J
Recently therehave been a few posts which may confuse (or indeed
originate from) uneducated reader(s) concerning the strength of solid
versus hollow axles.
The strength of any particular cross-sectional shape can be described
by quantity called the section modulus, which is kind of a combination
of the amount of material and the distance away from the bending line
- usually taken as the centre - of the section.
If we take a typical rear axle the OD is pretty close to 0.4 inch. A
hollow axle has an ID of pretty close to 0.2 inch - giving a tube or
column with a wall thickness of almost 0.1 inch.
Using these numbers:
The solid axle will have a section modulus of 0.00627.
The hollow axle will have a section modulus of 0.00588
For front axles, which are around 0.35 OD, the numbers are:
Solid - 0.00435
Hollow - 0.00390
Now, of course, these figures mean that a solid axle is stronger.
There are other factors which affect the strength of axles, of course;
the choice of material, the heat treatment, the surface finish,
etcetera.
A particular effect which cannot be eliminated from hollow axles is
that of the roughness of the inside surface of the hole. Solid axles,
of course, do not have this, and so also do not suffer from the chance
that a crack may start at an irregularity on the inner surface. Cycle
axles of the highest prices (and, the buyer may sometimes presume, the
highest quality) are finished very well on the outside surface; but
the inside is a hidden quantity, difficult to finish and inspect.
In the real world of cycling, there are axles made from different
material and to different qualities. Of course it is possible to find
a particular hollow axle which may be stronger than another particular
solid axle - but to claim in abstract that a hollow axle is stronger
(or even no weaker than) than a solid one is clearly wrong.
originate from) uneducated reader(s) concerning the strength of solid
versus hollow axles.
The strength of any particular cross-sectional shape can be described
by quantity called the section modulus, which is kind of a combination
of the amount of material and the distance away from the bending line
- usually taken as the centre - of the section.
If we take a typical rear axle the OD is pretty close to 0.4 inch. A
hollow axle has an ID of pretty close to 0.2 inch - giving a tube or
column with a wall thickness of almost 0.1 inch.
Using these numbers:
The solid axle will have a section modulus of 0.00627.
The hollow axle will have a section modulus of 0.00588
For front axles, which are around 0.35 OD, the numbers are:
Solid - 0.00435
Hollow - 0.00390
Now, of course, these figures mean that a solid axle is stronger.
There are other factors which affect the strength of axles, of course;
the choice of material, the heat treatment, the surface finish,
etcetera.
A particular effect which cannot be eliminated from hollow axles is
that of the roughness of the inside surface of the hole. Solid axles,
of course, do not have this, and so also do not suffer from the chance
that a crack may start at an irregularity on the inner surface. Cycle
axles of the highest prices (and, the buyer may sometimes presume, the
highest quality) are finished very well on the outside surface; but
the inside is a hidden quantity, difficult to finish and inspect.
In the real world of cycling, there are axles made from different
material and to different qualities. Of course it is possible to find
a particular hollow axle which may be stronger than another particular
solid axle - but to claim in abstract that a hollow axle is stronger
(or even no weaker than) than a solid one is clearly wrong.