Available for all British Sports Cars, American and Australian "muscle cars" and 60's and 70's performance cars such Cortina and Escort, Specials & limited production cars, race or road.......

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The advantages are:

1. Consistent spring rate, almost as good as a coil spring.  The spring rate can be found very easily using the Smithees "Bounce Test".

2. Better lateral location. This is particularly important in racing categories where lateral location aids, such as panhard rod or watts linkage, are not allowed (some Historic categories).

3. Reduce spring wind up and axle tramp, under acceleration.

With a traditionl spring pack, there is a certain amount of lost motion, as the axle moves in bump and rebound, and engages and disengages with the secondary leaves. There is also friction between the leaves.  These problems are not applicable with the single leaf.  Common fixes for racing are a rebound leaf over the top of the front half of the spring pack or to bolt the the front half of the the spring pack solid.  The rebound leaf has a problem with consistency, and the second method over stresses the rear half of the spring.

Side Load Location Without Panhard or Watts Linkage etc
(Where racing regs prevail, or attention to standard look is required)

Early leaf spring designs had a small metalastic bush in the front spring eye. When the car is cornering, the metalastic bush in the front spring eye twists the spring as the chassis rolls. This weakens the spring rate in bump and rebound, and reduces the lateral stability of the axle in cornering. Smithees single leaf springs have a large front spring eye. We use either a "type 16" (single piece) urethane bush, or our own proprietry steel rotating spring eye bush. With either type of bush, when the car rolls in cornering, the chassis rotates around the single leaf, and the leaf spring stays flat.

So a wide, flat single leaf has greater lateral stability then narrower multi-leaf pack. With quality U bolts that can be torqued sufficiently, the single leaf will be held at a permanent right angle to the axle. Given the sideways stability of our front spring eye bushes, sideways axle movement in cornering is much reduced. Note that we are not depending on the rear shackles for sideways location. The shackles will usually have "type 1" (two piece bushes)bushes - not good for stability.  If you do create stability at the rear of the spring, by using metal bushings or a "slider", then the rear would contribute to sideways location.

Consistent Spring Rate

Many early leaf spring designs allow the spring to go into negative arch ie the spring will start with positive arch unloaded, and as the weight comes on, go past flat and into reverse arch. This, in theory, should not affect the spring rate. But in practise we think it does (we have made and tested two sets of single leaves - one set that goes negative, and another set that doesn't). Also, in some road car applications we have seen, the spring actually bends in service, under the stress of continually going into negative arch. Therefore, in most of our designs we reverse the spring eyes to increase the positive arch. Reversing the spring eyes can also help lower the car, without using lowering blocks.

When the car is accelerating, the spring will be trying to wind up, causing axle tramp. The car looses traction. The usual way of reducing wind up is to fit a rebound leaf to the top of the spring pack.  It needs to be quite thick, and because of it's poor location, the affect on spring stiffness is not consistent. Track rods stop axle tramp. But they may restrict movement of the axle in bump and rebound. As the axle moves from it's static position, the suspension will get progressively stiffer. This could cause oversteer, and poor traction on corner exit. The problem of suspension bind is overcome in road cars by providing large flexible bushings. In racing, only the four link rear suspension with a "bird cage" (axle housing rotates with suspension movement) works properly - as in V8 Supercars. The Smithees single leaf rear spring will be stiff enough to avoid wind up, in most instances, without the use of track rods or rebound leaves.

"Forward Bite" and How to Get It

There are further considerations in relation to live rear axle set ups. The correct setting of pinion angle can help traction (forward bite) and reduce spring wind up. Lowering blocks that are too tall may create more leverage on the spring and increase spring wind up. A degree of compliance, inherent in the leaf spring set up, is good for forward bite. The angle of the front half of the spring could induce some anti-squat, although it is not as straight forward to calculate as a suspension with rigid links. With big horsepower cars, the torque steer will definitely be a factor, and can either be used to advantage, or the disadvantage offset to some degree, depending on the situation.

The Ideal Set Up

If you are running a live rear axle, the ideal is a 4 link suspension, but a leaf spring rear end can be nearly as good.  Some teams in American late model racing prefer the single leaf rear suspension.   The Watts linkage or panhard bar arrangement is best for lateral location, and allows adjustment for rear roll centre height.  In Austarlian road racing you usually run a leaf spring rear end because it is a requirement of the rules, but it need not be too much of a disadvantage.  We like to run a rear anti-roll bar where possible ie if limited slip diff is fitted, because this allows the use of softer rear springs.  Leaf spring set ups have a very poor motion ratio in roll.  There are calculations for this, and the ideas are explained in the Smithees Weight Transfer Worksheet.