Re: In a four-wheel-drive car which tire will get thinner?

Greetings Mayaard:
References:
Drawings and discussion of Castor, Camber and Toe on the following web sites:

1. http://www.whiteline.com.au/faqsusp01.htm#Camber%20-% 20What%20is%20it

2. http://hglmotors.co.uk/main/4w. shtml

Common tire ware problems:

3. http://www.mgf.ultimatemg.com/tyrewear.htm

Your question is related to wheel alignment. Wheel alignment is determined by
road design, the vehicle load, the average speed of the vehicle, the air
pressure in the tires and the type of suspension system used on the vehicle.
Without proper alignment significant wear will often occur on the inside or
outside edge of the front steering tires. The front wheels of a vehicle, which
are commonly used for steering, have three degrees of freedom in design
and alignment (adjustment). These degrees of freedom are called Castor,
Camber, and Toe. You can find drawings of these three degrees of wheel
freedom in References 1 and 2.

Castor is the backward or forward tilt of the axis of the wheels used
for steering. Most vehicles have forward tilting Castor such as that used for
the tilt of the fork on the front wheel of a bicycle. This Castor places the
axis of the front wheel ahead of the bicycle body. Castor is not a factor
in tire wear and on most vehicles Castor is incorporated into the design
and is not adjustable.

Camber is the tilt of the top of the wheel relative to the bottom of
the wheel. Because most roads are designed with a crown in the middle and
slope to the sides so that rain water runs off, most cars have the Camber
tilted outward to keep the tires flat on the road surface. Two lane country
roads may have a high degree of curvature in each lane and would require
greater Camber while multi-lane highways have less curvature in each lane and
thus require less Camber. The degree of Camber is related to the type of
roadway that is most used by the vehicle. Too much Camber will cause the
outer edge of the tire to wear more quickly while too little Camber will cause
the inner edge of the tire to wear more quickly.

Toe is a measure of the tilt of the front of the wheel relative to the
rear of the wheel. Historically most vehicles have had rear wheel drive and
front wheel steering which usually requires that the front of the tires be
tilted inward. This is called Toe-in. Toe-in is necessary to have a
vehicle with stable tracking so that it continues to travel in a straight
line with no force applied to the steering wheel (e.g. if the driver's hands
are off of the steering wheel). However, many vehicles with front wheel drive
require Toe-out for stable tracking. Toe causes the tires to drag
slightly sideways producing more tire wear than tires with zero Toe. More
recently, especially on vehicles with four wheel drive, the rear wheels are
also set with a small amount of Toe. This can be either Toe-in or Toe- out
depending upon the manufacture's specification. In order for the vehicle's
thrust line to be the same as the vehicle's center line, the Toe of the front
wheels and the rear wheels should be equal.

In answer to your question about a mid-engine vehicle with four wheel drive:
if you use Toe on the front wheels used for steering then they would wear more
rapidly than rear wheels with no Toe. If you have equal Toe on the front and
the rear tires the wear should be about equal. However, the front wheels also
wear when steering the wheels without the vehicle moving, as during parallel
parking. In all configurations of alignment, motionless steering causes the
front wheels to wear a bit more quickly than the rear wheels. You can
observe this as rubber marks on a garage floor caused by turning (steering)
the front wheels while motionless.

Best regards, Your Mad Scientist
Adrian Popa