Saturday, November 28, 2015

Should 2 new tires go on front or rear?


QUESTION: I have a 2011 Mustang GT 5.0-liter with 36,000 miles. It has the Brembo brake package and Goodyear F1 tires on it. I'm ready to replace the worn rear tires again with the new OE Goodyears. Last time I replaced the rear tires, I placed the new tires on the front and moved the older front tires to the rear. Now people are telling me that I should place the new tires on the rear drive-axle to reduce the chance of oversteer that might be induced by having too much fun in a curve. With an everyday car, I believe that understeer would be the greatest concern, but with 412 horsepower and almost 400 pound-feet of torque, maybe the new tires should be on the back. Which end of the car should the two new tires be placed on my Mustang, and is a nitrogen fill worthwhile?

ANSWER: On the back. Like you, my first thought was that new tires should be mounted on the front and the older, worn front tires moved to the rear. Because the front tires do a far higher percentage of braking and, of course, steer the vehicle, my instincts said put the best tires on the front.

But since loss of traction from the front tires - the "understeer" you mention - is easier to correct than "oversteer" - loss of traction from the rear tires - the recommendation for replacing just two tires is to mount the new ones on the rear.

In short, correcting understeer involves "breathing" back the throttle or modulating brake pressure to help the front tires regain traction. Oversteer, on the other hand, requires an instantaneous steering correction in the direction the back end is trying to go in order to keep the front tires pointed where the vehicle is headed, while at the same time neutralizing the throttle - not accelerating and not decelerating - to stop any wheel spin. If and when the rear end regains traction and wants to snap back - so-called "overcorrecting," a complete misnomer - the steering must be straightened in time with the rear end snapping back to keep the front wheels pointed where the vehicle is traveling.

In the racing schools I teach, we label the correction for oversteer as a three-step process: correct/pause/recover, CPR for short, an easy acronym to remember.

Nitrogen contains no moisture and is less prone to pressure changes with temperature changes, so filling tires with nitrogen makes some sense, but only if you continue to use nitrogen to top up tire pressures.


Q: I own a 2001 Dodge Dakota and would like to clean the engine. Is it OK to take it to a self-service car wash and power wash it?

A: It must be. Ever seen a used car on a dealer lot with a dirty engine? It's OK to clean the engine and engine compartment as long as you cover any sensitive wiring and electronics and don't aim the high-pressure spray directly at these components. Remember, engines and drivetrains get wet when vehicles are driven in the rain. The wiring harnesses and connectors used on modern automobiles are designed to be relatively weatherproof.


MOTORING NOTE: Regarding white smoke from the 2003 Honda, reader Paul Harvey offered a good suggestion. "I had a similar problem involving my 2002 Toyota Sienna. One mechanic told me it was caused by a sludge problem and would require an engine replacement. I had thought it was a head gasket problem and took the van to another mechanic, who diagnosed a bad PCV valve. After he replaced it, the problem was solved."

A stuck or clogged PCV valve typically forces oil into the combustion chamber, where it is burned, creating a bluish smoke. So checking the PCV system for proper function is a simple step to possibly explain the sudden appearance of smoke from the tailpipe. Thanks for the suggestion, Paul.

Star Tribune (Minneapolis)
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