Electronic Design

Bob's Mailbox

Dear Bob: Just read your recent "Mailbox" column (ELECTRONIC DESIGN, March 16, p. 20). You commented on a reader (Mike Smolin) who stated that his tire dealer put nitrogen in his tires. There are good reasons for this. The main advantage is that nitrogen is dry, whereas compressed air has a lot of water vapor in it. (Ordinary compressed air may have some water vapor in it. A little. It may or may not have much. /rap) When the tires get hot the water boils and increases the pressure in the tires, which affects handling, braking, etc.

  • Cliff Harris (via e-mail)
  • Pease: How often do you run your tires above 150°F? Racers may. I don't. I doubt if you do. So do you want to pay $8 or $12 for nitrogen in four tires? Yuppies may want to have "the very best," even if they can't tell the difference. Just like the Emperor's New Clothes.

    NOTE: Even at 220°F, water does not boil in an atmosphere with 30 psi of pressure. So how much "vapor pressure" will they add at 150°F? I'll guess 3 psi if you have a lot of water in there (1/4 cup) and perhaps 1 psi if you had 1/8 teaspoon. For sure, on my VWs, the tires don't get above about 150°F, no matter what. The temp rise is barely 25°F above ambient, and even in a desert, it rarely gets above 125°F. If they are that hot, your 30-psi pressure has already risen to 34 psi, and that's not a big deal (on my Beetle, from 27 psi to 31). If you are running your tires hotter than that, well, you probably have other problems. You said there were "good reasons." Good reasons for the mechanic? Like what—boat payments? I'll just keep air in there. For racers, balanced inflation is very critical, so they use dry nitrogen to prevent any 3-psi imbalances.

Dear Bob: My thoughts are that the new tires always go on the front. I put a very high premium on being able to steer. (Yeah, but these guys argue that being able to steer only a little, and to skid not at all, is better. Better for them, maybe. /rap) Only a real yahoo would put the new ones on the rear of a frontwheel-drive car. (That is exactly what these guys think is safest. /rap)

I miss rear-wheel-drive cars because they always wore the rear tires faster. Then you would put the new ones up front and have nice new ones for steering and half-worn ones on the rear for decent traction. I like a little oversteer! Always hated frontwheel-drive (a Saab, two Toyota Camrys) because of its sometimes abrupt understeer. (Yeah, but that is about all you can rent these days. /rap) Remember that stupid myth about frontwheel-drive: "If you go into a skid, floor it—it'll pull you out"? Talk about crapola.... (Check. Have you seen my pretty good book, How To Drive Into Accidents...? Check it out at www.transtronix.com. I put at least half of the chapter on skidding on the Web. Comments invited. /rap)

I've been driving Subarus for years, now, and you can't beat four-wheel-drive. Plus the torque is biased towards the rear, so you can kick the rear around if you want to.

  • Nick Allen (via e-mail)
  • Pease: Hello, Nick. Well, you and I agree that we want good traction under our drive wheels. But a lot of other people wrote in to say that it is dangerous to have worn tires on the rear, even for front-wheel-drive cars. They argue that most drivers are used to understeer and plowing. But under braking or under cornering with slack throttle (especially on rainy or slick surfaces), this can change to oversteer and a nasty skid. Maybe most drivers would be safer if they avoided that. Maybe they're terrified of getting into skids and don't know how to get out of skids.

    The chances that I will own a front-engine car or front-wheel drive are negligible. But that's what some worrywarts worry about, and apparently the tire companies do, too. So if you take in your car, and they insist on putting the new tires on the back, you will have to overrule them by rotating your own tires, front to rear, right in their parking lot and thumbing your nose at them! (And then you have to be extra careful, for a while, not to skid into any accidents.)

Comments invited! [email protected] —or: Mail Stop D2597A, National Semiconductor P.O. Box 58090, Santa Clara, CA 95052-8090

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