BY KEV ROONEY and GEOFF KREMER
Here we are going to deal with the fundamentals of springs. What they are how they work and how to chose them.
SPRINGS.
There are two basic ways of using a spring.
1 Axial.
2 Lateral.
Fig 1
A spring with a force applied Axially. ( a torsion bar)
Fig 2
A spring with a force applied Laterally.
Each of these bars of metal can be formed into the shape of a coil as below.
A conventional suspension coil spring is merely a torsion bar wound wound into the shape of a spring and the force applied at right angles to the bar’s axis.
The job of a spring is to store energy and release it when it can. The stored energy is described as poundage, and a springs rating, or SPRING RATE, is shown as;
Force or pounds per unit of deflection, or inches in our case.
Pounds per Inch.
Thus a spring rated at 100 Lb per Inch will require a force of 100 pounds to compress it 1 inch, 200 Lb to compress 2 inches, 300 Lb for 3 inches and so on until it becomes “coil bound” and cannot move any further.
Rule Number 1. SPRINGS ARE DANGEROUS.
If you have a spring loaded with 200 pounds, that energy will remain stored in the spring until you very carefully release it. If you are not careful, that 200 pounds may be released in a fraction of a second.
A LOADED SPRING CAN KILL!
Rule Number 2. NEVER EVER HEAT A SPRING.
A spring is designed to distribute the applied force uniformly along its length. Once you heat a spring, the heat will alter its crystalline structure permanently weakening it at that point. Any force applied will be concentrated there. It upsets the uniformity of the applied force and eventually it will fail.
A WEAKENED SPRING CAN KILL!
SPRING RATES AND HOW TO WORK THEM OUT.
We shall take as an example an unknown quantity in the form a newly built Pop.
First of all you need to prepare the car to be weighed. It should be set at its ride height.
In order to do this you may be lucky to have some springs that achieve this otherwise make up some dummy bars to take the place of the shock absorbers for coil over set ups. The bars should be set at the length the shock’s will be when the car is at ride height.
For other set ups, the bars should be fitted to ensure the correct ride height is maintained.
DO NOT ATTEMPT TO DRIVE A VEHICLE WITH DUMMY BARS FITTED!
The car needs to be at kerb weight, that is, as near a possible to the final driving weight and that includes all the fluids, battery and half a tank of fuel. Strictly speaking this should include you and a passenger sitting in it when weighed.
1 Weigh the car. Usually you can take it round to your local authority weigh bridge.
a weigh the whole car
b weight the front.
c weight the back
Example; a = 2000 Lb’s, b = 1250 Lb’s, c = 750 Lb’s. Note all this down.
You also need to know the stroke value of your shock absorbers and the amount they will need be compressed to when at ride height which should be half their fully extended length, so a shocker with a stroke of 4” should be closed by 2” when at ride height. More on shock absorbers later.
Jack the the whole car up and put it on stands.
Once again it should be at ride height or near enough to it for this purpose. Disconnect all the springs and shock absorbers and make sure all four wheels are resting on the ground…….no no, not leaning against the garage! On the car…cor I don’t know!
Now we are going to measure the unsprung weight.
This the weight of the wheels, hubs and back axle. This is necessary because the springs have only the weight of the body applied, not the weight of the bits resting on the ground.
Get a set of bathroom scales and put it under each wheel in turn, noting the reading.
Example; front wheel/hub = 50 LB’s each. Rear Wheel/hubs = 75 Lb’s each.
THE MATH’S.
We now have most of the information needed to work out the basic spring rates.
There is more to come but we’ll do this step by step.
The front of the car weighs = 1250 Lb’s divide by 2 = 625 Lb’s each side.
The rear weighs = 750 Lb’s “ “ = 375 Lb’s each side.
These are called the corner weights.
Now we subtract the unsprung weights from the corner weights.
Front 625 Lb’s minus 50 Lb’s = 575 Lb’s.
Rear 375 Lb’s “ 75 Lb’s = 300 Lb’s.
We know all four springs need to be compressed by 2” to achieve the ride height.
So the front springs are 575 divided by 2 (equal to the 2” compression) = 287.5 Lb’s/inch.
and the rear 300 “ 2 “ = 150 Lb’s/inch.
If the compression figure alters so the spring rate will also, thus 3” deflection on the front would need; 575 divided by 3 = 191.6 Lb’s/inch springs.
Having worked out the rate, now we need to fit the spring assembly.
If you’ve got an after market or ready fitted original set up, fine. If not you need to try to optimise your fitting position.
What follows is the ideal scenario but space constrictions and aesthetics can obviously affect this but try to keep it as close as possible. First a some points to keep in mind;
Scrub line. NOTHING on your car should hang down below your wheel rim edge.
This is so, in the event of a complete tyre deflation, there is nothing to stick in the ground and spin the car round, or worse still, over.
Bottom arm, front suspension alignment.
This should preferably be higher at the chassis end than the stub axle end, or parallel to the ground. It should not be lower at the chassis end as not only can you get into serious scrub line problems, but your camber changes during normal driving will produce very strange steering.
Top shock mount. This should be mounted SEPARATELY to the bolts or studs running through the top A arm. Yes, you’ve seen it on top show cars, from people that are supposed to know what they are doing, but it is wrong! If you mount the shock/coil assembly to this point, due to the angle it operates through changing it effectively lowers the spring rate as it rises.
Now, for normal fitting, the optimum angle is arrived at by the following method. If the bottom mounts need to be say 8” from the bottom A arm inboard mount, you need to draw a triangle with this length as your baseline. Draw the other side at the same length towards where the top shock mount would be. When you draw your other side in the final should produce 2 equal and opposite angles and this is the ideal angle to mount your shock, relevant to the lower A arm angle, not the parallel ground. However, in practice the shocker can be leant over to 30 degrees without causing problems.
Past this point and the spring rate starts to fall and the spring can start to collapse sideways.
Next we are going to find the ideal position for the mounting of the bottom shock mount.
Here we come to the use of leverages and ratios as seen in other tech articles on this site. If say the bottom arm is 12” overall length from chassis mount to swivel and the shocker mount is at 6” along then we have a ratio of 12:6 = 2:1.
If the bottom mount was moved inboard by 2”, giving a mounting length of 4” the ratio would be 12:4 = 3:1. This affects wheel travel within your arch.
If the shocker has 2” overall travel then in the first example the wheel would move 4” (2×2:1=4), and in the second example the wheel would move 6” (2×3:1=6”).
You would need to allow adequate clearance between your wheel and underside of wing/wing braces for this amount of travel plus safety clearance, and at all steering angles.
By altering the bottom mount position without altering the top positions the spring rates are all thrown out of kilter. Not only do you have to factor in the leverage effect but the alteration of the shock/coil assy angle reduces/increases the spring rate. The math can get very complicated to understand (even for us!!) so the practical way is the easiest. Back your shocker settings right off and drive at the nearest pothole, if the car thumps through it the spring rate is too high, if it wallows and continues to oscillate up and down it’s too soft. But if it absorbs the shock and settles down almost immediately you are just about right.
When using coil overs on the rear the ratio effect is still there. View your car from the side with the front of the location method (4 bar, ladder bars etc.) to the left. With the center of the axle as your end of leverage, you can see, using the formula for the front suspension, that if you place your shock assy to the far right i.e. behind the axle you will have less travel than placing them in front of the axle. So by choosing your mounting position carefully you can have the correct spring rate with the suspension travel (and therefore comfort) of a near standard car.
For better cornering characteristics the correct spring rate can be firmed up using the shock absorber settings and/or adding anti-roll bars, which are, in effect torsion bar springs, but are only operational under cornering conditions and have no effect on your straight line spring rates. Over stiffening the spring rate results in a very firm choppy ride that actually loses handling, as when you hit undulations in the road with the correct spring rate, the tyre surface stays in contact with the road where as the over sprung unit will lose contact. This in turn can cause braking problems as no grip, no brakes!! Again on over sprung vehicles the tyre sidewalls generally provide the only suspension movements and can soon start to lose their internal integrity when asked to perform beyond their design parameters.
Another form of springing is leaves.
Small strips of tempered steel usually put together in small stacks, and are used in a variety of mounting positions. They can be used as a single transverse (across), normally found on the front of beam axle’d rods, twin semi elliptical as on standard solid rear axle’d cars. They can also be halved in length, inverted and used as quarter elliptical’s when space to the rear of the axle is at a premium. Production cars using this method are MKII Jags and early Austin Healey Sprites. Jago’s used them on their very early T chassis. Single leaf springs are available on some production vehicles. When viewed from the side these have a tapered profile, which is set at the factory. These are known as, parabolic springs, and cannot be modified or even overhauled.
Spring rates on leaves can be altered by a spring smith by heating and pounding into shape. If you have a stack which you feel is “over sprung”, it is possible to remove springs from the stack to reduce the rates. It is worth bearing in mind that most early cars were over sprung, and when turned into a rod by removing wings, bonnet etc., the problem becomes worse.
Ride height can be altered by either having the spring eyes rolled the other way (reverse eye), which will lower the car by the thickness of the spring stack, or by having the spring re-tempered at a spring smith. Lowering the spring by this method will lengthen the spring and this can cause binding of shackles on a transverse front end or for the spring to hit the chassis on a rear semi-elliptic set up. Shortening a front spring causes no problems but altering the rear can cause the pinion flange angle to change, upsetting prop shaft operation. This can be remedied by tapered wedges inserted between the leaf spring & axle.
Spring efficiency can be improved by radiusing the underneath of each leaf where it bears against the next, allowing it to slide without digging into the next one. It is possible to buy Teflon interleaving which enhances this but adds to the thickness of the assembled spring, something to allow for when mounting or order U bolts.
To check whether your existing springs are tired, place one on its side and mark the position of the centre bolt on the ground. Now draw around the spring in chalk, pick it up, turn it over and replace on the centre line. The curvature should be the same if the spring is still in good condition. Also if the stack is dropped onto the ground in an inverted position, it should have a metallic ring, it if makes a clacking sound it’s time for an overhaul.
There are other springing mediums not covered here, such as air springs (still in its infancy in rod and custom circles, although used for ages in the coach industry) and torsion bars used on E types, Morris Thousands, Mazda trucks and VW Beetles etc. Whilst a good neat medium they are difficult to install and are difficult to re-rate for different applications.
A very informative site all about springs can be found herehttp://www.eatonsprings.com/Spring%20Tech%20101.pdf