Custom Bushes

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.

Rubber Bushes

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 bush.

Polyurethane Bushes

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 the bush).

(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 more quickly.)

Custom Polyurethane

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 sleeve

• Outside diameter of outer steel sleeve

• 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: 32.4mm

• 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.

Crush Tubes

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.

Machining Polyurethane

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 proud.

Lubrication

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.

Conclusion

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 application.

Contact: Greg Wright, http://www.superpro.com.au/superpro.html

The bushes used in our custom application were purchased at normal retail price.