1999-2009 [Subscribe to Daily Digest] |
Like any good engineer will tell you, it depends...
I am not sure at what point the Explorer goes from under to oversteer. If it requires a complete blowout, tread seperation, or _severe_ tire pressure drop than it is normal. Regardless of design, if one of the rear tires gets compromised you are always going to have less understeer. It is the sensativity that matters.
I have a minivan (probably about 70/30 wt distribution with seats out and tank empty) Coming around a corner quite briskly with seats out and tank empty it went oversteer due to a tire blowout on a highway exit ramp. Luckily I caught it in time and there was no damage. (except of course the tire)
So if a rear tire is severly compromised and a vehicle will go oversteer. I think this is fine. Let all the air out of the rear tires of a sedan and you will quickly see how oversteer a vehicle can get. You simply cannot generate the lateral force needed to keep the rear end in place if the rear tire is flat or has severe tread issues.
Problematic vehicles are ones that heavily rely on consumers watching tire pressures. I am not too familiar with the Explorer's numbers but if it is very sensative to small changes in tire pressure it is a poor design. (I don't think it is overly sensative, but I do not know) If it goes oversteer when the tread flies off, I can not fault the vehicle for that.
One of the ways to tune over/under steer is to have tire pressure differentials between the front and rear tires. The Corvair of the sixties ("unsafe at any speed") actually displayed understeer when the tires were inflated to the manufacturer's specifications. I do not know the numbers offhand but the front tires were barely inflated and the rears were inflated near their limits. Most dealers that read the manual to set tire pressures assumed the ~20 psi differential from front to rear was a misprint and inflated the tires all to the same pressure. (this was bad)
The new "smart" car plays this trick as well. If you open the gas filler and look at the tire pressures there is a heavy bias rear/front pressure differential. Infact the rear tires are of different size and style then the much less grippy fronts. All to keep the car understeer.
Personally I think that is plain dangerous. Most car buyers, especially US ones, do not check tire pressures often enough if at all.
-Joe
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