This article was first published in 2006.
If you’re doing any major suspension modifications
(like fitting a Watts link or Panhard rod, or altering the front suspension
pick-up points), chances are you’ll be looking for new bushes. But if yours is a
custom application, how do you source them?
But before we get into that, let’s take a look at
bushes in general.
Most factory bushes use rubber. It’s
important to know that the rubber is bonded to both the inner and outer steel
parts of the bush. The rubber consequently distorts in torsion as the suspension
arm pivots and it is this internal shear characteristic of a rubber bush that
allows it to be made accurately, means that it requires no lubrication, and
gives it excellent longevity.
The hardness of rubber bushes is
often different in different planes, with voids and other manufacturing tricks
incorporated to allow this to occur. This selective voiding of bushes allows the
manufacturer to facilitate compression in some directions but not others. For
example, a bush may be designed to allow some front toe-in to occur under
braking, improving vehicle stability. Other suspension bushes in a car may allow
some longitudinal deflection of the bush under low loads (absorbing small sharp
bumps) while resisting much stronger vertical forces. Some rubber bushes are
even designed so that different frequencies of vibration are absorbed by
differing amounts. In summary, most rubber bushes aren’t quite the cheap and
nasty design that they can at first appear to be.
So in a custom application, it makes
sense to first look at what factory bushes are available. You may well be able
to take advantage of their internal design, and if you select the bush early
enough in the design process, you can size the suspension components to suit the
A common replacement for rubber is
to use bushes made from polyurethane. The polyurethane deflects less under loads
than most rubber bushes, maintaining better dynamic suspension geometry.
However, urethane acts as an incompressible viscous liquid, rather than being
able to change its volume under compression as rubber does. This means that a
captive urethane bush transfers greater forces directly to the bodywork. The
result is greater noise and vibration. However, most enthusiasts are prepared to
trade this off for the better handling.
Also, when urethane bushes are used,
there must be sliding movement between the pivot (normally called a crush tube)
and the bush itself – the plastic bush will not internally deflect to allow for
the suspension arm movement. To allow the rotational motion to occur, generally
the crush tube is free to move within the urethane bush. A urethane bush
therefore requires sleave lubrication, accurate sizing to the crush tube and a
crush tube that preferably is extremely smooth (which allows free movement of
(There is one situation where the
new urethane bush does not require rotational lubrication, and that is when it
replaces a rubber bush in a sway bar link. In this situation the greater
hardness of the urethane has no drawbacks, and allows the sway bar to respond
Bush manufacturers make thousands of
differently sized bushes. So before looking at making something custom, it pays
to give a major manufacturer a call and see what they have available. Assuming
that you need a conventional suspension bush that is located at a pivot point,
the manufacturer will want to know these dimensions:
Diameter of through bolt
Inside diameter of outer steel
Outside diameter of outer steel
Inside width of the bracket to which
the bush is bolted (ie determines required bush length)
We recently needed a bush for a
custom application and the measurements were:
Diameter of through bolt: 8mm
Internal diameter of outer metal
sleeve (it was aluminium in this case): 25.4mm
Outside diameter of outer sleeve:
Length: could be varied (brackets
were not yet built but somewhere around 54mm)
I rang Super Pro suspension bushes
in Brisbane and after initially getting someone who not only didn’t know how
poly bushes worked (“Nahhh, they don’t rotate on the crush tube, maaaate”) but
also couldn’t be bothered looking up their bush listing, I got in contact with
Greg Wright who very much knew his stuff. Much to my relief he soon came up with
some suitable off-the-shelf bushes: SPF0107K which are normally used as
replacements in the front spring shackles of a ‘73-’85 Jeep (or the rear
shackles in a ’57-’65 Gordon Keeble or Tempest!).
These bushes are 25.4mm outside
diameter and have a total length including a single end flange of 30mm. The end
flange is chamfered (more on this in a moment) and including the chamfer, is
7.7mm thick. The diameter of the flange is 34mm. The bushes are designed to be
inserted from each end of the tube that holds them. The flange gives lateral
location and the chamfer reduces the amount of polyurethane which is in contact
with the bracket, reducing stiction and potential noise. Some design bushes have
flanges at both ends and are inserted with a press, one of the flanges
compressing sufficient to fit through tube before springing back out again.
In my application the bushes in use
have a small (~7mm) gap between them within the sleeve. This is of no
consequence as there’s still plenty of material to take the forces.
Note that poly bushes come in (at
least) three durometers, or hardnesses. Duro 70 is much like rubber in its
hardness, which duro 80 is a little harder and is most commonly used. Duro 90 is
harder again. Importantly, the higher the duro, the more easily the bush will
pivot on the crush-tube so if low stiction is needed when under load, a higher
duro should be used. Luckily, in our application where low friction was
important, the available bushes were duro 90.
In a poly bush, the bush material is
literally only one half of the story. The other half is the crush tube. As its
name suggests, the crush tube absorbs the forces developed when the through bolt
is tightened and of course, all the forces developed by the car on its
suspension are also channelled through the crush tubes. Most are made from
seamless drawn tubing with the bush ID very accurately sized to the crush tube
to give a good fit without being sticky.
The machining-down of crush tubes in
a custom application should be undertaken with caution: not only will a wall
thickness reduction decrease strength but the surface finish is very important
if low friction is to be maintained and the bush is not to be chewed-out over
time. One alternative approach – suggested by Greg Wright – is to find a shock
absorber shaft that is a good fit for the inner diameter of the poly bush. Using
a lathe, the shaft can then be shortened and drilled-out to take the through
bolt. The huge advantage of this approach is that the shock absorber shaft is
already hard chrome plated (for wear resistance) and is strong with a very
smooth surface finish.
If nothing is available off the
shelf, polyurethane can be bought from manufacturers in bar form and then
machined in a lathe. However, machining urethane is an acquired skill: a very
sharp tool, high lathe speed and quick and accurate work are needed.
Incidentally, when machining either inner or outer diameters, the bush should be
pushed onto a spigot and the spigot held in the chuck of the lathe. (When doing
inner diameters, two differently sized spigots will need to be used and the bush
turned around at the halfway point.)
In our application, the inner
shoulder of the flange was machined squarer...
...so that the flange sat flush
against the end of the tube...
...rather than sitting a little
Lubrication of a poly bush is very important: not
only to aid durability but also to avoid squeaks from the bushes. Use the
special grease provided by the manufacturer of the bush – this grease is
purpose-designed for the application and won’t generally run out or attack the
urethane. To avoid squeaks, make sure the flanges of the bush are also well
lubricated where they bear on the bracket. In some cases where squeaking is
impossible to stop, any paint that the bush contacts should be removed.
Whether it’s buried in the obscure catalog listing
of a suspension bush manufacturer or has to be custom-machined from polyurethane
bar, there shouldn’t be a bush that can’t be produced for any suspension
Contact: Greg Wright, http://www.superpro.com.au/superpro.html
bushes used in our custom application were purchased at normal retail price.