Spring Ding

Picking the right springs for your car's suspension.

By Michael Knowling

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This article was first published in 2008.

The function of the springs is to provide a cushion between road irregularities and the vehicle's body - along with its occupants. However, springs also help keep the tyres in firm contact with the road as much as possible - including during times of massive vertical and horizontal loads. As soon as we change the vehicle's mass, traction, engine power or braking abilities (to name just a few), it becomes increasingly necessary to change the car's springs to suit.

For example, the potential for body roll is increased when we fit sticky tyres or modify the engine for more power. One of the primary side effects of increased body roll is an uneven chassis balance, which can lead to various driving difficulties. Fitting stiffer springs can help to alleviate this problem, in addition to changing the car's braking ability and the amount of wheel traction available. The fitting of larger swaybars is of course also hugely important in controlling a vehicle's body roll.

Shock absorbers (dampers) are largely responsible for maintaining road adhesion. This is done through the controlling of the spring oscillations, and the damping of their movement through bump and rebound travel. It is very important to have the shocker closely matched to the spring. However, springs (not shocks) are needed first and foremost. There will always be road bumps and vehicle weight transfer, which will permanently necessitate some form of springing medium.

Determining Current Spring Specs

The first step in upgrading springs is to find out the specs of the springs currently fitted to your vehicle. The most accurate way to do this is to pay about A$5 to a spring company to measure the spring's rate on their coil spring test machine. A coil spring test machine is a device that compresses a spring down onto a load cell. The cell measures the force being applied and this value is then plotted against the corresponding amount of compression of the spring. This gives the spring rate, which is usually expressed in pounds per inch. The metric equivalent of Newtons per millimetre is sometimes used. To convert pounds/inch to Newtons/mm divide the pounds/inch figure by 571.

Alternatively (but less accurately) you can obtain a guide to the spring's characteristics by placing a measured mass on top of the spring and measuring the amount of compression. This mass (in pounds) divided by the compression (in inches) gives the spring rate in pounds per inch.

The purely theoretical method of determining spring rate is to measure the wire thickness, the number of active coils (ie those fully free to move) and the spring's coil diameter. All measurements must be in inches.

The equation for this method is:

Spring rate:

11,250,000 x wire diameter4
8 x number of active coils x coil diameter3

Once again the answer is given in pounds per inch.

However, the simplest method suitable for making comparisons is:

Spring rate:

wire diameter x wire diameter x wire diameter x wire diameter
number of free coils

In this case the answer is given as a number suitable only for comparisons between springs of the same overall diameter.

Variable Rate Springs

The majority of modern cars come equipped with variable rate springs which offer a soft, comfortable ride when the spring is being compressed by only small amounts, such as over small road bumps. As the amplitude of the bumps increase - or alternatively as the body roll of the cornering car increases - so does the amount of spring deflection. As the spring deflection increases, the spring becomes stiffer, opposing further spring compression. The result is less body roll and increased suspension control. The variable rate of a spring can be achieved in three ways. The wire thickness can be altered along its length, the overall diameter of the windings can be changed, or the spacing of the windings can be altered.

While variable rate springs can give advantages, some drivers do not like them. This is because the rate of roll linearity varies. That is, there is some initial body roll on turn-in that is then checked from proceeding further. To some drivers this amounts to a body lurch when the car first changes direction.

Spring Sources

  1. Off-the-Shelf Aftermarket Springs
  2. Many of the larger suspension companies sell ready-made springs to suit common makes of vehicles. Eibach, Jamex and Pedders for example, all distribute their own springs in a specification they've determined to be popular. This usually means approximately a 20 per cent stiffer rate and a 1-2 inch shorter compressed length. This is perfect if the owner wants a drop-in replacement, however the individual characteristics and the application of that particular car are not taken into account. On the upside, many of these springs can still combine effectively with the OEM dampers on road cars, thus much reducing the cost of a suspension drop. These springs are generally powder coated or have another equally attractive finish, and are available for around A$180 a pair (variable rate) or A$130 (linear rate).

  3. Designing Your Own Custom Springs

With this approach the first logical step is to consult a reputable custom spring manufacturer and ask for their assistance in designing a spring. You must know what you want to achieve, as the company will key your quoted details into a CAD computer program to determine the specs of the spring that you require. You will need to know the:

  • Type of coil ends (eg tapered)
  • Spring Free Length
  • Spring Outer Diameter
  • Spring Inner Diameter
  • Direction of Helix (optional)
  • Wire Diameter
  • Mean Wire Diameter
  • Number of Coils

Alternatively, you can simply take an educated guess as to what you want, for example, a 30 per cent increase in spring rate. You can determine the new spring specs by using one of the above formulas, looking at an increased wire thickness, different number of coils or a smaller coil diameter.

Spring Materials

Any type of steel used to form an automotive spring must have certain physical properties, with most being high-carbon steels. The most widely used spring materials we've found are oil tempered and hard drawn carbon steels, silicon chrome and chrome vanadium alloy steels, stainless steels (such as 302, 316 and 17/7) and nickel alloys (such as inconel). These come in a choice of square, oval, flat and common round wire sections. Round wire is universally regarded as the best, as it has no areas of weakness and is easy to work In Australia, the most common automotive spring material is BHP X4K92M61S micro-alloyed spring steel, which comes with a circular cross section.

Spring Manufacturing

The process for manufacturing "hot wound" coil springs is the same regardless of its material composition. There are some small variations in baking temperatures and shot-peening grade. The average price for a custom pair of linear rate springs is around A$110, while a variable rate version adds around 20 per cent to the price - depending on the spring's size and finish.

The required diameter steel wire is cut to the correct length, this having been determined using a dedicated computer program. The steel being used here is BHP's X4K92M61S grade. For customers wanting a variable rate spring with a tapering wire diameter, the length of steel is linished in fine graduations along its length. This can be a time consuming-process and the cost of a variable rate tapered thickness spring reflects this.

Most vehicles need to have at least one flat-tapered end of the spring, to enable it to fit securely in its mounting. This allows for slightly more spring travel also. This process begins with baking the end of the wire in a furnace at up to 1000 degrees C to soften the steel. The end is then fed into a machine with two large smooth-faced counter rotating wheels. These wheels squash the wire to the desired thickness, thus giving the tapered (squashed) end.

The straight lengths of steel are then laid out on a rack and fed length-wise into a massive furnace which holds them at over 1200 degrees C for up to half an hour. This is known as the forging process. Glowing red hot and adequately softened, a person then pulls one end of the wire out with a pair of (very long!) pliers and walks it over like a giant snake to a nearby mandrel former.

The wire is swiftly "hot wound" onto the mandrel, which has been pre-selected to give the desired number of coils and the correct diameter. A mechanical arm is used to guide the hot steel onto the mandrel, which is rotated as the arm moves along its longitudinal axis. A final hammering of the wire to the mandrel ensures the entire spring is at a constant diameter to the mandrel.

The newly coiled wire then enters another furnace at around 1000 degrees C, but only for a couple of minutes. When it comes out, it is checked against a jig for the correct free length and to ensure there are no form defects.

The spring is then dropped into a bath of oil for quenching. Quenching produces a hardened spring as it rapidly cools the steel to obtain the optimum blend of strength and ductility. Small flurries of flames erupt when the coil is placed in the bath.

Now the spring is removed from its oil bath and placed in an open-ended kiln for tempering at 480 degrees C for 1½ hours. This procedure re-heats the spring to produce a finished hardness that will resist sagging, but be ductile enough to prevent breakage.

The spring is then trolleyed off to be shot-peened. Using a "390" grade bead that is shot from all angles, this step induces a resistance to cracking is due to "favourable residual stress in the outside fibres of the material".

After shot-peening, the spring is scragged. This is a process that involves compressing the spring to a pre-determined height, to produce a permanent set. It increases the coil's elastic limit and induces additional "favourable" stresses in the coil.

The last (and optional) step is the final finishing of the spring. Many companies apply powder coating or enamel finishes to improve their appearance and to resist corrosion. A final coating is advisable to prevent the possibility future spring corrosion or degradation.

Spring Effects

Upgrading springs and/or dampers might give you the desired improvement you were after with no apparent ill effects. However, vehicle manufacturers spend thousands of hours developing the suspension design to combine with various other "hidden" characteristics of a car. For example, body rigidity is factored into their suspension design. Driveshaft angles are another consideration which may cause wear and reliability problems in a car with massively changed ride height. The tyres also supply a measurable amount of bump absorption, with the main contributing factors being their profile and side-wall stiffness. Fitting firmer bushes can give increased ride harshness, while seam welding a vehicle's body can also serve to increase rigidity.

So we amateurs (relatively speaking) have no hope of matching the overall development capability of manufacturer design teams. Therefore some problems may be encountered with extremely lowered and/or stiffened springs!

A lower spring (with no other suspension mods) wound with the same pitch will be much more likely to coil bind whenever there is a large amount of bump travel. Conversely, suspension droop is also negatively affected and the chance of the spring actually falling out of its mount increases in proportion to its shortness. The spring needs to have the ability to compress to the suspension's bump stops and to extend safely to the limit of the suspension's droop distance without coil binding or falling out.

All springs also have a natural frequency, measured in Hertz. This is essentially the spring's oscillating tendency once it has been compressed and released. Imagine a car driving along the road and suddenly the front wheel strikes a vertical step of 40mm. In this instant the spring compresses and absorbs this energy, but for the spring to return to its former relatively un-compressed state it must release this energy. The in-built natural frequency of the spring and the mass of the car acting through the spring will determine the period before this energy is released, and at what rate.

This is a very complex area, which for purposes of this article we simply cannot delve into. However, when you buy or design a certain set of springs and fit them to your car, you generally accept the natural frequency of the spring and its body accelerations that come as a result.

Final Fitment

There is a correct method of installing variable rate coil springs to vehicles.

The softest section should be located at the bottom mounting position (assuming the lower end is connected to the axle). By doing this unsprung mass is reduced, which increases the ability of the wheel to follow the contours of the road with a more uniform vertical force. Essentially it is desirable to have lightweight active suspension components to allow for faster suspension movements. During small amplitude bumps where the suspension travel is relatively minor, it is then only compressing the soft section.

As with any change of major suspension components, a full wheel alignment should be carried out afterwards, as castor, toe and camber can all be affected by different height springs. The vehicle should be inspected for clearance between the tyres, suspension and wheel arch. This should be observed through the car's full suspension travel and with full steering lock applied through each extreme.

Our Advice

It is always advisable to have a set of dampers that are closely suited to your new springs, and any good suspension shop should be able to recommend the appropriate set. Lower spec dampers simply cannot prevent the spring from oscillating and therefore they do not offer adequate wheel control.

Because suspension design is such a complex area, it is always advisable to talk to some qualified experts before making changes.

Contact:

Industrial Engineers and Spring Makers Pty Ltd
+61 8 8346 2276

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