A good poly bush read
Posted: Thu Feb 22, 2007 7:31 am
This was posted over on Northloop by a guy who works as an engineer for Jaguar. He may well frequent this place too as he is a beemer owner. Worth a read..........
So poly bushes are great right? I'm not part of the motorsport scene so I don't understand why they've become so popular. They seem to be sold on the pretence of being stiffer so the car is tighter and that must of course be better…
The merit of poly bushes has to start with the material, which is polyurethane, and how it compares to rubber. The base material offers better isolation properties than rubber because of its damping characteristics. Rubber increases its stiffness dramatically the faster you push it, or vibrate it, but it also has comparatively low damping for large, slow amplitudes. Polyurethane is better for this so controls larger amplitudes better and also transmits less vibration at higher frequencies. As a material it would be ideal to be used in place of rubber as it gives the option for improved isolation. This leads me to a question: if the advantages in the material are all about vibration and isolation then why is it sold as a stiff race upgrade?
Polyurethane is also compressible where as rubber is not. By this I mean its volume gets smaller where as rubber just changes shape when you squash it. It contains tiny air bubbles that allow this and the amount is measured by the density of the material. So stiffer polyurethane contains less air and would be of a higher density. Poly bushes as they are designed are very stiff in a material sense I think, almost as though they have no air.
So as a material everything seems positive but it has a rather fundamental flaw in that it can't be bonded to anything. This means it can only give stiffness in one direction —a when you squash it. Rubber on the other hand bonds to metal quite readily so it can give stiffness in both compression and in shear (when you try to push it sideways off a bit of metal it's stuck to). This is normally in the ratio of 5 to 1 I think off the top of my head, it's five times stiffer in compression than shear. But this can be manipulated so you can play tricks with metal form both for the inner and outer metals of the bush to control the stiffness as you want. Rubber bushes also give stiffness in a rotational sense as they are acting in shear in that direction too.
1. poly bushes can't provide rotational stiffness. Take a car and replace all the rubber with poly bushes and its wheel rates will be softer. Rubber bushes contribute on average about 4N/mm at the wheel I reckon, this can differ wildly depending on platform though. 2. poly bushes can't rely on shear to provide stiffness axially (along the direction of the bolt if you get what I mean) so have to have material around the sides that is compressed too. 3. poly bushes can't do either of these things yet the suspension still needs to rotate against the compressed material so you get lots of friction. Well you can just fill that full of grease —a that will last for ever. 4. poly bushes can't provide the same amount of pre-compression as rubber because of this friction. By this I mean squeezing an amount of material into a small hole —a it could mean poly bushes are actually initially quite soft and need load to stiffen up 5. depending on the platform you could be completely mucking up the suspension dynamics in terms of compliance steers and so on. The bmw suspension is a very good example as the rear trailing bush controls the amount of rear steer you get, fitting a poly bush changes this so the car could actually build lateral acceleration slower than it did before.
So in my opinion the technology is completely flawed and as an engineer I'm not comfortable with that. Certainly on the more advanced platforms where the wheel compliance behaviours are carefully controlled you could be making the car worse. And it will have more vibration as the material is so stiff.
You'll only find polyurethane in two places on most production cars (well three on Jaguars…) and that is for bumpstops and spring isolators as both suit the material properties perfectly.
There are a lot of people that have seen a benefit (possibly because they've replaced a knackered rubber bush with a poly bush —a how many have replaced with a new rubber bush?) and that's because some cars would suit a stiff bush for the track. Any car with a twist beam rear axle like a mk2 Golf for example. The twist beam is a very cheap way of keeping the rear wheels joined to the car but it has a major problem with compliance oversteer which is controlled by the two bushings. Most modern small cars now have these bushes orientated differently to try to counter act it but you can't cure it. On track it is better to make this bush as stiff as you can. But it would be better in rubber.
So my question of why it is used is I think answered by the fact that they are much cheaper to make than rubber bushes. A rubber bush needs a lot of tooling and manufacturing cost. If you do feel the need to make a bush stiffer then they are the only option.
So poly bushes are great right? I'm not part of the motorsport scene so I don't understand why they've become so popular. They seem to be sold on the pretence of being stiffer so the car is tighter and that must of course be better…
The merit of poly bushes has to start with the material, which is polyurethane, and how it compares to rubber. The base material offers better isolation properties than rubber because of its damping characteristics. Rubber increases its stiffness dramatically the faster you push it, or vibrate it, but it also has comparatively low damping for large, slow amplitudes. Polyurethane is better for this so controls larger amplitudes better and also transmits less vibration at higher frequencies. As a material it would be ideal to be used in place of rubber as it gives the option for improved isolation. This leads me to a question: if the advantages in the material are all about vibration and isolation then why is it sold as a stiff race upgrade?
Polyurethane is also compressible where as rubber is not. By this I mean its volume gets smaller where as rubber just changes shape when you squash it. It contains tiny air bubbles that allow this and the amount is measured by the density of the material. So stiffer polyurethane contains less air and would be of a higher density. Poly bushes as they are designed are very stiff in a material sense I think, almost as though they have no air.
So as a material everything seems positive but it has a rather fundamental flaw in that it can't be bonded to anything. This means it can only give stiffness in one direction —a when you squash it. Rubber on the other hand bonds to metal quite readily so it can give stiffness in both compression and in shear (when you try to push it sideways off a bit of metal it's stuck to). This is normally in the ratio of 5 to 1 I think off the top of my head, it's five times stiffer in compression than shear. But this can be manipulated so you can play tricks with metal form both for the inner and outer metals of the bush to control the stiffness as you want. Rubber bushes also give stiffness in a rotational sense as they are acting in shear in that direction too.
1. poly bushes can't provide rotational stiffness. Take a car and replace all the rubber with poly bushes and its wheel rates will be softer. Rubber bushes contribute on average about 4N/mm at the wheel I reckon, this can differ wildly depending on platform though. 2. poly bushes can't rely on shear to provide stiffness axially (along the direction of the bolt if you get what I mean) so have to have material around the sides that is compressed too. 3. poly bushes can't do either of these things yet the suspension still needs to rotate against the compressed material so you get lots of friction. Well you can just fill that full of grease —a that will last for ever. 4. poly bushes can't provide the same amount of pre-compression as rubber because of this friction. By this I mean squeezing an amount of material into a small hole —a it could mean poly bushes are actually initially quite soft and need load to stiffen up 5. depending on the platform you could be completely mucking up the suspension dynamics in terms of compliance steers and so on. The bmw suspension is a very good example as the rear trailing bush controls the amount of rear steer you get, fitting a poly bush changes this so the car could actually build lateral acceleration slower than it did before.
So in my opinion the technology is completely flawed and as an engineer I'm not comfortable with that. Certainly on the more advanced platforms where the wheel compliance behaviours are carefully controlled you could be making the car worse. And it will have more vibration as the material is so stiff.
You'll only find polyurethane in two places on most production cars (well three on Jaguars…) and that is for bumpstops and spring isolators as both suit the material properties perfectly.
There are a lot of people that have seen a benefit (possibly because they've replaced a knackered rubber bush with a poly bush —a how many have replaced with a new rubber bush?) and that's because some cars would suit a stiff bush for the track. Any car with a twist beam rear axle like a mk2 Golf for example. The twist beam is a very cheap way of keeping the rear wheels joined to the car but it has a major problem with compliance oversteer which is controlled by the two bushings. Most modern small cars now have these bushes orientated differently to try to counter act it but you can't cure it. On track it is better to make this bush as stiff as you can. But it would be better in rubber.
So my question of why it is used is I think answered by the fact that they are much cheaper to make than rubber bushes. A rubber bush needs a lot of tooling and manufacturing cost. If you do feel the need to make a bush stiffer then they are the only option.