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  #733  
Old 08-28-2013, 01:42 AM
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Re: Grand Cherokee + sand = FAIL

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Originally Posted by GMWK2 View Post
The sand was not that soft - I think 15 was fine.


Problem was the car - not the pressures.
Admittedly I wasn't there, however I went on previous advice and I'm pretty sure somewhere up the thread it was referred to as "very soft" or similar. Thanks for clarifying.

15Psi on 20"s will get you nowhere on Perth's northern beach tracks, where sand frequently ranges to a genuine "extremely soft" to "undrivable" on certain beach sections. Experience with "responsive" 4WD systems is that running the lower end of the viable pressure range possible will reduce the stress on the 4WD systems activation device. That may be electronic clutches, hydraulic pumps, etc... which I have seen many of call "time out".

Cheers,
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  #734  
Old 08-28-2013, 07:19 AM
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Re: Grand Cherokee + sand = FAIL

Yep, the trainer recommended 15psi. Actually I went with 16psi simply because that was the nearest setting to 15 on my deflator. Might try a tad lower next time. I had another 4x4 trainer once tell the group never to go below 20psi for fear of rolling the bead so opinions certainly vary even among the experts.
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  #735  
Old 08-28-2013, 07:36 AM
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Re: Grand Cherokee + sand = FAIL

I don't have enough experience to comment on whether I should have dropped lower than the 15 psi but I can say the only time the vehicle came to a stop was when it spat a dummy showing up the SERV 4WD light. And when we got the 4WD system going again (4WD low) it moved on without assistance.
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  #736  
Old 08-28-2013, 10:55 AM
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Re: Grand Cherokee + sand = FAIL

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Originally Posted by BobT View Post
Yep, the trainer recommended 15psi. Actually I went with 16psi simply because that was the nearest setting to 15 on my deflator. Might try a tad lower next time. I had another 4x4 trainer once tell the group never to go below 20psi for fear of rolling the bead so opinions certainly vary even among the experts.
Well the guts is Bob, he needs to be thrown the keys to something on 20"s so he can eat his historical arty-fact.

It'd be great to have a "rule" like this, but let's look at the variables which make these kinds of statements simply an easy "one size fits all" answer for trainers who can't be stuffed explaining how deflation works properly. So in brief, these things affect the required pressure:

- Tyre sidewall strength. Different carcass strengths will affect the amount of tyre deformation for a given pressure reduction. LT construction, 3 ply tyres will require a lower pressure to deform than the nice, soft road tyres they use to sell vehicles.
- Tyre sidewall height. With less height to deform, greater pressure reduction is necessary to adequately deform lower profile tyres.
- Vehicle weight. A heavier vehicle will deform a given tyre more at a particular pressure. Lighter vehicles will require a greater reduction to achieve the same deformation, however may not need it as much, as they are lighter.

Deformation is essential in sand as it lengthens the tyre's contact with the ground, therefore reducing the weight per square area. With an effectively lower weight on the ground, the vehicle will be less inclined to sink in. Sinking in is enemy to success, as the vehicle is always trying to climb out of the hole it has sunk into... pushing sand ahead and out. This robs energy and causes the bogging down, or loss of power, often experienced with older, underpowered 4WDs. With vehicles which have more power to apply, the driver can keep adding more and more energy until there is greater force being delivered than there is traction available. Wheelspin is the result... and the vehicle digs in, rapidly exacerbating the situation into an immediate end.

And regarding rolling tyres of the rim:

- First and foremost, it's a function of turning at speed. Lower your speed in turns, and widen the radius of them, are the simple ways to ensure you're safe. If you think a high speed turn is required to make that beach exit, you probably just need to lower pressures some more! Today's 4WDs are not asthmatic and do not need the massive "head of steam" needed previously.
- It's a function of leverage. It's way easier to roll a 15" than a 20" due to the additional sidewall height's increased leverage creating a greater force between the rim (the moving part) and the ground (the stationary part). It's the bead seal takes the brunt of this force... the guts of it is the force accumulates easier on higher profile tyres.

Don't think all trainers have taken interest in newer vehicles, with their changes in equipment and new technology. They tend to turn up in old school equipment... and deliver their old schooling.

Cheers!
Steve
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  #737  
Old 08-28-2013, 11:10 AM
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Re: Grand Cherokee + sand = FAIL

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Originally Posted by mm1964 View Post
I don't have enough experience to comment on whether I should have dropped lower than the 15 psi but I can say the only time the vehicle came to a stop was when it spat a dummy showing up the SERV 4WD light. And when we got the 4WD system going again (4WD low) it moved on without assistance.
MM I need to pull my finger out and check what's changed in Jeep's 4WD system over the last few years, but I think you'll find the reason behind these dummy spits is overheating of the "reactive' 4WD system. In "Low Range", the system is locked (proactive).

Cheers,
Steve
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  #738  
Old 08-29-2013, 01:00 AM
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Re: Grand Cherokee + sand = FAIL

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Originally Posted by Barboots View Post
MM I need to pull my finger out and check what's changed in Jeep's 4WD system over the last few years, but I think you'll find the reason behind these dummy spits is overheating of the "reactive' 4WD system. In "Low Range", the system is locked (proactive).

Cheers,
Steve
As I said I'm inexperienced in 4x4 and not exactly mechanically minded either but I wouldn't expect anything to overheat after being on the sand for only 60-90 secs (if that) which was when it first occurred to me. Another GC that wasn't part of our group on that day didn't even make it to the 60-90 secs.

Mark
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  #739  
Old 08-29-2013, 02:19 AM
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Re: Grand Cherokee + sand = FAIL

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Originally Posted by mm1964 View Post
As I said I'm inexperienced in 4x4 and not exactly mechanically minded either but I wouldn't expect anything to overheat after being on the sand for only 60-90 secs (if that) which was when it first occurred to me. Another GC that wasn't part of our group on that day didn't even make it to the 60-90 secs.

Mark
Understood MM. There's the situation where a system did indeed overheat, like the stinking hot clutches on an X-Trail left in "auto" 4WD in difficult terrain... to where a system protected itself from actually overheating, like numerous wheel-centric traction control systems in the same environment... to the potentially relevant situation where the forward drive system thought it was overheating.

To be fixed in software as it is, and to happen so fast as you've pointed out to me, this must be a glitch where the system is going into a protection mode due to miscalculation of the information from a sensor. This could be rate of temperature rise, absolute temperature, frequency of request for front drive assistance, etc. An incorrect threshold digit in code is able to put a system into protection mode perhaps when it simply got to operating temperature, so to speak.

It's amazing that they don't appear to have tested the sand function at all! I'd actually like it to happen to mine so I could poke around a few scenarios and work out what is behind the error prior to the patch being applied.

Oh, but lower pressures will always improve the situation No wheelspin means no call for forward drive means no glitch. No?

Cheers,
Steve
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  #740  
Old 08-30-2013, 01:56 AM
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Re: Grand Cherokee + sand = FAIL

Quote:
Originally Posted by mm1964 View Post
As I said I'm inexperienced in 4x4 and not exactly mechanically minded either but I wouldn't expect anything to overheat after being on the sand for only 60-90 secs (if that) which was when it first occurred to me. Another GC that wasn't part of our group on that day didn't even make it to the 60-90 secs.

Mark
I have never experienced the sand fault but before I got the sand fix, when driving on sand the traction control would grumble away almost continuously so I can well imagine something overheating quickly.
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  #741  
Old 08-30-2013, 06:46 PM
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Re: Grand Cherokee + sand = FAIL

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Originally Posted by Barboots View Post
Well the guts is Bob, he needs to be thrown the keys to something on 20"s so he can eat his historical arty-fact.

It'd be great to have a "rule" like this, but let's look at the variables which make these kinds of statements simply an easy "one size fits all" answer for trainers who can't be stuffed explaining how deflation works properly. So in brief, these things affect the required pressure:

- Tyre sidewall strength. Different carcass strengths will affect the amount of tyre deformation for a given pressure reduction. LT construction, 3 ply tyres will require a lower pressure to deform than the nice, soft road tyres they use to sell vehicles.
- Tyre sidewall height. With less height to deform, greater pressure reduction is necessary to adequately deform lower profile tyres.
- Vehicle weight. A heavier vehicle will deform a given tyre more at a particular pressure. Lighter vehicles will require a greater reduction to achieve the same deformation, however may not need it as much, as they are lighter.

Deformation is essential in sand as it lengthens the tyre's contact with the ground, therefore reducing the weight per square area. With an effectively lower weight on the ground, the vehicle will be less inclined to sink in. Sinking in is enemy to success, as the vehicle is always trying to climb out of the hole it has sunk into... pushing sand ahead and out. This robs energy and causes the bogging down, or loss of power, often experienced with older, underpowered 4WDs. With vehicles which have more power to apply, the driver can keep adding more and more energy until there is greater force being delivered than there is traction available. Wheelspin is the result... and the vehicle digs in, rapidly exacerbating the situation into an immediate end.

And regarding rolling tyres of the rim:

- First and foremost, it's a function of turning at speed. Lower your speed in turns, and widen the radius of them, are the simple ways to ensure you're safe. If you think a high speed turn is required to make that beach exit, you probably just need to lower pressures some more! Today's 4WDs are not asthmatic and do not need the massive "head of steam" needed previously.
- It's a function of leverage. It's way easier to roll a 15" than a 20" due to the additional sidewall height's increased leverage creating a greater force between the rim (the moving part) and the ground (the stationary part). It's the bead seal takes the brunt of this force... the guts of it is the force accumulates easier on higher profile tyres.

Don't think all trainers have taken interest in newer vehicles, with their changes in equipment and new technology. They tend to turn up in old school equipment... and deliver their old schooling.

Cheers!
Steve
Great post Steve,
and for anyone that has not been able to undertake some practical training, it should be read and re-read until fully understood.
There is definitely no formula or such a thing as a recommended pressure - sand conditions are far too variable.
Beware the "expert" encountered on the tracks.
I've seen conditions where it was necessary to let the tyres down until they stopped hissing - absolutely imperative when the tide was coming in and it was get that vehicle off the beach or incur considerable expense. With due consideration to Steve's points about the amount of steering input and power application, a very long tyre footprint will work wonders.
Drivers not understanding this simple principle are the reason beach exits and sand dune approaches get chopped up with big holes.
Anyone venturing on sand should have a quality air compressor on board and be willing to use it to protect their investment.
John
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  #742  
Old 09-01-2013, 01:48 AM
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Re: Grand Cherokee + sand = FAIL

Just want to add my 2cents on this one.
I take my GC on the sand at least every 2 or 3 months, Fraser Island, Moreton Island etc. Regarding the 20inch rims I have never needed to go below 20psi yet, and in pretty horrid conditions at times (very powdery, dry, deep, sand).

I agree totally with Steve that the lower your pressure the easier the vehicle will need to work. However with the 20inch rims there really is not a lot of sidewall, and even at 20 psi I have sustained a lot of damage to my rims from debris on the tracks, and at lower pressures would be more likely to.

Also driving over sections of coffee rock, etc, with the 20inch the rims are much more exposed, to the point you can almost feel the rims clunking against the rock in certain instances, it makes me uneasy driving over certain sections.

For these reasons I prefer to stick to 20 psi, and only go lower if needed...
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  #743  
Old 09-02-2013, 02:00 AM
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Re: Grand Cherokee + sand = FAIL

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As the posts above suggest, I was part of this gang at Stockton and had the 20inch at 15 psi.

Have contact Customer support again and expressed my anger that the fix was not applied when they knew I was specifically heading out on to sand. They are looking into it and are to get back to me within a day or two.

Not sure why they continue to play this game that this is all new to them!
As expected, no one got back to me. Called today and was told the bloke I spoke to is waiting on the technical group to get back to him and he ain't in until Monday so you'll have to wait until then.

Jeep Customer Care!
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Old 09-02-2013, 03:49 AM
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