Alloy wheels
The ultimate guide to maintaining and repairing alloys.
Offering enhanced looks and a touch of motorsport glamour, aluminium ‘alloy’ wheel rims are fitted to almost every new car. However their more delicate nature, compared to the structurally-efficient steel rim, means that harm from corrosion and impact have become major drawbacks. Even so, you can easily take steps to reduce the risk of damage, conduct some work yourself, or even replace the wheel completely to offer your car a more bespoke look.
The basics
Regardless of the material from which they are made, a car wheel is not just there to provide a link between the wheel hub and the tyre. They transmit the vital – and often considerable – forces involved in acceleration, braking and cornering, as well as transferring heat from the brakes, without weakening. While the strong pressed-steel wheel took over from the horsedrawn cart’s original wood-spoked rim in the early days of motoring, cast aluminium alloy wheels started to appear on sporting models of production cars from the late 1960s, offering a lighter alternative that reduced the car’s unsprung weight, providing a technical benefit by their installation. Provided that the rim can fulfil this basic requirement reliably, then secondary advantages, such as cost and visual appeal, can also be considered.
Race on Sunday, sell on Monday
As with many advances in automotive development, motorsports led the way. While the original die-cast magnesium ‘mag’ wheels, which appeared first in the 1930s, were extremely strong and
light, they were
expensive and prone to corrosion. A further problem was their tendency to catch fire in an impact and, as many O LEVEL/GCSE students will testify, magnesium is flammable and burns with a very bright flame at several thousand degrees Celsius, making a vehicle fire even more difficult to extinguish. Despite this, magnesium alloy wheels were developed further and were fitted to high-end motorsport exotica, especially during the 1950s and 1960s, creating an aura of desirability. Some companies still produce magnesium alloy wheels and have solved many of their traditional disadvantages but their reconditioning is excluded from this feature.
Despite being heavier and weaker, cast aluminium alloy wheels looked very similar to magnesium alloys, but could be made cheaply enough to enter the realms of the average motorist, who wished to ditch their boring ‘steels’ for more glamorous ‘alloys’. This led to rapid growth within the aftermarket scene, although every car-maker started to style their own Original Equipment (OE) wheels and fit them to the majority of their models.
The latest alternative is a milled alloy wheel that is fitted to certain high-end BMWS, which requires even more highly specialised repair methods.
What goes wrong?
While pressed steel wheels are stronger than alloys, good quality aluminium alloy flexes ever so slightly. Should the wheel strike a deep pothole, or even a speed-hump too quickly, it might be damaged but not enough to cause a sudden deflation. However, BMW proved recently that counterfeit rims shatter under the same forces that the company’s genuine wheels would withstand, so be on your guard if buying wheels unseen from a spurious online source on an auction/marketplace site. Be aware that, should your car be fitted with run-flat and/or very lowprofile tyres, the stiff sidewalls will transfer higher forces into the metal rim, increasing the risk of damage. As alloy compositions are more brittle than an equivalent steel rim, cracks are more likely to develop. Rim flat-spots also stress other parts of the wheel and a crack might open-up in a different area. A further problem can occur should the alloy blend be porous, which would allow air pressure to leak.
According to Darren Gardner, managing director of JP Alloys of Cannock in Staffordshire and our technical advisor for this feature, corrosion poses a greater problem than impact damage, even after the worsening state of British roads is considered. He explains that penetration of the protective clear-coat lacquer can be caused by many factors. While kerbing will scratch and gouge the paint, damage that is caused by a careless tyre-fitter can also chip the lacquer around the centre cap and tyre valve, resulting in white tentacles of corrosion that spreadout over time. Road salt accelerates the corrosion process, as does hot dust and metal particles from disc brakes which can become embedded in the finish. Thus, while keeping alloy wheels clean is critical maintenance, certain wheel cleansers, especially those that are acidbased, can damage the clear-coat. Some independent hand-wash attendants use brick acid to clean alloy wheels, which can make a car-cleaning bargain rather more expensively sobering.
Damage that occurs behind the wheel rim, out of sight, can also result in corrosion spreading to the front face.
Corrosion poses further problems for split rims, where the central hub section is bolted in place. Due to the titanium bolts running in an alloy thread, they can seize in place and break, when any attempt is made to unscrew them.
Even so, Darren reports that premium car brands do not guarantee a higher standard of alloy wheel finish – the majority of them, in his experience, seem to be finished to very similar quality standards. The exception rests with Asian car-makers, which appear to use a very thin grade of lacquer that cannot protect the metal sufficiently, when exposed to typical European winter conditions. However, he advises that car manufacturers have been more sympathetic in honouring warranty claims on corroded alloy wheels in recent years.
The repair process
Darren’s background as a body repair technician leads him to conclude that many drivers perceive that alloy wheel repairs are the preserve of a traditional bodyshop. However, he advises that readers should be aware about the three tiers of repair, the first of which resides with a typical smart repairer, who can rectify very minor damage, often on the owner’s premises. A visit to a typical bodyshop is not quite as convenient, although some outfits can strip, flat, fill, re-colour and re-lacquer the rim without having to dismount the tyre. Should you be on a severely restricted budget, you can carry out the main procedures at home and get a bodyshop to apply the final lacquer coats, although this increases the risk of a paint reaction and the need for the wheels to be stripped down again (Rob Marshall tried this himself and the result can be seen in Our Cars on page 90).
The third technique is the least convenient, but offers superior longevity, and is demonstrated by JP Alloys over the following pages. The bare rim is stripped completely of all coatings, prior to being painted with a polyester primer and water-based colour coats, before an acrylic lacquer is applied. The requirement to have the rims heated to temperatures in excess of 200°C means that typical bodyshop methods, which utilise plastic, or metal, resins to fill holes and corrosion pits, is unsuitable, because they will not survive the intense heat. Should any metal be filled, it must be done by welding.
The entire procedure can take as long as four days, but the end result will be wheels that are likely to be better protected than when they emerged from the factory, as the following steps demonstrate.
Remove the wheels annually and inspect them for flat spots and cracks. Clean away any brake dust and check for corrosion that could spread to the front face, or towards the edge, over the coming year.
1 With the wheel removed, a brief inspection of the inner and outer rim will give an idea about whether, or not, the rim requires cosmetic, or structural repairs. It could even be condemned at this point.
2 While smart repairers can carry out minor repairs with the tyre fitted, a comprehensive refurbishment dictates that the rubber is removed from the rim. The technician can then inspect the inner surfaces.
3 Storing tyres correctly is essential. They must be supported upright and not stacked, which could cause the tyres to deform, making them harder to inflate later. They also need to be identified so that they can be refitted to the same rim correctly.
4 Any impact damage, which results in run-out and wobble, can be confirmed by mounting the alloy rim squarely to a balancing machine that spins the wheel, making any distortion obvious.
5 While conventional tyre valves 5 are removed and discarded, expensive Tyre Pressure Monitoring System valves must be removed carefully, assessed for condition and stored safely, ready for later refitting.
6 For any large alloy wheel 6 reconditioning company, identifying the wheel is essential. JP Alloys does this by engraving a number close to the valve aperture, so that the wheel’s progress is monitored and tracked continually.
7 The base colour coat, lacquer 7 and any powder-coat layers are stripped chemically, with the alloy wheels being immersed in a warm, non-acidic bath of solution for approximately seven hours.
8 Some wheels require longer 8 stripping times than others, particularly those that have received previous repairs and have several layers of paint. The alloy pictured will require several treatments.
Several checks each day of the paint-removing bath are mandatory, including measuring the solution’s temperature. When the wheels are ready, they are raised out of the solution so that any excess drips back into the tank.
The wheels are removed from their cage by hand and rinsed with a hot water pressure-spray. They are then dried and inspected again for structural damage and to ensure that the IDS (see inset pic) are intact (see Step 6).
On its return from the paint-removing bath, the centre spigot is cleaned with a rotary wire bush. This is important because the spigot is not painted. If it were, the wheel would not locate squarely and evenly on the hub, posing a safety risk.
With the wheel mounted securely on a jig, a needle is placed on the top of the rim, prior to the operator rotating the wheel slowly. Should a gap appear between the needle and rim, the alloy is buckled.
The engineer marks-out the affected area as a reference point, before rotating the wheel through 360° twice, in order to recheck the marks indicating the flat-spots. Note the gap between the needle and rim.
Each affected area is heated with a blowlamp to 65°C, verified with a digital thermometer, which softens the material. The wheel used here has not had its paint removed, yet.
A dedicated hydraulic jack is used to push out the flat spot and restore the wheel’s uniformity. Care has to be taken not to apply too much pressure, which could distort, or even crack, the alloy.
In some cases, a very heavy impact not only damages the wheel but also flattens the edge. Thankfully, the wheel’s extremity can be restored to its original shape by applying some more heat and repositioning the jack.
At the time of our visit, JP Alloys was developing an in-house welding facility, so that structural cracks in alloy wheels can be repaired and deep gouges filled with weld to be ground flat, as pictured.
Light kerbing marks and gouges can be concealed by removing the surrounding material to a level that is equal to that of the deepest hollow. First, a body-file is used to get rid of the excess material.
Scratches left by the body-file are smoothed-out by using progressively less abrasive grades of sandpaper, starting at 240-grit, progressing to 320-grit and, finally, 500-grit.
To remove any exposed corrosion (inset pic), the straightened wheel is placed within a blast cabinet and shot-blasted with fine particles that will not pit the soft alloy material excessively.
Any cosmetic kerbing damage on diamond-cut external surfaces is assessed but not rectified at this stage. If the damage is not too deep, it can be dealt with at the diamond-cutting stage (see from Step 28).
The wheels are loaded onto the stoving track, prior to any dust being removed with compressed air. The rims are pre-heated to 208°C, before being painted with…
…several coats of polyester primer, applied to the hot, bare metal. After cooling, the 60°C surface temperature achieves a very strong bond between the high-build coating and the alloy material.
As corrosion can spread to the alloy wheel face from the back and the lip, these areas are painted to enhance the life of the finish. The painter can spin the wheel by hand, to ensure even coverage (see Step 36).
The wheel then re-enters the main oven and the applied primer is baked to 218°C. Once they emerge from the oven, every rim is inspected carefully for any defects.
Corrosion pits that remain (unlike steel, this will not cause the coating to be compromised) can cause flaws to become evident in the dried primer, which needs flatting-out with 500-grit wet-and-dry.
The wheel is then painted in the appropriate base colour. JP Alloys uses a water-based colour coat, which is applied inside and out. If the wheel is not diamond-cut, it will then be lacquered (see from Step 37).
Certain alloy faces are diamond-cut, to give a near-mirror finish. This is achieved on a specialist lathe and every pattern will have its own set coordinates – an archive of which is on JP Alloys’ server…
…and can be transferred onto the diamond-cut lathe’s computer. The wheel in question has to be mounted with utmost accuracy, so that the blade can remove the precise quantity of material.
With the wheel spinning, the robotic arm moves across the wheel’s face. Several passes are made, with the diamondcoated blade removing tens of microns' worth of material at a time.
The operator stops the machine several times to inspect the process; any error at this stage could ruin the wheel. The deepest cut is made on the edges, which should be sufficient to remove most kerbing damage.
It is common for tiny chips to emerge during the diamond-cutting process, spoiling the edge of the alloy. The operative not only looks for this but also checks that any impact damage has been repaired.
Just prior to the final cut, which removes 20 microns from the surface – a human hair is around 100 microns thick! – any minor chipping is removed using sandpaper to retain the spokes’ sharp edges.
This wheel has a flecksparkle effect and special paint is applied with a lowpressure spray gun after the final cut is completed. The wheel is then dried using a hot-air gun.
When the sparkles have dried and the wheel is dismounted from the diamondcut lathe, the sparkles refract light to display a prismatic range of colours. However, this light coating alone offers insufficient protection.
Corrosion protection is provided by the final clear-coat applications. Some shops hang wheels by a metal hook, but this introduces a spot for moisture and corrosion to enter, so JP Alloys uses a rotating centre stand.
The wheel is pre-heated to 178°C, prior to two coats of acrylic lacquer being applied to the front and rear faces. The wheel re-enters the oven and is heated to 208°C.
Once the wheels exit the oven, they cool naturally and undergo final qualitycontrol inspections. Either new tyre valves are installed or the original TPMS valves are refitted, using a service kit, if required.
The rims are reunited with their original tyres and balanced. JP Alloys also fits the rims to cars but recommends that the fixings are retorqued after several days have elapsed.