Tyring a 164

[C M Smith <cmsmith@domain.elided>]

Lower profile tires have a different shape of contact patch, but how can 
the effective area be much different? The size of the contact patch surely 
has more to do with the air pressure. X amount of vehicle weight requires Y 
amount of air pressure in the four tires to keep the car of the ground. The 
pressure in the tires exerts a force proportional to the area of the tire 
touching the ground. Where am I going wrong here?

I was surprised to read in European Car this month that the air pressure 
does not hold up the car, rather the tension in the sidewalls induced by 
the air pressure does. Duh.  And the hammer doesn't drive in the nail, my 
arm does.

Now I recognize that sidewall design affects the contact patch size, and at 
the limit, a tire can be designed which will run "flat" in that the 
sidewalls will hold up the car with no air pressure and still handle 
safely. (Why do they bother to pump them up then?). But for a given tire 
design a lower profile tire will be run at the same tire pressure as the 
taller tire on the same weight of vehicle and therefore, the contact patch 
area should be unchanged. I am assuming that the lower profile tire is 
sufficiently wider to be the correct replacement for the taller tire.

The contact patch on the lower profile tire will be shorter and wider. This 
will tend to make the wider tire easier to turn, not harder, discounting 
effort required to turn a stationary wheel (does anyone actually do that to 
an ALFA?). Steering effort is related to trail not tire width, i e the 
caster of the front wheels will produce a shorter lever arm (moment) with 
the lower profile tires, and therefore less effort required to steer. The 
lower profile will suffer more from nibble for the equivalent moment across 
the tread will be greater for the lower profile tire.

Even alloy wheel is heavier than rubber tire. Bigger wheels are heavier by 
more than the smaller tire volume can compensate for.

Lower profile tires may be less grippy in snow, but assuming the rubber 
compound and tread design are equivalent (often NOT the case) a lower 
profile tire should not be appreciably less grippy in the wet. Although 
water evacuation from under the tread area is affected by the width of the 
tire, it is also affected by the length of the contact patch. The snow 
problem comes from the characteristics of the snow. In dry fluffy snow the 
wider tire should not be at a disadvantage. In wetter heavier snow there is 
a significant difference because the lower profile tire cuts down to the 
firmer colder snow below the surface layer, sometimes cutting right down to 
the road surface. This same effect is the reason mud plugging 4x4s benefit 
from taller narrower tires.

Finally, I have noticed a dynamic effect from taller tires on slippery 
surfaces, not related to ultimate traction but to peak loadings during 
cornering and braking. Taller tires have more "give" in the carcass and 
therefore are easier to drive smoothly. Losing traction on slippery 
surfaces is about peak loadings far more than average total grip. Indeed, 
the deeper squishier rubber of a winter tread exploits this idea, and snow 
tires get more nervous feeling on snow and ice as they wear, just as they 
start to feel more solid and grippy on bare pavement.

Just as the Michael Schumachers of this world demonstrate their power over 
us mere mortals most evidently in heavy rain, (or when driving on three 
wheels!), the faster driver is the smoother driver because he can extract 
the closest possible to the maximum available grip, by not inducing peaks 
in the friction circle. Lower profile tires make that just a bit harder to do.




Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner

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End of alfa-digest V7 #1097
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