Wheel STIFFNESS matters! -We test Carbon Revolution's Carbon Fiber GT350R wheels vs AL wheels.

[50 Deep]

Good writeup @BillyJRacing . I will enjoy going through it over the next few days and really digest all the data.

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[datadatum]

I couldn't mention this at the time of writing the article but now it's public:

Production increasing 15X:

https://www.motorauthority.com/news...to-supply-150000-carbon-fiber-wheels-annually

So are the prices going to come down a lot???

 

[oldbmwfan]

@Epiphany you probably know as much straight from the FP horse's mouth as I do, but I was told that Ford was surprised at the performance of the CF wheels in their destruction and durability testing (which includes things like running into a 3- or 4-inch 90-degree curb at speed). Part of the performance was attributed to the wheels being so (relatively) light that the suspension could react faster, reducing the transient load the wheel experienced. I am getting all of this secondhand, so no data behind it. That said, I've seen enough aluminum wheels fail (both cheap and expensive) to consider basically any wheel a consumable item on track. I do think when/if CF wheels get down to ~$7500/set, we will see them popping up all over in amateur racing, at least in the big-wallet classes. When that happens, I wonder what will happen with ABS tuning and the like. Ford obviously felt the need to recalibrate the ABS for the GT350R because of the difference in rotational inertia.

 

[tracktardicus]

"That's why it's common to see people who switch to those lightweight wheels then say that "This car (GT350) needs 2.5, 3, 3.5, 4* of negative camber and push GT350 (and even R) owners to try to adopt those levels of camber that's unnecessary with a stiffer wheel."
Maybe I'm ignorant on a relationship on wheel stiffness and camber, but it's my understanding that you utilize camber to increase the tire contact patch and make the tire temperatures more uniform across the contact patch surface under the heavy tire loading that occurs on a track. This gives you better grip and more even tire wear for track use. To my knowledge there is no relationship between wheel stiffness and camber. Correct me if I am wrong.
<EDIT> So, I read the article, but there is no empirical data to support the claim. "ultra-lightweight aluminum wheels can often lose 1-degree of camber due to camber compliance PER cornering G." They say they tested a big-name aluminum wheel, but that could be a crappy $300 ForgeStar for all we know. Test it against the VIBE 7-spokes, or the Apex EC-7's, which is what enthusiasts are using in place of the CF wheels for track use. Otherwise, this is just another marketing ad. The CF wheels are terrific, but until they provide some verified performance numbers and offer a squared setup, I won't be convinced that the performance gain is worth 2-4x the cost.

 

[Forgedwheeler]

The suggestion that "Forged and Cast Aluminum wheels have the same stiffness" is just plain nonsense.
Which alloys are being used? Most cast wheels use A356/T6 (heat treated) with a nominal tensile strength of 34,000 psi.
Many forged wheel companies use 6061T6 with a tensile strength of more than 40,000 psi. Another common wheel alloy is 6069 with 60,000 psi.
A few companies use 7075 T6 with almost 80,000 psi tensile strength.
Equally important is the wheel design. Stiffness comes from spoke length, spoke spacing, cross section and a dozen other factors. Certainly not just the material.

I'm a big fan of carbon, but it is extremely expensive and very difficult to manufacture.
And the actual weight savings of carbon over a well-designed forged wheel is 10-15%, often less.

Much of what was presented came from the marketing folks. Engineering is built around facts and laws of science.

In the immortal words of John Adams, "Facts are stubborn things".

 

[Zitrosounds]

The suggestion that "Forged and Cast Aluminum wheels have the same stiffness" is just plain nonsense.
Which alloys are being used? Most cast wheels use A356/T6 (heat treated) with a nominal tensile strength of 34,000 psi.
Many forged wheel companies use 6061T6 with a tensile strength of more than 40,000 psi. Another common wheel alloy is 6069 with 60,000 psi.
A few companies use 7075 T6 with almost 80,000 psi tensile strength.
Equally important is the wheel design. Stiffness comes from spoke length, spoke spacing, cross section and a dozen other factors. Certainly not just the material.

I'm a big fan of carbon, but it is extremely expensive and very difficult to manufacture.
And the actual weight savings of carbon over a well-designed forged wheel is 10-15%, often less.

Much of what was presented came from the marketing folks. Engineering is built around facts and laws of science.

In the immortal words of John Adams, "Facts are stubborn things".

And the fact is CF wheels are better, albeit expensive. I don't think the article says otherwise. I work in test/prototype engineering for Army aviation. If somehow stating the benefits of CF over AL is propaganda, then my friends, the Army has been hoodwinked lol. I cant say much but i can tell you this, CF is superior in every application we have tested thus far. Every year we prototype/test/validate and field modifications made out of CF. In fact, it is the material of choice, for the obvious reason.

 

[BillyJRacing]

I didn't miss the "IF NEEDED" part. To be clear, the mere suggestion was yours, not mine.

You don't know IF it's needed so it's hard to criticize a design without that information.

Regarding any and all FEA analyses on these wheels and their respective hardware...my guess would be that you haven't done any and instead you rely on those that have. For that matter, so does the general public with each and every CF wheel that is out on the road. I'm a little shy of the resources of both Ford Motor Company as well as CR, so at this point I'm lagging in the virtual testing department. I'd love to see the data on any POF/destructive testing regarding the lug hardware or wheels themselves if you happen to have it and are able to share.

I do have most of that data, but unfortunately I cannot post them due to an NDA. I understand and respect your skepticism.

Not sure where you get anything about changing something simply for the sake of changing it. As far as IF the needs are different for motorsports use we aren't seeing it via Ford here in the US as neither the Continental/World Challenge GT4 cars or the race GT IMSA car are using them. I'd love to see CR test their best CF wheel in a 24hr race such as Le Mans or Daytona and see how they hold up.

I saw what was mentioned in the article. I'll ask around at the Continental GS finale if various teams would be alright paying the premium in order to run a CF wheel. Assuming CR did a run of 18" diameter wheels for the series (with the same thermal coating inside) do you think it fair to say the cost could be triple that of the Forgelines currently in use?

As mentioned in the article, CF wheels are not allowed in motorsports at the moment. Having been in Continental for over a decade (man, that makes me feel old) I can assure you EVERYONE would use them if they were allowed. You just can't make up 1-second per lap of a disadvantage like that. Heck, you can't make up 0.5 seconds a lap of an outright disadvantage.

Keep in mind that Carbon Revolution wheels don't cost that much more than the ~$10K Forgelines.

If you propose the question "do you want to spend more on blingy wheels" no team is going to say yes to that. Now if you tell them the carbon wheels have a 1 second advantage, they would be all for it. Some teams would be skeptical at first until their competitors start beating them by a big margin. At that point pretty much every team that wants to win and not just ride around, would convert to them. I know every top-running team would be the first to switch to carbon if/when they are allowed.

Those wheels have done 24 hour durability testing on the GT350R and Ford GT -which are almost as fast and faster than GS cars, and held up with no issues.

The one thing about CF is that when it does yield it gets ugly fast. Think: shards. Something most any sanctioning body would prefer to avoid, no? A different failure mode than what you see in a typical aluminum or steel wheel. Compared to metals, when composites are fatigue loaded the density of interlaminar and intralaminar "micro cracks" multiplies as opposed to that of growth of a single crack. That can clearly be an issue with even the most basic means of evaluating - a visual inspection.

I have yet to see CR share any data on this type of testing. You see snippets of testing to a certain standard for road "worthiness" but nothing detailed. Completely understandable, as the aim is to draw interest as opposed to scaring anyone away. In the context of CF versus the typical aluminum wheel, carbon fiber can be woven to be very strong in a given plane but make no mistake, it does not go through plastic deformation like various metals do.

When CF fatigues it does so in three stages (successive). First up is matrix cracking. Second, local delamination as a result of matrix cracking. Finally, local delamination consolidates leading to failure. No doubt, CR has optimized critical elements such as strand choice(s), weave pattern, resin type and dispersion, along with cure rates/temperatures, that in the end make for a wheel that shouldn't fail catastrophically under what could generally be considered "normal" use. Their inclusion on certain production vehicles as of late is indeed impressive. But the fact that their use to date in motorsports has yet to become the norm is telling. There are plenty of other CF components used in racing, but not wheels. Talk about shards at created via impact at speed...

Composites are an amazing technology. They can be designed to disintegrate (in your case of a crash structure) or they can be designed not to shatter into a million pieces (in the case of a monocoque). If carbon always blew up into shards in every application, it would be deadly to use in a monocoque. But the carbon fiber weave, pattern, epoxies, etc... are changed for different needs of different applications. The assumption that all carbon blows up like crash boxes and winglets on DTM or F1 cars is a common misconception, when they forget about the strength and design differences that carbon plays in monocoques.

I've seen a video of the Carbon Revolution wheel stressed to the point of failure and I can assure you it does not "explode" or fail like your video of a crash box, which is designed to explode to dissipate a straight-on load in the most effective way possible. The CR wheel failure is rather uneventful.

"That's why it's common to see people who switch to those lightweight wheels then say that "This car (GT350) needs 2.5, 3, 3.5, 4* of negative camber and push GT350 (and even R) owners to try to adopt those levels of camber that's unnecessary with a stiffer wheel."
Maybe I'm ignorant on a relationship on wheel stiffness and camber, but it's my understanding that you utilize camber to increase the tire contact patch and make the tire temperatures more uniform across the contact patch surface under the heavy tire loading that occurs on a track. This gives you better grip and more even tire wear for track use. To my knowledge there is no relationship between wheel stiffness and camber. Correct me if I am wrong.
<EDIT> So, I read the article, but there is no empirical data to support the claim. "ultra-lightweight aluminum wheels can often lose 1-degree of camber due to camber compliance PER cornering G." They say they tested a big-name aluminum wheel, but that could be a crappy $300 ForgeStar for all we know. Test it against the VIBE 7-spokes, or the Apex EC-7's, which is what enthusiasts are using in place of the CF wheels for track use. Otherwise, this is just another marketing ad. The CF wheels are terrific, but until they provide some verified performance numbers and offer a squared setup, I won't be convinced that the performance gain is worth 2-4x the cost.

Thank you for reading the article. The 'big-name aluminum wheel' brand and test is under an NDA due to issues with showing major issues in a competitor brand. Without concrete data that shows this deflection, the only proof from the article to show the camber compliance is the tire wear between the carbon and aluminum wheel. The excessive wear on the outer shoulder demonstrates the wheel flexing and overheating the outer shoulder. OEM GT3 wheels are very nice and expensive, and a bit heavier than the carbon wheel. If you take a cheaper wheel (especially a weaker 'stylized' wheel) that is even lighter than the GT3 wheel, it will have more flex -as discussed in the article, and will have even more camber compliance and outer shoulder wear.

The suggestion that "Forged and Cast Aluminum wheels have the same stiffness" is just plain nonsense.
Which alloys are being used? Most cast wheels use A356/T6 (heat treated) with a nominal tensile strength of 34,000 psi.
Many forged wheel companies use 6061T6 with a tensile strength of more than 40,000 psi. Another common wheel alloy is 6069 with 60,000 psi.
A few companies use 7075 T6 with almost 80,000 psi tensile strength.
Equally important is the wheel design. Stiffness comes from spoke length, spoke spacing, cross section and a dozen other factors. Certainly not just the material.

I'm a big fan of carbon, but it is extremely expensive and very difficult to manufacture.
And the actual weight savings of carbon over a well-designed forged wheel is 10-15%.

Much of what was presented came from the marketing folks. Engineering is built around facts and laws of science.

In the immortal words of John Adams, "Facts are stubborn things".

You seem to be confusing stiffness with strength. As mentioned in the article, they are different.

Yield strength is a measure of strength. Changing alloys and forging wheels does affect strength.
Stiffness is measured by Youngs Modulus which states that ALL aluminum has the same stiffness. This is discussed further in the article.

Agreed that wheel design (and weight) affects stiffness. But as mentioned in the article, there is only 1 'optimal' design for stiffness of an aluminum wheel (and a different, 'optimal' design for carbon) and given the best design, aluminum is on a completely different stiffness to weight scale from carbon. There's a nifty graph that shows this in the article. So given an already optimal design, aluminum can only be nearly as stiff as carbon by weighing SIGNIFICANTLY more. Or if its anywhere near as light as carbon, it will be SIGNIFICANTLY softer/less-stiff.

Indeed "Facts are stubborn things". FYI - in attempt to make it easier to read for those who don't have engineering degrees, this article was simplified from a lot of material data as well as confidential figures from tests that Carbon Revolution has conducted.

And the fact is CF wheels are better, albeit expensive. I don't think the article says otherwise. I work in test/prototype engineering for Army aviation. If somehow stating the benefits of CF over AL is propaganda, then my friends, the Army has been hoodwinked lol. I cant say much but i can tell you this, CF is superior in every application we have tested thus far. Every year we prototype/test/validate and field modifications made out of CF. In fact, it is the material of choice, for the obvious reason.

Agreed. There's a reason the military, aerospace, and motorsports uses composites. It's not because it looks cool.

 

[Forgedwheeler]

I'm not confusing stiffness with strength. I'm an engineer with 40 years of experience in wheel making.
Your assertion that there is one optimum shape for stiffness is nonsense.
Your assertion that any Carbon wheel is stronger and stiffer than any forged wheel is also nonsense.

Materials with higher tensile strength deform less under a given load. they are therefore stiffer. Sheesh.

An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region:[1] A stiffer material will have a higher elastic modulus. An elastic modulus has the form:

1. Young's modulus (E) describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain. It is often referred to simply as the elastic modulus.

I suggest you stick to marketing.

 

[JAJ]

You seem to be confusing stiffness with strength. As mentioned in the article, they are different.

Yield strength is a measure of strength. Changing alloys and forging wheels does affect strength.
Stiffness is measured by Youngs Modulus which states that ALL aluminum has the same stiffness. This is discussed further in the article.

Agreed that wheel design (and weight) affects stiffness. But as mentioned in the article, there is only 1 'optimal' design for stiffness of an aluminum wheel (and a different, 'optimal' design for carbon) and given the best design, aluminum is on a completely different stiffness to weight scale from carbon. There's a nifty graph that shows this in the article. So given an already optimal design, aluminum can only be nearly as stiff as carbon by weighing SIGNIFICANTLY more. Or if its anywhere near as light as carbon, it will be SIGNIFICANTLY softer/less-stiff.

Indeed "Facts are stubborn things". FYI - in attempt to make it easier to read for those who don't have engineering degrees, this article was simplified from a lot of material data as well as confidential figures from tests that Carbon Revolution has conducted.

Agreed. There's a reason the military, aerospace, and motorsports uses composites. It's not because it looks cool.

Thanks for the original article and even more thanks for your patience in responding to the follow-on posts!

 

[JAJ]

I'm not confusing stiffness with strength. I'm an engineer with 40 years of experience in wheel making.
Your assertion that there is one optimum shape for stiffness is nonsense.
Your assertion that any Carbon wheel is stronger and stiffer than any forged wheel is also nonsense.

Materials with higher tensile strength deform less under a given load. they are therefore stiffer. Sheesh.

An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region:[1] A stiffer material will have a higher elastic modulus. An elastic modulus has the form:

1. Young's modulus (E) describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain. It is often referred to simply as the elastic modulus.

I suggest you stick to marketing.

This is a table of elastic modulus, tensile strength and other characteristics of various Al alloys:

https://www.engineeringtoolbox.com/properties-aluminum-pipe-d_1340.html

Elastic modulus doesn't change much (less than 10%) from one alloy to another. Yield strength is all over the map though.

 

[BillyJRacing]

I'm not confusing stiffness with strength. I'm an engineer with 40 years of experience in wheel making.
Your assertion that there is one optimum shape for stiffness is nonsense.
Your assertion that any Carbon wheel is stronger and stiffer than any forged wheel is also nonsense.

Materials with higher tensile strength deform less under a given load. they are therefore stiffer. Sheesh.

An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region:[1] A stiffer material will have a higher elastic modulus. An elastic modulus has the form:

1. Young's modulus (E) describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain. It is often referred to simply as the elastic modulus.

I suggest you stick to marketing.

Thank you for giving your background, which correlates to your username. I understand your resistance to new technology that infringes and affects your livelihood.

Carbon Revolution has undergone extensive testing of strength and stiffness and meet or exceed all OEM requirements for a wheel. OEMs are a HUGE PITA to deal with and to meet their standards due to liabilities, so yes, CR have done a lot of testing and i've seen their results that show they are stiffer and stronger than forged wheels. If you don't have any NDAs, I would love to see your 3rd party testing results that disprove this. If you have them, please feel free to share here.

I apologize that you feel the need to be condescending and hostile to assert your superior knowledge in an open-discussion environment. I may not have an engineering degree but I work with a lot of world-class engineers and know enough to get by on a functional level. And yes I know what MOE is and how the stress-strain curve defines the stiffness of an object, as well as the yield point (strength).

Thanks for the original article and even more thanks for your patience in responding to the follow-on posts!

Thank you for the kind words. This article took months to write to take a bunch of white paper figures and Carbon Revolution's test results and not cause any issues with NDAs, while being technically correct, and while attempting to simplify it to make it easy to read for most people. It was quite a challenge and has gone through more revisions and technical checks than I would care to do again anytime soon.

 

[Forgedwheeler]

Carbon, like most other materials, has wide variances in strength and stiffness based upon a huge number of highly variable factors.
I'm just very uncomfortable with the assertion that "carbon is better" and "Carbon is stiffer" as unsupported glittering generalities.
I am absolutely certain that I can design and manufacture a forged aluminum wheel with 100% of any carbon wheels "stiffness " within 10% of its overall mass at one third of the manufacturing cost.
That reason is why carbon has made very little inroads into a share of the wheel market.

Carbon is lighter, yes. It has fantastic tensile strength, yes. But wheels have certain minimum thickness requirements that limit the advantages of carbon.

You have to tell the whole story.

 

[Lurker_350]

I'm not confusing stiffness with strength. I'm an engineer with 40 years of experience in wheel making.

Materials with higher tensile strength deform less under a given load. they are therefore stiffer. Sheesh.

An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region:[1] A stiffer material will have a higher elastic modulus. An elastic modulus has the form:

1. Young's modulus (E) describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain. It is often referred to simply as the elastic modulus.

I suggest you stick to marketing.

Wow. From an engineer no less......you are confusing stiffness due to geometry and stiffness due to material properties.

@BillyJRacing is exactly right. Young's modulus is a measure of how stiff a material is. You can change the stiffness of a created object by changing the geometry or material (Youngs modulus), but you CANNOT change the stiffness by increasing strength of a given material. A larger Young's modulus means the initial part of the stress/strain curve is steeper (and therefore the material is stiffer).

For example:

6061 aluminum O - 7 ksi YS, 17 ksi US, 10E3 ksi Youngs modulus
6061 aluminum T6 - 37 ksi YS, 42 ksi US, 10E3 ksi Youngs modulus

T6 heat treat is far stronger (both yield strength and ultimate strength are greater), but no stiffer. Both have a Youngs modulus of 10E3 ksi (which is far less than steel at 29.7E3 ksi).

 

[Hack]

@BillyJRacing is exactly right. Young's modulus is a measure of how stiff a material is. You can change the stiffness of a created object by changing the geometry or material (Youngs modulus), but you CANNOT change the stiffness by increasing strength of a given material. A larger Young's modulus means the initial part of the stress/strain curve is steeper (and therefore the material is stiffer).

For example:

6061 aluminum O - 7 ksi YS, 17 ksi US, 10E3 ksi Youngs modulus
6061 aluminum T6 - 37 ksi YS, 42 ksi US, 10E3 ksi Youngs modulus

T6 heat treat is far stronger (both yield strength and ultimate strength are greater), but no stiffer. Both have a Youngs modulus of 10E3 ksi (which is far less than steel at 29.7E3 ksi).

Well written explanation. Higher yield strength doesn't necessarily mean that the amount of deflection under load changes (within the elastic region). Some materials can be either more brittle or more ductile than another material, while still having a similar Young's Modulus.

 

[JAJ]

I have some sympathy for @Forgedwheeler though - his point is that while aluminum isn't as strong or as stiff as CF, you can still make a stiff wheel out of it. The stiffness of the finished wheel is determined by the design, not just the material.

The classic racing wheel design with five or seven y-shaped spokes (presumably) produces adequate stiffness in rotation to handle accel/decal forces while providing the best stiffness laterally for lateral g's. Making a wheel out of CF has to be more complicated because the stiffness may well be different in different directions depending how the fiber layers were built up.

Of course, the easiest answer is to build wheels out of the stiffest, strongest material you can find - steel is 6x stiffer and 4x stronger than aluminum but it's only 3x as heavy. Theoretically, an optimized steel wheel could be lighter than an aluminum one.

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