). Total 1000Nm, vehicle moves.

Here, where there is a big difference in the traction conditions, a locker increases the torque by a big amount - from a total of 200Nm to a potential total of 2100Nm (though not needed).

2) Assume same total torque requirement of 1000Nm

* Left wheel in sand, ground supports 400Nm before giving way.

* Right wheel in slightly firmer sand, ground supports 500Nm before giving way.

Open diff: torque on left wheel 400Nm, torque on right wheel, 400Nm, Total 800Nm, vehicle stuck.

Locked diff: Torque on left wheel 400Nm, Torque on right wheel 500Nm, Total 900Nm, vehicle stuck.

With the small difference in the traction, a locker increases the torque only marginally - from a total of 800Nm to 900Nm. Obviously I chose the specific figures to illustrate the point, but notice how in the second example there is only a very narrow range of traction conditions where the diff lock would make a difference.

With just a bit more traction under the left (spinning) wheel (100Nm extra), the vehicle would have driven out even without a diff lock, because the total torque would increase on both wheels - i.e. by 200Nm.

This may seem over-technical, but the salient points are that a locker provides the SUM of the two torques, whereas an open diff provides twice the LOWER torque. Therefore a locker offers a very small advantagee when the traction under both wheels is similar, and a great advantage when there is a big difference in traction condtions.

For the record, I have lockers in my vehicle, and have occasionally found them useful in sand. But they are much more value in mud, and absolutely indispensible in rocks or axle twisters, where wheels lift off the ground.

Anyway, like too many of my posts, I have rambled on... sorry! :-)

Regards,

Michael...