Tire Pressure Management

One of the most essential skills for any vehicle owner is the ability to manage air pressure.  Sadly with the status of nanny laws and the government mandating tire pressure monitoring systems (TPMS) on all new vehicles it’s turned tire pressure management from an active skill to passive nuisance.  Now people don’t check their tire pressure till the little yellow idiot light on the dash tells them to, and even then most don’t even know that even a slightly under or over inflated tire can cause problems long before an alarm goes off.

“Top of the World” trail in Moab, UT
You can see my air pressure is low by the bulge in the sidewalls.
Is that a good thing? Did I do it on purpose?

In this week’s post I’ll talk about the in’s and out’s of tire pressure management, why it’s important on road, and why it’s doubly important off pavement, and how things like tire deflators and onboard air systems earn their keep in just a few trips.  Read on for the good stuff…

Why should you worry about tire pressure?

Your tires are the most essential thing on your vehicle. Think about it, they are what connect your vehicle to the road.  Without them your vehicle couldn’t move – at all.  Take the motor out, and your vehicle can at least still roll.  No tires, no movement.  Your tires transfer engine power to the road to make you move.  Your tires transfer brake clamping force to the road to make you stop.  Your tires support the weight of your vehicle and soak up small bumps and imperfections in the road (or large ones if you live in a pothole prone state like I do). Also, outside of the purchase price of vehicle itself, tires are the largest recurring investment you make besides fuel.  So yeah, you’re tires are important.

If your tires are over inflated, meaning there is too much air in them, the tire bulges out at the tread and reduces the contact patch of the tire.  This can lead to premature wear of the tire and handling problems such as hydroplaning in rain and loss of traction under acceleration.

Ideally you want even wear across the entire tread.
This is even more important with blocky squared off treads common on A/T and M/T tires.

If your tires are under inflated, meaning there is not enough air in them, the tire bulges out at the sidewall.  While this increases the contact patch, which is something good when off pavement (more on that shortly), it can lead to premature wear of the outside edges of the tread as well as contribute to sidewall stress.  Too little air also reduces the cornering ability of the vehicle causing the tread to “float” under the center line of the wheel.  Take a turn too fast and you can actually fold a tire over causing the wheel to contact the road surface which can lead to things like vehicle rollover.  Low tire pressure, since it increases the contact patch, also increases road noise generated by the tire as well as increase friction leading to a decrease in fuel milage.

Needless to say, managing the air pressure in your tires is essential to maximize handling, fuel milage, and the tread life of your tires.  Those of us running larger more expensive all-terrain or mud-terrain tires can easily spend more than $1,000 for a set of tires.  That’s a big investment.  Wouldn’t you want to get the most out of your tires?  I know I do.

How do you establish optimal street pressure?

If you’re running stock vehicle with OEM type tires (same size and tread type as your vehicle was sold with when it was new) then this is really easy.  All you have to do is open up your owner’s manual or check the inside of the driver’s door.  There will be a label outlining the tire size, tread type, and recommended optimal tire pressure.

If you’re running a modified vehicle with aftermarket wheels and tires (different size and/or tread type than your vehicle was sold with) things get a little more complicated.  There are many variables that contribute to optimal tire pressure.  Things like the weight of the vehicle (which has probably changed with the addition of heavy duty bumpers, extra gear, and things like that), the size of the wheel (both in diameter and width), the size of the tire, as well as the tire type (all season, all weather, all terrain, mud terrain, etc) all have subtle impact on the pressure of the tire. Sadly there is no hard and fast rule for this.  There’s no simple equation for you to use.  There are just simply too many variables.  Luckily there is a rather simple method and all you need is a piece of sidewalk chalk.

An almost perfect example of using the chalk test.
My preference would be for a wider stripe over a few more lugs.

The chalk method for establishing tire pressure is something anyone can do and it only takes a few minutes.  You’ll want to start with a nice cool tire and a dry solid smooth surface (concrete is better than pavement, but if it’s smooth or sealed pavement you’ll be fine).

  • First, wipe off a section of your tire.  Remove any imbedded stones and wipe off any dust clinging to the tire.  Ideally you want the chalk on the rubber of the tires, not everything you’ve driven through for the last 100 miles.
  • Once you have a clean spot you’ll want to put a nice wide swath of heavy chalk on the tire.  Scribble on there nice a good.  Bonus points for getting your kids involved and extra points for colors like hot pink, pastel blue, and sunflower yellow.
  • Next drive your vehicle forward and backward across the chalked part of the tread.  Make sure to avoid puddles and deformations.
  • Finally check the chalk.  If it’s worn in the middle, your tire is over inflated.  If it’s worn on the edges, your tire is under inflated.  Adjust the pressure by 2psi accordingly and repeat the process.  Make sure to do this for both front and rear tires as well as trailer tires if you pull a trailer.
Once you’ve settled on an optimal tire pressure drive your vehicle for an few miles and check the pressure.  Most likely it’s gone up a few PSI.  You’ll want to let some air out to your optimal pressure. After the vehicle sits for a while the tires will cool and the pressure will lower, that’s fine.  Once you start driving again heat will build up relatively quickly.  If you drive for a long period of time (like an 8+ hour road trip) you’ll want to check tire pressure each time you fuel up to make sure your tires are not becoming over inflated.
Keep in mind tires do lose pressure over time.  Even if you don’t drive that often they slowly seep air out.  With that in mind you’ll want to check them every few weeks.  Most likely it will be a pound or two low over the course of a month.  They’ll also lose pressure in winter, and increase pressure in summer.  Oh, and elevation also affects tire pressure, although not a whole lot.  It’s just basic chemistry: pressure is a result of the temperature and volume of a gas, but I digress.
How do you establish optimal off-pavement pressure?

Establishing an optimal tire pressure for off-pavement travel is not an exact science.  All the same factors that impact optimal on-pavement pressure also have an effect on off-pavement pressure with a few extra additions.  The terrain (rock, sand, mud, snow, etc) as well as the length of travel (a few dozen miles verses 100+), and the speed (rock crawling verses Baja) all play a role.  The too biggest enemies to any tire when off-pavement are punctures and temperature.
Most people are familiar with punctures.  Even the most careful of drivers will puncture a tire or two in their time off road.  If they’re lucky it will be something small like a nail or stick that leaves a nice clean hole in the tread that can be plugged relatively quickly and easily.  If they’re not so lucky they’ll experience a sidewall puncture which pretty much kills the tire.  I’ve had my fair share of both, and everyone I know has as well.  It’s not if, but when.  Punctures can be mitigated somewhat by reduced pressure.  A softer tire will deform some before allowing something to pierce it.  Doesn’t help much against something sharp and metallic like a nail, but it does help a little against rocks, sticks, stumps, and other terrain things.

However, there is a danger in going too low.  This is where heat becomes an enemy.  Every time a tire flexes it generates heat.  As you drive down the tire this equates to hundreds and thousands if not millions of flexes.  Overtime this heat builds up.  Too much heat in a tire can cause things like de-lamination (how many times have you seen the tread of a semi-truck tire on the side of the road?), sidewall de-lamination (ever seen a semi-tire blow on the highway? I have) or tire bead failure (watch NASCAR for examples of this).  All of these things can, and do, happen to tires off road.  Also, even if the critical point of catastrophic failure hasn’t been reached it doesn’t mean the damage hasn’t been done.  Couple poor tire pressure management off-pavement with poor tire pressure management on road and you have a compounding problem that can lead an on-road tire failure.  It’s not all doom and gloom though.  Chances are you’re going to be within an acceptable tire pressure range and as long as you’re not driving like a total idiot (on road or off) you’ll be fine.

As tire pressure lowers the contact patch increases in size both in width and length.

When you “air down” for off-pavement travel you’re looking for two things.  First, to increase your contact patch.  By allowing the tire to deflate a little you’re increasing the length and width of the tire’s tread. This gives you both more grip and better floatation.  Grip is essential for tackling things like rocks, branches, and other obstacles in the trail. Allowing the tire to deform basically lets it grip the changes in terrain much like your hands and feet deform to pick something up or conform to changes in the ground.  Floatation helps in loose terrain like sand and gravel or in something like snow.  A small contact patch concentrates the weight of a vehicle within a very small surface area.  Too much weight in one area (also measured in PSI, but in this case we’re talking about physical weight per square inch rather than gaseous pressure per square inch) can cause a vehicle to sink.  By lowering the tire pressure and increasing its contact patch you’re lowering the effective weight per square inch of vehicle on the ground.  The perfect example of this are the big wide tires of arctic exploration vehicles.  Their super large super wide, almost comically so, tires run at a ridiculously low pressure (often in the single digest) allow the vehicle to generate less pressure on the snow and ice than that of a person standing on their own two feet.  Again, the math here can get pretty complex, but the principle is simple.  Larger contact patch means greater weight distribution and better floatation.
With all that said, there is no easy way to establish a singular optimal pressure for off-pavement travel.  You’ll have to become as much an artist as you will be a scientist when you figure out how far to air down.  There are some handy graphs and charts out there that are good starting points.  They take into account things like vehicle weight, tire size, and wheel size.  That usually ballparks a good starting point pressure. If you’d rather just wing it, a good rule of thumb to start with is airing down to half your street pressure for general trail riding.  From there it will just be constant monitoring of the tires’ pressure.  
  • If you feel the sidewall of the tire and it’s hot, you need to add some air. 
  • If you’re hitting bumps in the terrain and your kidneys are hurting, you tire pressure might be too high.
  • If your tires are not deforming around obstacles (rocks, trees, etc) chances are you’re pressure is too high
  • If your tire is “folding over” and the wheel rim is riding on the sidewall you REALLY need to add some air
  • If your tires are slowly losing air then your pressure might be so low you’re “burping the bead” and the tire is letting it’s own air out (or you have a puncture).
  • If you’re driving at a higher rate of speed in loose terrain and your steering feels unresponsive, then your pressure might be too low and the tire is folding over on itself.  Same goes for rear traction and handling.
Extreme example of tire deformation around a trail obstacle.
This is actually a good thing even though it looks pretty gnarly.
Arguably you won’t encounter this extreme an example while overlanding.

Simply put, this is something that is learned over time.  I’ll also be honest and say that selecting tire pressure for off-pavement travel could be an article in-and-of-itself.  I wanted to keep this article more conceptual to illustrate why tire pressure management, both on road and off, is important. The concepts are important to know, and what works for me and my Jeep with it’s tires, and my driving style, may not work for you.  This is where having the proper equipment comes into play. 

What do I need to effectively manage my tire pressure?
First and foremost, regardless of being a 4wd off-road vehicle or a 2wd street vehicle, every vehicle should have a tire pressure gauge in it.  Even if the vehicle is equipped with a TPMS, a pressure gauge is still a good thing to have.  Most basic TPMS only have a high/low threshold for their alarms and won’t give you an exact pressure.  That means you could be running as much as 5psi over or under and not even realize it.  If you’re lucky enough to have a TPMS that tells you actual tire pressures then you’re lucky, however it won’t tell you the tire pressure of your spare or your trailer tires.  Either way, a good gauge is essential — and not one of the el-cheapo mass-produced overseas built by the lowest bidder ones.  Spend a little coin on a good calibrated gauge you know is reliable and will last you for a few years.

Long before buying big light-bars, heavy bumpers, winches, or even bigger/better tires, I think every off-road enthusiast should buy their own portable air compressor.  I’m not saying it has to be the best one out there, or even something like my ViAir “Ultra Duty” Onboard Air system (although it is very nice if I don’t say so myself), but any kind of quality portable compressor that can run off 12volts while go a long way into not only making the process of tire pressure management easier, but also make it more likely you’ll actually do it.  Sure there are gas stations with coin-opperated compressors and some off road parks like Rausch Creek have onsite air, but last time I checked there weren’t compressors in the middle of the woods and deserts we as overlanders tend to frequent.  Self-sufficiancy is a quality I think is common to overlanders, so having ones own air compressor should be high on the list.  Much higher than LED light-bars, snorkels, or even a winch in my opinion.

Do you need them?  No.
Are they nice to have? Yes.
Do I think you should get a set? I do.
Why? They save time and make life a lot easier.

Tire deflators are a luxury item.  I’ll be honest, you really don’t need a set.  That said, once you have a set you’ll wonder how you ever went wheeling without them and every time you use them they’ll cement themselves are a worthwhile expense.  Luckily most quality sets are between $50 and $100 which doesn’t make them unobtainable.  Even if you used them once a month for a year a set of four will only end up costing you a dollar or two each time you use one — and that’s just for one year.  It’s a worthwhile investment it in my book.  Especially for a set of adjustable ones like the J.T. Brooks “Pro” Automatic Tire Deflators I reviewed last time.  Trust me: worth their weight in gold!
Beyond that a good tire repair kit is the last thing you’ll want to carry with you in your tire pressure management arsenal.  Punctures can and will happen and it’s always nice to be able to patch a hole and keep rolling than to forfeit your spare and be forced to continue down the trail ill-equiped and under-prepared for your next tire problem.

On trail repair of a ruptured sidewall.
He drove the rest of the way out with the plugs in place.
Couldn’t have done that without a repair kit and an air compressor.
Are you prepared for this?

When I roll out on an adventure I always start by checking my tire pressure.  This is part of a pre-trip ritual that includes checking fluids (fuel, oil, transmission, washer fluid, coolant, etc), washing my windows, stocking the cooler, and making sure I have a good playlist ready to go.  When I check my tire pressure I also make sure to give the tires a good visual inspection checking for punctures, sidewall abrasion, and any chunks missing out of the tread.  All of those things if left unchecked can turn a good day bad and make a long trip very short.  This isn’t just a once-and-done check either.  It happens every time I get fuel, the start of every day, and every time I transition terrain types.  Well, at least it should (do as I say not as I do – or don’t do – kids).
As far as off-pavement travel goes airing down usually happens while I disconnect my sway-bars.  I’ll go around and set my deflators on each tire, pop my sway-bar disco’s off, and by the time I have my sway-bar zip-tied up out of the way the tire deflators are usually done.  I always make sure to check the pressure with my gauge to make sure and air up/down as needed.  When I’m done wheeling for the day I’ll kick on the ViAir compressor and air up my tires back up to street pressure.

Of all the tools in my Jeep, my ViAir Onboard Air System gets used every time I hit the trails.
Sometimes it even gets used multiple times in one day.
You can’t say that about a 50″ light bar, a snorkel, or even a winch.
I know this whole thing has seemed rather gear-centric and all but requires you to spend money.  That said, it’s up to you as to how much you’re going to spend.  You can go balls-out and get the nicest most expensive set of deflators out there along with slick air compressor system capable of airing up your tires in seconds.  You can also take a budget minded approach and skip the deflators by airing your tires down manually and investing in a small light duty portable compressor.  I’d like to think I split the difference with my setup.
It also comes down to priorities.  I honestly laugh a little too myself when I’m at the off-road park and I see rigs with 50″ light bars, snorkels, and every bolt on accessory from the latest parts catalogue waiting in line to air back up.  I get people want to have rigs that look good, but as overlanders we need to have a different set of priorities.  Even just skipping one high-dollar low-function accessory would mean money available for a compressor.   We need to get the most out of every dollar we spend on our rigs.  We also need to only spend what we need to spend so we still have money left for food, fuel, and fees for our next trip.  No point having all the accessories from the catalogue if you don’t have some money left over to go out and play with them.  That’s why I put an air compressor all but at the top of my list for overland gear investments.  That might just be a future article right there.
Anyway, I hope you’ve been able to take something away from this three part series.  I know as overlanders we want our trips to be about experiences and not gear.  However having the right gear makes our trips possible, keeps us safe, and makes sure both us and our vehicles are around for many more adventures.  Tire pressure management is a high priority and I trust by now you realize that and what it means for tire life, and vehicle handling both on-road and off.


Author’s Note: The more I think about it the more I realize I should do a dedicated article on off-pavement tire pressure. So this three part series on tire pressure management will end up being four parts.  That said, I’m not sure if part four will happen next or not, most likely not.  I’d like to wait till I have new tires since my current tires are on their last leg.  Either that or I’ll see if I can convince a friend with more photogenic tires to help me out.  Either way, there will be another chapter in this saga.  It will just take me a little while to make it happen.