When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Wheel stiffness affects performance. -Testing Carbon Revolution's CF wheels vs AL
Hey guys,
(I wrote the article, please feel free to ask me any questions to discuss either the carbon wheels or the general principles that can affect ANY brand wheel -not just carbon).
Whether you own a GT350R, Ford GT, or Ferrari 488 Pista with Carbon Revolution's Carbon Fiber wheels or are just looking to buy new wheels for your car. Check out this article that explains the importance STIFFNESS plays in ALL wheels, the effects of reduced unsprung weight, rotational inertia, NVH, and strength (which is different from stiffness) has on ALL wheels.
You'll also learn that carbon fiber wheels are a game-changer, forged & cast wheels have the same stiffness, and lighter is not always better when choosing a set of wheels. Hopefully this article will provide you with more knowledge to make better wheel purchases in the future.
Don't have time to read? -Listen in where I join The Exotic Car Hour podcast to discuss the technical aspects from the MotoIQ article on the importance stiffness plays on ALL wheels, clarify some misconceptions of carbon fiber, and talk about my involvement helping to develop some of Ford's greatest cars like the Ford GT and the GT350/R Mustang:
This article is well written and very informative. It's a thoughtful description of a theory, but it doesn't provide any objective information about the theorized lack of camber compliance of aluminum wheels. The problem is that you have mixed a number of accurate statements like the ratio of lightness to stiffness in aluminum with some highly biased theory from the carbon manufacturer without providing any data on how the tests were performed. It's made more misleading by taking the theory of aluminum camber compliance and then performing a bunch of calculations based on this unsupported number to make it look "mathematically objective."
This is well written, but you haven't shown camber compliance is the factor. It may just be weight.
On the objective side, I did a quick statistical analysis on your lap times. The results are almost statistically significant when considering standard deviation (p-value = 0.07), so the pure math says there is probably some real advantage to the carbon wheels. I'm guessing it's likely just unsprung weight.
Haha. It was a challenge and very time consuming to take their data and Brett's far more in depth engineering jargon into a (hopefully) easier to read and understand format that didn't read like a white paper.
Earlierapex - thank you for giving it a read and your reply. I understand your skepticism on the unsupported camber compliance measurements. Unfortunately I was not allowed to post the photos of the rig or able disclose which brands deflected 0.5*, 1* or 1*+ per G of cornering force. I don't believe it was misleading at all to take measured results (whether believed or not) and put them into a real world example to make it easier to understand and backed up by the fact that I've personally ran those amounts of camber on the examples given.
If you are skeptical of camber compliance, and credit the lap time gain solely to the unsprung weight, what do you make of the tire wear pattern and amount between the carbon and aluminum wheels?
Thanks for the compliments and for contributing to the discussion!
I don't mean to be a jerk - this is intended to be an objective, devil's advocate approach. Not intended to offend.
Your theory hasn't yet been tested empirically with defined variable controls. You haven't empirically demonstrated that camber compliance even exists, much less that it is the primary variable for the lap time difference. In a strict statistical sense, you actually haven't even demonstrated that there IS a lap time difference (the p-value is 0.07). In other words, the faster carbon wheel lap times are lower, but they aren't "enough" lower to demonstrate that the lower times aren't just a random occurrence given the mean difference and standard deviations of the lap times (admittedly, they are almost significant).
You can't just say, "there's weird tire wear and CC seems like the best explanation, therefore camber compliance exists." That's a theory that needs hard data to prove. You almost show enough data to prove lower lap times, but you haven't shown any data whatsoever on camber compliance. I could state an equally plausible (and probably far more likely) theory - how do you know the tire wear isn't simply a function of better suspension control of lateral movement over bumps due to reduced unsprung weight?
There are a ton of questions here:
1) was the driver blinded to which wheels were on the car?
2) How did they calculate deflection to measure camber compliance? What are the exact details of the procedure?
3) Did they attach the wheel to some fixed device and apply a force at the rim? What was the exact force vector and how was the CC deflection measured? How was the force vector calibrated? Please show a picture (not a drawing) of an actual wheel being bent into this bizarre theoretical shape.
I'm in science, not engineering, but I have real trouble believing the theory of CC in terms of pure theory because you haven't taken into account the wheel barrel. If the lower half of the wheel were to deflect inward like your drawing: 1) the lower web radius would have to elongate, 2) the outer barrel rim radius would have to elongate, 3) simultaneously, the inner rim radius would have to compress, 4) which would require that the barrel elongate across the width of the barrel. There would also be rotational forces on the spokes in the horizontal plane that would twist as the lower half elongates. All of this would have to occur within the very narrow window of elasticity (where metal bends and returns to it's original form) before plasticity (where metal bends and stays bent) and without the tire popping off. If the barrel radius could change with 1G of force, then the wheels would have an oval (squished) shape when the car was just parked in the garage.
A (slightly) more plausible theory would be that the web would elongate by itself while the barrel keeps its rigid radius. If this were to happen, the hub would move perpendicularly to the barrel, which would shift the car's weight vector towards the outside of the wheel, which would change the tire patch dynamics, which would likely REDUCE grip and the lateral g-force window which would DECREASE overall camber changes across the suspension system.
And I don't believe this "more plausible" theory is even remotely possible. The problem is that the modulus of elasticity window is actually pretty narrow. If you get anywhere near that window, you are likely to periodically go over into plasticity, which means you would see guys at the track with formerly concave wheels that become permanently bent into a convex shape (the hub would permanently move to outside the outer rim). You would also see far more wheel failures (broken). Have you ever seen a wheel with a permanent convex bend? I haven't. Not one, ever.
If BBS were under-engineering aluminum wheels to a point that would allow them to get to the point of elasticity and be so close to the failure window that they would bend without warping or breaking, we would see multiple wheel failures at every DE weekend. And here's how we KNOW the wheels are engineered to a standard way beyond 1g=bend: if I could get so close to the failure point of AL wheels so that they would elongate at 1.5g with MPSCs, they would likely be permanently bent or broken with hoosiers and they would break on the first turn with slicks. I've seen tons of GT3s on pure racing slicks without permanently bent convex wheels. If they aren't permanently bending or breaking their wheels on slicks, how is even remotely possible that I could bend my wheels on mere street tires?
And there are lots of other practical and speculative challenges to the competing theories here:
1) have you ever seen the calipers on a 991 GT3 with PCCBs? My calipers are mere millimeters away from the wheel. If your theory were true, my calipers would contact the wheel spokes and the inner wheel barrel would also hit the lower suspension arm nut (which is even closer than the caliper). A 1.5 degree camber change that occurred solely within the body of the wheel itself would work out to about an 8mm reduction in wheel radius on the inner rim.
2) you haven't accounted for the serial failure rate of an engineering system. If the wheel is going to bend, the remainder of the system will have had to have exceeded the elasticity windows of compression and tension that are much lower than that of a rigid aluminum wheel. In other words, you are implicitly stating that the suspension bushings and springs can compress to a point that will bend aluminum without failing.
3) the modulus of elasticity changes fairly dramatically with temperature. If I could bend my wheels on a few laps on a warm day, what would they look like at 500C on a hot day during an endurance race?
In one simple statement to show the hyperbole of this whole theory of camber compliance: you are saying that you could use a rubber tire to bend an aluminum wheel. I have real trouble believing that, but I'm also completely willing to admit when I'm wrong. Just show us the data (and a picture of the bent wheel with a reasonable force vector that is applied through a rubber tire and with a wheel that then returns to its normal shape).
Part of my point here is to show how wild, speculative theory is just that. Theory can get silly without data (on both sides of the equation). Again, this is intended as a discussion, not an attack!
I couldn’t agree more with your statement. The likelihood of significant movement under 1 or 1.5g of side force in addition to centrifugal and a 1g stationary load, simply doesn’t make sense
Be interesting to see what the forces would look like on an aluminium rim given a rubber tyre is attempting to pull/bend the rim bearing on the barrel more than rim edge in any case.
if it ‘only’ takes 2-3 degrees of camber in most situations to counter the suspension geometry Changes, suspension compliance and tyre deformation it would suggest that wheel deformation is greater than the influence of body roll/ geometry, tyre and suspension - sounds unlikely
I have a bit of data to add here. I am running 19" rims on my 2018 GTS with RWS.
The OZ rims I am using just fits over the lower suspension arm, about a 4 mm gap, and
the rims narrows in diameter just 3 mm inside of the suspension arm.
Running at Chuckwalla, no problem. Running at Autoclub with its long high G turn 1 + 2, the suspension arm
rubbed on the inside of the rim ( a sideways movement of 3mm + digging into the aluminum rim).
I have since added a 7 mm spacer to eliminate this rubbing. So maybe not a 8 mm shift, but at least a 3-4 mm shift.
I have a bit of data to add here. I am running 19" rims on my 2018 GTS with RWS.
The OZ rims I am using just fits over the lower suspension arm, about a 4 mm gap, and
the rims narrows in diameter just 3 mm inside of the suspension arm.
Running at Chuckwalla, no problem. Running at Autoclub with its long high G turn 1 + 2, the suspension arm
rubbed on the inside of the rim ( a sideways movement of 3mm + digging into the aluminum rim).
I have since added a 7 mm spacer to eliminate this rubbing. So maybe not a 8 mm shift, but at least a 3-4 mm shift.
Thank you - very interesting, but could it just be the RWS? Looks like up to 1.5 degrees from centerline with more in-phase steer in higher speed turns - assuming a radius of about 11 inches, that's about a quarter inch (~6mm)
You haven't empirically demonstrated that camber compliance even exists,
On this point. Camber compliance is a real, very well understood phenomenon that OEMs routinely both measure and optimize. As a system it's typically measures with a K&C machine, which stands for Kinematics and Compliance:
Nothing is infinitely stiff, so every part in the system has its own spring rate; it will bend and deflect, the question is simply "how much?". Modern suspension is designed to minimize this deflection- it's one of the advantages of multi-link suspension over other types, also a reason the NVH of monoballs is tolerated in performance applications.
Simulation could provide a fair estimate of the amount of deflection on an aluminum wheel, something I have yet to find time to do, but at nearly 2k lbs of rear corner loading you can be certain the wheel does flex and deflect.
I do have K&C data from modern 911s that measure camber compliance with and without tires. Unfortunately it's not mine to share, but it suggests that the quoted 1 degree loss per G for aluminum wheels is for wheels that are significantly less stiff than Porsche typically uses.
That's very helpful. I suspect you could teach us a lot. Just a couple of comments 1) it appears that the K&C analysis looks at the camber of the "system," not the wheel. 2) your unshared data seems to validate the skepticism that an aluminum wheel will deflect by 1 deg per G.
This whole topic is a bit bizarre for me because you are the second resource to say: "I promise wheel camber compliance exists, but I'm not going to show you any data." I understand it's proprietary, but raw data isn't subject to the whims of interpretation and confirmation bias...
I did a little digging this weekend and found this "in silico" analysis of carbon vs. AL wheels (so still not empirical data) - it looks like max theoretical AL deflection under acceleration and cornering is about 0.65mm and 0.2deg (see last page), or roughly 1 order of magnitude less that what was claimed in the original article above.
I think the last comment was very interesting. Given the reality about system stiffness and manufacturer capabilities. I suspect
1. Porsche know and understand the system compliance very well
2. Modifications made to a cup car vs GT3 will show you where the first system gains are made
3. Wheels like all components flex, but the amount of flex will be I suspect small enough to be acceptable in the system. I suspect Porsche wheels only flex a very small amount, especially GT car wheels
4. Some manufacturers will knowingly sell cheap and in there system acceptably flexible wheels
5. The very expensive carbon wheel manufacturer has taken one point and made it a sell when in most cases it isn’t
09-19-2018, 06:58 PM
Wheel stiffness affects performance. -Testing Carbon Revolution's CF wheels vs AL
