Nitrogen Filled Tires

For anyone who has known me for any length of time, you might have heard me occasionally rant about nitrogen-filled tires.  I personally think that its the biggest scam inflicted upon the driving public in a generation... right up there with undercoating. My problem is not necessarily with the benefit of using nitrogen to inflate your tires, but more so with the overstatement of that benefit. First let me talk a bit about whats happening with the "air" that comes stock in your tire then I'll talk about how that's different with an all-nitrogen fill.

"Air" is actually about 78% nitrogen already, then 20.9% of it is oxygen, the last 1.1% or so is other gasses like argon. One of the characteristics of this mix of gasses is that it behaves pretty much according to the ideal gas law... which is the following description:

PV=nRT

where:

• P is the pressure of the gas 
• V is the volume of the gas 
• n is the amount of substance of gas (in moles) 
• R is the ideal, or universal, gas constant (just a number) 
• T is the temperature of the gas 

So not only does air behave according to this description, but each one of the components of air individually do also. One point to make here is that your tire pressure is directly related to temperature... so think about that when trying to figure out if you've "lost" air in you tire or its just colder. This is why you should check tire inflation when the tires are cold.

So now lets talk about the purported benefits of a nitrogen-only fill. The sellers of this service claim that it:
1. Reduces oxidation on the wheel and tires
2. Reduces molecular seepage and thereby under-inflation (which has lots of other effects)
3. Reduces moisture in the tire.

There are actually lots of other benefits that are claimed but all of them basically are derived from these three. So let me address them individually. Yep, it probably does reduce oxidation of the tire and of the wheel itself.  But that is almost never the cause of failure of a tire or wheel.  Molecular seepage also probably happens... but the claims are that it is "normal" for a tire to loose 1-3 psi per month due to permeation (seepage)!  However it has been shown that the more-realistic rate is more like 0.3 psi per month according to this ARPN Journal study.  Finally, yes it does actually reduce the moisture in the air... but again the amount of moisture in the air is already minuscule.  First off the air can only get into the tire as vapor so assuming 100% humidity there could only be a few ounces of water total in there.  Furthermore, most air compressors have the effect of drying the air used because of the over-compression and then precipitating out of water in the compressor.  This is why all air compressors have water drain systems.  Plus the effect of water in the tire is not that significant either.  It still behaves like an ideal gas.

Here is some math on the moisture content:

Let's first examine how much water is actually in the air that is used to fill tires. Generally the tire shop (or home shop) uses a piston air compressor and a pressure tank to source the compressed air for the tires. If we assume that the air compressed is 100-degrees f and 100% relative humidity starting at atmospheric temperature.… sort of a worst-case scenario. In terms of how much actual water is in the air, the moisture in this case works out to be 4.3% of the air mass. If we compress that mix up to 100 psi (what most shop compressors run at) then using standard psychometric charts, we see that the capacity for the air to hold water is reduced substantially. In fact the air can only then hold 0.53% mass of water.  In practicality we see that in shop compressors because you have to drain water out of the air tank periodically.  So then taking that air and cooling back down to 75-degrees (or basically the temperature of a normal shop), the water holding capacity goes even further down to 0.24% mass. This is also why industrial air compressors have dryers… because when the air cools down there will be more moisture dropped out. Luckily in a shop environment this happens mostly in one tank. The last step then is for the air to be inserted into tires. The pressure then changes back down to about 30 psi. So using the final mass of water in our air, the relative humidity of air in the tire will be 39% (remember starting at 100%), and the dew point will be 48 degrees.

But lets be real for a moment here… when was the last time your wheel rusted out? Or when was the last time you had a tire fail due to internal corrosion? The reason it doesn’t is because the air in you tire is already far dryer than the atmosphere. This is a simple case of saving drivers from a risk that is exceptionally low.

Pressure Variation Theory:

As noted above, pressure inside your car tires can be modeled using the ideal gas law.  In that law the relationship between pressure and temperature is identified as being a linear relationship. Using a 30 PSI tire, the relationship shows that for every 10 degree change in temperature, pressure changes by about 0.9 PSI. However, in this model, it doesn’t matter what gas is used. Air, nitrogen, water vapor all can be modeled with this formula to a reasonable level of accuracy. However, the nitrogen purveyors have constructed an interesting argument. That is that they assume that the water contained in the tire air is in a liquid state at normal temperatures and then goes to gas state at driving temperature. To figure out how much this effects the tire pressure, lets do the math. Using the data above you can see that is clearly is possible to have some liquid water in the tire assuming ambient temperature is below 48 degrees and you filled up at 100-degrees and 100% humidity. So how much does that effect pressure? Lets take our 0.24% water derived above and convert that to actual amounts. Assuming the air volume in a tire is about 12-gal, that would equate to about 0.368 Oz of water.  Using steam charts at 35-psi, the specific volume change in water is 696 times.  So our .368 Oz of water would be .01Kg which equates to 10 milliliters in liquid form.  Using our steam chart (and that assume that the water changes from 100% liquid to 100% vapor) the new volume would be 6,960 milliliters or 1.8 Gal.  But remember that it now complies with the ideal gas law so in order for the change so we compress that to 35 psi it becomes 0.5 gal or 4% of the compressed air volume. Using the fact that proportionality applies to pressure mass relationships this would increase or decrease pressure about 1.4 psi.  So even in the most extreme example I can think of the state-change theory of nitrogen promoters has a very small effect.

1. There probably is a benefit to having only nitrogen in you tires vs "just" 78% nitrogen.

2. The benefits however are way too small to matter especially if you're paying upwards of $70 per tire for this service.

3. You would be far better served by just checking the pressure on your tire regularly.

Don't be afraid of your car

Technology can be scary, but fear not.

I cant tell you the number of times I've heard the complaint from car owners that modern cars are too high tech these days and that it's too complicated for the average home mechanic to effectively fix things.  I disagree to an extent.  First lets review the historical perspective of the technological advances of cars.  When I was in high-school (late 1980's) I took several years of auto shop.  At that time computers were beginning to be more common in cars but we were trained to tune and maintain cars that were truly without computers.  That is to say we adjusted points, timing, carburetors, and a variety of other analog adjustments.  However even then, there were things that were just out-of-scope for the average mechanic.  For example if you spun a bearing, it would have been very unlikely that the average mechanic would have the equipment and knowledge to repair that themselves.

Fast forward a few years and in the early 1990's I started working on cars that had actual computers in them.  That's when I discovered the wonders of the diagnostic trouble codes (DTC).  The earliest versions of these required the home-mechanic to jumper the diagnostic port and record the number of times the "check engine" light flashed.  It would translate to any stored error codes that the car recorded (most likely a sensor of some sort).

Well things have gotten much easier these days, and I would argue even better than before computerization.  This has a lot to do with the OBD2 standard that all modern cars now comply with and governs how computer error codes are communicated to the mechanic.  It is possible to plug in a code reader or computer to your car and not only view all the trouble codes, but also see in real time dozens of individual sensor measurements in real time.  The reality is that you cant set timing or dwell on a modern car and that's a good thing.  Let the computer figure out the best operating parameters.  I'm not saying that every thing that goes wrong with your car can be understood by your computer interface, but diagnosing a car problem is much different today that in used to be... but it still requires diagnostic skills.  For example when I look at a error code generated by my car, I also reference the factory service manual and in fact online discussions of the same issues... experience still matters.

The car companies sure don't make it easy.  Most of the time when you pop the hood of your car the first thing that you see is usually a large plastic engine cover.  Its there mostly to keep the owner from seeing and fiddling with the engine.  That said don't be afraid of plugging in and figuring out what's going on.  There still will be a time and place for bringing it to your mechanic.  I have no problem taking my car in for an alignment, recalls, and plenty of other things that I don't have the equipment or knowledge to do myself... however jumping in and working with technology doesn't scare me and it shouldn't deter you either.

A Clean Battery is a Happy Battery

I first heard this statement from a fellow Marine while we were driving across the Jordanian desert. He was cleaning a battery on the 5-ton truck he was operating at the time and that phrase kind of always stuck with me.  In this modern world of high tech vehicles I still believe that battery maintenance is a vital part of overall car maintenance.  So as a part of the regularly scheduled vehicle maintenance, I will do a battery maintenance as well... typically every 30k miles or so.  So why does a batter get so gunked up even if your good about keeping other things clean?  It has to do with gasses venting from the battery reacting with the lead posts.  It could happen to either post but I've seen it more on the negative side (anode pole) because that's the side that is susceptible to corrosion during charging.  Here's what I do to service the battery:

1. Remove the battery and connecting brackets.
2. Pour a little distilled water on the top of the battery
3. Sprinkle the top with baking soda
4. Using a wire brush, scrub the top and sides of the battery
5. Rise the top and sides with more distilled water
6. Dry with paper towels
7. Using a standard screwdriver, pop the cell caps off (See Note below)
8. Top off the cells with distilled water (only to the bottom of the plastic)
9. Dry off the top again with paper towels.
10. Replace the cell caps
11. Using a battery post cleaner, clean off the lead posts
12. Similarly clean the connector leads
13. Reinstall the battery
14. Spray a little bit of corrosion preventer on the posts.

Notably, some batteries are considered "maintenance free" (which isn't really true) but actually means that the cells are not accessible.  Simply skip steps 7-10 but by all means still do the rest of the service.

before

after

First Wash

Finally got the car out of the garage from its hibernation and gave it a good bath.  I had been planning to hit it with the pressure washer for some time now, and just had to make the time for it.  I was really being careful to get my other vehicles far away as to not be splattered with any grease or funk.  It was easy enough to roll out of the garage, but I had to wire up the front of the differential tube to keep it from dragging.  By the way, the differential / rear end unit in this vehicle is huge.  It has a torque tube setup which means that the differential assembly actually begins at the back of the transmission.  It looks like a big "T". Can't wait to get that out so I can put a picture of it here.  I washed inside and out and removed a large quantity of junk from the interior and the undercarriage mostly.  I wasn't quite as pleased with the results of the pressure washing on the front suspension.  The front independent suspension had (has) years worth of dried grease built up on the control arms and knee-action shocks...  pressure washer didn't stand a chance.  Its going to have to be a more manual process getting them cleaned up.  The brakes in one of the drums locked up on me when I tried to get it back into the garage and it took some serious pushing to get it back in.  Now to get the driveway cleaned up.

Motor

I know it’s been a while since I’ve posted any new updates, but rest assured the work goes on.  It’s somewhat of a challenge to make time for working on the car AND posting about it.  To complicate things, I’ve taken on new responsibilities at work, and so for the time being I’m dong both my old job and my new one.  Excuses aside, I did manage to get the motor and transmission pulled from the car and have all of the body panels off to the point where it’s just really the chassis and the body left.  My plan is to pressure wash the car, then start fabrication work on the firewall.  I’ve got the engine block ready to dummy up, now just looking for the right transmission... seems everyone is looking for a T56 that bolts up to an LS1 block.  At any rate, work continues.

Quirks - Part 1

As I’ve indicated on earlier posts, I am very much amused by all of the little design quirks of this vehicle.  So, I thought that I would start a regular feature detailing the interesting, different, and odd things I find as I disassemble and build this vehicle.  I do realize that this might be entertaining for only me, so please bear with me.

As I was unloading the car from the trailer last summer, I noticed something that puzzled me for several months.  There were straps underneath the running boards... two on each side, four total.  On each end they are held by rubber mounts and they are about an inch wide thin sheet metal running down the underneath side of the car.  I though that I was looking at some storage straps or even the remnants of some compartment.

I wondered about these things for many months.  Particularly I wondered what kind of negative effect this would have if and when I do not put them back on the finished car.  As I was minding my own business, and just cruising the internet, I ran across the options manual for this vehicle (on the “old car manuals project”).  So on this page, I found the answer to this question.  They are not straps after all.  For $5, a new car buyer buyer in 1939 could add the option of under car radio antenna.  On the Master Deluxe model, it was standard.  “For car buyers that prefer an antenna that is not visible.”  Of course I had never heard of such a thing, but makes sense to me.  I wonder how it worked... The other odd thing is that there is actually an external antenna mounted in front of the driver side door.  Until now I had assumed it was stock, but now I guess it must have been added later.

I’ve got a link to the Old Car Manual project on my “links” page, and they have tons of great information including the original factory service manuals, marketing flyers and booklets, as well as some very early technical service bulletins.  I love that site just for the history, but has proven very useful so far on this project.

Fenders

Obviously when you intend to build a car, you have to take it apart first.  As I indicated earlier, my first goal in this project is to get everything disassembled. Then cleaning and start building.  The process for the next several months will be disassembling from front to back.  Then when its down to the body and frame, I’m going to power wash the whole thing.  From there, I’ll start working on the motor mounts and the firewall modifications.  Once that’s complete I’ll then pull the body off and start setting up the chassis.

First things first, getting the front end apart. The fenders are major players in that department.  After removing the hood cowls, I started unbolting and found two fun facts about the fenders:  First the fenders on a 1939 Chevrolet are bolted to nearly every other component imaginable.  The second is that most bolts installed in 1939 do not want to be removed.  Thank God for super rust penetrant.  I’ve been using it to spray the bolts before attempting to remove.  I wound up using the cutting torch on the bottom bolts of the fender (connecting to the running boards), but as you can see below, they are finally off.