Suspension bushing thoughts

[scalino65]

Cheap Casting Grade Polyurethane - all the wrong properties to do the job of a suspension bush.

Try PU bushes on a lightweight car if you want to find out how they ruin ride characteristics.

I fitted a set of Powerflex Bushes to a Lotus even a few years ago and it shook like a wet dog. kept them in place for a week and threw them away.

The real problem with PU is that the casting grades behave very much like an elastic solid and have very limited visco-elasticty at even modest frequency.

This means their damping characteristics are poor which is evident

The frequency which creates a glass transition of a typical rubber bush is around 10^6 Hz which means that an NBR bush will provide good damping across the range of frequencies normally found in suspension components.

PU will behave like an elastic solid well within the frequency range found in most suspension systems so doesn't really achieve the basic requirements of a well engineered bush.

You have to ask why the Automotive industry spend billions of Pounds every year on NVH work and sticks to materials such as NBR and HNBR for virtually all production cars.

The PU bush industry relies on cheap casting grade materials manufactured at room temperature in cheap tooling and I believe the materials used have been selected for this capability rather than the properties that are required for a suspension bush.

If you are only interested in the basic stiffness of the bush then they may do the job but then you have to muck around with grease because the sliding friction is so dreadful.

Nylon is poor because it swells up and binds when wet but there are many other engineering Polymers that have very low friction and work well

We have been using Tivar 1000 for about 10 years now and we don't have to grease bushes as the sliding friction levels are very low and they align suspension even more effectively than PU.

Many thanks for this, its very interesting. Sorry to be thick but does that mean for a 911 you'd stick with Porsche standard stuff? Can you get Porsche bushings in this Tivar stuff ? Whats the standard 911 busing made of anyway- NBR / HNBR?
cheers
Will

[fetuhoe]

We only use TIVAR 1000 on Track Day cars which will be driven hard and some increase in transmitted vibration is acceptable.

The standard bushes are NBR and are around 60/65 Shore A as standard but the design is quite smart.

The front control arm bushes are not bonded but 'compressed into place. The Radial Compression used and significantly to the static stiffness which helps with alignment but doesn't reduce the damping capacity.

They are quite a trick to fit but we have made a hydraulic jig to make it easier.

It is possible to buy these bushes in a harder material from either Rennline or ER. Rennline say that their bush is HNBR which is slightkly more dense but I think they use the same manufacturer as ER.

Spring Plate bushes weren't bonded until 1968 and are also available in an uprated NBR.

The Neatrix bush seems about the same hardness as a standard bush but I don't think they last as well as a factory supplied part.

Trailing arm inner bushes are also available in an uprated NBR and these work quite well or can be changed for a Monoball.

We have made trailing arm Monoballs for years and they work well and don't have too much influence on NVH.

We make the housing from 6AL4V Titanium, the bearing is a high quality NMB unit, they are weather seals and fitted using a Smalley's Spirolox clip rather than a circlip.



Depending on the age of the car the front camber plate bushes are also able to be upgraded.

We make new harder than standard rubber bushes for the camber plates used on a 1965-1970 car bit not for later cars.

Engine and Transmission mounts also have an impact on gearchange and we used an uprated mount from a Cabriolet.

We buy them from Hamburg Technic and have been using them for about 10 years.

We plan to introduce a complete range of HNBR Bushes by Spring this year.

[911hillclimber]

All this may be why Bob said for the road and hill climbs the stock rubber bushes were perfect...
I put plastic bushes everywhere in my 360 bhp Impreza, squeezed like mad at the front, nowhere else.

[fetuhoe]

We use Tivar 1000 for several reason

We initially tried a Graphite Loaded Nylatron and custom machined the bushes to fit. Initially they worked well but started to wear quite quickly - we put this down to volume change when they were damp.

We then moved to Delrin which is virtually impervious to water and which works quite well. The problem is that this material has absolutely no 'give' so if you make a batch of bushes the tolerance on the steel parts means some bushes are a tight fit and others can be loose.

Custom fitting is not a sensible option as it makes supplying them to anyone without a lathe quite a problem.

Tivar 1000 is also impervious to water but unlike Delrin will undergo a small initial deformation when compressed.

The degree of stress relaxation that occurs is repeatable and also produces a slight hardening of the surface skin of the material.

We selected this material after discussions with the engineering team at Quadrant Plastics.

This means we can make the bush with a small initial interference fit and after a very short time the bush develops a small ‘set’ and operates smoothly with minimal clearance.

The only lubrication needed is very light smear of assembly grease during initial build up.

We made the first set of these bushes about 8 years ago and the car with them fitted has just come back to our workshop and they are still in good order.

They also have a very small amount of damping and the brittle transition temperature is around -150 degC.

There is a new variation on this material with a higher density and we have made an initial trial set of bushes to see if it offers any benefit.