NASCAR314

Are the C5 Magnesium wheels as strong as the aluminum thin spoke Speedlines as far as using for autocrossing on race tires?

I run 18" x 9.5" C5 wheels on my C4 for autocross and they work great, just wondering about the integrity of the Magnesium ones.

I do run sticky race tires and abuse the hell out of the car, never broke a wheel, but those wagon wheels are pretty tough.

froggy47

I am not aware that true "made of magnesium" wheels were ever offered on a c5 (unless it was an overseas option).

Are you referring to a made of magnesium wheel or a color?

Hat_Trick_Hokie

I believe OP is talking about the optional magnesium C5 wheels, option N73 I believe:



Very rare, and very light.

NASCAR314

Yes, those are the ones I am asking about.

ltborg

The finish certainly is more delicate than the aluminum. The powder coat doesn't adhere to the magnesium very well. That said, they are super light. Mine were mid 18 lbs for the rears. I never had any issues racing with them.

I don't know the details of the alloy used, but magnesium (and the alloys of it) are not as strong and are softer (more compliance) than aluminum. That said, the spokes on these wheels are bigger than the aluminum so they might actually be strong. I don't know of any evidence either way.

Mine have held up just find to multiple seasons of autocross with slicks.

froggy47

Are there markings on the back that indicate the wheel/material?

Were they available all c5 years?

Speedline or other mfg? Alcoa?

17 and 18?

How wide?

Scooter70

http://forums.corvetteforum.com/1565843633-post7.html

ltborg

Factory option on non-Z06's. I believe they were manufactured by Speedline. Stock (non-Z06) sizes and offsets (17x8.5, 18x9.5).

You can tell what they are by the looks. The mag wheels have wider spokes and a much smaller center cap. They also have a rubber gasket on the back side and in the lug nut areas. These gaskets seal moisture out from the joint between the main wheel (magnesium alloy) and the hub piece (some other material) to prevent corrosion.

Magnesium:

Aluminum:

ZedO6

C5Z Speedlines are only 2-3 lbs heavier per wheel and come in better widths...spun cast wheels are infinitely more durable. You can bend a true mag wheel on the tire machine just by being a little sloppy.

parkerracing

But they burn real nice...

ltborg

Just for reference, here are the weights for the rear wheels I measured:

Mag (powder coated) - 18.8 lb
Mag (painted) - 18.4 lb
Aluminum (OEM finish) - 20.1 lb

The aluminum style is the painted (not polished) with the fillets where the spokes meet the outer rim (like the second aluminum picture I posted above).

Pumba

Magnesium filings will burn. Magnesium wheels will not!

Crepitus

Magnesium strips burn like an arc welder. A little experment with that got me expelled from school...

parkerracing

Get them hot enough they will...in about the color of the font you used.

NASCAR314

OK, well I have aquired some, they had been scratched up and are now good for race tires.

Question: They have a rubber strip around the center section on the hub side, it is removable and held on by a small lip in the wheel center.

"Any idea what this is for?" See attached pic. I don't even know if it's needed, or if it has something to do with corrosion protection? or what?

Anyone have any idea's?

ltborg

The hub area is a different alloy than the wheel outer and spokes. The gasket on the back side and the gaskets in each lug hole are to prevent moisture from causing corrosion between the dissimilar metals.

SouthernSon

And to think, the accessory case on the back of most Continental and Lycoming light aircraft engines are made of the stuff.

I used to burn underwater in my commercial diving days using broco rods. Magnesium is explosive!



Also, sacrificial anodes for water heaters are made with the stuff. Thus, the reason for the gasket on the wheels.

NASCAR314

I'll leave the gaskets in place. They are in great shape, as are the wheels, (except for scraped off powder coating)

froggy47

Burning old mag VW engine block

edwardo99

I did some research trying to figure out what the magnesium alloy is in the N73 option wheels in case I had to buy some weld rod and have a cracked or damaged wheel repaired. Here is what I found.

At exactly the same time that Speedline was making the wheels for Chevrolet, they were making a quite similar wheel for Ferrari. Unlike the N73s, that wheel was marked with the alloy. AZ91. The A means Aluminum, the Z means Zinc, the 9 means 9% Aluminum, the 1 means 1% Zinc. It appears that AZ91 was the typical alloy used for car wheels. I suspect that Speedline would be using only one alloy, at that time, for any Magnesium wheels they were making, no matter the customer.

The aluminum and zinc basically cured the old cracking problem that earlier magnesium wheels exhibited by making the material more ductile and tougher. The AZ91 specification went through many revisions, each one tightening up the allowable impurities. It is the impurities that cause/allow/promote corrosion. By the time of the N73 wheels, corrosion was basically no longer an issue. (Not counting galvanic corrosion… the reason for the seal ring that comes with these wheels, as noted elsewhere in this thread.)

As for strength, I found a reference that quoted the ultimate tensile strength of AZ91D (the D means revision D of the spec) at 33.3 Ksi. This is in the range for the cheaper, weaker aluminum alloys. It is almost exactly the same strength as A365, an aluminum alloy typically used for cast wheels.

I've been running mine for 2 years, mostly autocross, some track, with good results. They were a little warped when I got them and I had them straightened.