From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Spare Tire (sewup) Valve Stem Corrosion
Date: 26 Mar 1999 22:09:30 GMT

Sheldon Brown writes:

>> I think it has been shown that the tire cannot come free of the rim
>> because it is constrained by the brake to lie between the fork crown
>> and rim even if the tire is not glued on and will jam the wheel.

> This may be true of newfangled bikes with the idiotic tight clearances
> currently in fashion, but isn't true of reasonably designed bikes.

Brake pads on bicycles have always been adjusted to not much more
than 1/8 inch (3mm) and that is too small for a tire to squeeze
through.  Nothing new fangled about that.  The tubular cannot get off
the rim because the brake caliper makes a closed envelope around it.
The other side of the tire is outside the axle and the whole thing
makes the scenario of the tire falling free of the wheel a myth.

I suppose one visualization of the event would be that the tire falls
off the rim for lack of any glue while riding straight ahead and that
it falls to the left side of the rear wheel remaining passively
hanging from the axle as it drags loosely on the ground.  This is pure
wishful thinking.  The tire must receive a side force to be dislodged
and that side force does not vanish the moment the tire comes off,
Hence the bare rim will slide on the road and cause a crash.

Tires can roll while riding straight ahead if, for instance the rider
lifts the bicycle while sprinting, causing a side hop.  This has
occurred and the inflated tire either got pushed back on when the
dislodged section reaches the brake or it jams and blows out the tire.
I have witnessed both of these results for front wheels.  Neither had
anything to do with valve caps and couldn't have.

>> If there is no tire on the rim, this makes no difference, the rider
>> will fall anyway.  This has been tested.  After a rider rode home on a
>> flat tubular for lack of a spare, we tried riding the bike on the
>> street and found it about as difficult as riding with a tire on ice.
>> On a hardwood floor this was even worse.

> Front wheel or rear?  I'd readily believe that a front rim would be
> unrideable, less ready to believe that of a rear.

This was a front wheel.  The point is that lateral traction is so poor
that it can't be ridden reasonably and I believe this is good proof
that when rolling a tire in a curve, the bicycle will fall.  It is not
intended to prove that one cannot ride a bicycle on the rim under
ideal conditions.

> I must grant that the scenario of a tubular rolling and the valve
> popping out, allowing the rider to ride to a quasi-controlled stop
> is an unlikely one, but I remain unconvinced that it is impossible.

I also believe it is not impossible but find the probability so
remotely small that the concept cannot reasonable be supported as a
reason to not use valve caps.  In that event one could say the same
thing about clinchers although the likelihood is even more remote.

>>> ...Rolled tubs are mainly caused by improper gluing, perhaps also
>>> by blowouts.  Poor cornering technique ("lean the rider not the
>>> bike" style) would also seem to be a contributing factor.

>> Ouch!  We keep getting new misconceptions into this thread.  The
>> propensity for a tubular tire to roll does not change with lean of
>> the bicycle in a curve.

> Oh?  They don't roll when you're riding straight.  They don't roll
> when you're rounding a gentle curve at low speed so that the lean
> angle is very small.  Are you saying there is some specific lean
> angle where tire rolling switches from an impossibility to a fixed,
> finite risk?

To put this into perspective, in the days when we (people who rode
bike for sport) rode only tubulars, I was descending on the local
sub=per descent rode with a bunch of bikies some of whom liked to male
like motorcycles don the hill.  I and another rider changes the lead
several times and on one turn my friend passed in the last moment into
a turn so that he had to lean what surely looked like he wouldn't make
it.  When we got to my house, he decided to change his front tire
because the sidewall had a nick.  The tire came of with hardly any
resistance because it had been a spare and the glue did not adhere,
being too dry.

That event gave me a great understanding of rolled tubulars.  It takes
a side force that occurs only when the tire momentarily loses
traction, and when recovering, receives a load that is not in the plane
of the wheel.  As I mentioned, tires roll when a sprinter lifts off or
when in a curve, a side slip occurs.  Neither of these circumstances
is generally recognized because the first conclusion is that the tire
came off to cause the crash when in fact it was a loss of traction.

>> Here again the image is an upright bicycle in a curve with the
>> rider leaning in so that the tire is pushed off the rim from the
>> side force.

> Some cyclists actually ride that way.  You and I have both written
> extensively on the unwisdom of this, but the
> lean-the-rider-not-the-bike style is widely recommended in books and
> articles by noted racers who don't understand the dynamics of
> cornering.

That's a whole can of worms in itself but is mainly discussed on
wreck.racing.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jobst.brandt@stanfordalumni.org
Subject: Re: Tubular Tire opinions wanted.
Newsgroups: rec.bicycles.tech
Message-ID: <EjSC9.49823$Ik.1294465@typhoon.sonic.net>
Date: Wed, 20 Nov 2002 20:32:36 GMT

Mark Janeba writes:

> Low inflation pressure is a common problem with tires that have
> flatted, yes.

How nice, but are you interested in the thread or is snide comedy your
aim?  Most rolled tires (not flat tires) are caused by low inflation
pressure, something that occurs on MTB's more often because their
tires are often used with low pressure on soft ground.  However, the
phenomenon also occurs on road bicycles and is not something
restricted to poorly glued tubulars as is often suggested.

>> It is not cornering but rather low inflation pressure, or
>> recovering from a side slip when returning to good traction.
>> Cornering forces themselves do not induce tire roll, there being no
>> side forces.  Tubulars can do that better than clinchers.  We have
>> seen it often enough that checking whether tubulars were glued
>> securely was a standard inspection for race officials in the days
>> of these tires.

> I'd like to hear the reasoning for this claim also.

You may have meant "Could you explain how this occurs?"  "also?"

If you look at the cross section of a 25mm tire on a rim and rotate
the force vector (road to hub) around the tire cross section center,
you'll not that the tire is pushed into the rim even when the lean
angle is 45 degrees, the maximum reasonable use of a road tire.  This
became evident to me when I discovered that a tubular spare tire had
not adhered to the rim when I got home after a high speed descent
leaning hard into the turns.

I have seen tires roll after a rider side-slipped across gravel and
returned to clean pavement.  This can cause a tires to roll but it is
not from leaning into the turn.  However, if the tire is so soft that
it deforms enough for the center of pressure to move outside the width
of the rim the tire can roll.

Jobst Brandt  <jobst.brandt@stanfordalumni.org>  Palo Alto CA