Tips for Offroading on Sand?

robcarync · Aug 28, 2014

I have gotten my X on the beach once before, back in May for my Bachelor Party!!!! It was awesome, I didn't get stuck, but some sections of the beach were very loose sand, and there was not a lot of forward momentum at all. My engine did start to overheat...which was a bummer. Engine still running strong though. I was driving in 4 Lo the entire time and had my auto transmission lever in 2...but did not air down my tires.

I am going out to the same spot again this weekend, and am trying to make sure I am driving the way I should. Google gives interesting debates and inconsistent answers as far as whether to use 4Hi or 4Lo.

I know the common sense stuff: don't spin your wheels and get yourself buried to your frame, etc...but this time around I will:

1) Have my radiator relocated so it is more in line with the fan and able to use fan shroud.
2) Lower tire pressure like I am supposed to.
3) Give my cooling system a once over to make sure it is in good condition and filled with proper fluid levels.

So question is:

1) What is the actual best transfer case gear to be in? 4Hi? 4Lo?
2) What is the best automatic transmission gear to be in? 1? 2?
2) Any other tips for navigating through the sand?

I am not so much concerned about getting stuck. I have a front Lokka, straps and shackles, a shovel, and a hi lift...just looking to reduce the potential for over heating and be efficient in my navigation.

bottobro · Aug 28, 2014

I always air down that always helps.
I don't go into 4 low unless I'm close to stuck or going through some really loose/wet stuff
Gear 2 keeps the rps down and doesn't let u go past a certain speed.

At least this is what I do. But what do I know I was only driving on the beach every other weekend this summer

robcarync · Aug 28, 2014

bottobro said:
I always air down that always helps.
I don't go into 4 low unless I'm close to stuck or going through some really loose/wet stuff
Gear 2 keeps the rps down and doesn't let u go past a certain speed.

At least this is what I do. But what do I know I was only driving on the beach every other weekend this summer

Well you sound like an expert on beach driving, then!

Sounds like 4Hi will work, and keep it in gear two!

I was thinking that the increased torque in 4Lo made it easier to kick the sand back and probably made it lose traction easier.

Thanks for the tips!

Just checked out your build thread...love your X man! The 33s make all the difference in the world ... *regretting only having 32s*

I like the straight red too...I wish I had that color instead of burgandy

TJTJ · Aug 28, 2014

For sand, the 4 high vs 4 low is a misleading question, as there are many variables that make one right and the other wrong...hence people arguing over "which is right".

If you have ENOUGH TORQUE, then 4 high can be better...but, if you need low wheel speed and more control then 4 lo is better.

What GEAR to be in also depends on the same question, as well as what speed you'll be in.

As you've surmised, airing down is about the best way to increase the contact patch of your tires....which increases flotation, which helps to avoid sinking into the sand and bogging down, etc.

I'll add a bit -

Think of these three concepts:

1) Flotation - A small surface area penetrates better, think pointy nail vs flat ended log, etc. So, the larger the contact patch is, the harder it is to make it penetrate down into the sand.

No matter how large your contact patch is, you will STILL sink down at least a little bit...and the smaller the patch, the more you'll sink.

2) When the tire has sunk down from just the truck's weight, there is a wall of sand IN FRONT of the tire that the tire must CONTINUOUSLY climb/overcome as it tries to roll forward. This is like having a very small wheel chock in front of all 4 tires at ALL TIMES. The deeper you sink, the larger those wheel chocks are.

If the tire has a larger diameter, it will be hitting that wheel chock at a higher point, so the wall of sand in front of it is PROPORTIONALLY SMALLER. (Its hitting at a less acute angle, so its more like a gentle ramp and less like a wall in the way...) So taller tires have a longer contact patch AND an easier time overcoming the sand chocks. A shorter tire faces a steeper wall of sand, and, will have a shorter contact patch. The term is "Angle of Attack" that the tire's leading edge is making with the terrain.

Generally, a LONGER contact patch helps the most with forward traction (Think "tank tread shaped"). A shorter but fatter tire tread can help compensate for the total contact patch area, a lot, but, not for the angle of attack.

3) When you make a turn in sand, with an open diff, it tends to turn using only the OUTER tire on the turn if too sharp, so you lose the inside tire's contribution to pushing you forward and climbing all those sand chocks, etc. IE: You now have HALF the traction.

The thing that happens is that that outer tire might slip, as sand is somewhat slippery being a lot like a lot of wee ball bearings. If the outside tire slips, its going to spin and dig a hole...increasing the size of its sand chock dramatically.

You want to make GENTLE turns, wide sweeping changes in direction instead of sharper cuts that will be more likely to spin the outer tires and bog you.

When you consider all three concepts, you will discover that, depending upon HOW soft the sand is, its compressibility, and your speed and directional choices, that the resistance that you are trying to overcome will be analogous to when you are driving on the road...and that ONE t-case and tranny choice is less likely to cover all of the possible variations....than simply being able to use more rpm when you need more torque, and less wheel speed when the tire's traction is marginal, to avoid spins...via gear selection on a rolling basis, etc, and use of the gas pedal to not apply too much or too little power, etc.

Higher horsepower rigs are the ones who generally NEED to use 4 hi in sand, as their abundant torque tends to get them into hole digging trouble. They can also SPIN paddle wheeled tires - sending rooster tails of sand skyward as they skitter along the dunes, etc.

Xterras...no so much...we might STALL trying to spin paddle wheel tires, and might fare better with AT type treads in sand for example. (An MT, when it spins, throws MORE sand out from under your tire, making the hole deeper, faster.) In SOME sand, the MT tread works OK where you avoid spins, as the tread elements tend to grab deeper ridges of sand to push against.

The deeper the tread element can sink in, generally, the deeper the tire is sunk in...so, its a balance between flotation (Deep tread elements mean the tire HAS TO sink deeper to support the same weight) and traction (Deeper elements give more traction). So for some sand the MT will work better, and for some the AT will....but for example at 2 of the 3 basic NJ Pine Barrens sands, or the east coast shore sands, etc, the AT tread TENDS to work more often than it doesn't.

The PSI you air down TO also changes due to the same factors...as the more you air down, the shorter your tire gets...and TOO short means your attack angle is too acute, etc. The heavier your rig, the higher the PSI you need to get the SAME flotation, etc...and, the more weight is pushing you down INTO the sand.

You really need to get out and experiment....and SEE what works on YOUR rig/tire combo.

Food for thought.

:tunes:

robcarync · Aug 28, 2014

TJTJ said:
For sand, the 4 high vs 4 low is a misleading question, as there are many variables that make one right and the other wrong...hence people arguing over "which is right".

If you have ENOUGH TORQUE, then 4 high can be better...but, if you need low wheel speed and more control then 4 lo is better.

What GEAR to be in also depends on the same question, as well as what speed you'll be in.

As you've surmised, airing down is about the best way to increase the contact patch of your tires....which increases flotation, which helps to avoid sinking into the sand and bogging down, etc.

I'll add a bit -

Think of these three concepts:

1) Flotation - A small surface area penetrates better, think pointy nail vs flat ended log, etc. So, the larger the contact patch is, the harder it is to make it penetrate down into the sand.

No matter how large your contact patch is, you will STILL sink down at least a little bit...and the smaller the patch, the more you'll sink.

2) When the tire has sunk down from just the truck's weight, there is a wall of sand IN FRONT of the tire that the tire must CONTINUOUSLY climb/overcome as it tries to roll forward. This is like having a very small wheel chock in front of all 4 tires at ALL TIMES. The deeper you sink, the larger those wheel chocks are.

If the tire has a larger diameter, it will be hitting that wheel chock at a higher point, so the wall of sand in front of it is PROPORTIONALLY SMALLER. (Its hitting at a less acute angle, so its more like a gentle ramp and less like a wall in the way...) So taller tires have a longer contact patch AND an easier time overcoming the sand chocks. A shorter tire faces a steeper wall of sand, and, will have a shorter contact patch. The term is "Angle of Attack" that the tire's leading edge is making with the terrain.

Generally, a LONGER contact patch helps the most with forward traction (Think "tank tread shaped"). A shorter but fatter tire tread can help compensate for the total contact patch area, a lot, but, not for the angle of attack.

3) When you make a turn in sand, with an open diff, it tends to turn using only the OUTER tire on the turn if too sharp, so you lose the inside tire's contribution to pushing you forward and climbing all those sand chocks, etc. IE: You now have HALF the traction.

The thing that happens is that that outer tire might slip, as sand is somewhat slippery being a lot like a lot of wee ball bearings. If the outside tire slips, its going to spin and dig a hole...increasing the size of its sand chock dramatically.

You want to make GENTLE turns, wide sweeping changes in direction instead of sharper cuts that will be more likely to spin the outer tires and bog you.

When you consider all three concepts, you will discover that, depending upon HOW soft the sand is, its compressibility, and your speed and directional choices, that the resistance that you are trying to overcome will be analogous to when you are driving on the road...and that ONE t-case and tranny choice is less likely to cover all of the possible variations....than simply being able to use more rpm when you need more torque, and less wheel speed when the tire's traction is marginal, to avoid spins...via gear selection on a rolling basis, etc, and use of the gas pedal to not apply too much or too little power, etc.

Higher horsepower rigs are the ones who generally NEED to use 4 hi in sand, as their abundant torque tends to get them into hole digging trouble. They can also SPIN paddle wheeled tires - sending rooster tails of sand skyward as they skitter along the dunes, etc.

Xterras...no so much...we might STALL trying to spin paddle wheel tires, and might fare better with AT type treads in sand for example. (An MT, when it spins, throws MORE sand out from under your tire, making the hole deeper, faster.) In SOME sand, the MT tread works OK where you avoid spins, as the tread elements tend to grab deeper ridges of sand to push against.

The deeper the tread element can sink in, generally, the deeper the tire is sunk in...so, its a balance between flotation (Deep tread elements mean the tire HAS TO sink deeper to support the same weight) and traction (Deeper elements give more traction). So for some sand the MT will work better, and for some the AT will....but for example at 2 of the 3 basic NJ Pine Barrens sands, or the east coast shore sands, etc, the AT tread TENDS to work more often than it doesn't.

The PSI you air down TO also changes due to the same factors...as the more you air down, the shorter your tire gets...and TOO short means your attack angle is too acute, etc. The heavier your rig, the higher the PSI you need to get the SAME flotation, etc...and, the more weight is pushing you down INTO the sand.

You really need to get out and experiment....and SEE what works on YOUR rig/tire combo.

Food for thought.

:tunes:

Good info, as always TJTJ. I instinctively knew all of what you just said, but never really thought that detailed about it. Only thing is that I have a front Lokka, so the open diff in the front isn't applicable to my situation.

I think, as you said, I just need to get out there and experiment. This is only the 2nd time I will have gone offroad in the sand, so I didn't know if there were general best practices for gear selection.

granitex · Aug 28, 2014

The most important things to remember are
1 air down
2 air down
3 air down
4 momentum
5 try to coast to a stop
6 don't park below the high tide mark
7 air down

mudchet · Aug 29, 2014

Regarding your comment on overheating...

I've seen a number of Xterras have over heating problems at Windrock here in TN. Turns out the warm outside air, difficult climbs and slow speeds heat up the engine. Often times the trucks that find themselves overheating are ones that have previously been in lots of mud and have clogged the radiator with silty dirt. If this might be you as well, spray a hose onto your radiator and give it a good rinse. If you see brown muddy water draining out then keep spraying until the water draining out is clear.

TJTJ · Aug 29, 2014

robcarync said:
Good info, as always TJTJ. I instinctively knew all of what you just said, but never really thought that detailed about it. Only thing is that I have a front Lokka, so the open diff in the front isn't applicable to my situation.

I think, as you said, I just need to get out there and experiment. This is only the 2nd time I will have gone offroad in the sand, so I didn't know if there were general best practices for gear selection.

Well, as the Lokka differentiates in turns, it will be better than an open diff....in that a spin will not force the other tire to spectator status. If there's not enough traction to differentiate though, it WILL tend to make the inside tire rotate faster than its ground speed...but not enough to rooster tail as the open diff would when it lost traction, etc....and the outside tire will be AT ground speed under those conditions, so, overall, a big improvement.

As Granite pointed out, along the same lines as all above, airing down is job #1, avoiding sand chocks with gentle turns, momentum instead of spinning tires, and coasting to a stop to help avoid a pile of sand in front of the stopped tire...and, the excellent point about NOT parking where the tide may roll in and swamp you.

:tunes:

robcarync · Aug 29, 2014

So what I am hearing is that airing down is necessary and proper. I won't skip that step this time. Last time I was in a rush and probably underestimated how much airing down would help...I just figured it would help, but not make a huge difference. Which, I never got stuck or anything last time, but it was definitely tougher to keep going than I thought it would be. I have to take a 30 minute ferry ride, so that gives me plenty of time to air down before hitting the sand.

I would say I need to rinse out my radiator pretty badly, I am sure. I have never replaced it, or rinsed it, and have been offroading for about 3 years in it now. Eventually I want to upgrade to the SC radiator, but it wasn't in the cards for this Labor Day trip. That is a great idea (something I had thought about...but never got around too...)...and I am SURE will help with keeping the engine cool.

Thanks for all the tips!

granitex · Aug 29, 2014

An additional tranny cooler is always a good idea, you can add one for about $30-$40 and they will always help out when heat is an issue. Remember that your tranny got good and hot at the same time.

robcarync · Aug 29, 2014

sooo just sprayed out my radiator...let's just say I need a power washer now to clean my driveway with how much mud came out of it.

Mental note made...add it to my list of post wheeling clean up activities.

Temp never got above center on normal roads...sand was the only time it got close to the red. Even then, once I got to the harder more compact sand and parked, it went back down very quick. Never thought to check the radiator...

mudchet · Aug 29, 2014

Nice!

robcarync · Sep 1, 2014

Pleased to report that with airing down to 20 PSI (I know, could have gone lower, but this was better than 35 PSI)...and leaving the transfer case into 4 Hi, I experienced zero over heating, and felt like I had great control in the sand. Never had any issues, even when having to come to a complete stop in the softer sand, and could easily start back up and keep moving.

Not sure if it was the airing down, or the 4Hi instead of 4Lo...or maybe I was just more confident and comfortable the 2nd time around, but the Xterra performed flawlessly in the sand and made it through with ease.

Prime · Sep 1, 2014

I typically air down to 15 or so for the beach. No need for 4 low unless you get into some deep soft sand.

Glad it went well!

TJTJ · Sep 2, 2014

The best psi for sand depends on what tires you have and how soft the sand is.

Essentially, you air down to make the contact patch larger....but more than that doesn't add abilities, and, merely decreases ground clearance with no benefit, etc.

As every tire is different, I always recommend parking the tire on a rock or curb, etc, something that pushes the tire at a small point, and airing down while looking at that point. As you lower the PSI, the tread will start to wrap that point, and your point of diminishing return will be when the point is wrapped but the curb, etc, is not going to put the rubber against the rim....you want as much sidewall height as possible.

IE: Start airing down - Watch the tread sink onto the ground, and when the contact patch stops GROWING, you're done.

Bringing along a compressor/way to air back up/adjust psi on the trail, is also a good idea.

TJTJ · Sep 2, 2014

By coincidence...my buddies on Nexterra posted a thread on airing down with some graphics. (Thanks Dirtysocks!)

Comparison of contact patch at 8 psi vs 25 psi.

Notice the point of widest width was the same, but the length increased dramatically, which also increased the length of the widest section, etc.

Notice how much less total pressure per square inch you get when there's more contact patch...so you're less likely to sink.

Also notice the tread patterns are not th same between the top pic and bottom row of pics....so these are for different tires.

The specific changes will be tire specific.

robcarync · Sep 2, 2014

More great info, TJTJ...the change in footprint of the tire was very dramatic. I have rarely aired down simply because I didn't think it would make THAT big of a deal. I usually am not doing anything that difficult where airing down is necessary to get by, but this clearly shows you can get quite a bit of tire flex and extra contact area by airing down.

mudchet · Sep 4, 2014

TJTJ said:
By coincidence...my buddies on Nexterra posted a thread on airing down with some graphics. (Thanks Dirtysocks!)

Comparison of contact patch at 8 psi vs 25 psi.

Notice the point of widest width was the same, but the length increased dramatically, which also increased the length of the widest section, etc.

Notice how much less total pressure per square inch you get when there's more contact patch...so you're less likely to sink.

Also notice the tread patterns are not th same between the top pic and bottom row of pics....so these are for different tires.

The specific changes will be tire specific.

Great post. Thanks for the info.

robcarync · Sep 4, 2014

TJTJ said:

One thing that is awful with that chart: The graphics start with 15 PSI on the left, and the chart starts with 40 PSI on the left. Totally backwards!

On a serious note: One particular observation is that the biggest increase in tread area is the jump from 25-20 PSI and from 20-15 PSI. Obviously that changes with different tire constructions and materials...but I would bet that the general relationship is pretty similar.

For instance, I typically run my tires at 35 PSI on the street...and now I know I won't really get a whole lot of benefit unless I air down to 15-20 PSI. We all know here that 15-20 PSI is the popular air down target, but I know I have on a couple of occasions gotten impatient airing down and thought "ahhh whatever, 24 PSI is low enough, it will still help out". Which, partially airing down obviously helps some, but since it is not a linear relationship, you are REALLY missing out on a lot of the advantage if you don't go to at least 20 PSI.

TJTJ · Sep 4, 2014

robcarync said:
One thing that is awful with that chart: The graphics start with 15 PSI on the left, and the chart starts with 40 PSI on the left. Totally backwards!

On a serious note: One particular observation is that the biggest increase in tread area is the jump from 25-20 PSI and from 20-15 PSI. Obviously that changes with different tire constructions and materials...but I would bet that the general relationship is pretty similar.

For instance, I typically run my tires at 35 PSI on the street...and now I know I won't really get a whole lot of benefit unless I air down to 15-20 PSI. We all know here that 15-20 PSI is the popular air down target, but I know I have on a couple of occasions gotten impatient airing down and thought "ahhh whatever, 24 PSI is low enough, it will still help out". Which, partially airing down obviously helps some, but since it is not a linear relationship, you are REALLY missing out on a lot of the advantage if you don't go to at least 20 PSI.

You are correct in that the specifics of the individual tire itself will change the relationship, but it is actually VERY variable as to how low is low "enough"....as some tires are simply inherently stiffer and stop deforming sooner, and others are essentially flat tires at the PSI that others still have a few inches of sidewall left, and so forth.

The load range of the tire is a clue that can be used to make a guess for example.

Example: Your X has a GVWR of ~ 5,400 lb, so, each tire is supporting roughly 25% of that, or, ~ 1,350 lb.

The max psi and max load supported at that max psi are roughly proportional, IE: At ~ 50% of the max PSI, it will support ~ 50% of the max load, and so forth.

Tires deform when weight is applied, which flattens the contact patch, making it typically grow longer as the tire becomes "less round" and more of it is flat against the ground. The pressure resists this deformation, so a tire inflated harder is "rounder" and has a smaller contact patch than a flat tire with no psi, etc. If one tire simply supports more weight at a given psi, it will remain rounder at that psi than a tire that can't support as much weight at that psi, and so forth.

To illustrate this:

I quickly picked two actual tires of the same size but with different load ratings. I didn't spend time seeing if they were the most extreme examples, just picked two that were obviously going to be different:

So, If I have one tire that is rated to support 2,270 lb at 30 psi, and another than is rated to support 3,415 at 80 psi, the first tire is supporting ~ 59% of its max load (1,350 lb/2,270 lb)

and the second tire is supporting only ~ 39% of its max load (1,350 lb/3,415 lb).

That means JUST to support the weight (1,350 lb), I only need ~ 59% of the first tire's max psi and 39% of the second tire's psi.

59% of 30 psi = 17.7 psi.

39% of 80 psi = 31.2 psi.

IE: The second tire, proportionally, needs about TWICE THE PSI to support the same weight.

So, that's an example of how the actual psi you might choose for a given foot print could differ by a factor of ~ 100%, for the same sized tire....JUST from a load range difference.

The sidewall construction, the weight of the rig, use of bead locks, and other factors, will FURTHER change what works, and so forth.

This is why I don't like to "Just say what to air down to"....I want people to experiment, and WATCH the patch as they deflate the tire, and SEE when it stops growing, etc...as that's more likely to be "right" for THEIR tires on THEIR rig, etc.

I saw a guy flip his jeep in the NJ Pine Barrens of all places on a hill climb, because he aired down to what someone told him THEY aired down to....and, it was too little for his tires, and they more or less just spun off his rims and he crabbed sideways on the hill and went over.

He had his dog's leash tied to the roll bar, and I still remember that poor dog being flung around by the neck as the rig rolled, looking like one of those airplanes that circles around you on a string....but with no propeller. :zomg:

robcarync · Sep 4, 2014

I was more thinking that the non-linear relationship might be generic to all tires, not specifically the 15-20 and 20-25 PSI jump. Meaning if you put a fixed weight on any tire and incrementally air it down from max PSI to when the bead breaks, the contact patch will progressively make bigger jumps in size as pressure gets lower.

But now that I do some math, I realize that it should be linear for a theoretically perfectly uniform tire...Area = Weight / Pressure... any deviation from this linear relationship is due to sidewall and tire construction supporting the weight instead of the contact area.

This is just my engineering brain trying to over analyze things. I like thinking.

Agree experimentation is best

/Science Pondering

TJTJ · Sep 4, 2014

Yeah, the engineering stuff is applicable. Two with the same load range simply address ONE variable, but, the numbers of sidewall plies and or stiffness, etc, can make a lighter rig not flatten a tire with NO pressure in it for example...more like a "Run Flat" tire in final effect, etc.

As you saw in the charts, for one individual tire, the effect is at least proportional if not linear....with points of diminishing return at either end of the chart if you were to continue it. The other factor is the tread face that forms the contact patch itself.

If the psi is low enough, the MIDDLE of the contact patch can actually be concave, and/or not be pushed as hard onto the ground as the outer tread area of the patch. Think about what the tread of a FLAT tire would look like for example...there is a point at which LESS pressure is not additionally elongating the tread, and, there is a point at which, despite the tread being as elongated as possible, (When the RIM hits the rubber/ground, the tire will not be able to flatten further...), and, there will be insufficient internal air pressure to maintain ground pressure.

granitex · Sep 4, 2014

A few years ago I ran into a guy with a sammi running a new set of boggers, on a very easy trail without valve stems just trying to get them to flex. Now he was running a very light rig, with some pretty huge boggers, and bead locks. but on flat ground you could not tell. So the tire construction, site conditions, and vehicle weight all have to be taken into consideration.

TJTJ · Sep 5, 2014

granitex said:
A few years ago I ran into a guy with a sammi running a new set of boggers, on a very easy trail without valve stems just trying to get them to flex. Now he was running a very light rig, with some pretty huge boggers, and bead locks. but on flat ground you could not tell. So the tire construction, site conditions, and vehicle weight all have to be taken into consideration.

That's a perfect example btw.

robcarync · Sep 5, 2014

I understand all of that perfectly...talking about the effective spring rate of each tires. Sure thing.

My pondering aloud was purely based on the observation that the data previously shown actually showed not diminishing gains, but increasing gains in contact patch at lower and lower pressures. Meaning if you aired down 5PSI from 40-35, you did not gain as much contact area as if you aired down 5 PSI from 20-15. Meaning losing 5 PSI at the top end of the pressure spectrum did less for you contact patch than losing 5 PSI at the bottom end of the spectrum. Not looking at the overall SIZE of contact patch, but the incremental CHANGE in contact patch relative to the incremental change in pressure.

TJTJ, I know you like physics

A tire is essentially like a spring, where you apply a force, and the spring compresses a certain distance, but we are talking about applying a force (pressure), and the tire deflects (contact patch).

A typical spring is directly linear. Apply 10 pounds, the spring compresses 1 inch. Apply 20 pounds, it compresses 2 inches...30 pounds, 3 inches, etc

The data on the tire patch showed (to use the spring analogy)...apply 10 pounds, compress 1 inch, apply 20 pounds, compress 3 inches, apply 30 pounds, compress 6 inches...etc

You got more and more gain for the same change in pressure, the lower you go...like a variable spring rate. Obviously the tire construction and weight of the vehicle play into the actual application of airing down...but again, this is just an engineer pondering too much about the mathematical relationships between variables....

hurting anyone's brains yet? :yawn:

TJTJ · Sep 6, 2014

robcarync said:
I understand all of that perfectly...talking about the effective spring rate of each tires. Sure thing.

My pondering aloud was purely based on the observation that the data previously shown actually showed not diminishing gains, but increasing gains in contact patch at lower and lower pressures. Meaning if you aired down 5PSI from 40-35, you did not gain as much contact area as if you aired down 5 PSI from 20-15. Meaning losing 5 PSI at the top end of the pressure spectrum did less for you contact patch than losing 5 PSI at the bottom end of the spectrum. Not looking at the overall SIZE of contact patch, but the incremental CHANGE in contact patch relative to the incremental change in pressure.

TJTJ, I know you like physics A tire is essentially like a spring, where you apply a force, and the spring compresses a certain distance, but we are talking about applying a force (pressure), and the tire deflects (contact patch).

A typical spring is directly linear. Apply 10 pounds, the spring compresses 1 inch. Apply 20 pounds, it compresses 2 inches...30 pounds, 3 inches, etc

The data on the tire patch showed (to use the spring analogy)...apply 10 pounds, compress 1 inch, apply 20 pounds, compress 3 inches, apply 30 pounds, compress 6 inches...etc

You got more and more gain for the same change in pressure, the lower you go...like a variable spring rate. Obviously the tire construction and weight of the vehicle play into the actual application of airing down...but again, this is just an engineer pondering too much about the mathematical relationships between variables....

hurting anyone's brains yet? :yawn:

You are on the right track.

Its because the "round tire" is starting with a small patch, as, a REAL toroid's contact point with a plane would be a LINE....so as all tires are essentially going to look like a toroid that's a bit flattened on the end resting on the plane (In use), the decrease in pressure will cause the contact point to stretch from that theoretical line, to "fatter line", and so forth.

The point was that this relationship may be progressive as described in your formula, but, the actual ratios will be tire specific. Load Range, ply types, carcass construction, rig weight, etc, will change the actual ratios.

There will also be a point of diminishing return for a toroid, as the 'hole in the middle" will theoretically touch the plane so to speak when fully deflated, and the gains in contact patch seen approaching that limit will then grow more slowly as the inner toroid/RIM is not going to also deform as it approaches the plane, as the tire/outer toroid did, and so forth.

The distance from the inner to outer toroid is essentially the sidewall height in this example. A lower profile tire (Shorter sidewall) will approach the point of diminishing return sooner than a taller side walled tire, and so forth. Many lower profile tires have stiffer sidewall construction, and, airing down looks differently on them than it does for a tall side walled version.

There's a LOT going on in a tire.

robcarync · Sep 6, 2014

TJTJ said:
You are on the right track.

Its because the "round tire" is starting with a small patch, as, a REAL toroid's contact point with a plane would be a LINE....so as all tires are essentially going to look like a toroid that's a bit flattened on the end resting on the plane (In use), the decrease in pressure will cause the contact point to stretch from that theoretical line, to "fatter line", and so forth.

The point was that this relationship may be progressive as described in your formula, but, the actual ratios will be tire specific. Load Range, ply types, carcass construction, rig weight, etc, will change the actual ratios.

There will also be a point of diminishing return for a toroid, as the 'hole in the middle" will theoretically touch the plane so to speak when fully deflated, and the gains in contact patch seen approaching that limit will then grow more slowly as the inner toroid/RIM is not going to also deform as it approaches the plane, as the tire/outer toroid did, and so forth.

The distance from the inner to outer toroid is essentially the sidewall height in this example. A lower profile tire (Shorter sidewall) will approach the point of diminishing return sooner than a taller side walled tire, and so forth. Many lower profile tires have stiffer sidewall construction, and, airing down looks differently on them than it does for a tall side walled version.

There's a LOT going on in a tire.

That's what im talking about TJTJ

just thinking about things on the deeper level and admiring the complexities in a tire

metzican · Sep 7, 2014

If you do some simple math from that data provided. And calculate how many Lbs the air pressure in the tire is supporting according to the area you will find out that the lower the pressure the less weight the air pressure is supporting. Which would indicate that the side wall of the tire as it is being flexed more supports more weight. I'm sure there might be a turn around point but for a good healthy tire and side wall it will act as a leaf spring. Also with spring compression most springs may be advertised as linear but in fact they are far from it. Which makes sense to me.

Along with this as you deflate the tire further the length of the "Bulge" on the tire starts to increase since a larger area is sitting on the ground and the "bulge" on the tire as to move further up and closer to the diameter. and this also could correlate to the tire side wall increasing its load support of the vehicle.

Also even though the tire contact area to the ground may not increase linear to the pressure. if you actually take the horizontal surface area that the air pressure is acting on I'm sure the corilation is much more linear. You can have the previously mentioned "bulge" and that horizontal plan of it could not be touching the ground but the air will be pushing down and up on it respectively to help support the vehicle. In another words the PSI of the tire contacting the ground could be significantly differnt then the PSI in the tire. As in the way many hydraulic cylinders work, larger area with low psi (air or oil) to create high pressure on a small area other side.

TJTJ · Sep 9, 2014

metzican said:
If you do some simple math from that data provided. And calculate how many Lbs the air pressure in the tire is supporting according to the area you will find out that the lower the pressure the less weight the air pressure is supporting. Which would indicate that the side wall of the tire as it is being flexed more supports more weight. I'm sure there might be a turn around point but for a good healthy tire and side wall it will act as a leaf spring. Also with spring compression most springs may be advertised as linear but in fact they are far from it. Which makes sense to me.

Along with this as you deflate the tire further the length of the "Bulge" on the tire starts to increase since a larger area is sitting on the ground and the "bulge" on the tire as to move further up and closer to the diameter. and this also could correlate to the tire side wall increasing its load support of the vehicle.

Also even though the tire contact area to the ground may not increase linear to the pressure. if you actually take the horizontal surface area that the air pressure is acting on I'm sure the corilation is much more linear. You can have the previously mentioned "bulge" and that horizontal plan of it could not be touching the ground but the air will be pushing down and up on it respectively to help support the vehicle. In another words the PSI of the tire contacting the ground could be significantly differnt then the PSI in the tire. As in the way many hydraulic cylinders work, larger area with low psi (air or oil) to create high pressure on a small area other side.

Yeah, the mass balance equation of course allows you to keep one constant...the weight the tire is supporting is not changing, merely what part(s) of the tire are supporting what proportions.

The total ground pressure in pounds remains constant, but the pounds supported per square inch of contact patch drops as the area of the patch increases, etc.

The sidewalls are ALWAYS supporting 100% of the weight though, if its being supported, as they are all that separates/supports the rim from the dirt, etc.

The difference in how much they can support is related to the PSI preventing their collapse...and, as mentioned, the carcass stiffness, etc, that dictates how much it will support with even zero psi, etc.

The concept to remember in contact patch consideration is again that the width of the patch is fairly constant, with some round shouldered tread designs (Balloon Treads, etc) being an exception in that regard, and that its the tread LENGTH that is primarily growing when the PSI is reduced.

There is a point at which the tread's effective length stops growing at the interference chord of the inner/outer toroids. When the rim approaches the tread as the PSI drops, there is a limit at which the angle between the rim and the remaining tire sidewall height, stops lengthening the tread.

So, the total weight the tire supports remains constant, but, the distribution across the tread's contact patch changes with PSI and tire construction characteristics...and, the proportions supported by the tire construction and PSI will vary as well.

If the patch is not evenly supported by ground pressure (Too little PSI/tread stiffness/conformity, etc), so that SOME of that patch is supporting more weight than other parts, the SPECIFIC ground pressure at these points in the contact patch will exert different psi on the ground. Ideally, the tread elements that can sink into the terrain for traction will have enough specific ground pressure to penetrate the involved substrate, and NOT be concaved at these points due to inadequate penetration, etc. And, ideally, the elements that are providing flotation will be spread out enough to distribute the weight on them adequately to NOT sink in, and so forth.

BKxterra718 · Sep 9, 2014

All us Brooklyn boys runnin Goodyear Wrangler MT/r's 33x12.50 we play on the sand no problems with no airing down. Always 4Lo and go never 4hi. Just sayin... Well except for Rubicon runnin 35's he guns the sand dunes like like he owns it :/

www.justinandersdenmark.com

TJTJ · Sep 9, 2014

BKxterra718 said:
All us Brooklyn boys runnin Goodyear Wrangler MT/r's 33x12.50 we play on the sand no problems with no airing down. Always 4Lo and go never 4hi. Just sayin... Well except for Rubicon runnin 35's he guns the sand dunes like like he owns it :/

www.justinandersdenmark.com

You can get away with that on the wetter/well traveled beach/packed stuff, etc.

Get into some sugar sand, etc, and things change.

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