LIGHTER WHEELS, HEAVIEF LOADS AT A PRICE
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Alloy wheels are lighter than steel, of course, which boosts payloads. They're also strong and good looking — but virtue doesn't come cheap. We examine the pros and cons.
• What looks pretty and gives you an extra half-tonne of payload? Aluminium wheels of course — but not everyone is convinced, mainly due to a few scare stories in recent years. The aluminium wheel's advantages, include a high strength-to-weight ratio, but its main drawback is cost. For an eight-wheeler, which is the sort of vehicle that can offer the half-tonne payload improvement, the price premium will be around £3,500. Even so, according to your type of operation, aluminium wheels could be cost-effective.
First a few basics, Aluminium wheels are not made of pure aluminium, but from an alloy of the metal with various other substances such as magnesium, which impart both strength and durability. This alloy is also highly resistant to corrosion of all types. Tales of alloy wheels which corrode when used near the sea are usually traced to users of some very early, and inferior, car wheels, or to other non-structural aluminium, which can indeed be affected by salt water.
The only thing that happens with good quality alloys for road use is that the salt on the roads can ruin the highly polished sheen which it is possible to give this metal. The shine can however be restored with some proprietary cleaners. Aluminium alloys are used in marine applications and so stories of salt corrosion are largely fanciful Other myths include: "If it's so light it must be weaker." The alloy is about one-third the density of steel, but depending on the composition it can have a tensile strength of up to 900MN/m2, which is as good as many structural steels.
Alloys which contain zinc and copper as well as magnesium have the highest strengths, in the region of 600MN/m2. They are widely used in the aircraft industry and in military armoured vehicles. They do not catch fire easily, as has been alleged, although this was once a problem with alloys containing a very high proportion of magnesium. indeed, structural aluminium alloys have a far better thermal conductivity than steel, which can help the wheels run cooler, and thus extend brake life.
There are some problems with alloy wheels, but you would have to be pretty determined to find them. For example, any aluminium compound is attacked by alkalis to a greater or lesser extent, so pouring concentrated potassium hydroxide over them would be damaging, and would give off inflammable hydrogen gas. Aluminium also does not like hydrochloric acid either, or the rather more common sulphuric acid which is used in batteries.
These alloys are not new tangled, as some would have you believe. The best known name in the alloy wheel business is Alcoa, which has been making truck wheels in the USA for more than 40 years, and the material itself has a much longer history, having been used as a frame for the Zeppelins!
Two types of alloy wheel are available, forged and cast. Both offer weight savings, and the cast versions are the cheaper of the two. In the UK most of the demand for alloy wheels seems to have been for forged wheels marketed by Alcoa, though Aspect wheels have made some impact in Holland on Dafs. Both makes are offered by Leyland DAF, which quotes the Aspect at 2160 per wheel. The Alcoa wheel is offered in two versions, machine finished or polished — which does look very smart. A polished Alcoa wheel costs up to about £275 per item according to whether it is a line fitting, as with Leyland vehicles, or is retro-fitted as with Dais. A Leyland line-fitted Alcoa wheel costs £210.
The saving in weight for this money can really add up, even with what appears to be a modest amount of about 20-30kg per wheel. A four-wheeler, which of course has a total of six wheels including the twins at the rear, will be 120kg lighter. A vehicle with 22 wheels, such as a 6 x 4 tractor plus tri-axle trailer with twins will get very close to the half-tonne mark. Whether 20kg per wheel is worth around £200 only the operator will know. The obvious candidates are those with bulk loads to transport where every kilogramme counts, such as tankers for fuel and powders, but really any vehicle where payload is important can benefit.
Payload and weight calculations are tricky at the best of times, and there is little sense in having nice weight saving wheels if you are doing half your runs empty, but with most operations payload is the main earning factor. Actually buying the wheel is not, unfortunately, the only expense, however, as the wheels also need special studs in the case of Alcoa (to cope with the thicker nave) and special nuts for the Aspect wheels. For a 4 x 2 vehicle the Alcoa studs are listed by Leyland DAF at 2260, and for the Aspect nuts the same company asks 2175. Going to Aluminium is not a cheap option, despite the benefits, and it is no surprise that the majority of fitments tend to be seen in large fleets. Shell, Ferrymaster, Silcock Express and London Carriers among them.
There are probably few owner-drivers who would consider the expense worthwhile, but one who did was Len Featherstone, who runs a Foden eightlegger. He did it for the strength. Len has been using Alcoa wheels for six years.
His problem arose when in four year's aggregate work he used up eleven rims on his tipper. The wheels had been cracking at the rims, and sometimes "you could hear them go". So he went over to alloy wheels and, he says, has had no further trouble. Since then he has worked on M25 construction which was tough on any vehicle. Apart from a couple of gouges, which were smoothed off fairly easily, the alloy wheels have shown no sign of damage.
II Five-year guarantee
Wheels from Alcoa are now guaranteed for five years, and in case anyone is still in doubt about durability the company points to its winning of the tough German type approval. Those tests were concerned with fatigue properties and rim strength, and the results showed that the durability of forged alloy wheels is very high. One of the advantages of forging the wheels is the consistency of material density throughout the wheel, and the all-over machining makes them very nearly perfectly round — an achievement that is notoriously difficult with most ways of making wheels.
This makes balancing less of a problem with alloy wheels, and many users also claim large increases in tyre life for the same reason. Any true-running tyre should have the same life, irrespective of the type of wheel, but the mass production of conventional steel wheels sometimes leaves a lot to be desired. One user, Lothian Chemicals, quotes a drive axle life of 150,000km on tanker work, where 80,0001an had been the typical life before fitting aluminium.
Servicing of alloy wheels has sometimes been a problem because of tyres sticking to rims, but Alcoa says that this was caused by using the wrong tyre lubricant. A non-water-based product is needed, such as Freylube or Michelin BIB grease. Spigotted wheels can sometimes stick on the hubs too. The solution is a combination of good maintenance and the right grease. Alcoa recommends cleaning the spigot with a wire brush and then applying a non-copper-bearing grease such as Rocol MG. When a wheel is stuck you do have one advantage with aluminium — it expands at a different rate from steel, so it is worth pouring nearly tibiling water on the wheel, which can be enough to unstick it: be careful though.
A specially-adapted puller is usually necessary to allow for the thicker (22mm) nave, and some basic handling care is also needed in the workshop if the aluminium is not to get badly marked. On the plus side, the wheels are a lot easier to handle. Special clip-on alloy balancing weights are available from Alcoa — do not use ordinary steel weights. All Alcoa wheels are spigot-centred with a 281.2mm centre bore diameter.
In the end the decision is a simple one, of whether that extra payload matters enough to pay for it. Pay-back periods of as little as two years are possible, but remember that you also get high strength and low maintenance — you will never need to paint them: they just need washing with clean water or mild detergent. Although the current demand for alloy wheels is not high — under I% of sales according to Leyland DAF — there are areas where the benefits are really beginning to convince operators. A surprising 8% of eight-wheelers ordered from the Anglo-Dutch company are being specified with alloy wheels, and the company is about to put a demonstrator on the road with very fat singles on the rear which is said to offer a full 500kg payload advantage over standard specification.
0 by John Parsons