There are a
number of ways a car manufacturer achieves a better fuel economy and one
of them is by using a fuel-efficient tyre. In a very simplified picture
all the tyre manufacturer has to do is to reduce the rolling resistance
or minimise the energy loss, which is most clearly reflected in the distance
a car will freewheel on a level road before coming to a stop from a particular
speed.
Unfortunately the solution is not that simple, as during each cycle of
a tyre travel, part of the tread is compressed as it passes through the
contact patch, which again relaxes as it lifts clear. If one were to plot
a graph of load versus compression, the relaxation line would not track
the compression line, but would run parallel to it, until it finally eases
sideways to the origin. This area within the loop formed, represents the
energy, which is turned into heat, which in turn is either radiated into
the atmosphere or passed to the air inside the tyre. This is the reason
why a tyre becomes hot (and their pressure rises) in any car after it
has run a long distance. Eventually, a time is reached when the tyre remains
at a certain temperature and all the energy absorbed by rolling is radiated
away.
 Unfortunately
in actual running of a vehicle, a tyre does not behave in the manner just
described as at least two of the wheels are transmitting a driving force,
or else all four of them are transmitting a braking force along with each
tyre generating a cornering force from time to time. Cornering forces
create a complex situation as each tyre carries a different load thus
accepts a different share of it. All these forces are exerted through
the tyre contact patch, resulting in a tyre being regarded as one of the
greatest mathematical challenges in engineering to provide performance
in every respect of grip, wear, noise, ride comfort and steering feel,
while still achieving a better rolling resistance and thus better fuel
economy.
Rolling resistance can also be controlled by varying the load or by altering
the inflation pressure. Increased loads increase rolling resistance, while
higher inflation pressures decrease the rolling resistance. This is one
of the reasons why some car manufacturers recommend higher inflation pressures
in their cars to achieve better fuel economy. For vehicles with heavy
loads it is suggested that they increase their inflation pressures to
achieve better rolling resistance but to drive at reduced speeds to avoid
heat built up in the tyres. The other problems encountered by increase
inflation are tyres getting more prone to impact damages or having a rough
ride.
 The
other alternative is to put an additional belt just under the tread as
in a bias belted construction or in a radial tyre, to ensure that it has
to flex only in one sense rather than two, as the tread remains flat all
the way round the circumference, even during the contact patch. This is
one of the major advantages a radial ply tyre has over its bias ply counterpart.
Secondly this unique flex action of the radial makes it more economical
to use as its construction has more volume of air at the same pressure
giving it a better flotation effect, better load carrying capacity as
well as a lower rolling resistance than the bias ply tyre.
The other problems encountered by tyre manufacturers in creating a low
rolling resistance tyre are the complexities of the various compounds
used in a tyre, which I would like to discuss as a subject on its own.
This brings us to the topic of this article as to how car manufacturers
along with the tyre manufacturers choose the right tyre for your car.
Though the cost of a tyre is of importance and will depend upon how a
car is economised on, again it is a subject I do not wish to comment on.
When a new model is to be launched by a car manufacturer, whether it is
a completely new car developed from the drawing board or is an improvement
over a model already in their current range, the first emphasis is on
the ride comfort and handling of the car. The second major decision is
the choice of the tyre size. There were days when even the type of tyre
(whether bias or radial ply) played an important part. Next comes the
dialogue between the car manufacturers and the tyre suppliers. Obviously
the tyre manufacturer would prefer a tyre size and design, which is in
their current range, but sometimes it may be necessary to design a tyre
exclusively for the car.
 Car
manufacturers generally choose a size for their basic model from the range
of size easily available by different tyre manufacturers, both as original
equipment as well as in the replacement market. They generally believe
that whenever a customer has bought a new car, it feels great, handles
nicely and after the 30-35,000km mark when the need to replace tyres rises,
it should behave identically or if not better after fitting the replacement
tyres from the market. Keeping in mind that some users might want to customise
their cars to a more sporty looks by fitting alloys, wider or bigger size
tyres as an option, car manufacturers generally look for the largest outside
diameter tyre one can fit, then vary the rim diameter and tyre profile
to subtly change the characteristics of the car to suit comfort and handling
while preserving the gear ratios and keeping the tyre package within the
wheel arch, by using a 13- or 15-inch wheel and by juggling about with
the aspect ratio maintain a near constant tyre outside diameter (OD).
In India, most car manufacturers choose a tyre with a large air volume
with high load carrying capacities, whereby they can achieve the maximum
isolation from potholes and rough road conditions, also by lowering the
air pressure one can more or less tune the tyres to a more comfortable
ride. Thus one will notice most of our cars be it the Maruti, Lancer,
Honda, or Ford, all come in the 13" rims with an option of upgrading
to a 14" one. Even the aspect ratios chosen vary from the conservative
80 per cent to the comfortable 70 per cent ones with the option of lowering
to 65 or 60 per cent.
Therefore the suspensions are also tuned to such options as well as the
track and thrust angles are set for the same. Most cars are front drive
- torque steer equipped with a possibility of excessive under steer. Here
again a larger wheel with wider tyre contact patch may help in somewhat
a better grip but will increase torque steer, which relatively may again
effect the fuel economy. Apart from that fitting a very aggressive sporting
tyre to the basic car, without having changed other aspects of the suspension
geometry may get you a lot of grip but will also may the car lurch about
causing the car to 'fishtail', which at high speeds is not a very health
feeling.
My case study is about one such problem. |
February 2002