What are polyurethane bushes and are they worth the expense of fitting, what are their benefits and pitfalls, and what should I know before investing in a set for my vehicle; all common and reasonable questions as many purchase with little knowledge.
Suspension components, mountings, and many other items on a vehicle have bushes fitted, they serve several purposes, the main one is to provide shock resistance and improve the NVH (noise vibration harshness) of a vehicle. This essentially means they remove a substantial number of intrusions to a vehicle to make it a nicer environment to operate in, they do this by removing vibrations from engines, transmissions, and suspensions; as well as making a vehicle quieter.
Traditional bushes work well in most automotive applications and are generally made from rubber and rubber/synthetic compounds, these are competitively priced, easily installed or replaced, and have a reasonable working life. Vehicle manufacturers specify the rating of these various bushes, this is usually using the shore rating as it is a standard rating for vehicle manufacturers, supply chains, and materials suppliers to work to for reliability and quality. Rubber based components deteriorate over time in general automotive applications, this generally means they wear, many other vehicles bushes are subject to other forms of attack, these are vehicles operating in environments in which rubber is not a suitable material. 4X4’s are a prime example as all rubber bushes dry out over time, but off road vehicles are subject to accelerated wear from the enormous variety of abrasive mediums encountered by 4X4 operating environments as well as petroleum based materials such as oil, diesel, or petrol. Many other automotive fluids attack rubber based bushes, the weather also plays a part as warm weather makes them become soft, cold weather can make them become brittle.
Polyurethane bushes are made from polyurethane, and are often abbreviated to PU bushes, their modern compound is a much better alternative to rubber based products, this is why many commercial vehicle manufacturers are switching to them. They may cost more than a traditional rubber bush, but, when the considerable working life of many times that of rubber bushes are taken into account, they work out considerably cheaper.
PU bushes are resistant to abrasion, vehicle fluids such as oil or petroleum products, and general road dirt such as salt used during the winter months, this makes them the ideal alternative for off road or commercial vehicles. Many people are switching to them only to find their vehicle characteristics suddenly alter, this is mainly due to the fact that their replacement bushes are of the uprated variety, this allows considerably more noise to enter, and the ride suddenly becomes harsh.
Vehicles have a Shore Rating for every flexible mounting, these vary from component to component, as an example the suspension bushes may be a shore rating of 80, the shock absorbers 70, and the anti roll bars may be 100, this is where we encounter our problems. Many suppliers of cheaper bushes use one Shore rating for all their bushes, modified off road vehicles may be fine with this as the ride and noise is not as much of an issue as a vehicle in mixed use, or used predominantly on road as family vehicles. Selecting the correct Shore Rating is the most important consideration when switching to PU bushes, so a decision has to be made as to which characteristics are required for the vehicle, do you want the original ride and noise quality to be maintained. If your answer is yes, you need to find a supplier who supplies replacement bushes to the original equipment specification’s, most quality manufacturers supply these for specific vehicles in sets, to the original manufacturers specifications. Each component has the correct Shore Rating for each individual suspension bush, it is important that they are easily identifiable so they can be correctly fitted to the correct location.
Uprated bushes are available for those looking to fit harder bushes, these are considerably higher than the manufacturers original Shore Rating specifications and will be harsh on a road orientated vehicle. Most reputable manufacturers supply kits which are midway between the original specification bush kits, and the highly rated bushes for pure off road work, this allows road vehicles to trade up to slightly harder bushes.
Many cheaper manufacturers and suppliers will supply standard sized bushes in one Shore Rating, these will be supplied to fit any vehicle that they will fit, these, are to be avoided as they can adversely affect a vehicle characteristics.
PU bushes must be fitted into clean metal, rusty metal with flakes of metal and rust must be cleaned properly prior to installation, they must have any washers fitted to the correct locations, and any central metal tubes fitted where bolts pass through them. Care should be exercised with any Metalastic type original rubber bushes, these are bushes with an outer sleeve of white metal, this, often separates from the rubber and must be removed before installing new PU bushes.
Installing new PU bushes is a straight forward job, easily tackled by the home mechanic as they can be lubricated prior to installation; in cold weather they can also be dropped into hot water to soften them prior to installation.
For most suspension bushes, simply remove the old bush and any white metal sleeve, clean any corroded mating surfaces back to clean metal and coat with copper grease, insert the new bush and lubricate any washers or metal centre tubes with copper grease. Lubricate any bolts with copper grease before fitting, and where PU bushes wrap around components such as anti roll bars, clean and lubricate the mating surfaces.
PU is now being introduced for more arduous applications such as coil spring top and bottom mountings, this is an ideal material for such applications, the same rules apply when installing them in such applications, clean and lubricate.
As a general rule of thumb, these are essentially fit and forget components, the only problems may arise is with vehicles covering significantly high mileages, or actual physical damage from off roading.