PSIlanimous

This started with Fillyjonk having to wrestle with the tire-pressure monitoring system again:

I decided to haul out my little pump and bring them back up to 40 psi (max inflation is 44, but I thought “given the changeability of our temperature, maybe it’s best not to go to the very max on a chilly day” and also it’s hard to read the marks-between-the-tens on the little gauge). So that took maybe 10 minutes to make sure all four tires were up to the right level, but it shut off the light, so that was good.

Now to me, 44 psi, which is what it says on the sidewall of the tire, seemed awfully high, and I wondered if maybe the mandatory label inside the vehicle might call for something thirty-ish. But no, she confirmed: 44 is where it’s supposed to be. Not the first time that she was right and I was wrong.

And of course, this wasn’t any of my business in the first place, but I do get antsy about Ford tire pressures:

On March 6, 2000 the NHTSA began a preliminary inquiry and on May 2, the NHTSA began an investigation (PE00-020) concerning the high incidence of tire failures and accidents of Ford Explorers and other light trucks and SUV’s fitted with Firestone Radial ATX, ATX II, and Wilderness tires. On August 9 Firestone recalled all ATX and ATX II tires and all Wilderness AT tires manufactured in Decatur, IL. On August 31, 2000 the Office of Defect Investigation (ODI) upgraded the investigation to an Engineering Analysis (EA00-023) to determine whether Firestone’s recall covered all the defective tires.

Ford and Firestone both issued root cause analyses to the NHTSA. Firestone argued that vehicle weight, tire design, low recommended inflation pressure, and lower tire adhesion for tires manufactured at the Decatur, IL factory contributed to the tire failures. Ford argued that the tire design led to higher operating temperatures compared to similar tires manufactured by Goodyear and that differences in manufacturing at Decatur led to weaker tires that were more prone to failure. Ford also argued that the size of the wedge, a strip of rubber between the first and second belts, is smaller in Firestone tires than Michelin tires making them weaker than comparable Michelin tires.

Publicly Firestone argued that Ford’s recommended 26 psi inflation pressure was too low and should have been 30 psi. In addition Firestone argued that the Explorer was abnormally dangerous and prone to rollovers in the event of a tire failure, leading to more injuries and fatalities. In the words of Firestone CEO John Lampe, “When a driver of a vehicle has something happen such as a tread separation, they should be able to pull over not rollover.”

One could go back further, to the first-generation Chevrolet Corvair. A rear-engine car with an obvious rear weight bias, the Corvair was fitted with swing axles out back, creating a tendency to oversteer, to which General Motors provided the cheapest remedy possible:

As with the Renault Dauphine and pre-1968 Volkswagen Beetle, Corvair engineers relied on a cost-free tire pressure differential to eliminate oversteer characteristics — low front and high rear tire pressure‚Äďa strategy which induced understeer (increasing front slip angles faster than the rear). Nonetheless, the strategy offered a significant disadvantage: owners and mechanics could inadvertently but easily re-introduce oversteer characteristics by over-inflating the front tires (e.g., to typical pressures for other cars with other, more prevalent suspension systems). The recommended low front tire pressure also compromised the tire load capacity.

The General’s recommended pressures: 15 psi front, 24 psi rear. It was in the manual, but your average pump jockey, while he was filling your tank up with Good Gulf, wouldn’t see that as he pumped up those low-looking front tires. The second-generation Corvair had a much better rear suspension, possibly even better than Corvette would get for 1965, but by then, the damage was done, and by model year 1969, so was the Corvair.

Still, none of these were really pertinent to FJ’s TPMS issue, and I guess I’m sorry I brought it up in the first place.

1 comment »

  1. fillyjonk »

    8 December 2018 · 4:31 pm

    Naw, no, I don’t mind. It helps to know I’ve got people looking out for me. I have to look out for myself* so much of the time it gets tiring.

    (*And I also seem somehow to get saddled with looking out for students and sometimes even colleagues, who are grown-@$$ adults and should look out for themselves)

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