The apparently humble F1 component with remarkable attributes

In this column we sometimes cover quite complex subjects and try to explain them in a Formula 1 context. At other times the subject may seem simple but the nuances add complexity. Equally, I always try to answer questions put to me by readers. So, let’s delve into a subject which occupies the second category: torsion springs.

Before answering that question let’s examine just what springs are all about. A spring is simply an elastic component that stores mechanical energy and exerts a force when it’s deflected.

The form of spring most people know is the coil spring which can be found in everything from a vehicle’s suspension to a retractable ballpoint pen. If the spring is made of wire of a constant diameter and the coils are evenly spaced, then the spring has a constant stiffness or spring rate, normally expressed in Newtons per millimetre or pounds
per inch. So, if you had a spring with a rate of 175N/mm and you put 175 Newtons (about 18kg) on the end of it, it would deflect 1mm.

What many people fail to realise is that although the spring may deflect in a straight line, it does so because each part of the coil is actually being twisted. Therefore the material property that determines stiffness is the shear modulus rather than the elastic modulus. This is why the weight saving from using titanium for springs isn’t as great as one might hope – while the strength to weight ratio of titanium is significantly better than that of steel, the shear modulus to weight ratio of titanium relative to steel is only slightly better.

Having seen that a coil spring achieves its stiffness by the twisting of the wire from which it’s formed, we can understand that a torsion bar spring is merely an unwound coil spring. There are some other subtleties such as end conditions, but a torsion bar has two distinct advantages.

The first is that if one considers any device that’s being twisted, the maximum deflection occurs at the outer surface, and at some point around the middle of the section no twisting is taking place. So it makes sense to remove the material from the centre, which is doing little work.

The nuances of springs are little understood, but what cannot be underestimated is their contribution to effective kerb-riding

Photo by: Simon Galloway / Motorsport Images

With a straight torsion bar this is a relatively simple machining operation, but one can imagine that with a helical wound spring it would be much more difficult. This can give the torsion bar a useful weight saving over a conventional coil spring.

The second advantage of a torsion bar is in packaging. All cars use some form of push or pullrod operating a rocker arm to activate the spring and damper mechanisms. Putting a torsion bar through the centre of the rocker and splined to it while anchored to the chassis or gearbox at the other end is a far neater installation than a bulky coil spring attached elsewhere on the rocker.

But these aren’t the only form of springs to be found on an F1 car. One will often see disc springs or Belleville washers used on central heave springs. These, while slightly heavy, have the advantage of being compact in an area where space is at a premium. They can also easily be made progressive or rising rate.

The car was suspended on disc springs and we wrote computer codes that allowed us to calculate how to stack the springs to give us our desired ride heights at different speeds to best exploit the aero map

Aerodynamic loads increase with the square of the speed of the car and hence it is useful to have a spring that gets progressively stiffer as it deflects. This can be done geometrically by judicious positioning of the spring and push/pullrod attachments on the rocker, or by having a progressive rate spring. Disc springs achieve this by a stack of varying-thickness washers arranged such that the less stiff springs progressively lock out, leaving the stiffer springs to pick up the load.

One of the advantages Michael Schumacher’s Benetton had in 1994 and 1995 was that the car was suspended on disc springs and we wrote computer codes that allowed us to calculate how to stack the springs to give us our desired ride heights at different speeds to best exploit the aero map. All teams had been doing this with active suspension – but I think we were the only ones to exploit this to such an extent on a passive suspension system.

Of course there are other ways to get a progressively stiffening suspension and one that’s been in use for many years is the use of bump rubbers. These are shaped elastomeric devices that are gradually compressed but, due to their shape, produce a non-linear force as a function of their displacement.

The final type of spring used on F1 cars is the gas spring. This is a personal favourite of mine and was used on both active and passively suspended cars.

Following the ban on active suspension, Symonds' Benetton team gained an advantage with disc springs in 1994

Photo by: LAT Photographic

It has the great advantage of being very smoothly progressive and easily adjustable by altering the gas pressure, and sometimes the gas volume, to achieve just the rate increase required. It has some disadvantages, mainly the fact that the spring rate changes with temperature, but this is easily compensated for. It was in common use until 2022 when the FIA decreed they were too complex and capable of ‘hiding’ unwanted characteristics.

I never felt this was the case but, nevertheless, teams are now obliged to revert to an inventory of heavy and expensive mechanical springs. The gas spring isn’t completely dead, however, since every F1 engine uses them instead of coil springs to close the inlet and exhaust valves. In this context they’re infinitely superior because they’re not prone to valve bounce which, for a long time, was the limiting factor for engine speed.

The humble spring, at first sight one of the more simple parts of an F1 car, hides attributes that when used correctly do much to enhance performance.

Gas springs as seen on the 1993 Williams F15C's active suspension were in common use in suspensions until outlawed in 2022

Photo by: Sutton Images