<VV> Tire Pressures

[Brent Covey]

Hi Van,

> why doesn't
> the Fiero, for example, have to run a big tire pressure differential? It's
> almost idential weight ditribution-wise, and yet the door sticker calls
for
> 30 psi both front and rear. What's the skinny here??

Weight distribution is just part of the picture-

For a very good explanation of this topic, Jim Hall wrote a couple articles
(Why Rear Engine Cars Handle Best) in Car and Driver magazine during spring
1965 you might be able to unearth. Jim Hall was the smart guy out of
Midland, Texas who built the Chapparal race cars.

All tires have a slip angle when they're turning corners, theres a
difference between the actual path they take and the exact angle they're
being steered, the wider path taken by the tire is determined by the
additional 'slip' and the difference between the two is slip angle. Slip
angles are carefully controlled on cars because its important they're very
similar on both ends of the car, a slightly larger slip angle on the back
will turn into ferocious oversteer at speed and a small amount of extra slip
on the front deteriorates into extreme plowing understeer at high speed. The
ideal is to minimize slip generally, and maintain a slightly greater amount
of front slip under most circumstances so that the driver has the option of
modulating slip angles himself with the throttle. Because at high speeds the
car is travelling hundreds of feet per second small changes make gigantic
differences to what side of the road you end up on and which way you're
facing that wouldnt be an issue at modest speeds.

Tire slip angles are determined by the load carrying abilities of the tire
compared with the load they are seeing at a given moment and job they're
being asked to do. Bigger tires with more air pressure reduce slip angles
where they're installed- smaller tires or lower air pressure increase slip
angles. The more weight or cornering, acceleration and braking loads you're
expecting a tire to carry the greater its slip angles. In a car with a lot
of weight at one end you have some options for reducing the slip angle of
the tires on the 'heavy' end;

-larger tires
-higher tire pressure
-reducing roll stiffness (resistance to body lean in corners)

Roll stiffness is the resistance of the car body to lean in corners- you can
design a suspension to not resist the effect of the body leaning over in
corners, or to strongly resist this. Adding roll stiffness (resistance to
leaning) increases slip angles on the end of the car thats resisting the
roll. Because roll is proportional to cornering forces this allows the
suspension engineers to tune the chassis to make the most of the cornering
power at both ends of the car by progressively shifting loads in corners to
the end of the car with the smallest slip angles; the roll is carried on the
end that has extra traction. The effect of resisting the body lean by a
stiff
spring or sway bar is the tire on the outside of the turn carries an
ever-increasing share of the load, which increases its slip angle, and
promotes neutral handling.

GM got thier fingers burned bad enough on the Corvair viz tire pressure
differentials that I would imagine they
were loathe to suggest staggered tire pressures on the Fiero. So, they found
other ways to equalize the slip angles on the Fiero, namely putting most of
the roll stiffness in the front suspension to force it to carry roll loads
and generate higher slip angles when the car was pushed in corners to make
sure the tail didnt get away. The early Corvair has tremendous rear roll
stiffness and swing axles dont let you reduce it much, even with softer
springs and considerably increased front roll stiffness, so tire pressures
were pretty nearly the only tool that would really make a difference on
earlies- the lates have very weak rear roll stiffness and this is mostly why
they are inherently better handling cars. Most efforts to tame the Corvair
are focussed on increasing front roll stiffness to transfer cornering loads
onto the front wheels to minimize the possibility the rear will generate
larger slip angles than the front.

GM makes a few other vehicles with very similar weight distribution to the
Corvair that are generally regarded as safe and not inherent oversteering
machines- the small block Corvette for many years was pretty close to the
Corvair weight distribution, most full sized stationwagons also are, and any
moderately loaded pickup truck is. The 1971-1976 GM B body stationwagons
have virtually identical suspension geometry to the late Corvair despite a
very different looking suspension (leaf springs in the back). Its designed
to minimize roll resistance and transfer loads to the front wheels in
cornering, and has about the same roll axis as the 1965 Corvair.

I would encourage you to track down Jim Hall's article if you can find it, I
imagine someone on VV might be able to scan you a copy, for a fuller
understanding of the subject,

Hope that was some help,
Brent Covey
Vancouver BC