<VV> Cantilever tires

[Padgett]

>Ah you never ran Formula Ford or V. A  5" rim in  Formula Ford with 7" of
>tread. the sidewall was an inch or more out from the rims.

I described this earlier in a PM and slightly different form since did not 
know it was of general interest. Apparently it is.
...
Sure, before the into of cantilever tires (around 1965 - for those of you 
with good libraries, see the Car and Driver "2+2 vs 2+2" road test) treads 
and sections were narrow to prevent reduction of the contact patch in a 
corner. The physics are fairly simple is you think about a tire and a 
narrow rim. As you increase the lateral force, the tire moves in 
response   from / \ to something more like / | and the side away from the 
corner lifts while the sidewall on the side into the corner becomes almost 
a straight line and the tire tries to roll the sidewall under, decreasing 
the effective contact patch and the traction. OTOH if you begin with a 
wider rim / \  tread movement straightens the outer edge, re-enforcing the 
contact patch. You can also design for minimal contact in a straight line 
and increasing traction in a curve.

This is why in aulden times race tires were narrow and inflated to high 
pressures. (Was also because generations of teachers taught that friction 
was a constant and the surface area did not matter. They also taught that 
more than 1 gee acceleration through tires was impossible. Got very upset 
when I pointed out that it meant you could never turn a 1/4 in under 9 
seconds).

In the early 60's, drag racers found that as the contact patch increases 
(within limits - see "unladen understeer") so did traction but it took '80s 
fractal math to show why however they had no limits on wheels and no 
lateral forces to contend with..

Tire companies grabbed the new technology and developed tires that were 
designed so that side forces did not lift the outside edge when cornering 
despite a relatively narrow rim. This was particularly important because 
both the SCCA and NASCAR had strict limits on rims but not on tires. These 
tires were designed so that tire distortion in corners did not reduce the 
contact patch despite rims considerably narrower than the tire section. 
This geometry was called a cantilever tire since the acting surface (edge 
of the tread) was some distance beyond the support (rim).

A "low profile" was necessary to gain this effect and led to the first 70 
(1967) and 60 (1970) (years from memory so no promises) series production 
tires that were the distant ancestors of tires today.

Even so, for a good handling street car you want a rim as wide as the tread 
and for a race car, the rim should be the same or more than the section 
width of the tire.

Part of the reason early Corvairs had a tendancy to spin was because the 
geometry of the swing axle just made the decreased contact patch problem 
worse - the harder you conered, the less traction you had until breaking 
loose was inevitable.

Once you know it is there, you can feel it happening.

Padgett