Thread: Bias Ply T & F!
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Old 10-28-2016, 11:09 AM   #5
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Cool Re: Bias Ply T & F!

Originally Posted by CapriRacer View Post
There is quite a bit of variation between tires and what you described sounds like you are dealing with some of the extremes. And while it is true that radials are generally more sensitive to toe-in, this property can be changed with some simple changes in tire construction, so it is highly dependent on what 2 tires are being compared.

Oops! I always get that backwards. Yes, it is positive caster that adds stability. I, too, have wondered why some vehicles spec'd negative caster - but I also noticed that they changed to positive caster before they changed to radials. If there was such a fundamental difference between bias and radials (and there are!), why wasn't this a change made EXACTLY when the changeover took place? I suspect it has more to do with how sloppy the suspensions were (maybe I should use the term imprecise here!) I know it was somewhere in the 1960's when Calspan started measuring the forces on tires and maybe that influenced the car makers and their suspension settings - instead of going by the feel of a test driver.

On Toe-in: It's not that the radials were more
sensitive to toe-in, it's that they probably tracked
straighter and were just as stable with less. The
amount of toe-in required for stability with bias
tires would probably cause premature wear on the

On Caster: I suspect the way the bias-ply tires
'gave' under dynamic stresses and weight was
different than how radials give, so negative caster
provided directional stability to them, as opposed
to simple trail caster(positive) used with radials.

I don't know the specifics on that, perhaps someone
with more engineering prowess could contribute.

All I'm trying to suggest here is that millions of
everyday people - not race car drivers - drove on bias
ply tires from before one war to well after another,
safely, without having to 'saw' the steering wheel back
and forth - just a movie and TV trope used to indicate
driving and being in motion while in the safety of a
sound stage, with a giant projection of a highway
behind it. And aligning properly for bias-ply tires, I.E.
negative instead of positive caster, and different toe
settings than we might spec today, were part of that.
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