Springs are common devices. Your car's suspension has a
few, probably 4. The most common type of spring is the
coil compression spring. This is used by almost everyone,
even my nearest pen has one inside. Coil springs are an
example of a torsion spring, compressing the coils together
Another type of spring is the torsion bar. This works in
exactly the same way, by twisting a bit of steel about its
longitudinal axis, but it looks, and is used very differently.
The coil spring is much easier to handle in the car
environment. Essentially, you sort of squidge it in between
two circular-ish holes, one in a strong part of the chassis,
the other in some manner attached to the axle carrier, and
you're done. You can go about your life bouncing around as
you like, the coil shape takes care of all that twist-to-
compression conversion for you. A torsion bar has to be
grabbed very securely at each end, then you have to work
out some linkages to make the suspension load twist your
bit of steel. Essentially the packaging can't be an
There are advantages however, you can use a much
smaller, lighter bit of steel. You can change the car's ride
height by changing the amount of starting twist. New
torsion bars with different spring rates are pretty cheap,
and the packaging CAN be very compact, built right into
the overall design. For this reason, some of the most
successful cars or all time use such a suspension, including
the Volkswagen Beetle <link>, Citroen Traction Avant
<link> Morris Marina <link> Porsche Beetle Sport <link> M1
Abrams <link> Audi R15 <link>.
Even F1 cars make use of such springs, built into the pivots
for their complex bell crank and anti-roll linkages <link>.
The main disadvantage with torsion bars is a linear spring
rate. You twist it 1 degree, an extra 500N force, another
degree, another 500N force. It's very dull. Many
applications require a progressive spring rate, with coil
springs, this is easy... have wimpy floppy coils that
compress easily, they run into each other and become a
lump of steel, leaving beefier coils to provide a beefier
Now, if we are a bit sneaky, I think its possible to make a
progressive torsion bar. Take a tubular torsion bar, then,
put a smaller one inside it. Now you're free to pre-twist in
the inner torsion bar any way you like before locking it to
the outer one*.
If you use inner and outer bars of the same length, Say a
500Nm outer bar with a 50Nm inner bar pre-twisted inside
then as you twist, one unloads as one loads and you get the
sum of +ve and -ve forces, but this is just a complex way of
making a boringly linear torsion bar. Ugh.
Now, if you could make the inner bar shorter, then it
twists to a greater degree as a fraction of its length -
Progressive! Unfortunately, gripping a torsion bar inside a
long tube is a mechanical mess. You can use a longer
internal torsion bar, this will twist less as a fraction of its
length, now you have a regressive spring. If you have a use
for that, great.
Now, the solution. Large torsion tube, connected to Ring
gear of epicyclic gearbox. Small torsion bar coaxial,
connected to Sun gear of epicyclic gearbox <link>. Both
large and small torsion elements fixed together at the
other end. Construct the gearbox to be drive the sun gear
at 2:1 of the ring gear, done. Progressive torsion bar.
* I imagine the inner bar to have a square-section recess
for a socket wrench to pre-tension it. The inner face of the
outer tube and the outer face of the inside tube will be
splined with an adapter collar to slide in an lock them
together. Anti-seize compound recommended.