Poly bushes

[c7trp]

Not 7 related...

 

A guy in the office has bought some poly bushes for a VW Bora for £10 on ebay.

 

Link to similar

 

Looking at them the casting quality is quite poor, but more of a concern, the metal bush running down the centre appears to be aluminium.

 

The metal centre has a rough hole drilled in it for the mounting bolt. (i.e. drilled with a blunt drill, and never been near a reamer!)

 

It seems to me that the bolts bearing surface should be something a bit harder than aluminium.

 

Anyone any experience of these bushes?

[mav]

Proper powerflex poly bushes have steel inserts...

[c7trp]

Thought so,

 

Mav

 

[6speedmanual]

A poly bush and a metalastic bush work in different ways.

 

With a metalastic bush the centre tube is gripped between the cheeks of its location by the clamping force of the bolt. The outer metal sleeve is gripped in the housing into which is is pressed. The pivot action comes completely from the flexibility of the rubber. Having thin inner and outer tubes is beneficial as more rubber can be packaged between the tubes in any given space available. Steel centre tube can give more strength against crushing in less space than aluminium.

 

The poly bush centre tube is also gripped by the clamping force of the bolt. The poly bush however rotates about the centre tube. It acts as a bearing. The poly bush material can be self lubricating or can have some lube added to help it move with low friction. Having an aluminium centre tube is not a problem so long as it can take the clamping load from the bolt without crushing (hence why in the bush in the link the tube is quite thick). Having a bigger diameter centre tube also makes for a bigger bearing area and reduces wear and friction when the bush pivots. If a thick centre tube is specified to acheive the bearing surface required, then better to make from ali for lightness.The poly part of the bush has to press into its housing tight enough that it always grips harder than the friction in the bearing surface. Some poly bushes have a cross hatched finish to the poly part of the bearing surface. This can improve retention of grease to lube the bearing. A rough drilled hole down the bolt hole of the aluminium tube is not necessarily detrimental to performance. (although preferably the manufacturers should strive for a neat job!)

 

One of the reasons why poly bushes sometimes last longer is that there is no twisting of the material as there is is a metalastic bush. A poly bush tends to become worn out in the bearing surface whereas a metalastic bush tends to tear apart the rubber section through twisting of the material.

 

In neither application does the bolt act as a bearing or hinge pin. If it is doing, the bolt is too loose or the cheeks to stiff to pull in and clamp the centre tube.

 

As the two type work differently and the materials are very different, they give quite different effects on suspension behaviour.

 

 

Peter

 

 

BRAWNGP green SUPERLIGHT

FCITW 2009 😬