We had a bit of snow this weekend in the northeast.

I was going up a bit of an incline which is called my driveway and started slipping.

Instead of trying chains, I tried something I read about getting out of desert sand. I let a bit of air out of my tires to get additional traction.

It worked beautifully! Something else to add to my bag of tricks. BTW, I have a 12volt pump in the car.

Conway Yee

Yes, that's a well-established trick for icy roads. It does work.

As hinted at in the OP, there are some safety limits to observe. Tires that have been partially deflated are only safe for low speed operation. At high speeds, they are distinctly hazardous -- poor handling and possible blowout (loss of bead seal) under the stress of emergency steering/braking.

But it's still a good trick to have up your sleeve. It's pulled me out of a pickle more than once.

Not just for sand and snow, but most off-roaders will air-down their tires as soon as they leave the pavement. The lower the tire pressure, the longer the footprint made by your tire, providing more traction. It also allows the tire to conform to obstacles such as rocks and downed trees, again giving better traction. But if you go too low you risk spinning separating the bead or causing damage to the sidewall from pinching the tire between the wheel and the ground.

On normal-profile tires (65-80 profile, as in 225/70-R16) it is generally safe to go down to 15psi, which should double the size of your tire's footprint. Just keep your speed low, generally under 15MPH. Always air back up before driving on pavement (though I have driven as fast as 45MPH at 15psi for short blacktop segments between trails, but not generally recommended).

Don't try this trick if you have very low-profile tires; you will damage the tire.

From what I remember of high school physics ... the amount of friction is determined by the 'coefficient of friction' and the 'normal force' (downward force), and it has nothing to do with the contact surface area ...

So ... that might lead me to wonder if deflating a tire ...

(1) changes the coefficient of friction - which seems kind of odd

OR

(2) doesn't really increase friction, but it does makes the tire less likely to sink into a soft surface, which really does help.

OR

(3) just works, and I clearly remember too much from high school physics for my own good.

Sigh. My brain hurts. I think I'd better stop posting here for the rest of the year ;-)

What you might not be remembering is that friction is affected by surface area. Partial deflation of the tire increases the surface area of the contact patch.

Actually no. The classic physics formula for friction is independent of surface area. Given the same normal force and the same coefficient of friction, the frictional force is the same no matter what the surface area.

The reason that deflating tires helps traction is that in materials like snow and soft sand, distributing the load over a bigger area actually changes the coefficient of friction because the snow/sand is less compacted. See Friction and Normal Force

Get "BFGoodrich All-Terrain T/A KO Tires - The tire that invented the all-terrain category, setting the standard for on- and off-road performance" on your vehicle,you will sneer at ice and snow where mere mortals fear to go.And still drive in the dry with high mileage to boot. Really.Tires are the name of the game IMO.Heavy vehicle is second on list for ice IMO.No science facts to back up,just what works here,real well.

Thats what I have BFG AT KO. I posted in the other thread we just got back from my parents in WV. Going up slick mountain roads seeing AWD vehicles spinning front and rear tires and I'm still in 2wd.

Yup,insanely good all season tires,my experience mirrors yours to a tee. I missed your post,what thread? edit-never mind,found it.

OK, now its the new years so I'll post again :-)

Lets talk All-Wheel Drive (AWD) vs 4-Wheel Drive (4WD), and the influence of limited slip differentials (LSD).

From what I understand, AWD vehicles will get stuck when just one wheel loses traction ... UNLESS they have LSD somewhere in there (front axle, rear axle, and/or front-to-back). I think they'll typically have a LSD front-to-back, which provides a minimum of two drive wheels.

4WD vehicles typically have no front-to-back differential, which is why they "buck" when doing a tight turn on a dry surface. This means that for them to get stuck they need to lose traction with one of the front AND one of the rear wheels ... UNLESS they have LSD somewhere.

My 4WD Chevy Suburban with the option code G80 has a limited slip differential in the rear axle. When the two rear wheels spin at a differential rate (difference) more than 100 RPM (1.7 rotations per second) the rear axle will lock up side-to-side. This provides drive from a minimum of 3 wheels.

So I need to get the rear wheel spinning at a fair rate to lock the rear axle - and keep it locked long enough to get me going. I still need some time in 2WD on an icy parking lot to get a feel for that. I wish my truck has a way to manually engage the rear differential lock, so the rear wheels would be locked BEFORE the wheels start slipping.

BTW, my John Deere 3320 tractor with mechanical four-wheel drive (MFWD) works in a similar way to my 4WD Suburban. In MFWD mode the there is full differential on the front axle and the rear axle, front-to-back is locked together, and it has a manually engaged rear axle differential lock. So can provide three drive wheels.