(previously published in
'de Wereldfietser', 1-1999)
(c) m.s.gerritsen 1999
A standard bicycle-frame
consists of 11 tubes, fried together in an intelligent way so that several
functions can be achieved. You should be able to attach all parts in their
proper position (fairly important for handlebars, saddle and crankset),
the construction should have sufficient stiffness and strenght, and
the contraption should handle safely even when fully loaded up. And it
will be nice if it doesn't look to bad, and isn't too heavy. The frameset
is a deciding factor in the overall quality of the bicycle as a whole.
You can hang a fortune in parts of a poor frame, but it will never be a
pleasant bike. If it fits, you'll have more fun with a better frame and
simpler parts. Quality in a frame depends on:
Touring bicycles are invariably
made from steel or aluminium alloy, although on tradeshows you might occasionally
encounter a titanium or carbon frameset.
longivety, achieved by a design
which takes account of the material properties, has carefully designed
joints and a properly executed construction,
the proper mix of strength and
resilience, again a function of material properties and the choosen dimensions.
A frame should be stiff enough so that it will handle faithfully in fast
descents, or on a steep climb. But too much stiffness isn't a good thing
either because tyre adhesion will suffer during cornering and bumps will
be transmitted even more.
good geometry, for handling
a good finish, with all braze-ons
in their proper place and a paintjob which will last longer than the first
Steel is the classic choice,
but from the MTB-scene you see aluminium more and more gaining a foothold..
the weight of a material by volume. A liter of steel will weigh 7.9 kgs,
aluminium 2.7kgs, and Titanium 4.5 kgs. Within practical limits the alloying
af the basematerial has no influence on the specific weight.
the tension (e.g force per crosssection) where the material breaks. Most
metals will exhibit flow before breaking. At a tension below the tensile
strenght the material wiill flow, i.e. deform permanently. Below this elastic
stress limit the material will spring back, if you go higher you have a
bent frame! The elastic stress limit is thus more important than the final
breaking strenght. With alloying, heat treatment and coldworking the strenght
of the alloys can be changed.
Stiffness: If you
apply a load to a specimen it will deform. Keep the stress below the elastic
limit and the deformation is lineair with the stress applied. The E-modulus
is the coefficient which decriibes how much a given material will deform.
For steel the modulus is three times as high as for aluminium, eg a given
crosssection in aluminium will be three times more elastic than the same
shape in steel.
Alloying elements have little
influence on the stiffness, so a more expensive steel tube will be just
as stiff as a cheap one. On the other hand it is quite possible that the
more expensive tube can be stretched 4 or 5 times further before it takes
a permanent set, because the permissible elastic stress limit is so much
Steel is a group of alloys
which are based on the element Iron (Fe) By the insertion in the cristal
lattice of several exotic elements with funny names ( Chromium, Molybdenum,
Vanadium) and by coldworking the elastic limit can be raised. The weight
and the stiffness of a given tube remains the same, but the difference
in strenght between a cheap tube and a high-class bicycle tube can be a
factor of five. If you keep the strenght the same, an expensive frame with
the best tubes could weigh a factor of 5 less. Unfortunately although this
frame wouldn't fail, it would also be 5 times as flexible, as the E-modul
In practice the better frames
will weigh about half, and be a bit more flexible than their cheaper brethren
and quite a bit stronger. This reduction in stiffness is acceptable, because
nobody worries about stiffness when designing a cheap frame, sufficient
strenght is the priority. A gaspipe frame is thus too stiff and the excessive
stiffness is all in the wrong place too.
Apart from more strenght
there is also another reason to choose a more expensive tubeset, and that
has to do with the dynamic material properties. That all steels are not
the same becomes obvious when you pick up a tube, as they sound a lot different.
I will not bother with a discourse on energy dissipition, micro strain
or the area of the hysteresis curve, but you can remember that a higher
strenght material will result in a better riding and more responsive frame.
There are quite a few manufacturers
which offer (or have offered) a great many tubesets in varying alloys.
The best known are Columbus and Reynolds, but there are quite a few more
with Falck, Dedacciai, Oria, Mannesmann, Poppe & Pothoff (Stainless),
Tange, Ishiwata, Vitus and True Temper etc. Usually you will find a sticker
explaining what you have spent your money on, usually with a fancy name
and sometimes the specification. Most manufacturers start with a tubeset
from 25cromo4 with a 0.8 mm straight wall, followed by several series in
butted 25cromo4, some higher end versions with modyfied cromo (up 20 %)
and some high-end version of such strenght that the wallthickness can be
reduced to 0.4 mm.
If the sticker says 25cromo4,
it will decribe a steel tube (the 95% Iron content is so obvious it doesn't
need mentioning) 2.5 % carbon (again carbon is always used, so we will
leave out the C) 4/4=1% chromium and some molybdenum. But the alloying
elements sound wonderfull (I love Oakley with Plutonite and (where did
they get this) Unobtanium) so copywriters will make the most of it and
then some.Titanium is used in stainless steels to improve the weldability,
but ad-writers will let you believe that you're getting a Ti frame. I'm
waiting for the day 'they' discover it is also an important ingredient
in white paint!
The specific weight of aluminium
is one third of steel. Unfortunately the same goes for the E-modul. If
you copy a steel design you have to increase the wallthickness by a factor
of three to achieve the same stiffness, so you will win very little. Often
you will find you have compromised longivety, because the permissible stress
in aluminium alloy welds is more than three times lower than the permissible
stress in a steel joint. Weightsavings can only be attained if you increase
the tubediameter instead of the wallthickness. (this is called oversizing)
An oversize tube will be stiffer in bending (because more of the material
employed is at a greater distance from the neutral- or centerline). The
same effect can be observed with a rolled up newspaper, this is a more
effective weapon than when folded flat. Because the tube is stiffer, it
is strained less and longivety is improved. Compared to a steel frame a
welded aluminium job will be heavier or stiffer to be able to achieve the
same life expectancy. For this reason the current crop of aluminium frames
all have fat but thinwalled tubes.. However there is a limit to how much
the tubes can grow, because the maximum diameter to wallthickness ratio
is limited. Overdo it and you get a beercan, which is very light, but also
very easily dented. Ofcourse in a bicycleframe that would not permissible.
As a rule of thumb, an oversize
aluminium frame will be stiffer than a steel counterpart in order to survive.
But also the transmission of roadshock and the frequency response will
be different. Do not buy an aluminium frame without riding it first, if
all you have ridden are steel frames. It could be quite different, although
fat tyres or a carbon fork might mask this entirely.
A theoretical aspect of
aluminium is the fatique limit, which it doesn't have as opposed to steel.
Fatique is the forming of cracks under cyclic stress, at levels well below
the elastic limit. In cycling circles this is well known as the broken
carrier in Nowhere. For steel there is a well defined stresslimit: stay
below and the construction will last forever, but aluminium will always
break if you keep loading it long enough. However neither steel or aluminium
bikeframes are designed for an infinite life, the designer aims for long
enough life with the envisaged use. To build that undestructible steelframe
you would have to add more weight, which no-one wants to lug around, and
is difficult to sell. For both materials the question should be what standards
did the designer apply, and as you usually don't know it is not practicable
to condemm alumnium on this aspect. However if you do break a frame, a
steel job is more easily fixed than an alumnium one.
Butted frametubes feature
a variable wallthickness, taking into account the expected loads. The middle
of the tube is usually the thinnest part, and the ends are about 0.2mm
thicker. The reason for this is that the stiffnes of a frameset is not
only determined by the stiffness of the tubes, but also by the deformation
in the joints. With thicker tube-ends, the stiffness is increased at very
low cost in terms of weight. Longivity is increased as well, and the dressing
of the joint is less critical. Butted tubes are made one by one, so they
are more expensive than the plain gauge stuff which arrives at the factorydoor
in 7m lenghts. A frame with butted tubes will be a bit lighter and a bit
more responsive than a plain gauge version.
To lug or not to lug
Steel frames are usually
brazed (with or w/o lugs) or welded. Aluminium frames are either welded
(usually necessitating post-heat treatment) or glued with lugs.
Lugs are the fittings which
can be used to join the tubes in a frame. The lugs are made in large series
and are very convenient for series production or the small builder. The
tubes are fitted in the sockets, making the assembly is self-jigging, and
necessitating very few fixtures. Downside of the proces is that the tubediameter
and the angles are determined by the lugs available. It is also possible
to join tubes without lugs. In that case the tube-ends have to be mitered
precisely, and the assembly is held in a jig. Steelframes can thus be brazed,
or welded as we see with aluminium frames. By doing away with the lugs
the designer gains absolute freedom with regard to tubediameters and angles.
With steel lugged construction, you are usually limited to a 1" or 1.125"
horizontal toptube, and a drop dependent on chainstaylenght, seat-angle
and the available bracketshell. This is the reason you can often find small
frames with a higher bracket than is used in the versions for longer legs,
or the ghastly double toptube, in cases where a single large diameter tube
would be more more efficient and less obtrusive.
Due to the lugs used, the
tubediameters were fixed for a long time. If you wanted a stiffer frame
the only way was to increase the wallthickness. This increased weight and
stiffness in equal amounts. If you use an oversize tube, where the wallthickness
is kept constant, but where the extra material is used to increase the
diameter, things change. Look at the following table: a single 35 mm tube
is twice as stiff as two 28.6 mm tubes, but weighs 30% less.
diam. x wallthickness
22.2 x .8
25.4 x .8
28.6 x .8
28.6 x .9
28.6 x 1
31.7 x .8
34.9 x .8
Step through frames.
I'm not in favour of the
step through design. You give away stiffness for free, which you have to
try to recover with fat and heavy tubes. On the other hand, if the hips
aren't what they used to be, or if you want to use the bike outside the
holidays you might not agree. The worst thing to do then will be to buy
a Dutch roadster, the opoefiets (grandma-bike) This frame is made
with small diameter tubing, curved for extra lenght, and the tubes are
quite close together, making a very flexible frame. What you want is a
frame where the wheels will still track on the same line, even if you are
honking up a hill and pulling on the handlebars. Torsional rigidity is
achieved by using a stout toptube instead of two thin laterals, by raising
the step-through height as far as possible and by using large diameter
and thick tubing. Any attempt at design and elegance will usually result
in a poorer frame.
Frames to measure
Do not buy a custom frame
if you can be happy with an off-the-peg frame. This saves money and a long
wait. However a custom frame will be sensible if you have special requests
with regards to the construction, or if your bodydimensions do not conform
to the averages used by the frame-factories. That will be the case if you
are very large or small, or a woman, but there are also quite a lot of
people with short legs-long bodies and vice versa. For instance if you
have unusual short legs you are better served with a 'dachshund' frameset
than by a small frame with a very long stem, where you will put an excessive
amount of weight on the front wheel.
Whether a steel or aluminium
alloy frame survives your holiday depends on the design, not on the material.
However steel is more easily repaired. A frame from high-quality tubing
will ride nicer than a cheap version with more heavy metal. An aluminium
alloy frame will probably be stiffer than a comparable steel frame. Make
a long testride and buy what you like. A decent fitting frame is worth
more than lots of shiny parts.
Ranking of available -steel-
753 853 631