Thursday, May 08, 2008


Local television news has a story about filling car tires with 100% nitrogen. Allegedly, nitrogen doesn't leak down as quickly so tires stay at a higher pressure for a longer period of time. This increases gas mileage. There's also a claim that nitrogen extends the life of the tire, but I suspect that if they're properly inflated – regardless of the gases inside them – they'll last longer too.

For a fleet operation, filling tires with nitrogen probably makes sense. If the claimed 3% increase in gas mileage is true, it translates into large cash savings. But the local business offering this service charges $39.95 to fill four tires. From where I sit, that may not be a cost-effective savings for consumers.

There's a solution, of course. I use a cheaper mixture that contains only 80% nitrogen in both car tires and bicycle tires. The only drawback is that I have to check the pressure regularly. That means using a pressure gauge on the car tires about once a week. But for a bicycle, the pressure has to be checked before every ride.

That brings us to the ABC Quick check. (You didn't think I'd skip a BikeEd moment, did you?) A is for air. Check your tire pressure before every ride. Yes, it reduces rolling resistance, but it also reduces the chance of a pinch flat. Spin you wheels and look for tire cuts or a rim wobble that could indicate a loose or broken spoke. B is for brakes. Squeeze the brake lever and ensure that there's a fingers width of space between the lever and the handlebar. C is for the crank and chain. Try to move the crank arms in and out, feeling for any looseness. Feel for a loose pedal, and see that the chain is in place. Q is for quick releases. Get in the habit of putting them in the same position all the time so you can tell at a glance if they've been moved. Returning to C a moment, when you first start out pedal gently to ensure that the chain is in place properly and no one has tampered with your derailleur levers, particularly if the bike was parked outside in a public place.

There are some costs associated with ignoring the ABC Quick check. Once, someone who looks remarkably like me fixed a flat tire, then pushed off down a hill. hadn't closed the brake release lever, so the caliper barely touched the rim. Naturally, the lever went all the way to the handlebar and he discovered the brake was pitifully ineffective. Doing the ABC Quick check would have prevented this mishap.

Another time, this same rider had a rear quick release partly open. The wheel was installed in an old frame with horizontal frame ends. When he stood to power away from a stop light, the wheel popped out and tacoed. While it's possible to straighten a wheel by supporting it on a curb and forcing the rim back into alignment, it's not a practice I recommend. Some of the neighborhood kids learned a wide variety of new and exciting words that day.

(Snark Alert!)

Here's a fitness idea that would also save money for big fleet operators, like cities that have a large number of police cars – get cops to check their air pressure at the beginning of each shift, and give them a bicycle pump for inflation. It's a win-win situation! Proper tire inflation leads to improved gas mileage and increased tire life. And as a side benefit, we get physically fit police officers with fewer health problems. (Except for TCSO who would need extensive training in how to use a bicycle pump in the first place.) As a taxpayer, I'm 100% behind this idea!



Blogger Fritz said...

If nitrogen indeed leaks more slowly than the other components of air, then the other components will leak out of the tire faster than nitrogen and eventually you'll end up with close to 100% nitrogen in your tire, right?

12:40 AM  
Blogger Ed W said...

I asked one of the smart guys at work about it. He said that nitrogen doesn't expand and contract as much as other gases, but I didn't have time to research any of it last night.

I'm working overtime AND we had a planning meeting for tomorrow's Tough Kids event, but I'll try to do a search for nitrogen information.

5:17 AM  
Blogger MikeF said...

I have been studying this topic for over four years now and am a proponent for nitrogen filled tires. The reason nitrogen stays in tires longer than plain air is that the molecular diameter of nitrogen is larger (approx. 3.2 Angstroms) while the oxygen molecule is 2.8 Angstroms. An angstrom is 1/250 millionth of an inch. Since tires are made of permeable rubber, both of the principal gases of air leak out over time. Plain air leaks out on average at a rate of about 1.5 PSI (pounds per square inch) per month. Tires inflated with nitrogen would take 3-4 months to leak down this much. You are correct that if a tire were properly inflated with air or nitrogen, the benefits of better tire wear and fuel efficiency would be about the same. There is physically very little difference in the thermal coefficient of expansion between air and nitrogen. All elements, whether liquid, solid or gas expand and contract with temperature. The reason nitrogen does not expand and contract as much in tires is due the moisture content in the tire. Nitrogen is so dry that there is less than 5 PPM of water vapor present compared to up to 10,000 PPM with air. The largest contributor to premature tire failure is under inflation. According to a study a few years ago by a division of the DOT, around 90% (to the best of my recollection) of the tires in America are underinflated. Nitrogen filled tires help people like you and I keep their tires within the pressure tolerances acceptable for good tire wear. For me it is a convenience thing. I do not have time to check my tire pressure (or even think about it for that matter), so filling my tires with nitrogen made sense, even at a $20-$30 price. I paid just under $600 for a set of Michelins for my car and getting an extra 30% tire wear was appealing to me. Depending on how big of an under inflation offender you are the fuel savings can be significant. At the very least you get your money back for the nitrogen inflation. I drive approximately 12000 miles per year, and my car gets about 22 MPG. Right now our unleaded fuel costs $3.75 per gallon. That means I will spend around $2,045 per year on gasoline. With fuel savings as low as 1.5% I would save $30.68 which is more than the $29.95 it costs in your area for nitrogen fill. The real benefit is safety, which you can't put a price on. With properly inflated tires your car handles better, stops better and are less likely to blow out. Overheating due to severe under inflation can cause a blowout. It is also good for our environment. Each year millions of tires go to the landfills. Since nitrogen helps tires keep their pressure longer and properly inflated tires get longer life, the load on the landfills will be less. I could go on, but I need to get ready for work. Hope this helps.

7:40 AM  
Blogger CGHill said...

My tire store of choice - actually, I buy online from the Tire Rack and have them installed locally - charges six bucks a tire for nitrogen, and I had them pump it into a fresh set of Dunlops last year. I'd say there are too many variables to assert that there are substantial fuel-economy gains, except to the extent that the tires are indeed less likely to be underinflated. (I still check mine on a regular basis, out of force of habit; they never seem to go down.) At the moment, I consider the $24 well spent.

5:04 PM  

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