Thread: When to lock CDL

In 4WD with an open centre diff all four wheels are driven unless one or more wheels spin. Then the amount of power going front to rear can vary wildly
With the CDL locked the front to rear split is fixed at 50:50. Even if say a rear wheel spins the amount of power going front to rear is still 50:50.
TC then stops that wheel from spinning.
I often, but not always, lock the CDL on unsealed roads. Having TC helps. In high range I might lock it on skatey or corrugated roads. In low range I usually lock the CDL.
A Defender in low range with CDL locked, TC and anti-stall doesn't have much of a problem with wheelspin, in my experience.

Oh this is so funny some need some education in CDL operation obviously Mr T

Umm...1nando, i watched the vid ...in that situation you would defiinately have the CDL locked anyway.

...On bitumen I reckon it would be a pretty rare occurence to have two wheels in the air LOL - on bitumen front and rear wheels are being driven, enabled by centre diff (different to a Tojo with front hubs unlocked - 2WD).

Originally Posted by 1nando

With centre diff unlocked power only ever goes to either the front or rear axle not both at the same time. .....

Well, yes, but at the same time no!
When one wheel is unloaded like AStPW showed(wheel in the air) of course this happens, but it doesn't always happen on a gravel road, as the guys have already noted.
Many moons ago, my dad had an 8t fridge van truck that used to play up in the morning. To get him going he enlisted my help in the RRC(LT95 back then, but same difference when I fitted a LT77+LT230).
The driveway was about 50m all gravel(red scoria to be exact). The first 10m pulling the truck, diff unlocked 2nd low, always had the RRC spin one front and one back wheel. But it would pull and spin and pull and spin .. etc.
There was just enough traction to get some momentum, but also not quite enough to have tarmac like grip. But definitely used to get one front AND one rear spin up every time.
What you relating is when you definitely have a situation where one wheel is going to have far less grip than any of the other 3.
That argument is not about gravel road driving.
What AStPW showed in that video is '4WD', not 'gravel road driving' as discussed by many and sundry.

It was Rick130 that commented on the understeer when the CDL was locked.
In my Rodeo ute, I had the same annoyance on gravel. Long wheel base gave understeer all the time, unless a lot of right foot was used to deliberately minimise it, but with the transfer set to 4WD(and front hubs locked), it became pig understeer. I hated it.
Same with the Frontera I had after that too. Much less pig understeer than the rodeo, as it had a shorter wheel base. In 2WD mode(it had a rear LSD) it could be balanced OK(nothing like a Disco or RRC) but with the 4WD switch set, a bit too much understeer.
Frontera and Rodeo are not renown for handling excellence too tho!

On a flowing gravel road where the chances of lifting a wheel is pretty much zero, you will get pretty much 50/50 drive split between axles with centre diff unlocked.
On a 4WD track, where you find boulders that can lift a wheel, obviously an open centre diff isn't the ideal way of tackling the obstacle.

Originally Posted by rangieman

Oh this is so funny some need some education in CDL operation obviously Mr T

I reckon the humorous turn in the thread is due to the possible misunderstanding as to what type of gravel road driving is being discussed.
I took it to relate to fast flowing gravel road driving, not the type that may throw up an obstacle like a boulder that could see a wheel lifted.
To me, the second example wouldn't usually be described as a gravel road, but more as a track .. 4WD track.
Hard packed gravel road, understeer, and references to that effect usually equate to fast flowing gravel roads.

Originally Posted by AK83

Well, yes, but at the same time no!
When one wheel is unloaded like AStPW showed(wheel in the air) of course this happens, but it doesn't always happen on a gravel road, as the guys have already noted.
Many moons ago, my dad had an 8t fridge van truck that used to play up in the morning. To get him going he enlisted my help in the RRC(LT95 back then, but same difference when I fitted a LT77+LT230).
The driveway was about 50m all gravel(red scoria to be exact). The first 10m pulling the truck, diff unlocked 2nd low, always had the RRC spin one front and one back wheel. But it would pull and spin and pull and spin .. etc.
There was just enough traction to get some momentum, but also not quite enough to have tarmac like grip. But definitely used to get one front AND one rear spin up every time.
What you relating is when you definitely have a situation where one wheel is going to have far less grip than any of the other 3.
That argument is not about gravel road driving.
What AStPW showed in that video is '4WD', not 'gravel road driving' as discussed by many and sundry.

It was Rick130 that commented on the understeer when the CDL was locked.
In my Rodeo ute, I had the same annoyance on gravel. Long wheel base gave understeer all the time, unless a lot of right foot was used to deliberately minimise it, but with the transfer set to 4WD(and front hubs locked), it became pig understeer. I hated it.
Same with the Frontera I had after that too. Much less pig understeer than the rodeo, as it had a shorter wheel base. In 2WD mode(it had a rear LSD) it could be balanced OK(nothing like a Disco or RRC) but with the 4WD switch set, a bit too much understeer.
Frontera and Rodeo are not renown for handling excellence too tho!

On a flowing gravel road where the chances of lifting a wheel is pretty much zero, you will get pretty much 50/50 drive split between axles with centre diff unlocked.
On a 4WD track, where you find boulders that can lift a wheel, obviously an open centre diff isn't the ideal way of tackling the obstacle.



I reckon the humorous turn in the thread is due to the possible misunderstanding as to what type of gravel road driving is being discussed.
I took it to relate to fast flowing gravel road driving, not the type that may throw up an obstacle like a boulder that could see a wheel lifted.
To me, the second example wouldn't usually be described as a gravel road, but more as a track .. 4WD track.
Hard packed gravel road, understeer, and references to that effect usually equate to fast flowing gravel roads.

All this dick waiving is just making it way to complicated when it is not rocket science at the end of the day .
Just lock the cdl when traction is likely to be lost and that icl wet skid pads FFS

Originally Posted by rangieman

.... Just lock the cdl when traction is likely to be lost and that icl wet skid pads FFS

Going by Dave Ascrosft's comments, I now just do just as a matter of forcing myself into a habit.
if the gravel section is short, I don't.
eg. some months back up the Silver City hwy, it's basically all bitumen, but one small section near a research station was gravel. Even tho I have it in my mind to set the CDL to lock on gravel, for the 5-10klms of gravel not worth the effort.
But doing hundreds or thousands of klms on, hard packed, fast flowing gravel .. even tho I know I don't really need it!, I still lock it anyhow.

Now(the question needs to be asked)!! .. where do we find these skid pads to have some fun on?

1nando says...True 4wd drive spilts power 50/50 between the front and rear axle. It is impossible to have 50/50 power split between axles with a open centre diff.

I am a bit hesitant to add fuel to the fire here, but will anyway. I dont think the above, and some other posts are perfectly accurate

Physics...

Power = torque x speed

Ignoring internal friction in the drivetrain, I actually think that with no front or rear locker and CDL unlocked, the vehicle shares torque equally amongst the 4 wheels and speed in each is independent. In conditions where a wheel may slip, the torque in the system is limited to the torque that the wheel with least traction can apply, and all others receive this much torque. In a circumstance where one wheel is no longer in contact with the ground, and assuming no application of the brakes either by man or machine, then all wheels receive zero torque. So if in slipping conditions one wheel has some traction but is slipping, then this will be going at higher speed, with same torque, so will get more power. The 3 others get same torque, less speed, less power.

With the CDL locked, the speed in the front and rear propshafts is equal, and with no other locking diffs, then torque is shared equally left and right in each of the axles, but torque in front may be different to rear. Within each axle torque is same left and right, but speed may be different

And, my driving, which is based on opinion, and less experience than others here....

Depends on the circumstances. For example on sand, when travelling alone, and with no winch, no maxtracks etc, ( a long time ago) I always preferred to drive my RRC with diff unlocked, so that if i started to get into trouble I would get some warning of trouble, and have something to "deploy" i.e. CDL, rather than drive with CDL locked, get bogged and then have nothing but a shovel to attack the problem with.

And a closing thought

I think there are too many differing circumstances in the real world to be able to reduce this a simple blanket rule, which is why there are differing opinions in this thread. Its also why you can't learn all this stuff with 5 minutes of experience, and thats what keeps us all interested.

Mark
edited 'cause I can't quote competently

Depends on the circumstances. For example on sand, when travelling alone, and with no winch, no maxtracks etc, ( a long time ago) I always preferred to drive my RRC with diff unlocked, so that if i started to get into trouble I would get some warning of trouble, and have something to "deploy" i.e. CDL, rather than drive with CDL locked, get bogged and then have nothing but a shovel to attack the problem with.

I found (From experience) that waiting until you need 4WD to engage it is a recipe for disaster in sand because by the time you have engaged 4WD/Locked the CDL it is usually far too late.
I see examples of this every holiday season where people get bogged and THEN engage 4WD to try and get out of trouble, The sands on WA beaches are Not that forgiving and I have seen people hopelessly bogged in sections of sand where if they were in 4WD mode they would have had absolutely no difficulty getting through.
I would think that everyone who lives in the Geraldton area that has been to the Greenough river mouth has seen this sort of thing happening at one time or another.
The same applies for many of the popular beach areas, Inskip point is well known for its bogging's and I wager that many of those bogging's are a direct result of failing to engage 4WD until it is too late.

If you are doing any beach driving ALWAYS engage 4WD/CDL and drop your tyre pressures to suit the conditions.
YES it is a PITA pumping up your tyres again when exiting the beach But it is No hardship putting the truck into true 4WD mode and doing this is much more preferable to swinging on a shovel in the middle of summer and retrieving the buried maxtracks or worse Begging someone to pull you out of the mess (That's IF you can find anyone).