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.......
Contact Us for Details
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
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
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
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
"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.