What Goes Up, Should Come Down
…or vice-versa, when it comes to tire pressure. One could call tire pressure control “the poor man’s winch”, since lowering tire pressure tends to elongate its contact patch or footprint on the ground. That increase in gripping surface area increases traction on difficult surfaces. Airing down tires in off-road situations is old hat to 4WD enthusiasts, but new to me. It is considered at least as effective as jamming traction boards under the tires, if not more so.
I normally wouldn’t consider it because of its drawbacks:
- While you’re airing down or pumping tires back up, you can be sitting beside the road for considerable periods of time.
- If you hit a perfect patch of ground for making time in the middle of badness, you cannot pick up the pace on it to gain time – going too fast on a deflated tire can cause overheat and handling issues.
- Heavy vehicles on high-pressure tires benefit less from lowering pressures – but do still benefit.
- Play Baja Racer, and the doughball handling can put you in a ditch or over an embankment, pronto.
- Go too far with lowering pressure, and you can unseat the tire bead, effectively dismounting the tire.
- There is a risk on rocky ground of compressing a sidewall enough to pinch it, resulting in damage or puncture.
- Overall, operating a vehicle on underinflated tires is a direct trade: increased traction in trade for increased odds of tire failure or vehicle mishap.
I have to admit, I’ll occasionally be lowering my tire pressures not to conquer new trails, but as a last desperate act when I’ve underestimated a trail’s traction difficulty or roughness. Roughness? Yes, and I’m not talking about climbing over grapefruit-sized rocks. Idling over one-inch pebbles in the 10,000# GVWR Mighty Furd can be physically punishing, due to its 65-80 PSI tires underneath very stiff springs, with a family-sized front antisway bar in front. Those full-support foam seats play a part as well, conveying any violent shifting to the body immediately, instead of launching it in a trampoline-style acceleration common to old-school coil spring seats. The violence which results from such minor road imperfections is difficult to convey in print – I’ve often disbelieved it while sitting behind the wheel, wondering what a ruptured kidney feels like – but I hope to present it effectively here and there in future posts. It does not ride like an SUV. Those are wonderfully floaty. The Super Duty is more akin to The Green Hulk grabbing you by the ankles and flailing a brick wall with you. As the medicos like to say, “there may be some discomfort”. I’m hoping that the tall weight of the Four Wheel Camper takes some of the starch out of the experience.
Having to air down and then take the time to pump back up in order to cross a single deep sand or gravel-filled wash is not my idea of a good time, but waiting until you’re buried axle-deep is not that convenient, either. That’s also an issue with the diesel version of the Ford – its engine does not bog down or speed up to signal that you’re losing traction. It just pocketa-pocks along like a metronome with jangled nerves, and it’s up to you to notice that ground speed is dropping off. If you bury the tires, even airing down may not save you. So although I may not air down as a preemptive move, you can bet I’ll be paying attention while rolling across certain traction-challenged areas. So far, I’ve managed to avoid mud entirely, but that’s likely to change as the Intrepid goes on tour.
The main reason I have never aired down my tires is because I have not known how low I could safely take them down, nor owned an air pump capable of bringing them back up to 70 PSI anytime this century, if at all. Your typical auto parts store 12V air pump is handy for air mattresses and bike tires, but pressing it into taking even one E-rated tire up to highway pressures will commonly lead to one of two results in short order: it will melt its cigar plug connector and/or damage the power port it’s plugged into (without blowing the fuse), or it will get hot enough to fry eggs, embrittling its rubber hose to the point that it snaps off next time the pump is unpacked for use. With the pressure on, so to speak, that can be disappointing. I’ve done both with mine.
The “how low to take them down”, I’ve mentioned in a previous post. That is, measure the distance from the ground to the wheel rim on your vehicle. If you reduce that by 25%, the new distance is your target. Whatever pressure it takes to get to that target is your lowest “safe” air-down pressure. In the case of the Mighty Furd, a starting pressure of 65 PSI yielded a height that, when reduced by 25%, took 25 PSI to meet it. That’s quite a reduction, and the 10-ply tire sidewalls did not bulge out as much as I expected. Then again, that thickness makes them pretty stiff. I also let my distance reduction be a little on the conservative side, so I wouldn’t doubt that I could err as low as 20-22 PSI without much risk. The main issue here for my own usage is the heat buildup from sidewall flex, which is aggravated by speed and wall thickness.
In a thick E-rated tire, hot dogging a trail in race mode can become a new learning experience. The “Well, how fast can I go?” question is best played conservatively, unless you are in a real race bankrolled by somebody else, and which has a sizable purse for the winner. For us mortals, it’s best not to rely on personal anecdotes on some enthusiast’s forum. I once came across a Ford Diesel enthusiast’s forum where the question came up of what kind of sustained speeds a maximum load on a gooseneck trailer could safely be towed at, centering mainly on tire limitations. (Tire ratings go down radically as speed increases.) Predictably, three participants appeared who seemed to link masculinity with speed, and none would admit to averaging any less than 80 MPH while hauling 20,000-24,000 pounds cross-country. Brilliant. In the same vein, the question of how fast one can go after airing down tires will always elicit, “I always go 35-45 MPH, no problem.” There’s always at least one, and it may prompt some other insecure male to return the volley in a bragfest. Pay no attention. Such things are emotional jousting challenges, reflecting bravado more than technical knowledge or fact. Taking them seriously can put you on YouTube.
The most common recommendation among those more qualified seems to be 20 MPH for 20 PSI, reducing it 5 MPH for each 5 PSI lower. That’s for light SUV-level rides, so the best option for the Ford is to use 20 MPH as a maximum, and get out occasionally to feel the sidewall heat. In short burst situations, it comes down to just how squirrely a 9,000-pound truck handles on severely under-inflated tires. I’m not sure what the point is in squeezing out a few more miles per hour anyway, when airing up will approach half an hour. As Martyn of Adventure Trailer likes to say, “You’re four-wheeling. Are you suddenly in a hurry to get back to the office? While the pump is running, break out a chair, relax, sip a beer and take in the view.”
The second method I’ve come across as a guide for how low tire pressures can safely go is 25% of the maximum pressure shown on the tire sidewall. In my case, 25% of 80 PSI means a 20 PSI minimum, which sleeves in pretty well with my conservative measuring of wheel rim height reduction at 25 PSI. This method is generalized, while the measurement method is specific to the vehicle. Your choice.
How to take the tires down? Many if not most off-roaders use a tire gauge to let air out. Based on past experience, they count the seconds it takes to get into the ballpark, then stop and start until they finally get the exact pressure they need. Then they go on to the next tire. This isn’t a bad approach if you’re dressed to suit and won’t ache from kneeling for long periods. Besides, it doesn’t take all that long to drop 35 PSI down to 15 PSI. Dropping from 70 down to 25 PSI takes awhile longer, and the thought of accidentally undershooting pressure does not appeal. Since I’m inattentive and dependent on the Mighty Furd’s operation, I’ve opted for using automatic tire deflators. You can use just one, but they come in sets of four so that you can apply them all and be nearly ready to go by the time you’ve completed your first lap around the vehicle to begin retrieving them. There are several brands and types out there. Most folks feel that they’re too expensive. As a confirmed cheapskate, I see them as a Godsend.
The particular ones I picked are Coyote 4-56 PSI units, which can be set for any pressure within that range. I don’t need that range, myself. They can also be safely left on the tire while deflating or driving aired down, though I can’t imagine wanting to do that unless I discovered a wall of water barreling down the wash, or a bear appearing out of nowhere to demand that I hand them over – as I understand it, they neither use them nor understand the concept behind Craigslist. I like these units in that they are consistently right on the money when they automatically stop the bleed-down, and they absolutely do not seep air after that point. They are one of those pieces that quietly does what it’s supposed to when you turn it loose and walk away, with no caveats or compensations to keep in mind. If the pressure at start is only a little higher than the target, they can be manually triggered to start lowering pressure – which is not a universal ability. Getting all four adjusted to the target pressure you want is easy, but time consuming. Once accomplished though, that’s it forever. They all tuck into a small and slender leather case that can go in the glovebox or stay with your air pump.
Ah, the air pump. I flip-flopped a lot on this one. Adventure Trailer advocated a fast and reliable ARB twin-pump unit that cost as much as all of my tires put together, but their more plebeian and affordable unit seemed to collect a multitude of user complaints on poor quality and inoperative behavior. Since I will have no winch or other backup measures available other than a small shovel, I won’t be airing down my tires in order to deliberately advance onto more difficult terrain. My airing down will be the only available backup when extra traction proves to be necessary (or in punishing ride conditions). It will be the emergency response to allow escape, rather than serve as an enabler to advance things to the next level. The other reason for not airing down willy-nilly is that the engine must be idled to run the pump, and prolonged idling (more than 10 minutes) on Ford’s Navistar diesel for my year promotes gumming up the valves and clogging its overly-complex and expensive emissions system. The only way to partially undo this is to run it hard at high speed soon afterward to get some heat up, which isn’t going to happen on a trail or dirt road. It’s one of those things that is a non-issue short term, but can have a cumulative effect if you intend to hang onto the truck for the long run. Thus, I won’t be using a more exotic pump often enough to justify its much greater cost.
Viair is a trusted and popular brand more commonly used by RVers, and the first-blush choice came down to fairly quick speed with a huge amp draw and a limited duty cycle. Duty cycle is a somewhat standardized rating of how long an air pump can run before it overheats and must sit and cool down before turning it on again. That cool-down time is generally thirty minutes, so if anything screws up your desperate charge toward 70 PSI on all four tires, you’ll be extending your inflation time quite a bit. Looking at some fill time results on these Big Boy pumps, it looked like this little contest might be doubtful as to whether final pressure could be reached before the emergency temperature shutoff would occur, and I’d potentially be cooling my heels while the pump cooled its own. Once again, this is a non-issue for lighter vehicles using lower highway tire pressures. In the case of their 400P model, which can push 2.3 CFM at no load and 1.44 CFM at 70 PSI, its duty cycle is 33%, which means that it will need to rest 30 minutes for each 15 that it runs, to avoid damage. Continuous use at a modest 30 PSI is limited to 40 minutes, and it is not reasonable to assume that 60 PSI will merely cut that time in half – the higher the pressure, the slower the fill. It pulls up to 30 amps in operation. That’s a lot, which explains the short electrical leads and long air hose. Auto parts store pumps reverse that, since thin-gauge wire is cheaper than high pressure hose.
One of my mantras is, “If there is doubt, there is no doubt.” I chickened out on the Big Amp model. After much gnashing of teeth, I opted for Viair’s less productive 450P-Auto – which has no duty cycle for the pressures I’m pushing: 100% at 100 PSI. It’s a more leisurely fill rate, but one which will not be forcibly interrupted for any reason. Even so, it still pulls some 23 amps maximum, which obviously requires clamps to connect directly to the vehicle’s battery terminals – while the engine is idling. Just as a nerdy test, I hooked it up to a deep cycle 104Ah AGM with an 8-amp battery charger trying to keep it up. No go. The pump managed to get one tire up to 60 PSI before it stalled from lack of voltage. This thing relies on a stout alternator, pure and simple. It pushes 1.8 CFM at no load, and 1.09 CFM at 70 PSI. Not all that bad a volume decrease at working pressures, over the bigger pump.
Unlike the 400P, the 450P-Auto is what it implies: automatic. That does not mean that it reaches a target pressure and shuts off. What it does mean is that if you let up on the handle’s fill trigger, the pump stops running so you can look at the included air gauge to read pressure, or move on to the next tire. There’s no need to hit the power switch to get an idea of where you are on pressure, or to remove the fill tip so you can move on to the next tire. It’s one of those little conveniences that in practice isn’t so little. Both models can deflate tires. While the 400P can be set to deflate hands-free, the 450P-Auto handle must be held in order to keep a button pushed down. For me, that is a non-issue.
It is a large and heavy air pump, weighing in at 13 pounds. It comes in a rather impressive heavy duty carry bag with numerous pockets, the kind of bag that lets you know that if they skimped on something, it sure wasn’t the bag. The spare air filter elements and tip adapters take up a couple of pockets, leaving many more for things like the Coyote deflators. The zippers are strong and engage as smooth as glass. The pump runs at a comfortable noise level, the only real racket being made by the rattling of its foam-wrapped carry handle. But, after a couple of minutes, that rattling disappears as the pump heats up. So while the Viair 450P-Auto is a far cry from the types of pumps that adventurous and well-heeled off-roaders use, it’s still a serious pump, and one that’s a better fit for my very occasional desperate moments. You know, those moments when a gizmo absolutely, positively has to work.