POLYURETHANE BUSHES

POLYURETHANE BUSHES

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, and 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, and 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 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 and these are considerably higher than the manufacturer’s 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

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.

Assassin (Away on business)

thanks for that - interesting - I had been wondering myself as i already have bushes "going" at 28k! So now my next challenge will be to find Poly bushes for a Terios!

Blimey, some of those are bigger than a Terios!

Tom Mc wrote:

Blimey, some of those are bigger than a Terios!

I looked at getting some SuperPro bushes about 12 months ago for my truck, the price was not too bad, a little bit more expensive than original equipment but what put me off was I needed someone to install them as I did not have the tools or the time, and that's when the £££££ starts to add up.
A complete kit for Terracan (shogun mk2) came around £200 I think, & It would have supposedly taken 20+ Hours for a mechanic to fit and replace the bushed costing around £400.

If I looked to do it again, I would probably look to do the works myself and probably an axle at a time.


On a TD5 Discovery I owned in my earler years it had a set of bright orange bushes added just before I purchased the vehicle, I cant recall what brand or make, but they did stiffen up the ride and it cornered alot better than any previous discovery I had driven. Also looked nice, orange bushes on black waxoyl chassis.

Jas

They are an ideal job for the DIY or novice as installing them is basic common sense and requires little knowledge, and very few basic tools.

In my view polybushes are a very bad idea for serious overlanding. Unless you have driven gravel tracks or stony regs you have no idea of the pounding the suspension takes. Polybushes can split and be immediately useless. They are rigid and pass all the severe shock loading from the track straight to the steering and suspension components which were not designed for such increased loads. When rubber bushes fail they do so slowly rather than catastrophically. If you are heading out into sandy, as opposed to stony desert, just make sure your rubber bushes are in reasonable condition, as sand gets into any small splits or lesions and acts like abrasive, hastening the wear. Leave polybushes for the kind of tarmac only show offs who drive, and look ridiculous in, Range Rovers with cotton reel rims.

This is totally incorrect, each suspension bush, or indeed any other rubber busg has a "Shore rating" which is the rating for compliance and flexibility for any flexible mounting and not just suspension bushes, if you replace the rubber bushes with poly bushes from a reputable supplier, and specify OE ratings they will be exactly the same.

Problems arise from a couple of areas, mainly the cheaper component suppliers who supply a set of poly bushes which are cheap and all have the same shore rating to people who don't understand this, they fit them and sometimes get several problems.
Many people deliberately fit uprated bushes, and many go too hard, in this instance it can also lead to problems.

Splitting bushes? never seen any poly bushes split, seen them chewed up through incorrect fitting, serious off road damage which could happen to any type of bush, and even fall out when they have been cut to fit rather than being fitted correctly.

Im under the opinion like with most standard equipment to leave it well alone.
The Manufactures don't spend Millions on R&D and supply naff Components.
When I first bough my Terracan I modded it lots. At the Time of sale, apart from the snorkel and uprated Dampers it was standard. The Standard components do work quite well IMO, are cheaper to replace and you defiantly know that It will work on your vehicle.

Jas

This is why I like to stick with the manufacturers original Shore ratings when I fit poly bushes, you are right about manufacturers spending £££Millions on R&D, but they also compromise in areas such as durability versus price, often price wins.

Compromises such as fitting rubber bushes are always down to price as rubber bushes are cheaper to manufacture, thus cheaper to vehicle manufacturers; if a component is £1 cheaper than a competitors and you build 1 million vehicles a year its £1M saved. Imagine this with all components on a car, procurement in the motor industry is all about where a compromise can be made and savings made.

Many vehicles, particularly commerical vehicles such as trucks or buses are having Poly bushes fitted as standard because operating time and down time is an important criteria for an operator, imagine a bus having to be off the road for one day to replace worn bushes, this costs in time and spares; but also in lost revenue and operating costs for vehicles are tighter than ever before. Our new trucks all have Poly bushes fitted as standard because they are more reliable, and I asked our supplier if they were uprated, only to be told NO, they were the manufacturers original shore rating.
Since out trucks came with Poly bushes fitted as standard by the manufacturer we have never had an issue with them, traditional rubber bushes would normally be replaced every two years in their operation, so a significant advantage to us as operators in time and costs.

Thanks for your detailed briefing Assassin. If I do decide to go polybush I will get you to fit them, since you clearly know what you are talking about.

I am always wary of any vehicle modifications. I would never go into the bush for instance with an engine that had been chipped. Standard tune is the most reliable. As a pilot you are taught always to operate your engine within the manufacturer's operating parameters - anything else will get you in trouble at some point down the line. Even suspension can be overdone, unless the mountings are reinforced at the same time.

I have seen a split poly on a Defender. Admittedly after several hours of terrain that would test most suspension to destruction. (The leaf springs on a new Hilux went over centre on the same track.) This was some time ago, so maybe things have improved or maybe it was the wrong rating bush, or the vehicle was over specified Gross Weight (another parameter you step outside at your peril.)

I can quite see why bus and truck operators would use them. But that's on tarmac; have Toyota started fitting them as standard on Landcruisers destined for Australia or Africa? (I don't know, but it would be interesting to find out.) My Patrol has been on the current set of bushes for quite a few years. Took it down into Western Sahara last October and no problem. They will need replacing soon, but they've done well, although admittedly I don't use it much in between overseas trips.

It would be interesting to hear from anyone who has used them for long overlanding trips.

thanks again for the useful advice

They have been used extensively on many off road vehicles such as heavy plant, and this is where they originally came from for use in 4X4's, and this is what tested them to their limits in pure off road conditions of all types ranging from mud to pure abrasive rocks.

Such machines are never used on roads, and it as predominantly the mining and quarrying industry machines which originally had them fitted as these conditions encountered in places such as lime quarries totally destroyed rubber bushes, not just suspension, cab mounting bushes, engine mountings, and even flecxible rubber mountings were destroyed in very short periods. To counter this they moved away from rubber to synthetic coumpunds such as nitriles which were much more resistant, but lacked the flexibility of runner to suppress vibrations, and then onto butyl compounds which were more flexible, but inconsistent in their shore ratings, then came polyurethane. Poly bushes could achieve the correct shore ratings and resist the conditions, and proved to have a very long lifespan.

Engines are a different issue, they have what are called duty cycles, the duty cycle is when the engine is working at its maximum power for a prescribed amount of time. Basically an automotive engine works at full power for less than 10% of its time due to speed limits, etc, and the only time it is at full throttle is usually when a vehicle accelerates at full power away from a standing start or for a 4X4 it would be constant hill climbing for sustained periods.

If we take an actual example of a Cummins diesel engine its automotive duty cycle offers us up to 325 BHP from that rating, if we take the same engine and look at its industrial duty cycle which is the engine running at full power, such as on a generator, its maximum power rating is 150 BHP, less than half its automotive duty cycle rating. Chipping is not an issue in itself if its done correctly, automotive engines are a compromise between power, torque, durability, and emissions, often its beneficial for many diesel powered vehicles to be chipped for torque as all its actually doing is altering the parameters slightly, but keeps them well within the designed duty cycle of its application.
Where chipping comes unstuck is when people chip for maximum power, BHP in simple terms is Torque X Revs, if you increase one or the other it theoretically increases the BHP, but you feel no difference in driving as BHP is a theoretical figure designed solely for comparison.

If we look at this in action we see the old Mini 1000, it came with a single carburettor in manual transmission configuration, developed 40 BHP and did its job; the automatic version was exactly the same engine which had its maximum engine revs increased from 5250 RPM to 5750 RPM so same engine, but on paper it actually developed 45 BHP as an increase in engine revs increases the maximum calculated figure of Torque X Revs.

When you chip solely for power you get issues, to get maximum torque you don't get as much heat as chipping for maximum power, and heat is the biggest killer of chipped engines which are chipped solely for power, basically the heat developed is much higher than the engine is designed for, for long operating periods, this, along with other issues such as oil problems is what kills most powewr chipped engines.

Aero ratings are different again as they are usually based on an engine operating at three times its maximum power output, and duration. In simple terms a typical helicopter operates with a 190 BHP engine, and for 195 minutes, but its design is actually for an engine capable of operating at 3 X its maximum power, or 570 BHP for a period of 195 minutes, and obviously this is a safety critical area for the designers, which won't be compromised on by anyone.

Thanks once more Assassin for your detailed and knowledgeable briefing - a pleasure to read. I think the real issue in all this is summed up in a phrase you use about chipping - "if fitted correctly". Finding anyone knowledgeable enough to provide really sound advice and equally sound vehicle improvements is not easy. People often want to sell you what sounds good in the publicity or what they have in stock or can easily obtain or what some Australian/South African supplier is pushing as the latest thing. It will be interesting to see if I can get Nissan UK to talk about Shore ratings. I bet they can't do that where the Patrol is concerned. The minute you mention the word Patrol they go into stunned ignorance mode. This is why I am wary of anything other than simple modifications. I know my Patrol is reliable from my own experience; if I did something like chip it I would never really have that level of confidence in it again. And I don't need the extra torque. I have been stuck in dunes just once and it wasn't torque that got me out it was dropping the tyre pressures to 10 psi. Even on rocky mountain tracks I have never felt I needed anything more out of it. As with so many extras, you have to ask yourself "Do I really need it?" Where the polys are concerned I shall seriously reflect on your advice.

Many engines are chipped, but by the manufacturers to achieve higher or lower power output's or higher or lower power output's, particularly as most manufacturers now use "generic" engines which are used across a range of car and light commercial vehicles, and they always incorporate development potential within an engine; but why.

If you are a manufacturer you know saving a penny on a component soon adds up, particularly on high volume vehicles, so why make 10 different engines when you can make only 1 engine to suit all applications, you get quantity discounts from all your component suppliers so vast savings, and you can build one engine instead of ten and you only want one set of tooling instead of ten sets to build these engines. You have already simplified your parts suppliers systems and you can rationalise your aftermarket supplies of consumable and non consumable components for your dealers, meaning they have not got large sums of money tied up in rarely used components which they rarely sell, but have to keep a certain stock level of as main dealers, so a cost saving for the dealers as well.

If we take another example of a BMW 3.0L diesel engine.

This engine is used obviously for BMW vehicles, but its also used for many other applications such as the base engine for marine convertors for marine engines, in marine use its rated from 100 BHP to the newly announced 376 BHP variant in triple turbo form in the newly launched 5 series Motorsport version, this is over 125 BHP per litre for a diesel engine, which is very rare these days for any automotive diesel in production form.
If we compare this to our earlier Cummins 5.9 litre diesel it is way behind the BMW diesel which is half its size and in top power specification still delivers over 50 BHP more, yet any 3.0L 6 cyl diesel engine is going to be cheaper on fuel than a 5.9 litre engine.
BMW designed this engine correctly from the start, they used only quality components and incorporated a considerable development potential into the original design, and now just one engine delivers over twice the power it was originally launched with, and its performance competes with petrol sports cars. Where was this engine developed? here in the UK.

Vanity and fashion is a common problem and trends often dictate people fitting something because its in vogue, yet, fortunately for this site most members are more experienced to fall for this bling type existance, most would sooner not spend money on bling and have an extra weeks expeditioning instead.