TIre pressure and winter traction.

[Zaedock]

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It appears to me that Capri's main point is that for road applications excluding solid ice and very deep snow, decreasing air pressure does not give an advantage.

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That's not true. In addition to off-roading, I also drag race. I lower the pressure in the rear tires to increase traction. I lower to the point where the rear end becomes slightly unstable and go up in 1/2 lb increments until I find the sweet spot. Even off road, climbing up the smooth, flat granite rocks/slabs we have in New England, airing down adds traction.

I completely agree with Drew. Lowering your pressure whether it's 2" or 2' will add traction. For those who think it's good to add pressure in a snow storm to "cut-thru", please stay home. Your theory has merit, but's it's not going to happen. If you want to "cut-thru", then go to a skinny tire. If you stay with the same diameter, you'll have the same contact patch but it will be long & narrow instead of short & wide.

 

[EK]

Many have personal experience that matches my experience - lower tire pressures most often provide better traction in snow and ice.

Beyond that generalization, the best thing to do is find a test area and experiment with various tire pressures for your own equipment and conditions to see what works best. The differences can be significant.

 

[WalterMitty]

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It appears to me that Capri's main point is that for road applications excluding solid ice and very deep snow, decreasing air pressure does not give an advantage.

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That's not true. In addition to off-roading, I also drag race. I lower the pressure in the rear tires to increase traction. I lower to the point where the rear end becomes slightly unstable and go up in 1/2 lb increments until I find the sweet spot. Even off road, climbing up the smooth, flat granite rocks/slabs we have in New England, airing down adds traction.

I completely agree with Drew. Lowering your pressure whether it's 2" or 2' will add traction. For those who think it's good to add pressure in a snow storm to "cut-thru", please stay home. Your theory has merit, but's it's not going to happen. If you want to "cut-thru", then go to a skinny tire. If you stay with the same diameter, you'll have the same contact patch but it will be long & narrow instead of short & wide.

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Ok, so your previous emphasis on Harsh Conditions was spurious and you basically believe that lowering air pressure improves traction in all conditions. I guess I understood your first post on this subject after all. I'm not sure I understand the reference to lowering pressure 2 [inches] or 2 [feet] but it probably makes sense to most other folks. Perhaps you are referring to snow depth (?). If it does refer to snow depth, I still see a difference in those conditions and would take different actions (and vehicles!

)

I won't of course be staying home this winter, but it is interesting to see folks who air down for rock crawling, drag racing, and off-roading extrapolate that application (increasing contact patch to increase traction or flotation)to all other applications. Then, without a hint of irony, you say that going to a narrower tire would improve traction *and* have the same sized contact patch.

I guess we'll just agree to disagree and let the observers draw their own conclusions. It is my opinion that airing down increases the tendancy (for a given tire that is not changed for road conditions) to ride up on snow/slush/water and decrease the stopping/turning/starting performance of the vehicle; a form of hydroplaning if that clarifies. The effects would be negligible at walking speed, but would increase as vehicle speed increases.

As a couple of other posters have appropriately stated, the full gamut of driving conditions does not lend itself to a one-size-fits-all answer.

So for me, as a snow event passes through my area I start out in my sedan with tire pressures at normal. If conditions worsen to the point that the sedan is no longer appropriate (due usually more to ground clearance issues than anything else) I switch to the Z71 (again at normal pressures). If conditions continue to deteriorate, I may air down the AT tires on the truck so I can stay on top of drifts and deep snow. THEN, I head for home or someplace I can weather out the storm. YMMV

 

[Drew99GT]

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and would take different actions (and vehicles!

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A 92 Wrangler with 35 inch tires that has the capability to run 6 psi and rock crawl on slippery granite??? (beadlocks/lockers front and rear perhaps)? Over a Chevy Z71 pickup?

Think I'd take dudes Wrangler in "HARSH CONDITIONS"

, but that's just me!

 

[WalterMitty]

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and would take different actions (and vehicles!

)

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A 92 Wrangler with 35 inch tires that has the capability to run 6 psi and rock crawl on slippery granite??? (beadlocks/lockers front and rear perhaps)? Over a Chevy Z71 pickup?

Think I'd take dudes Wrangler in "HARSH CONDITIONS"

, but that's just me!

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Hey, if we can go that far off the subject I'll just take a 1999 M1A2 Abrams tank with the NBC package for "HARSH CONDITIONS".

Oh, and I want air support and an LCAC for road trips.

Beam me up Scotty..

 

[Drew99GT]

Hey, that's what Zaedock said he's got!

An M1A2 would be nice though, you could just blow up the snow drifts and cruise right on through.

 

[MGBV8ROADSTER]

WalterMitty

Aquaplaning is dependant upon speed and tyre pressure, increasing tyre pressure reduces footprint.

http://www.easa.eu.int/doc/Rulemaking/NPA/NPA_14_2004.pdf

 

[WishIhadatruck]

I think when the person was writing the VW owners manual they were assuming you would never drive on the road after a snow unless the road had already been plowed and salted. Then if the only thing left on the road was a very thin layer of slush I would say airing UP would be a good idea. I spend a lot of time on unplowed roads like everyone else and so I don't think airing my tires up a few psi would make much sense most of the time.

 

[WalterMitty]

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WalterMitty

Aquaplaning is dependant upon speed and tyre pressure, increasing tyre pressure reduces footprint.

http://www.easa.eu.int/doc/Rulemaking/NPA/NPA_14_2004.pdf

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Right MGB. Although if we now bring aircaft data into the thread one of us is going to have to bone up on our Space Shuttle Tire specs and how returning from space impacts tire durometer and deflection rates. Surely someone here has heard of the space walks conducted to air up or air down the Space Shuttle tires based on whether it's raining in Florida or not.

To your point, a quote from the paper for those not inclined to view it:

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An aeroplane will aquaplane at high speed on a surface contaminated by standing water, slush or wet snow. For the purposes of estimating the effect of aquaplaning on contaminant drag, the aquaplaning speed, VP, is given by -
VP = (V sub p = 9 times the square root of P)(The math symbols didn't paste right)
where VP is the ground speed in knots and P is the tyre pressure in lb/in2.



Predictions (Reference 5) indicate that the effect of running a wheel over a low density liquid contaminant containing air, such as slush, is to compress it such that it essentially acts as high density contaminant. This means that there is essentially no increase in aquaplaning speed to be expected with such a lower density contaminant. For this reason, the aquaplaning speed given here is not a function of the density of the contaminant.

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So in the formula, as tire pressure decreases, the speed at which hydro{aqua}planing occurs also decreases. Wow. That almost sounds like a conclusion based on science and verifiable references. Which doesn't mean it can't be proven wrong, but it would be interesting to see a similarly documented alternate hypothesis.

Of course, if you run your airplane off the runway into the wet grass I'll be right out there with everybody else airing down the tires and off loading weight to try to float it back onto the hard top when the tow vehicle arrives. Further, when I win the lottery and go into the bush pilot business, I'll definately put the biggest tires money can buy on my DeHavilland Beaver and air them down as low as I can when landing on swampland.

Gosh, I wonder if the OP is still around.

Did you get anything useful stang5?

 

[Zaedock]

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Hey, that's what Zaedock said he's got!

An M1A2 would be nice though, you could just blow up the snow drifts and cruise right on through.

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Actually, I don't run beadlocks. I run 15x8" cheap steel wheels with 35/12.5" Mickey Thompson Baja Claws. I do have lockers and a granny low transmission though

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...going to a narrower tire would improve traction *and* have the same sized contact patch.

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Walt, I never said a narrow tire would increase traction. I said if your goal was to try and reach the road surface, a narrow tire would work better. Also, a narrow tire will in fact have the same size contact patch as a wide tire of equal diameter and pressure. The shape of the patch will be different. The narrow tire will have a long and thin contact patch and the wide tire will have a short and wide patch. If a Jeep with 33x12.5 tires on the drivers side and 33x9.5 tires on the pass side were up on a glass platform, the patches would look different, but if you measured the areas, they would be the same. I know it's funky, but it's true.

Anyway, I will agree that there is no on-size-fits-all answer. It would be interesting to run some pressure tests during the next snow storm. There are probably way too many variables though, which is why this discussion is older than dirt. In the end we can only do what works best for us.

Just remember to drive safe

 

[y_p_w]

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Walt, I never said a narrow tire would increase traction. I said if your goal was to try and reach the road surface, a narrow tire would work better. Also, a narrow tire will in fact have the same size contact patch as a wide tire of equal diameter and pressure. The shape of the patch will be different. The narrow tire will have a long and thin contact patch and the wide tire will have a short and wide patch. If a Jeep with 33x12.5 tires on the drivers side and 33x9.5 tires on the pass side were up on a glass platform, the patches would look different, but if you measured the areas, they would be the same. I know it's funky, but it's true.

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I don't know about that. I thought that contact patch size is dependent on weight and inflation. However - doesn't individual tire construction also have a lot to do with the contact patch? When I contacted Pirelli regarding why their P Zero Nero M+S was "all-season" and their 3-season P Zero Nero was not, I was told it was a combination of a rubber compound that stayed soft at colder temps and a carcass/belt construction that decreased the contact patch. Honestly - I wasn't that impressed with how well they did in the snow. They're not heavily siped and the only large biting edges are on the inner and outer edges.

Certainly it's hard to say anything about "all things being equal". I was under the impression that tire manufacturers often vary their materials (thickness of belts for instance) for different sizes of the same tire model.

 

[Auto-Union]

My take is that lowering pressure is better with tires less suited for winter trction, like an hi-mi a/s.(to gain "surface" traction) With good snows that already have good/adequate snow grip (what I'm calling "surface traction") might benefit fron additional air, to enhance and add the "cutting" effect described.

 

[eljefino]

It's my impression the lower inflation lets the voids flex which throws snow out better. This would apply to all season tires as well as snows.

Best answer might be to bring a 12v inflator, run normal pressures, and deflate if one gets really stuck.