From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Rolling Resistance
Date: 1 Apr 2001 02:26:28 GMT

Phil Holman writes:

>> That tire casings have a complex shape change is demonstrated by
>> the business card test between tire and tube.  A paper business
>> card will be shredded to square confetti on the diagonal by the
>> cord angles.

> Is the orientation of the threads a refined consideration in tire
> design.  Hoop stress is double longitudinal stress in any
> unsupported pressurized cylinder. A tubular tire will pick up
> additional longitudinal stress by being stretched around the rim.
> The orientation looks like 45 degrees on a Conti Sprinter.

If the cloth were not on a bias, it would have to be manufactured in
the final toroidal shape that bias ply accommodates easily.  You'll
notice that folding tires are flat belts in the package, yet they can
be inflated to be toroidal rings.  Beyond that, the bias ply is what
keeps tubular tires pressed on the rim glue.  Without constriction,
caused by a 45 degree bias, the tubular tire would not stay in place.

Whether rotating a cloth with fibers crossing at right angles
increases stress is probably not so in this case.  I think the
ancients have been through this and arrived on a useful casing.  After
all, cars use it too although the radial has taken over in many
applications.  The radial was not obvious and not easy to make.
Michelin designed it for rubber tired railways and only after, found
it had benefits for cars as well.

The squirming of a bias ply tire on a car is large and causes tread
scrub that wears them out rapidly.  That is mainly a result of the
donut shape with the ratio of major to minor diameters being less than
4:1, something that is not the case with a bicycle where a radial
would not change the contact deformation significantly, the tire being
basically an infinite radius hose (straight) being squashed at the
contact point.

If you want to see the difference, look at a loaded bias ply truck or
car tire and you'll notice that he narrowest point is at the load
point and bulges appear on either side, there where the cords that
pass though the load point bulge out.  In contrast a radial has a
belly at the load point.  That's how you can detect a radial tire, by
its conspicuous belly.  When radials were first introduces, many folks
though their tires were under inflated because they could see the
belly.  On a bias ply bicycle tire, the bulge is at the load point
because the supporting cords do not extend significantly beyond the
load point.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Rolling Resistance Measurement
Date: 18 Jun 2001 22:09:30 GMT

Terry Morse writes:

> Hmm, could that be caused by pneumatic trail, where the shape of the
> tire distorts at speed? That would make sense. I wonder. Maybe
> someone who knows more about pneumatic trail on car tires can
> comment.

That's "dynamic trail" and is caused by RR which shifts the center of
pressure of a tire forward with increasing RR, the restitution
pressure at the rear of the contact patch not being the same as the
compression pressure, against the road.  The effect first became
apparent when radial ply tires first became common on cars instead of
bias ply tires that had higher RR.  Cars having been designed around
bias ply tires, they had odd handling characteristics as though there
was too much trail when changing to radial tires.

The RR effect was readily apparent when I pushed my car in the garage
on a slick concrete floor.  With bias ply there was much squealing and
tread squirm that went away when I switched to radials on which the
car could be pushed with one hand effortlessly.

Jobst Brandt    <jbrandt@hpl.hp.com>