Highwheelers used steel cranks, not aluminum.
A clever fellow figured out that he could twist round steel cranks to
make them stiffer. Late-era highwheeler racers liked the twisted
cranks because the pedals didn't give as much, even though the cranks
weighed only about half as much:
"Known as 'Southard's Twisted Cranks,' they were an invention of a
Southampton man of that name, under his patent No. 17408/1889, and
embody an extremely clever principle. Each crank was forcibly twisted
exactly half a turn--when cold--the claim being that, after this
treatment, the metal would not twist any more on violent pedal
pressure being applied to it. Obviously left and right cranks were
were twisted in different directions and were not interchangeable.
Southard's cranks were stated to weigh only 7 ounces [~200 grams] the
pair, against 16 ounces [~455 grams], the alleged weight of the
standard cranks of the period. For some time after their introduction
they enjoyed considerable popularity."
--"Bartleet's Bicycle Book," p. 52
The upper highwheeler is the late-model Humber racer (a radial
re-spoked to tangent, which Humber despised) with the weight-weenie
Southard cranks:
http://i31.tinypic.com/8ysvuq.jpg
Thin Southard crank detail:
http://i25.tinypic.com/11aecyc.jpg
Hideously overweight ordinary crank:
http://i32.tinypic.com/vcudcl.jpg
The Southard cranks are noticeably thinner than the cranks on the
lower highwheeler.
(Yes, those are toe-clips on the Humber with the thin Southard crank.
Naturally, both racers lack mounting steps.)
"Fig. 26 is an illustration of 'Southard's' patent crank fitted by the
same cotter pin. This is truly a beautiful crank; its specialty
consists in the mode of manufacture. They are made from soft steel, a
bar of which is twisted _cold_ , after the fashion of a coil of rope;
when a certain stage is reached, the steel offers enormous resistance
to being further twisted in that direction. The cranks are supplied in
left and right, and, although they weight just half the weight of old
crank, the utmost strain a rider can put on them will never bend or
break one."
--"English Mechanic," 1893, p. 445
http://books.google.com/books?id=DIUAAAAAMAAJ&printsec=titlepage#PPA445,M1
Here are the cranks, the thin Southard being second from the bottom:
http://i27.tinypic.com/2it361z.jpg
Sharp mentioned the Southard crank:
"122. Raising of the Elastic Limit.—Let a bar be subjected to a
stress—represented by the point k (fig. 111)—considerably above its
elastic limit. If the load be removed and the bar be again tested, it
will be found that it is elastic up to a stress as high as that
indicated by k. Thus the elastic limit in tension of a material like
mild steel can be raised by simply applying an initial stress a little
above the limit required."
"An important application of this principle occurs in the case of
Southard's twisted cranks. Here the cranks are given a considerable
initial twist in the direction in which they are strained while
driving ahead ; their strength is considerably increased thereby. A
twist (sec. 109) is equivalent to a direct pull along certain fibres,
and a direct compression along other fibres at right angles. The
initial twist in Southard's crank is, therefore, equivalent to raising
the elastic limit of tension of the fibres under tensile stress, and
the elastic limit of compression of the fibres under compressive
stress."
--"Bicycles & Tricycles," 1896, p. 138-9
http://books.google.com/books?id=gFMN3-srupsC&printsec=titlepage#PPA139,M1
Sharp points out that the twisted crank is weak under back-pedal
pressure, emphasizing why the cranks came in left and right and were
for racers:
"These considerations, when applied to the case of Southard's
cranks, detract from the value of the initial twist. The line t t
(fig. 109), which is the tension line when the rider is pedalling
ahead, has had its elastic limit in tension artificially raised, and
its elastic limit in compression artificially lowered by the
initial twist. When back-pedalling, t t becomes the compression line.
A twisted crank is therefore weaker for back.pedalling than an
untwisted crank of the same material."
--"Bicycles & Tricycles," 1896, p. 140
http://books.google.com/books?id=gFMN3-srupsC&printsec=titlepage#PPA140,M1
Cheers,
Carl Fogel