This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: firstname.lastname@example.org (Jobst Brandt)
Date: Thu, 08 Aug 1996 15:31:33 PDT
Although there are many arcane folds that people devise, it boils down
to pragmatism. Most spares are used tubulars because those who use
them typically ride together and for a new rider someone offers a
spare that gets returned or not at some later time. Therefore, we are
talking about a previously glued tubular and the point is to prevent
the whole tire from getting goo all over the tread and sidewalls, so
you flatten the tire against itself lengthwise with the sticky base
tape stuck to the sticky base tape. Now you have about a 40 inch long
flat tire that when folded in half twice makes the typical wad that
riders carry under their saddles secured by a footstrap.
Footstraps being nearly extinct, I don't know what people use today,
but whatever it is, it must be tight and secure. If it isn't, the
tire will jiggle enough to abrade the sidewalls to become a
pre-packaged blowout, to be installed when you get a flat on the road.
Don't do it. Most spare bags sold today are not good places to put a
tubular tire because they will allow the tire to vibrate too much.
It's bad news to ride alone with one spare anyway, so you ought to
ride with other tubular riders when you go any significant distance
from appropriate tire service. It's not like carrying a tube and
patch kit that can go until you run out of patches (you can cut
patches in half too). The advantage of using tubulars is so marginal
that the little weight saved is best applied to track and criterium
racing where its minuscule reduction in rotational inertia can at
least be argued to have some significance.