New-York tribune., 1919-10-12
Slow Speed ON
Hills Adds to
Wear ON Tires
c. E. Manierre Shows How
Slippage Multiplies Strain
ON Castings When Power Is
Used but Progress Is Slow
To the Automobile Editor OF The Tri-

To the Automobile Editor Of The Tri-
bune.
Sir: 1t will be generally conceded
that at racing speeds the wear Of the
rear tires is very rapid, and also that
those who maintain fairly high speeds
on the highway are using up their
tires in the process. A thing by no
means so obvious is that the driver
may rapidly wear the tread from the
tires without at any time exceeding
q speed of thirty miles an hour.
The extra wear upon the tires by
reason of steep grades, just capable Of
being assembled in high gear, is neces
sarily considerable even though the car
proceeds at as low q speed as fifteen
miles an hour. In fact, even at fifteen
miles an hour the slippage of the tire,
which is necessarily involved in driv
great as it would be driving the car
at thirty miles an hour on the level
and is equal to that which would occur
in driving forty-fve miles an hour on
the level
This necessary slippage is doubled
when the speed Of the car is increased
from forty-fve to sixty miles an hour
on the level, and the engine energy
required to drive thirty miles an hour
/,
that required for sixty miles on the
level
This somewhat roundabout reasoning
shows the slppage and consequent
wear Of the tire to be eight times
greater on such grades If the touring
speed Of thirty miles an hour Is main
tanned up the incline than 1t Is on the
level at the same speed.
1t Is a general practice to increase
the speed before reaching hill in
order to accumulate momentum for the
ascent. Usually the idea Of this pro-
cedure is that with q short hill this
momentum may avoid going into second
gear. But it Is a good practice, even If
the hill Is long. for the accumulated
momentum is paid for with less tire
wear.
1t is a less common practice, but
sensible and economical, 10 allow the
car to slow down on the grade as the
momentum is expended and to stop
on the grade with speed just sufficient
to prevent the engine from laboring.
The driver should have in mind that
whenever he has to exert the full power
of the engine or anything approximate
ing to it, the wear on the tires pro
ceeds in q rapidly Increasing ratio.
and for economy in that respect 70
drive the car so that at all times the
throttle shall be only partly open con-
tributes materially to the length Of life
of the tires.
From the point OF tire economy. it
is quite correct to keep up the average
touring speed on down grades, al
though it should not be forgotten that
moving car on an upgrade can stop
more quickly than one which is de
scending the grade, and therefore. cir
cumstances may make it prudent to
descend more slowly than the fore
going information would indicate as
proper speed for driving.
The extremes OF grade may be CON
sidered as those below per cent
down which car would not coast
cwng to the fact that it's rolling re-
slstance is greater than the effective
pull of gravity, and the 80 per cent
grade, down which, if the car was in
motion, it could not be stopped at all
by reason of the fact that gravity
be met by brakes, or rather by the
braking action of the wheels against
the road.
For each car there is critical
Grade somewhere in the neighborhooc
Of 2h per cent which has been the
cause of many accidents.> Its exact
amount depends upon the rolling re-
sistance OF tho individual car. Its
popularity is that if the car is brought
20 stop on the grade by the use OF
he foot brake. it may remain standing
after the brake is released. but the
slightest accident, as of an urchir
jumping on the running board, or ever
gust Of wind, may be sufficient Il
start it down the grade. On sleeper
grade it would not stand at all. On
ower grade it would not coast.
/. is therefore prudent where there
is any possibility of such accident <
both set the emergency brake and turr
the wheels toward some obstruction, t,
meet the possibility of some meddle
some person releasing the brake.
CHAPTrS T .IANIEPPE