|Light alloy rims are generally heavier than steel rims|
For a long time, the most important argument for the purchase of aluminium wheel rims, was their low weight. Nowadays, it seems that this is no longer the case, because they are mostly heavier than steel wheel rims, particularly if they are wider and have a larger diameter. Whereby, the saving of weight, in particular with the wheels, makes the most sense, not only must less weight be moved on the road, but also when accelerating, less effort is needed to turn the wheels. Apart from anything else, a lower unsprung mass is very important for a good chassis.
|Forged rims have an advantage and are reasonably priced|
Apart from die-cast wheel rims there are also forged rims. They were first developed around the time of the Porsche 911. Due to the fact that forged alloy wheels nowadays, are no longer as expensive as they were in the past, their advantages are available for more car drivers. Their extraordinary rigidity allows a more slender construction method and lower weight than with comparable steel wheel rims. Nevertheless, advantages are still maintained with the rigidity, yield strength and toughness. Let's have a look at the process of both production methods.
|Various die-casting forms and demands on the alloy|
All aluminium wheels are a result of gravity die-casting at approx. 750°C. Should the wheel be produced by forging, the mould is shallow and resembles the final product only in its diameter. If it is only cast, the mould is produced precisely according to the calculated form developed in the computer. Die-cast wheels have mostly, only a very low amount of foreign metals (strontium, titanium), forged wheels are alloyed much stronger with magnesium, silicon and titanium. As a rule the low pressure casting, with just 1 bar of pressure, occurs from below, upwards. After being released from the mould, the still hot moulding blank must have the burr removed, and be X-rayed thoroughly. At this point, it can still - without having caused high labour costs, be remelted again - with only relatively small damage to the environment.
|Centering, drillings, coatings|
There now follows the usual fully automatic treatment, by which the exact centering, the drillings for the wheel studs, the valve hole and the remaining wheel rim geometry are carried out. Finally the quality is once again checked, important here, e.g., is possible imbalance and the concentricity. Following this, is the relatively complicated coating and degreasing process and the application of the etch-primer and the undercoat. Whereby, a process is applied by laying on a DC voltage, similar to the process used when re-spraying a vehicle. Finally, the mostly silver coloured paint and the clear-varnish are applied, one shortly after the other, in the so called wet-on-wet method. In between, in each case, are drying processes which, at approx. 200°C, are a little hotter than those in the spraying of vehicles.
|Rim-bowl by press-moulding, rim-well by roller-milling|
The forging of the wheel rims is a little more complicated. In this case, the blank, made from a special forging alloy, is pressed, step by step, at approx. 500°C and with up to 2000 tons of pressure force into a form which is not however, as versatile as that used in die-casting. At this point, by the way, the drillings also occur. The actual rim bowl is now finished, the rim-well however, is only now produced by roller-milling. It is important that the temperature is maintained throughout the entire manufacturing process and the solution annealing at the end. The following sliver removal treatment (burnishing) and spray-painting is, once again, similar to that in the die-cast rim production. The lower weight through less wall thickness is decisive. Besides, the rigidity, if anything, has been increased.
Certain metallic components - also wheel rims - can look as if they are chrome plated and have however, only been polished. 05/07