Safety In The Wet : Danger Of Hydroplaning New Tyres In Front Or At The Back?
Most vehicles are equipped with the same size tire at every wheel position.
Ideally all of these tires should also be of the same type and design, have
the same tread depth and be inflated to the pressures specified by the
vehicle placard or owner's manual. This combination best retains the
handling balance engineered into the vehicle by its manufacturer.
However due to the front tires' responsibility for transmitting
acceleration, steering and most of the braking forces on front-wheel-drive
vehicles, it's normal for front tires to wear faster than rear tires. If
the tires aren't rotated on a regular basis, it's also common for pairs of
tires to wear out rather than sets. And if the tires aren't rotated at all,
it's likely that the rear tires will still have about 1/2 of their original
tread depth when the front tires are completely worn out.
Intuition suggests that since the front tires wore out first and because
there is still about half-tread remaining on the rear tires, the new tires
should be installed on the front axle. This will provide more traction, and
by the time the front tires have worn out for the second time, the rear
tires will be worn out too. However in this case, intuition isn't
right...and following it can be downright dangerous.
When tires are replaced in pairs in situations like these, the new tires
should always be installed on the rear axle and the worn tires moved to the
front. The reason is because new tires on the rear axle help the driver
more easily maintain control on wet roads because new, deeper treaded tires
are more capable of resisting hydroplaning.
Hydroplaning occurs when the tire cannot process enough water through its
tread design to maintain effective contact with the road. In moderate to
heavy rain, water can pool up in road ruts, depressions and pockets
adjacent to pavement expansion joints. At higher speeds, the standing water
often found in these pools challenge a tire's ability to resist hydroplaning.
Exactly when hydroplaning occurs is the result of a combination of elements
including water depth, vehicle weight and speed, as well as tire size, air
pressure, tread design and tread depth. A lightweight vehicle with wide,
worn, underinflated tires will hydroplane at lower speeds in a heavy
downpour than a heavyweight vehicle equipped with new, narrow, properly
inflated tires in drizzling rain.
If the rear tires have more tread depth than the front tires, the front
tires will begin to hydroplane and lose traction on wet roads before the
rears. This will cause the vehicle to begin to understeer (the vehicle
wants to continue driving straight ahead). Understeer is relatively easy to
control because releasing the gas pedal will slow the vehicle and help the
driver maintain control.
However, if the front tires have more tread depth than the rear tires, the
rear tires will begin to hydroplane and lose traction on wet roads before
the fronts. This will cause the vehicle to begin to oversteer in which the
vehicle wants to spin. Oversteer is far more difficult to control, and in
addition to the initial distress felt when the rear of the car starts
sliding, quickly releasing the gas pedal in an attempt to slow down may
actually make it more difficult for the driver to regain control, possibly
causing a complete spinout.
Members of The Tire Rack team had the chance to experience this phenomenon
at Michelin's Laurens Proving Grounds. Participants were allowed to drive
around a large radius, wet curve in vehicles fitted with tires of different
tread depths - one vehicle with new tires on the rear and half-worn tires
on the front, and the other with the new tires in the front and half-worn
tires on the rear.
It didn't take long for this hands-on experience to confirm that the"proving grounds" name for the facility was correct. The ability to sense
and control predictable understeer with the new tires on the rear, and the
helplessness in trying to control the surprising oversteer with the new
tires on the front was emphatically proven.
And even though our drivers had the advantage of knowing we were going to
be challenged to maintain car control, spinouts became common during our
laps in the car with the new tires on the front. Michelin advises us that
almost everyone spins out at least once!
Experiencing this phenomenon in the safe, controlled conditions of
Michelin's Laurens Proving Grounds rather than in traffic on an Interstate
ramp in a rainstorm is definitely preferred!
In case there is any doubt, when tires are replaced in pairs, the new tires
should always be installed on the rear axle and the worn tires moved to the
front. |
