Beware cast rims?

[sacoffee]

Pulled the tires off to work on cleaning up brake calipers and wheel wells ... Found this on one of the rims. Obviously a PO knew about this, see the grind marks, but left it on the car and passed it on. These are aftermarket cast rims.

The issue with cast is that the metal is not work hardened through the forging process. Any metallurgists here that can talk about the semis forged and low pressure cast system and the resulting strength of the wheel vs forged?

I am p.o.'d beyond belief that a PO knew about this and left the rims on the car.

Anyone running these aftermarket turbo twist made in Italy should do an inspection. Any suspicious ares should be ground clean and dye penetrant tested.

edited - leaving text, see comment below on my foul up

[911SauCy]

Is that a crack? Otherwise idk wtf im looking @

[Rodsrsr]

It looks like left over doublestick tape from the lead weight to me.

[McLaren-TAG]

Originally Posted by sacoffee

On car, scares me that I drone 500 miles with my wife in the car

That's not a very nice thing to say about your wife.

[JAR0023]

Originally Posted by Rodsrsr

It looks like left over doublestick tape from the lead weight to me.

+1. I've been staring at the photo for a couple of minutes and all I see is tape residue. -J

[sacoffee]

Originally Posted by McLaren-TAG

That's not a very nice thing to say about your wife.

good one

Well, I look like a fool, I posted the wrong picture, I uploaded from my iPhone without glasses .. it looked like the photo but now on my desktop with a 30" screen I can see this is obviously tape residue.

I edited my original post

[911SauCy]

Originally Posted by JAR0023

+1. I've been staring at the photo for a couple of minutes and all I see is tape residue. -J

I thought it was the cigar and tequila impairing my vision

[80-911SC]

well ....... how bout the picture ?????

and there is the idea the PO didn't know .... alot of people do not work on there own cars ...and may have been done a long time ago ...

and just a side thought ... quite a few of these cars have the wheels you are talking about ...

but your expert opinion is that they should all take them off and dye test them ???

what exactly are the credentials that you hold to make this opinion ??

and if you had or have them how come you bought a car with these wheels on it and did not dye test them in the PPI process ???

also you are neglecting to mention how old these wheels are but they likely could be 20 plus years .... was the car ever tracked or driven hard ??? guess what its a sports car ... you are supposed to drive them hard and the way you find the edge is to pass it and back up ...

I have said wheels on my car ... smacked straight in a pothole at 100 plus and dented it ... had a wobble ....found another set and mounted and went down the road ... live with it and do a better PPI next time ......


After reading this you will notice .... I have made a broad opinion off little information ...

just as you have about the wheels on your car ....

look thru the car .. I am sure you will find something else to whine about ...

unless of course the PO told you that the car had never touched a curb or any wheel or tire failure ... which is a doubtful statement .......

Have a good night ...

Steve

[Rodsrsr]

I dont think there ever was a crack, just evidence that they were made in Italy and probably cast. So the seller should have disclosed that the rims were cast?
I'm really having a hard time understanding your point here.

[chris_seven]

Originally Posted by sacoffee

The issue with cast is that the metal is not work hardened through the forging process. Any metallurgists here that can talk about the semis forged and low pressure cast system and the resulting strength of the wheel vs forged?

The vast majority of wheels used by the Automotive industry are cast and are perfectly safe from any sensible point of view.

The legendary Minilite magnesium wheel was sand cast and could be tricky to balance due to the uneven distribution of porosity but they seem to have worked well.

The 911 wheel is forged from a 6082 alloy with, I believe, a T6 Temper. This results in a typical tensile strength of around 350MPa with a typical elongation of around 10%.

Traditionally cast Aluminium wheels use LM25TF - although I am sure there are more modern variations that will provide better properties.

The tensile strength of this alloy is around 280MPa with its elongation being around 5% so it is a little more brittle and may crack more readily under impact conditions.

The strength of a wheel is dependant on both the alloy and the cross-section being considered and cast wheels tend to be thicker than forged wheels so they compensate for the slightly weaker material.

The small amount of porosity that is always present - around 0.2%- in either gravity die or low pressure die cast wheels will also make them more prone to long term corrosion although good quality powder coating will tend to eliminate this issue.

There is no reason to assume that a 'cast' wheel is dangerous.

It is incorrect to assume that a forged wheel has had the benefit of 'work hardening' - assuming that this process does confer a benefit.

The majority - but not all - materials will work harden when cold formed and this will increase strength. It will also reduce ductility so there is a pay off.

If metal is 'hot worked' which means it is worked above its re-crystallisation temperature, then work hardening will not take place. It is my understanding that Fuchs wheels are hot forged and then heat treated.

The idea behind forging cast metals is to break up the As-Cast structures rather than to work harden them.

The solidification process in most metals is reasonably straightforward and generally results in fairly coarse grain structures - although there are modern inoculation techniques that can improve this situation - It also produces grain boundaries that tend to be rich in impurities as these are the last regions to solidify.

If you can achieve a deformation of around 50% then these poor structures tend the broken up and the high temperatures usually involved will cause re-crystallisation to occur and improve the basic grain structure.

It is possible to forge cast materials but there are issues.

A typical die cast Aluminium Alloy has a relatively high Silicon content. There are two main reasons for this.

The first is that the viscosity of the molten Aluminium/Silicon alloy is quite low so it pours well and fills mould cavities well.

The second is that at around 12% Silicon the alloy has a Eutectic Composition which has the lowest meting point of any Aluminium/Silicon Alloy and this helps to reduce cost due to lower heating costs.

Eutectic alloys also have invariant freezing points which is to say that they freeze at a single temperature.

LM25 is about 7% Si so it melts at about 60degC lower than 6082 and has a freezing range of around 50 degC.

The limited freezing range has a significant impact on cycle times.

If you try to forge LM25 or other high silicon alloys then there are issues with regard to the flow of material due to the presence of hard, acicular silicon particles which do make life difficult.

I am sure is possible to make a cast wheel and then subject it to a 'light forging' operation and gain some small improvement but I would like to see some good data about strength, impact behaviour and fatigue life before I was convinced it was worthwhile.

If I had to treat a cast wheel to obtain an improvement in its properties I would almost certainly considered Hot Isostatic Pressing of the casting prior to subsequent heat treatment and machining.

I am dubious about Semi-Forged components and would like to see a more detailed explanation.

There is a process which involves the forming of semi solid materials which was initially described as thixocasting and involved the production of slurries (partially solid liquids) and there has been much development of these techniques.

There has also been much development of Squeeze Casting techniques and these may eventually prove interesting.

I think this process is now described as semi-solid forging.

I would be surprised to find that this type of process was economically viable and I could see costs being similar to a forged component but I would be happy to be proven wrong.

http://www.wiley-vch.de/books/sample/3527322043_c01.pdf


If originality is not an issue then correctly heat treated die cast wheels should be structurally safe and provide a very cost effective wheel.

[COLB]

Very nice write up. I even understood a fair bit of it!

[sacoffee]

Originally Posted by 80-911SC

well ....... how bout the picture ?????

and there is the idea the PO didn't know .... alot of people do not work on there own cars ...and may have been done a long time ago ...

and just a side thought ... quite a few of these cars have the wheels you are talking about ...

but your expert opinion is that they should all take them off and dye test them ???

what exactly are the credentials that you hold to make this opinion ??

and if you had or have them how come you bought a car with these wheels on it and did not dye test them in the PPI process ???

also you are neglecting to mention how old these wheels are but they likely could be 20 plus years .... was the car ever tracked or driven hard ??? guess what its a sports car ... you are supposed to drive them hard and the way you find the edge is to pass it and back up ...

I have said wheels on my car ... smacked straight in a pothole at 100 plus and dented it ... had a wobble ....found another set and mounted and went down the road ... live with it and do a better PPI next time ......


After reading this you will notice .... I have made a broad opinion off little information ...

just as you have about the wheels on your car ....

look thru the car .. I am sure you will find something else to whine about ...

unless of course the PO told you that the car had never touched a curb or any wheel or tire failure ... which is a doubtful statement .......

Have a good night ...

Steve

Sorry to hit a nerve Steve ... the thread topic has a ? mark for a reason. No updated picture, been on other things this weekend. Fortunately you don't need credentials to have an opinion. Off now to find something to whine about.

[sacoffee]

Originally Posted by chris_seven

There is no reason to assume that a 'cast' wheel is dangerous.

It is incorrect to assume that a forged wheel has had the benefit of 'work hardening' - assuming that this process does confer a benefit.

The majority - but not all - materials will work harden when cold formed and this will increase strength. It will also reduce ductility so there is a pay off.

If metal is 'hot worked' which means it is worked above its re-crystallisation temperature, then work hardening will not take place. It is my understanding that Fuchs wheels are hot forged and then heat treated.

Nice write up.

My comment is based on assuming most wheel manufacturers use a cold forge process. The concern expressed here in the original thread is based on layman knowledge that cast wheels will crack more than bend.

This rim in question has a crack and its obvious it was known by someone before me.

[chris_seven]

http://forums.rennlist.com/rennforums/attachments/911-forum/536056d1305344659-heads-up-on-wheel-availability-fuchs-fuchs-wheels1142645046.jpg

I have seen other descriptions that describe the process as hot forging although there is a description where one of the 8 stages is described as 'coining' which is usually a cold process.

If subsequent stages involve hot work then the effects of cold processing and work hardening will be eliminated by re-crystallisation.

It is fair to say that it is possible to solution treat Aluminium Alloys and then cold work prior to Age Hardening but this is usually applied to products such as bar or strip.

A T6 temper produces the strongest condition for a 6082 type alloy.

A T561 temper can used for bar products that will be machined to very exacting tolerances and wheels would really fall into this category.

This temper involves solution treating followed by a cold deformation of 1-2% which will have a very small effect on tensile strength but will reduce the likelyhood of distortion during any machining operations.

T3 and T7 tempers also involve cold work and are used with 2000 series alloys to reduce sensitivity to Stress corrosion cracking which can be a problem with alloys that have high Copper contents.

Ii is unlikely that a wheel could be cold formed without several interstage annealing operations to allow ductility to be recovered.

There is some basic information here:

Aluminum and Aluminum Alloys Subject Guide | Information Resources | ASM International

[HawgRyder]

There is one other possible way of making a wheel...machining from a billet.
I suspect the billet would have to be normalised in some way to start with...and then all machining operations performed...and perhaps heat or cold treated afterwards.
This is how custom motorcycle wheels are made...and they seem to hold up to a lot of abuse.
I don't know if they are stronger or lighter compared to forged.
Bob

[techweenie]

Virtually all Porsche wheels since 1990 have been cast wheels. Many versions in Italy (the original Turbo wheel was a design copyrighted by an Italian company).

With no picture, it's hard to know what the issue is, or if there is an issue, or if this is just a waste of time.

[chris_seven]

Originally Posted by HawgRyder

There is one other possible way of making a wheel...machining from a billet.
I suspect the billet would have to be normalised in some way to start with...and then all machining operations performed...and perhaps heat or cold treated afterwards.
This is how custom motorcycle wheels are made...and they seem to hold up to a lot of abuse.
I don't know if they are stronger or lighter compared to forged.
Bob

A wrought alloy has a broken up cast structure and very good properties.

there could be some anisotropy but a good quality 7000 Series Alloy such as a 7075 would be quite good.

There could be some residual stress issues following machining but there are well established stress relief techniques for these type of alloy.

They could be made at least as light as 6082-T6 wheels and if the full strength of a 7075 were used (Typically 540MPa) they could be lighter.


Could be expensive.

[Arne2]

Originally Posted by sacoffee

My comment is based on assuming most wheel manufacturers use a cold forge process.

That is a faulty assumption, and if that is the basis for your logic you will need to start over. Most alloy wheels for automotive use are cast. Forged or other processes that use extra force to form the structure (rolling, and such) are not common, and are limited to high-end wheels.

I'd still like to see pictures of the flaw. After 25 years in wheels and tires (prior career) I've seen all manner of wheel problems first hand.

[McLaren-TAG]

The picture was taken down by the OP because he realized it wasn't a crack, but lead weight tape residue which looked like a crack when he saw it on a small screen (I guess on his phone) but was clearly not a crack when viewed on a large screen.

Didn't stop the discussion from proceeding though, and I guess if nothing else I now know lots more about the processes of making wheels and why there's a valid reason that high performance models cost more.

[sacoffee]

Originally Posted by McLaren-TAG

The picture was taken down by the OP because he realized it wasn't a crack, but lead weight tape residue which looked like a crack when he saw it on a small screen (I guess on his phone) but was clearly not a crack when viewed on a large screen.

Didn't stop the discussion from proceeding though, and I guess if nothing else I now know lots more about the processes of making wheels and why there's a valid reason that high performance models cost more.

Thanks