THE GOING RATE.

Have you ever driven a street rod with suspension that was too hard, or raced a car that refused to launch?
Chances are that, somewhere along the line, you might have chosen the wrong rate springs...

In the not-so-distant past, it was considered cool in the street rod world to visit your local breaker’s yard, hunt out an old Jaguar Mk10 - or maybe even an E-Type - and cut out the rear suspension assembly. Once home, the unit would be stripped down, cleaned, chromed and detailed. Next, it was bolted in place under the back end of a glass fibre rod.
Seductive
The end result looked neat at a time when the alternative appeared to be an Austin Westminster axle painted bright red. Those chromed half-shafts twinkled seductively in the evening light, the home-made bottom tie link proudly displayed the legend ‘Ford’. Everything seemed perfect - except for that mouthful of loose fillings on every journey. See, nobody told you about spring rates.The complexities of suspension and spring rates is not something you would find explained in any street or hot rod resource guide.

In many ways, however, the Jaguar suspension set-up was too seductive. Mounted on its own sub-frame, all the budding rodder had to do was remove it from one car and bolt it on to another, coilover shocks and all. The problem was that the spring rates - the stiffness, if you like - had been chosen with a car weighing a ton and a half in mind, whereas the average street rod weighed about half that. This resulted in a teeth-jarring ride that took the fun out of any journey. Some rodders soon realised that the ride quality could be dramatically improved by the simple expedient of removing one shock from each side (the Jag IRS system has four coilovers in all),
although on a heavier car that would often make the ride too soft. However, it was a step in the right direction.
Fast-forwarding to the present day, by and large the Jaguar IRS has fallen out of fashion in favour of home-brewed independent set-ups or, more commonly, four-link mounted live axles. In drag racing circles, the four-link, or ladder bar, reigns supreme.
If you drive a car that retains its factory suspension, then rarely is there much cause to make radical changes to the spring rates. Only when you start putting massive amounts of horsepower, or removing a whole load of weight, does the matter of spring rate start to rear its head. On a race car, weight transfer is heavily affected by spring rates, so careful consideration must be given.
 
Above:
When a spring is put under load by the vehicle weight,the difference in length from the maximum extension to length at ride height is the 'rebound' height.
 

Above:
If a softer spring is fitted, the ride height is restored by pre-loading the spring. This is done by winding up the bottom cup. Softer springs can allow a race car to react more quickly. This is NOT recommended for street use!!!
Spring Rate
If however you throw away the factory suspension and hang the rear axle on a four-link or ladder bar assembly, then you must calculate what rate springs you need. OK, so you can guess - or copy what someone else has done - but, to get things right for your car, it’s worth spending a little time doing some basic mathematics. The end result will be worth it.

The stiffness - or rate - of any spring is measured in lbs/in. As this suggests. it is the measure of the amount of load in pounds necessary to compress the spring by un. For example, a 200lbs/in spring will compress by 2ins if a load of 400lbs is exerted on it.
The first thing to do - and here we will talk largely about four-bar or ladder bar installations
- is to establish what length shock you will need. This will frequently be governed by space restrictions but, as a general rule, coilover units such as those in the Koni SPA-1 range (the drag racer’s favourite) are available in fully-extended lengths, measured eye-to-eye, ranging from 15/ins to 19/ins. An important measurement is the stroke - that’s the difference in length of the shock from full extension to full compression. This doesn’t vary as much as the fully extended length - something between 5ins and 7ins being the norm.
In our hypothetical installation, we’ll take it that the car needs a shock which is approximately 16ins long. Now, one thing to make clear at an early stage is that a shock is best mounted at a tangent to the arc of movement of the suspension components.
In a ladder bar situation, this is easy to establish as the suspension pivots about the front mounting point of the ladder bar. In a four-bar installation, the suspension pivots about the Instant Centre - that’s the imaginary intersection point of the upper and lower four-bar links.
Corner Weights
To work out what rate springs we need, we must establish the true corner weights of the car. The corner weight is literally the weight of the car as measured at each wheel. Add all four corner weights together and you get the total weight of the vehicle.
Note, however, that we said ‘true’ corner weights. What we need to throw out of the equation is the ‘unsprung weight’ - that’s the weight of all the suspension components that actually move. This will include the wheels, tyres, hubs, bearings, brakes and a proportion of the suspension links (wishbones, four-bar etc). The true corner weight is, therefore, the total corner weight minus the unsprung weight.
With the chosen shock unit bolted in place, you need to measure the available travel. This is not the same as the full travel of the shock from fully open to fully compressed, but the travel from fully extended to the chosen ride height. On a shock with 7ins of available travel, this should normally be about 5ins, leaving 2ins for ‘bump’ travel (that is, upwards movement of the suspension components.
To establish a ballpark spring rate for our hypothetical installation, let us say that the corner weight is 1,080lbs. If the unsprung weight measures up to 80lbs, the true corner weight will be 1,080 -80 = 1,000lbs. Divide this by the 5ins of travel (1000÷5) and the result is 200lbs/in.
You could replace that spring with a softer one - say 150lbs/in - but then the suspension would settle more, reducing the ride height. In our first example, shock travel from full extension to ride height is: corner weight÷spring rate (ie 1,000÷200) = 5ins. In our second example, this would be 1000÷150 = 6.66ins, meaning that the car will sit 1.66ins (6.66 - 5) lower than we want. To re-establish the correct ride height, we must pre-load the spring by moving the lower adjustable spring cup up by 1 .66ins.
The advantage of using a softer rate spring is that the suspension will react more quickly and in a drag race application, that could be just what you want - especially in a heavy Super Stock type of vehicle where rapid weight transfer is necessary.
However, you must be aware that, whereas with a 200lbs/in spring, it will take a load of 400lbs to compress our’ shock that final 2ins of bump travel, whereas with a 150lbs/in spring this figure will drop to just 300lbs.

Left: The Motion Ratio is calculated by dividing the distance from the suspension pivot point to the bottom spring mount (A) by the distance from the pivot point to the tyre contact patch (B). This is true for both A arm and strut suspension systems.


Above: If spring is not mounted on a tangent, calculate spring rate after measuring vertical distance between pivot and top of spring (C), and length of spring at ride height (D).
Ideal World
In an ideal situation, a softer spring will always benefit reaction and 60ft times. It will also help the vehicle to cope better with an undulating track - and we've all raced on one of those at some time!
However, the disadvantage is that spring life will be reduced (ie the spring will settle) and this can be detected by a change in ride height. Softer springs are also more prone to coil bind -the condition where the coils of the spring are forced into contact with each other.
On any suspension set-up, the bottom mount of the coilover - and here we must include strut type front suspension systems - will not be directly over the centreline of the wheel and tyre.
Almost without exception, the mounting point will be inboard of the centreline. What we need to calculate is the Motion Ratio.
To do this, it is necessary to measure the distance from the pivot point of the suspension to the bottom mount of the coilover
(length ‘A’) and from the pivot point to the centre of the contact patch of the tyre (length ‘B’).
The Motion Ratio is calculated as A÷B.
From this we can now calculate the true spring rate required which is:

True Corner Weight x Motion Ratio
0000000 Available Travel

This same equation can be used to accurately work out the required spring rate of a four-bar system as long as you remember that the length ‘A’ will be measured from the Instant Centre (IC) to the lower shock mount and length ‘B’ will be measured from the IC to the centreline of the rear axle.

Calculations
If for any reason the shocks are not mounted at a tangent to the arc of suspension movement, then we can still calculate the required spring rate, although things do get a little more complicated. Here we need to make another measurement: the vertical (or ‘actual’) distance between
the upper shock mount and the pivot point of the suspension system - distance ‘C’.
To calculate the required spring rate, we must now use the following formula:


Corner Weight x Spring Length x Motion Ratio
0000Available Travel x Vertical Distance

Let’s put some figures to this:
True Corner Weight = 1,000Ibs
Length of Coilover = 15ins
Available Travel = 5ins
Vertical Distance = l2ins
Motion Ratio = 0.75
Spring Rate = 1,000 x l0x 0.75 = 187.5lb/in
..........................5 x 12


We can see that if such factors as Motion Ratio are ignored then we would get a different answer altogether:

Spring Rate 1,000 x 15 = 2501bs/in
........................5x 12

The final consideration when choosing springs is to ensure that the front and rear spring frequencies are different. If they were exactly the same, every time a car hit a bump, it would want to ‘porpoise’. Most of you will have at some time seen a drag race car bucking and heaving back and forth off the line - this is often the result of having ill-matched springs front and rear. By ensuring that one spring does not compliment the other, you can make certain that this porpoising is eradicated.

You cannot so readily calculate the frequency of a spring, but you can read it from the graph!
The frequency is expressed in cycles per minute (CPM).
Hopefully this will have given you an insight into how to work out what rate springs you need to give your car a fighting chance on the street or strip.

Below:
If a car has front and rear springs of the same frequency, the result is ‘porpoising’- a tendency to buck back and forth. If you know the corner weight, you can read off the spring frequency - measured in cycles per minute (CPM) - from the, graph below. What you need to calculate is the Wheel Rate (a measure of the actual force and travel seen at the wheel):
Wheel Rate = Spring Rate x (Motion Ratio)²
Front and rear spring frequencies must differ.