Transverse leaf spring

By Kev Rooney

Getting your beam front end to sit and ride correctly is dependent upon getting the correct spring rate and operating angle.   The angles are governed by spring lengths and the rate by the thickness and number of leaves.   A lot of you will be fortunate enough to have a matched assembly, but many will need a custom made spring.   To have a spring built a springsmith will require a spring width, spring eye inner diameter, centres between eyes and the distance between the bottom of the leaf next to a line drawn across the bottom of the spring eyes.  The width and eye diameters will be governed by the shackles and perches that you are using but the other two measurements need to be worked out.

In order for the front spring to function correctly and not have a shimmy, the shackles should never exceed a 45º inclination, with a ride inclination of 30-35º, and an installation inclination of 0º.  Might sound tough to calculate but it’s relatively simple.   If you first install your shackles at 45º you then measure centre to centre in a straight line.   As this is a straight line the spring can never go past this length and hence the shackles cannot either.   Should the spring ever try to exceed this length it will simply start to bend the other way thus reducing the shackle angle.

Next place the shackles in a horizontal position (0 degrees) and measure centres.   You now have the two figures necessary to calculate the spring set (arc).

Now some of you may be saying “I’ve always understood that the shackle angle should be 45º at ride height”.   This is correct if you are using an original unboxed chassis.   These early springs are a much higher rate as the chassis absorbs a lot of the road undulations.   As the spring is a lot stiffer it will hardly ever move beyond its 45º set up, unlike the softer more modern spring which is designed to give more suspension movement.

There are monoleaf units out there for front ends.  These give a weight reduction plus more of a drop as the thickness of the spring nest is removed, lowering the ride height by the same amount.  These are really what are known as parabolic springs and when viewed in head on profile are thin by the spring eye, thickening up towards the middle and then reducing again.   These are made for specific applications and cannot, and should not, be altered from original spec.   Alterations can cause failure through metal fatigue.   I have the feeling that some of the aftermarket units are altered to suit as I’ve heard of several failures.   Either that or they are carrying far greater weight than the original design specification.   The original intended use of most monoleaf/parabolic is in pairs on the rear of light delivery vans, i.e. Escorts, Astras etc.

The multispring can be fettled to operate better.   Rust is an enemy as not only will it weaken the spring, but prevent easy movement thus giving a stiffer ride.   The nest is held together by a single central bolt and riveted clamps.   Once the clamps are removed clamp the stack either side of the central bolt with G clamps.   You will see that the central bolt is normally quite cong to facilitate easy assembly.   Turn the nut to the end of the thread and release the clamps until all tension if off and finally remove the nut.   Derust all the leaves and radius the underneath of each spring leaf where it bears down on the underneath leaf.   If the leaf underneath is too recessed it will need replacing as it will prevent easy movement and will weaken the spring due to metal erosion.

When reassembling do it dry as grease will attract all the road dirt unless you are fitting old style leather gaiters.   The other option is using Teflon interleaving, but the extra thickness generated will need to be allowed for in ride height.   As with coil springs chroming is a no-no again unless the spring has been built with a 30% upgrade to allow for the spring rate reduction caused by chroming.

Before starting any leaf spring work its worth checking if the spring is in need of an overhaul.   Two quick checks are to drop the spring, both eyes facing down, onto the floor.   It should give a metallic ringing, a dull thud indicates the need for an overhaul.   Then place the spring on the floor, mark the central point, and draw round the profile.   Now turn the spring over and realign the centre point and the profile should match exactly.   If not you are in need of an overhaul.   Please note that on rear semi-ellipticals the locating pin is not always central.

The final tuning option available is spring/ride height.   If the rate is correct the car can be lowered by having the spring eyes re-rolled upwards rather than down.   This will lower the car by approx the diameter of the spring eye.  From the point of rate reduction the shorter springs are stiffer.   The long ones are more compliant to movement due to the extra leverage exerted on them as the central clamping point acts as the pivot.

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