One-piece alloy wheels can be either cast, or forged and milled from a billet. The latter process supposedly makes for a slightly denser and stronger structure and eliminates any bubbles that may have formed during casting; they're also more expensive.
But in every day, real-world use is there really any advantage? Forged wheels are generally more expensive than cast (separate question), so is the extra expensive really worth it? I'm kind of thinking that a well-make cast wheel is every bit as good as a forged wheel in every day use.
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Forged are RACING. :)– JoErNanOOct 13, 2016 at 17:49
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I know this wasn't your question, but as with almost all things related to vehicle performance, the answer is tires. What Paulster mentioned about decreasing rotating mass definitely rings true, so long as your tires have the traction to put that engine (or braking) power to the pavement.– MooseLuciferOct 13, 2016 at 19:31
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Fun fact: Google Translate thinks that stock steel disks are forged too.– DaerdemandtOct 14, 2016 at 1:44
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2 Answers
I think bubbles are less likely to have anything to do with it. The forging process makes the metal more durable by aligning the grain of the metal. After forging, there is less of a chance for stress risers to occur in the metal. This means it will stand up to a lot more abuse than you'd see a cast wheel stand up to. The forging cut from billet, can then be made lighter to handle the same stresses which a cast wheel could take.
Why is this important? A wheel makes up part of the weight on the car considered "unsprung weight". This is basically all the weight located below the springs on a car. In addition to it being unsprung weight, the wheel also is a rotating mass as you travel down the road. Due to both of these factors, every pound of wheel/tire weight equates to about 10 pounds of vehicle weight. If you could take a total of 10 lbs off of the wheel/tires, it would be like taking a 100 lbs out of the trunk. A while back, Centerline Wheels did this with their ConvoPro line of wheels, making them much lighter (I'm not sure if they are cast, forged, or what ... but they are lighter).
Now, with that said, does this really matter for real world use? Probably not. If you are a racer looking to shed an extra 100 lbs (or the equivalent of 100 lbs) from your car, this is a great place to look for that reduction. As with all things, it comes at a cost. The pocket book is where you're going to take it when you go this route, as you've already suggested.
ADDITIONAL INFORMATION:
I did some further research about the difference in sprung weight vs unsprung weight. On Centerline's website, they have a FAQ question which spells this out. I'll take their answer to be authoritative. The answer is a little different than what I have above, so I'll cut and paste it here:
Q: Why is the weight of wheels important?
A: Wheels are rotating mass/unsprung weight. Every 1 lb reduction in rotating mass is equivalent to an approximately 8 lb reduction in static weight. With lighter weight wheels, you will benefit from increased fuel savings, quicker braking, improved tire wear and better acceleration. Always ask about the weight of wheels. Your vehicle is not engineered for heavy wheels. Therefore, it will not perform well if you effectively put four anchors on it!
answered Oct 13, 2016 at 17:52
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1"every pound of wheel/tire weight equates to about 10 pounds of vehicle weight" just curious, do you have any reference for how this effect is calculated?– LukeOct 13, 2016 at 19:44
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@Luke - I'm looking for what I read back when Centerline created the ConvoPro wheels, but here's an article in HRM about what you're asking. Dropping 82lbs of wheel weight gained them over a 1/10th in the quarter. That's real world results. If I find exactly what I'm talking about, I'll share it up.– Pᴀᴜʟsᴛᴇʀ2 ♦Oct 13, 2016 at 20:58
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The conclusion from both those articles is that lighter wheels = faster acceleration, which I accept. But I was more interested in the claim that reducing the wheel weight has a 10x greater affect than reducing the sprung weight. The first article you link draws a parallel to the "100lbs of sprung weight = 0.1 lower E.T." accepted theory but then says that theory doesn't always hold up, and even if it did, the results they obtained indicate a 74lb drop in wheel weight would be equivalent to 100lb of sprung weight (-0.1 E.T.).– LukeOct 14, 2016 at 14:49
For the average user, the answer is no. There have been tests done to determine which one actually works better, and it was determined that the forged wheels did provide for better acceleration because of being lighter and denser than the cast wheels. However, in the real world, there is very little if any difference.
At the conclusion of the test results, you find these words:
So is there a best wheel? In theory, yes, but in the real world the difference comes down to your piggy bank. We like the Volk RE-30 for its uncompromised engineering and the SSR Type-F for its balance between price and function. But who wouldn’t like them if we could all afford them. If you had a dedicated track car that sees plenty of trackside curbing, potholes and off-roading, maybe the AME or 5Zigen is a better fit since there’s a high likelihood of banging them up. But a stronger wheel also might mean the difference between finishing a lap after bending as opposed to being towed off. And if you just need to look good and get more rubber under your first daily driven tuner ride, then the AXIS wheel might be the right bang for your buck. In the end, while the theory tells us the über-strong, expensive and light wheels will perform better, our real-world tests and wallets beg the question, How much is this margin worth to you? Our only regret was that we didn’t get to destroy some nice new pretty wheels. We’re sure the difference in price would have easily been justified by that test.
[Emphasis added]
answered Oct 13, 2016 at 17:45
