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Ron Stygar Carl Buckland Dale Beuning Forums Help
From digest.v7.n515 Thu Oct 30 16:25:01 1997
>From: "Sherman, David (NJAOST)" <DSherman_at_NJAOST.ML.com> Forging is the metalworking process in which a slug of metal (usually heated: i.e. hot forging) is forced into the desired shape over a die by the impact of a mating die driven by a machine. The metal of the slug draws in a predictable way, and the resulting grain structure adds strength. This is the industrial equivalent of a blacksmith hammering a red-hot piece of iron to make a horseshoe. Casting (gravity, low pressure, or high pressure) is where molten metal is poured or forced under pressure into a mold to create the desired shape. The metal alloy must be chosen as much for its flow characteristics as its strength. It can be difficult to avoid porosity in the casting from dissolved gases, and contamination from oxidation of the molten metal. However casting complex shapes such as wheels is relatively easier and cheaper than forging. The BMW M3 cast rims have a seemingly well deserved reputation for fragility: they tend to bend in the face of potholes or road obstacles. One of the monthly car mags bent no fewer than 5 of them in the course of their long term test. Several Digest members have reported similar issues. On the other hand, I'm sure that plenty of cast wheel users have never had a problem. The forged wheels are a $1450 upgrade option as I recall, which is a not inconsiderable sum (although you do get 5 of them). Ben Liaw recently reported bending even a forged wheel, but then of course he does live in New Jersey. <g> I have the forged wheels on my 96 M3. They are light in weight, good looking, easier to clean, and have proven strong for me. If you can afford the extra, I'd recommend them.
Neil |
