At 07:57 PM 3/5/2003, Kevin Stevens wrote:
>Full-time locks the center diff, and you're fighting the turn difference
>between front and rear.
Correct! To put it in very basic terms, when a four-wheeled vehicle goes
around a corner, _each_ wheel describes a different arc and turns at a
different speed.
This is why four-wheeled vehicles usually have an open differential in the
drive axle so that the two wheels on that axle can turn at different speeds
as the vehicle corners. I say "usually" because if the goal is to maximize
straight-line thrust without regard to tire or mechanical wear, the two
drive wheels can be mechanically locked. In a four-wheel drive vehicle,
thrust can be maximized by locking the drive wheels together on each axle
_and_ locking the two axles together.
On the street, open differentials are used on each drive axle to reduce
tire and mechanical wear. Some vehicles have various types of
"limited-slip" or "locking" differentials designed to lock the drive wheels
together during acceleration in order to keep the lightly-loaded wheel
(usually the inside wheel if the vehicle is turning) from spinning.
In the early days of four-wheel drive vehicles, the connection between the
front and rear axles was solid. Driving one of these vehicles on dry
pavement with good traction would cause increased tire and mechanical wear
and possible breakage of parts. Later four-wheel drive vehicles often use
some type of differential between the front and rear axles thus making the
vehicles driveable on dry pavement with four-wheel drive engaged.
The big lie about four-wheel drive is that often times, neither drive axle
has any limited-slip or locking feature. Even though both axles may be
locked together by the design of the transfer case, if both axles have open
differentials, you may find yourself in situations where one rear wheel and
one front wheel (usually on the opposite corner) spin and the other wheels
do nothing. Go trail riding with your buddies in Jeeps and other light,
short wheelbase four-wheel drive vehicles and you will see this a lot. I
remember one time on a trail ride when a Suzuki Samurai got hung up on a
rough part of the trail. Opposite tires were on solid ground while the
other opposite tires were either in the air or on loose dirt. The Samurai
just sat there with opposite wheels spinning. A couple of guys jumped on
the bumper of one of the corners that was spinning and the Samurai dug
itself out.
Sorry for the long-winded explanation on a topic really not suited for this
forum. Hopefully, the folks not familiar with four-wheel drive got
something out of it.
Bob...
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