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[ANSWERED] Conventional 5 lug nut wheels vs. forged center lock wheels

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[ANSWERED] Conventional 5 lug nut wheels vs. forged center lock wheels

 
Old 05-18-2018, 10:59 PM
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jvp
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Default [ANSWERED] Conventional 5 lug nut wheels vs. forged center lock wheels

The original question is here.

ltomn asked:
Knowing that there are a few sports car manufacturers that utilize center lock, forged wheels versus conventional, 5 lug nut wheels, please give us the positive, or negative, reasons for using or not using both wheel types? Porsche does have a center lock, forged wheel option on some of their models that are sub 150K. Since Corvette is approaching 150K with the ZR1 would Corvette consider forged, center lock wheels on it, or any of the Corvette models, as a option? Further, since the ZR1 and ZO6 have massive, broad torque bands, and much higher horsepower, would that affect the reasons for using forged wheels in general?
Tadge answered:
Thanks for the question, Itomn. We get this question fairly often from people who think it would be cool to mimic the race car execution. As you may be aware, Corvette once had a factory option for an aluminum knock-off wheels back in 1963 on the C2, which was an early variant of the center lock wheels now offered on some of our competitors.

The chief benefit of center lock wheel attachment (vs our 5 lug nut arrangement) is speed and efficiency of a wheel change in the pit lane at a race track. Simply put, with the correct tools, changing a single lug is faster than changing 5. Also handling a single large nut is a lot more manageable than 5 small ones with much less likelihood of losing one. We usually hear that the center lock look is cleaner and transmits the identity of the race car to the street. However, on nearly every other metric important to our customers, center lock attachments come up short vs. the conventional 5 lug. As you can imagine, wheel attachment is one of the most critical aspects of corner assembly design - particularly on a high power, track-capable vehicle - and GM has spent many hours optimizing this joint. From a safety perspective, wheel attachment is one of the most sensitive joints on the car, and yet it must be easily and frequently serviceable (and potentially in remote locations with less-than-ideal tools). As a case in point, to achieve the same robustness of wheel attachment that 5 Corvette wheel lugs are tightened to 100 lb-ft (140 N-m), Porsche requires 444 lb-ft (600 N-m) on their center lug attachments - a torque wrench capable of 444 lb-ft of torque is nearly 4 feet long! Whether at our assembly plant, our dealers, or at home in the garage, a center lock requires specialty tools for removal and installation. Furthermore, depending on the implementation of the center lock attachment, there may actually be a mass increase! Typically, a bolt or stud/nut is still required to hold the rotor in place to the bearing hub, and frequently this also holds an adapter with the threaded nose that the wheel is attached to - which is likely added mass over a conventional attachment. Finally, in the real world of mass-production cars, where environmental factors, build variation and service incompetence can affect customer safety, having 5 lug nuts offers a form of redundancy making the wheel attachment more resistant to those variables.

To the second part of your question relative to forged vs. cast/flow-formed aluminum wheels with respect to power (and wheel attachment) - Regardless of wheel construction (We have both forged and cast/flow-formed - which is a forging-like process for the rim-section of the wheel), the wheel is rigorously designed to survive our high structural and stiffness requirements (inherent with the broad torque and high horsepower you mention) with the lowest mass. The difference in material ( between cast/flow-formed or forged) is really seen in the stress limits (for yield and fatigue) that the wheels are designed to, but both types of wheels could be properly designed to meet the loading pattern caused by center locks. In our case, the wheel design limitation is frequently wheel stiffness, for which both forged and cast/flow-formed aluminum are actually equivalent (same modulus of elasticity), which makes either wheel construction viable for Corvette. In fact, Z51s have forged wheels, while GS, Z06, and ZR1 have cast/flow-formed wheels, all of which had the wheel construction chosen for optimal wheel design, styling, and robustness.

So in summary, it's actually very possible to run a center-lock wheel attachment for either a cast/flow-formed or forged wheel. In the case of Corvette, we are confident that we have chosen (and will continue to choose in the future) the best combination of wheel attachment and wheel construction for Corvette.
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Old 05-19-2018, 12:50 PM
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Default Factor of Safety - 5 lug attachment

As a consulting engineer occasionally asked to assess the liability of major retailers selling and installing tires or performing brake maintenance, after which a wheel comes off the hub, I once ran the following test:

My old Silverado with aluminum wheels with 5 lugs had a 100 lb-ft torque spec. I torqued two of the non-adjacent lug nuts to 100 lb-ft, and the other three to 40 lb-ft, and marked all of the nuts to determine if any would move. I drove for 4,000 miles with this configuration, and none of the nuts moved from their original positions.

Thank you Chevy engineers for the robust factor of safety against less-than-perfect maintenance practices.
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Old 05-19-2018, 11:18 PM
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Having worked on Porsche center hub wheels, I would never want to own a set. Getting them torqued is a workout requiring two people (car started and someone standing on brakes, other person on massive torque wrench) and then there's a safety lock that has to lock down over the hub bolt. It's a royal PITA

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Old 05-21-2018, 06:27 PM
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I believe that at one time center lock wheels were called "knock offs" because you removed the nut by striking it with a large hammer.., on a wire wheel of the era, they looked great, but today outside of a museum or a concourse event, you rarely see wire wheels or "knock offs" anymore.
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Old 05-22-2018, 08:42 AM
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Great answer, very informative read! Cool fact that Z51 wheels are forged while the rest are cast. The comment about service incompetence also made me chuckle...
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Old 05-22-2018, 01:49 PM
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jay leno talks about his center lock wheel on his Big cube ZR1, need to check every 100 plus miles, carries a torque wrench behind the seats.
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Old 05-22-2018, 02:16 PM
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Hmmm...cast Z wheels might explain the damage occurances?
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Old 05-26-2018, 09:31 PM
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Originally Posted by Newdude View Post
Hmmm...cast Z wheels might explain the damage occurances?
More likely, the weight of the car...combined with the insistence on using such huge-diameter wheels with such low-profile tires. On a car intended for road use, the wheels shouldn't be any larger than required to clear the brakes. The Z06 has brakes from a 747, so there's your answer. There is probably no solution short of using smaller brakes, which in turn would require a reduction in the car's mass, and since the car is already made largely of unobtainium, the only way to reduce its mass would be to make it smaller so it could be lighter.



I read a thread over on one of the Porsche forums where a guy with CLs on his car was sick of dealing with the inconvenience and had converted back to the conventional 5-lug setup. During the process, he had weighed everything and published the results. From a mass standpoint, it was so close to a wash that it wasn't worth discussing the difference. Porsche doesn't use adapters on their hubs, they use different/special hubs...and IIRC, the 5-lug setup still came in at less than 1 lb per corner heavier. The whole exercise came down to convenience vs cosmetic preference. How badly do you want to look "racy"?

Last edited by 1analguy; 05-26-2018 at 09:42 PM.
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Old 06-06-2018, 05:40 PM
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I was watching a guy torque a center center lock at Laguna Seca a few months ago. The 4ft torque wrench actually caused the front wheels to slide on the smooth concrete. A person had to sit on the driver seat and hold his foot on the brake to hold the wheel stationary while torqueing.




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Old 06-07-2018, 01:17 AM
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Thanks Tadge! Your wisdom is clear and obvious to we neophytes. Now I know why you have the job you have and not me. I truly appreciate your response.
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Old 06-07-2018, 08:35 PM
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I'm not sure if it wasn't a consideration or he didn't mention it, but generally, with center lug, one side of the car is clockwise to tighten and the other counter clockwise.

A lot of customers/mechanics, etc would be torquing their center lugs to hell, jumping up and down on the 4ft breaker bar, trying to get the center lug to "break free." Also, they typically have cotter pins for safety and could easily see some idiot not using it when he lost/broke it.

I don't think many GT3RS will be going to the local shade tree mechanic.

Last edited by village idiot; 06-07-2018 at 08:35 PM.
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Old 07-10-2018, 05:24 PM
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Default Centerline lugs look cool but are a PITA

I have been driving Corvettes and Porsches for many years. I currently have two Corvettes and a 911 GTS. In a two year period I have picked up a nail twice in my 911. Nobody can get the wheel off for tire repair. You have to be towed to a local Porsche dealer for service. It may look good but it is not customer friendly. Just two weeks ago I picked up a nail in my right rear tire.
Limped slowly to a Porsche dealer just because Mr. Tire cannot get the rim off the car for repair. I will never buy center-line lugs again. It cost more and now you are dependent on Porsche for simple tire repair. They are not cheap labor as you can expect.
Total monopoly over the customer. I am very happy that my Corvettes do not have this style rim. Keep it simple is best for the Corvette. Best sports car in the world. Thank you Tadge!!!
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Old 07-10-2018, 05:33 PM
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Just as I suspected! Personally, I wouldn't own a center lock hub myself after doing a bunch of homework on them and hearing from Tadge. Imagine living in the Mountain West of Montana and Wyoming where distances are vast and there is no Porsche dealer between Denver and Salt Lake City and Spokane. Tadge made a believer out of me.

Last edited by ltomn; 07-10-2018 at 05:34 PM.
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