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[Penguino]

Lately, I have seen a lot of people asking questions about wheels, their strength, weight, manufacturing method, which wheels are good, what to buy, etc and I’ve seen some very good responses but I have also seen some misleading opinions so I decided to write this series of articles to make sure you know everything you need to know about wheels.

This information comes from over 5 years of experience in the automotive wheel industry. I’m an Industrial Engineer with a minor in Mechanical Design, have dealt with every aftermarket forged wheel manufacturer in the world, have designed, engineered and manufactured very strong and light wheels in both cast and forged variants and have very close relationships with some of the leaders of the wheels industry in the USA including members of the SAE Aftermarket Wheel Committee and the SEMA Wheel Industry Council.

Article 1

Here is the first article. Hope you like it and could use it to form accurate opinions about your future wheel purchase and can properly and accurately assist others.

What are Alloy Wheels?

Alloy wheels, or most commonly known as “rims” are automobile wheels that are manufactured from aluminum or magnesium alloy metals. They are called alloy wheels since they are not manufactured from pure aluminum or magnesium but using alloys (mix of metals) usually 6000-Series for high end aluminum wheels which contains less than 1% Si and about 1% magnesium.

6060-T6 Aluminum is very good because of its properties. It is very ductile, durable and lightweight, making it very good among wheel manufacturers. The “T6” comes from a Heat Treatment technique called precipitation hardening and an artificially aging process to alter the properties of the alloy

Manufacturing:

Automotive wheels can be manufactured using several process. Setian (DTMRack.com) and crimsone90 made very good articles referring these methods. This articles can be found here (http://www.m3post.com/forums/showthread.php?t=357430) and here: (http://www.e90post.com/forums/showthread.php?t=295274).



Casting

Most of you are familiar with this process.

By definition, “Casting is a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify.” There are two main techniques to cast aluminum wheels, gravity casting and low-pressure casting.

Gravity casting is a method of manufacturing a cast wheel of an specific metal alloy, typically aluminum, although Magnesium is also used, which involves supplying molten metal to a mould cavity via a feeder through a running system where the molten metal entry point is located above the top of the mould cavity. In this manufacturing process all the metal entering the mould is subject to turbulence. This turbulence can cause severe defects such as oxide inclusions and entrapped gas porosity and in some cases mould erosion and hotspots.

Low-pressure casting in the other hand takes care of the disadvantages of Gravity casting by filling the mould by injecting molten metal at about 25psi through an in-gate just below the top of the mould cavity from a source under the mould through a method which allows complete filling of the mould. The force of gravity acts against the upward flow of the metal and eliminates the possible turbulence caused by the falling liquid metal. The molten metal is stored in a pressured container and by increasing the pressure metal is then forced into the mould.

In low-pressure casting the method in which the metal is forced into the mould is the main difference/advantage over gravity casting and is absolutely controlled. As explained above it results in a low turbulence or turbulence-free mould filling with little to no corrosion.

The molten metal in the furnace is in a closed container under protected atmosphere. Because of this the metal absorbs less hydrogen and any other impurities as well as the oxide formation being greatly reduced. As opposed to gravity casting, the metal surface is not interrupted constantly since it is been forced from under the metal surface. What you get is a very clean quality metal.

Basically low-pressure casting uses positive pressure to move molten metal into the mold quickly thus resulting in a finished product with an Aluminum that is denser than Gravity cast.

The advantages of low pressure die casting process are several:
- higher yield achievable
- reduction of machining costs
- excellent control of process parameters
- good metallurgical quality
- leads to an optimal use of the aluminum
- excellent mechanical and technological properties of the castings.



Forging:

Out of the three, Forging is the manufacturing process that creates the lightest metals. There are several ways to forge alloy wheels, but the most common is press forging.

By definition “Forging is the shaping of metal using localized compressive forces. It is to form or to make by heating and pressing.”

The forging process uses immense amounts of compressive force to shape the metal. During this process a wheel blank is placed between two shaped plates and the press slowly shapes the wheel using continuous forces. During the process the internal grain is deformed and rearranged to the shape of the part. This deformation due to compressive force improves the strength uniformly throughout the entire structure of the wheel. Press Forging allows closer tolerances as well. The wheel is typically heated to about 400-Farenheit. The forging process typically uses anything between 4,000 to 10,000 tons of pressure, 6000 being the most common, and reaches up to 16,000 tons to produce forged magnesium wheels. Heat treatment and artificial aging can further improve the wheel mechanical properties.

It is important to understand that forging does not compress the metal. Forging alters the grain structure of the metal and makes it stronger. For example, a 19” aluminum wheel blank that weights 30lbs will still weight 30lbs after the forging process. The major difference is that after the forging process, the 30lbs forged aluminum wheel blank will be much stronger. Since the aluminum is stronger it allows the engineers to manufacture a lighter wheel by removing excess aluminum.

To make it easier to understand, imagine a structure that requires a beam with a diameter of 5-inches. If we manufacture that beam out of forged aluminum, since the mechanical properties have been improved, the beam diameter could now be reduced to around 2-3” in diameter helping to make the structure lighter. You could do the same and keep the diameter of the beam at 5-inches if your purpose is to make it stronger.

In other words, Forging does not make the metal lighter, it makes it stronger. Forged wheels are also formed via flow-forming which is explained below.



Flow-Forming:

Flow Forming is a precision metal forming. It is commonly known as a forming technique. It is the newest player and is only available by a limited number of manufacturer.

By definition: “Flow Forming is an incremental metal forming technique in which a disk or tube of metal is formed over a mandrel by one or more rollers using tremendous pressure.”

In other words, a disk, or in this case an aluminum wheel blank, is placed between two rollers and the wheel spins while the two rollers form the barrel at high pressures and extends the barrel of the wheel to the desired length. This deformation arranges the grain structure of the metal in the direction of the rotation of the wheel. The blank is typically made using metal casting. This process helps reduce barrel thickness. The blank barrel is usually around 4.0-9.0-inches wide and is extended to 7.0”-11.0-inches depending on the application. Although the piece is cold formed, during the forming process, due to friction, the wheel blank is heated to around 400-Farenheit. Because of the high pressure and high temperature the metal mechanical properties are changed. The mechanical properties of the metal became very similar, if not the same, to that of a forged wheel. No material is lost during the forming process, resulting in less aluminum used to create the finished wheel.

Flow Forming in essence forges the wheel barrel by spinning and forming. Forming can be done hot, warm or cold and is accomplished on mechanical or hydraulic press, or on special forming equipment. Heat treatment and artificial aging can further improve the wheel mechanical properties. Flow forming process can be a cost-effective alternative to conventional press forging.

Because of its properties flow forming is also known as liquid forging.

Please let me know if you have any questions regarding this and I`ll try to reply them as soon as possible.

On my next article, I`ll be covering the Design and Engineering aspect of a wheel. It`ll include information regarding wheel weight, strength and fitment. It's nearly complete and will be posted in a few days.

[Penguino]

Article 2 - Design and & Engineering

This is my favorite area. Engineering is the most important part of the wheel developing process and is where a lot of wheel manufacturers fail. I’ll cover only some of the key factors under the design and engineering stage.

Weight & Strength

The weight and strength ratio of aluminum wheels is key when developing performance wheels. As explained above, aluminum does not become lighter through forging or forming. It only becomes stronger. Since it’s stronger, we can use LESS material to achieve the same level of strength. This is very important to understand.

For the purposes of explaining this, let’s assume we are designing our own wheel. We’ll call this wheel, the “M92”. This wheel will be 19 x 8.5” in size. We’ll assume we are designing this wheel and manufacturing in all three manufacturing methods: casting, forging and flow-forming. Please refer to the chart below:

As you can see, the same wheel design manufactured in the all various manufacturing processes will require the same weight (amount) of aluminum and therefore will have the same weight BUT different strength expressed as load ratings (explained below). In other words, since the design is the same, it’ll required the same amount of material to make it, however, since the mechanical properties of cast, forged or flow-formed aluminum are different the the strength level (load rating) will be different with cast being the heaviest, and forging leading in weight and forming coming in as close second. Please keep in mind different aluminum materials have different density so the weight might be slightly different.

Load Rating is the required load the wheels are capable of withstanding. Every organization has their recommended load ratings per application for which they test against. This might be done for safety reasons. For most BMWs this number is around 650kg – 730kg.The load rating requirements are determined by taking the vehicle's heaviest gross axle weight rating (GAWR) and diving that by 2. This number might be significantly lower than the recommended load rating established by international wheel quality standards.

Now, let’s assume that we are interested in having our M92 wheel rated at exactly 660kg per corner. Please refer to the chart below:

As you can see, since we are interested in achieving no more and no less than a load rating of 660kg per corner, via casting we’ll need to increase the weight of the wheel (add material) in order to achieve our desired load rating bringing the final weight to 26.2lbs. For Forming and Forging, since the aluminum is stronger than casting and the required load rating is LOWER than what our wheel is capable of holding at the current weight of 25.0lbs, then we are able to remove material from the wheel to achieve our required load rating and at the same time lose weight. In this case we are able reduce the weight to 23.6lbs and 22.8lbs accordingly, to achieve a load rating of 660kg.

Please note that “adding” and “removing” material can only be done during the design process. It cannot be done AFTER the wheel has already been made.

In conclusion, forged/formed wheels are not stronger than cast wheels. If rated at the same load, they will be lighter and equally strong. This is assuming they are all engineered equally.

Fitment:

I am a firm supporter of manufacturers that make wheels “model-specific”. Fitment on a vehicle is very important and can drastically change the dynamics of your car. Here we’ll discuss “model-specific” wheels vs “custom” wheels.

#1 – Load Ratings and Weight Reduction

Every vehicle requires its own specific offset, width, bolt pattern, fender, strut and brake clearance, back spacing, etc.

For example, lets say your jacket size is a 42L. Making a “Custom” wheel is like buying a 44R and “customizing it” in order to make it fit. It’ll never fit right and the quality won’t be the same. It is always better to buy a jacket that is already 42L!

The same goes with wheels. In order to built “custom” wheels, manufacturers take a blank and machine the back pad as much as they need to -sometimes up to 30mm --, require you to run spacers if the fitment is more aggressive, and provide centering rings in order to make it fit. It has gone to the extreme that not even your OEM centercaps and bolt lugs fit! What is custom about that?

A cast wheel should not be machine more than 3-5mm IF NECESSARY. A Forged wheel I wouldn’t shave more than 10mm of the back pad. This can SEVERELY deteriorate the integrity of the wheel! Removing material from the back pad can be a disaster. Adjusting offsets by milling several millimeters of the back pad is not the proper way to do it. This safety hazard is more present with cast wheel, but even with forged wheels there is a margin you cannot exceed. This is a potential safety hazard and could result in a complete wheel failure. The consequences of exceeding these limits are balancing issues, excessive run out, weaker structure and severe vibrations at high speed.

Using the same blanks across platforms (Same blanks for different brands) can limit your design due to brake clearances and force you to use under rated wheels (weak) or over rated wheels (heavy) for your vehicle.
Let’s use our M92 wheel as an example. Let’s say we make this wheel as a “generic blank” and then we would like to customize it for a Porsche.

In this case, since the wheel is being “custom made” for a Porsche application, requiring a lower load rating, and the blank used is a generic blank then the weight stays the same since you can no longer remove weight since the wheel is already made. Right now you are mounting a wheel on your vehicle that weights a few pounds more than what it should. You’ll also be required to use centering rings since the center bore is larger than the Porsche center bore.

Now let’s say you use a Porsche specific blank.

In this case since you are using a Brand-specific blank you have a lower weight (sine the actual load rating = required load rating) and the fitment will bolt on without any centering rings or spacers since is meant for the vehicle that is going into.

The other case is where the “custom” wheels is for an application that requires a higher load rating. In this case, your “custom” wheel will have a LOWER load rating than the recommended for your vehicle and could potentially be a safety hazard.

[Penguino]

Reserved #3

[Penguino]

Reserved #4

[remmib]

Very interesting.

[Penguino]

Originally Posted by remmib

Very interesting.

Article 2 posted.

[Mr. ///M3 RD]

Originally Posted by carlitosz

Lately, I have seen a lot of people asking questions about wheels, their strength, weight, manufacturing method, which wheels are good, what to buy, etc and I’ve seen some very good responses but I have also seen some misleading opinions so I decided to write this series of articles to make sure you know everything you need to know about wheels.

Thank you so much for this great article Great Job!

Now may I ask you a few question?

1) What do you think in your opinion makes 19 Inch BMW Alloy Rear Wheels crack ?

2) What effect if any do RFT mounted on the rear wheels have or do they contribute to the cracking i.e. added stress to the inside rim of the wheel?

3) Why do front wheels never or shall we say very seldom crack?

and finally ...

4) My Rims are BMW 293 would you know (from a strength point of view) if they have been Cast, Flow Formed or Forged? I assume they are cast

My 2009 sDRIVE Z435i after 18,000 KM has suffered cracks in both rear wheels (driver side to the extent that it caused a flat and during changeout of the wheel it was found that the passenger side has also 2 hairline cracks). I am awaiting resolution from my BMW Dealership. I believe runout checks will be made to determine if they are within BMW TIR tolerance (I personally fail to see how BMW can perform a accurate runout check on a wheel that has cracked as shown in the attached picture ~ you see I also have a very strong mechanical engineering background some 40 plus years, 25 of those years field hands on experiences solving field problems).

For your information I am attaching a link of photos on the Driver Side Cracked 19 Inch Wheel ...

For Photo Album Please Click Here

Thanks in advance for your help,

Rolf-Dieter

[remmib]

Originally Posted by carlitosz

Article 2 posted.

Great info.

[Penguino]

Originally Posted by Rolf-Dieter

Thank you so much for this great article Great Job!

Now may I ask you a few question?

1) What do you think in your opinion makes 19 Inch BMW Alloy Rear Wheels crack ?

2) What effect if any do RFT mounted on the rear wheels have or do they contribute to the cracking i.e. added stress to the inside rim of the wheel?

3) Why do front wheels never or shall we say very seldom crack?

and finally ...

4) My Rims are BMW 293 would you know (from a strength point of view) if they have been Cast, Flow Formed or Forged? I assume they are cast

My 2009 sDRIVE Z435i after 18,000 KM has suffered cracks in both rear wheels (driver side to the extent that it caused a flat and during changeout of the wheel it was found that the passenger side has also 2 hairline cracks). I am awaiting resolution from my BMW Dealership. I believe runout checks will be made to determine if they are within BMW TIR tolerance (I personally fail to see how BMW can perform a accurate runout check on a wheel that has cracked as shown in the attached picture ~ you see I also have a very strong mechanical engineering background some 40 plus years, 25 of those years field hands on experiences solving field problems).

For your information I am attaching a link of photos on the Driver Side Cracked 19 Inch Wheel ...

For Photo Album Please Click Here

Thanks in advance for your help,

Rolf-Dieter

You are making several generalizations on your post. I have personally not seen any tendency of BMW 19" wheels cracking, especially the rear. The only case of cracked BMW wheels I have seen was the BMW 19" 225M which was probably due to manufacturing defects. RFTs do contribute to this since they are stiffer than regular tires and do not absorb impact as much, thus transferring the force of the impact directly to the wheel. Regarding front or rear wheels cracking, front wheels crack/bent as much as rear wheels, in fact, the majority of the cracked wheel cases we see in E90Post are for front wheels.

If you are having issues with your wheels, I recommend switching to non-RFT as a start. Also , your roads might be the one to blame here. If you have bad roads where you drive, it might pay off to pay a little more attention and try to avoid potholes as much as you can.

In most cases, 19" Wheels + RFTs is a very bad idea.

[Mr. ///M3 RD]

Carlos, if there was an e-mail I would reply to you in private. Since there is not I just Deleted ....... my response to you.

here is a link you may like to explore .... Plenty of cracked wheels here

you might even Google "Cracked wheels UK Watchdog"

Happy reading.

[Mr. ///M3 RD]

Looks like Carlos has run out off time with is follow up reports

... looks like there be no one becoming a wheel guru soon

[tony20009]

Thanks very much, OP, for this information. Very clearly written and understandable by us laymen.

[tony20009]

Originally Posted by Rolf-Dieter

Looks like Carlos has run out off time with is follow up reports

... looks like there be no one becoming a wheel guru soon

I hope not...

[thurston]

Great articles - thanks for the info! Any recommendations on wheel manufacturers that do "model-specific" fitments? Trying to find a set of 18" wheels without having to worry about spacers, etc.

[Penguino]

A lot of things on my hands right now. Will update this within the next couple of weeks.