Does Wheel Stiffness Affect Performance?  We test Carbon Revolution’s Carbon Fiber Wheels

Factory OEM Porsche Aluminum Wheel:

The OEM 997 GT3 wheels are 19×8.5 +53 and 19×12 +68 and are wrapped in 235/35-19 & 305/30-19 Pirelli P-Zero Trofeo R tires. The fronts weighed in at 46.2lbs (21.0kg) while the rears came in at 57.2lbs (26.0kgs).

First up was the OEM Porsche wheels. After doing a sighting lap in another car to ensure track conditions were good, we re-checked the cold pressures and I buckled myself in the car and did two nice, easy out-laps to scrub the mold release agent off of the tires. As expected, the front tires took a bit of sawing at the wheel to get them to come in but as soon as they were cleaned off, the front of the car switched-on and we were good to start our first flying lap.

As expected from the Trofeo-Rs, the first flying lap was the fastest, but as the lap times wore on, so did the left-front tire on this primarily right-hand track. Turns 8 & 9 are a long, double-apex right-hander that requires a lot of entry speed and trail-braking to make it far into the corner before rotating the car and powering out of. Even with a very late-apex, Turn 9 is so long that you can’t escape from the Porsche’s typical on-throttle understeer, which wore away at the LF tire.

Adjusting the line with a longer trail-brake to further reduce understeer in turn 8 helped to put down a good time half way though the run, but there was no escaping the excess wear on the LF that quickly felt worse and worse as if you’re dragging the tire across a cheese grater.

After the 10 laps we came in for a quick tire and wheel change.

Carbon Revolution CR-9 Wheel:

The CR-9 wheels were identical to the OEM 997 GT3 wheels at 19×8.5 +53 and 19×12 +68 and were also wrapped in identical 235/35-19 & 305/30-19 Pirelli P-Zero Trofeo R tires. The fronts weighed in at 38.5lbs (17.5kgs) each while the rears came in at 46.2lbs (21.0kgs). That makes for a 7.7lb & 11lb weight savings front and rear per wheel for a total of 37.4lbs of unsprung weight savings.

Once the CR-9 wheels were mounted and after double checking the tire pressures, I headed back out on track. Once I left pit lane, the difference in the carbon wheels caught me by surprise. Just making the slow left turn leaving the pits (not cornering any harder than you would on the street), the response from the car and how quickly it ‘pointed’ in to the corner and how connected the steering was apparent when I was not even considering paying attention until the start of the first timed lap.

Sure enough, by the time I made my second turn in the long Turn 2 sweeper, the improved steering response and direct feel remained consistent, backing up what I just felt in the previous turn and responding very quickly even on cold Trofeo R’s! The difference was so apparent that it felt like I was on completely different tires, as if the baseline was done on soft sidewall all-season tires, and I was now on R-compounds. It really was that noticeable and I wouldn’t believe it if you told me.

During the 2 laps of scrubbing in the front tires, the steering felt more direct and connected to the tire. I don’t want to say the steering was necessarily lighter, since I don’t like overly light steering, but the steering response felt cleaner and more exact in the way a Lotus Elise/Exige or original NSX with no power steering gives you that feeling through the steering wheel of being directly connected to the road, the carbon wheels gave this Porsche that same character. It is eye-opening to see how this ‘enabling-technology’ really did help the steering feel of the Ford GT, which already has hydraulic steering and one of the coolest suspensions of any road car, but especially the GT350R that’s still mechanically (steering wise) a Mustang.

After the front tires switched-on, I was ready for my 10-lap timed run. Lap 1 was a good 0.38 seconds faster than the old baseline and what really suck out was how sharp turn-in was. I could be smoother and slower with my hands to get the same chassis reaction out of the car, meanwhile I didn’t need as much steering input to get the car to turn. Another thing that greatly stuck out was the improved front grip, especially when trail-braking. Trail-braking is the hardest load on an outside front wheel because it’s both braking and turning. If the wheel is going to flex, trail-braking is when it will flex the most.

Sure enough, the stiffer carbon wheel had more front grip in trail-braking, allowing me to brake even later, carry more entry-speed and getting the car rotated sooner before putting the power down. Putting the power down was also immediately more responsive.

The curbs are not necessary to use at LVMS but I picked a few to hit consistently on all the laps just to see how the wheels react over bumps since otherwise, the track was billiard-table smooth. The carbon wheels soaked up bumps without launching off the curbs like it did with the aluminum wheels. The car landed with a single hop recovery, while the aluminum wheels had two undulations of recovery.

The Carbon Revolution wheels enabled our car to go 0.38 seconds faster on a 50-second lap. To put that into perspective, on a normal 2-minute lap, that difference would be 0.85 seconds. On a 3-minute track like VIR’s grand course, that difference would be a substantial 1.27 seconds. Short of Lime Rock Park, there aren’t too many tracks with 50-second laps, but over the course of the 10 laps, the time difference totaled 1.68 seconds. Race teams would spend a lot of money for those kind of gains.

Overall, the improved response of the car made it feel like I was on a more aggressive, stickier tire. Or as if either the car had stiffer sway bars or suspension bushings. The difference was quite noticeable and you don’t have to be a racecar driver to feel it. I’m sure pretty much anyone who enjoys driving would be able to tell a difference.

As the laps clicked off, the Carbon Revolution wheels were consistently faster and more importantly, consistent than the baseline run. Where the front tires were all but gone by the end of the 10-lap run on the baseline, I had plenty of front grip and probably should have pushed the LF tire harder in the long sweeping right-handers.

It was quite eye-opening to experience such an improvement in the steering feel, front grip, and nimbleness of the car. I was very interested in seeing how the left-front tire held up. Once I got back to the pits, I hopped out of the car and immediately looked at the LF tire. I was blown away to not see any of the blistering like we saw on the OEM aluminum wheel.

These are BOTH Left-Front tires from the comparison test run on the same car with the same camber for exactly 12 laps on each tire. The Carbon Revolution CR-9 equipped wheel (left) had very little feathering on the surface of the tire, and minimal wear on the outer shoulder (it’s flipped around for the image). Meanwhile, the OEM aluminum wheel (right) had a lot more understeer-induced feathering across the tire and more importantly, the outer shoulder is severely blistered from over-heating due to a lack of camber from the wheel deflecting under load.

Now Porsche is a great company and they know how to make a lightweight wheel. They also know how to make a strong wheel. These OEM aluminum wheels cost well over $5K and are excellent, especially compared to most aftermarket wheels. However, there was an obvious and visual amount of camber compliance going on here. With no other difference than the wheels, for the tire to roll-over and blister like it did, the wheel had to deflect and lose dynamic camber, which focused the heat on the outer-edge of the tire and punishing it beyond its operating range to fail and blister.

By comparison, the Carbon Revolution wheel is much stiffer, has far less camber compliance and thus, effectively has more camber mid-corner to spread the load across the tire more evenly. This allowed the tire to work better and explains our increased front grip. Remember, the only change was the wheel. We were on the same size wheels and tires and started with the same cold pressures and ran within minutes of each other. A comparison test doesn’t get much better than that.

Carbon Revolution themselves were quite surprised as well and didn’t expect to see such an obvious and empirical evidence of the difference of camber compliance. But it makes sense that having such little static camber (factory camber is around -0.5* in the front) on a very sticky and aggressive tire would be a great method to show camber compliance. If the car had more static camber, the only way we could have shown camber compliance is by looking at tire temperatures with a probe-type pyrometer.

CONCLUSION

This visit to Las Vegas Motor Speedway was quite eye-opening. We learned a lot regarding the importance wheels play on the performance of the car and how stiffness, light weight, lower rotational inertia, and unsprung mass improve steering feel, tire wear, grip, and lap time.

We walked away with real-world visual tire wear evidence of the increased stiffness Carbon Revolution wheels have over high-end OEM aluminum wheels. We also gathered data that showed a lap time improvement of 0.38 seconds on a 50-second lap, which would translate to close to a second on most tracks with 2-minute laps.

Carbon Revolution has shown us the future of wheels and how the use of carbon fiber technology can improve every aspect of how a car rides, performs, and ‘enables’ every aspect of the car to work better. Carbon truly is a revolutionary material that shifts the paradigm of what wheels can do the same way carbon fiber monocoques changed the face of aircraft and racing.

Going back to the original question: Do your wheels flex? -Of course they do, they all do. But depending on the material, design, and weight of your wheels, it may be a little or significant. When buying a new set of wheels, be sure to consider not only the brand and quality of the wheel, but the trade-off between stiffness and weight. The only way to have a significant amount of both is to go to a game-changing carbon fiber wheel, which has countless performance and ride benefits above and beyond simply bragging rights and bling factor.

SOURCES:

Carbon Revolution
Dream Racing
Pirelli Tires
Ford Performance
BillyJohnsonRacing.com

Table of Contents:

Page 1 – Intro
Page 2 – Carbon Fiber Wheels used by OEMs
Page 3 – Benefits of Carbon: Lightweight – Rotational Inertia
Page 4 – Benefits of Carbon: Lightweight – Gyroscopic Effect
Page 5 – Benefits of Carbon: Strength
Page 6 – Benefits of Carbon: Stiffness
Page 7 – Benefits of Carbon: Stiffness Continued – Camber Compliance* (Must-Read)
Page 8 – Benefits of Carbon: Durability, NVH & Cost.
Page 9– Important Factors for wheels on Racecars & Track Test Procedure
Page 10 – Wheel Weights, On-Track Testing, Data/Laptime/Tire Analysis, and Conclusion

40 comments

  1. Good article covering lots of different areas. It would be interesting if in the 911 test, the camber was optimized in each case and THEN compare lap times, tire wear, and what the static camber numbers ended up being. Perhaps your lap time differences could be equalized with more static camber for the aluminum wheels. But yes, the tire wear was a good demonstration of the difference in wheel stiffness and camber change.

    In your opinion, is there any downside to carbon fiber wheels on a race car aside from cost? They would be fine with the heat for a couple hours stint? No concerns about impact loading or how they would fare in a smaller crash (e.g. an aluminum wheel would bend and get you back to the pit, carbon fiber would shatter)?

    1. More static camber might equalize things in steady state cornering but being able to get away with less camber means a flatter contact patch for better braking and acceleration in addition to not having to deal with camber issues like tramlining and surface mu sensitivity under hard braking.

  2. This is one of the most interesting articles I’ve read on this site in a while! I’m still amazed by how much the aluminum wheels can deflect.

  3. regarding camber compliance; How much influence does wheel size itself have on camber compliance? the article states that wider tires require less static camber, but what about different diameters of wheels? for example would 17’s require less static camber than 19’s?

    1. Diameter does not have as big of an influence on camber as width does. But as the diameter of a wheel increases, it becomes difficult to maintain stiffness without proportionally more weight. An ultra-lightweight large diameter wheel (say 19″) will be far more flexible than an 17″ wheel of similar weight and will require more static camber due to the increased camber compliance.

  4. Interesting, but more an ad than an actual review. Carbon fiber is nice, but still years away from being actually affordable, if it ever is. No mention of the cons anywhere. Like what happens if you hit a big pothole? You can always have an alloy wheel fixed. Try that with CF.

  5. In conclusion, make sports cars a proper weight, like 2300-2800lbs and equip with 16in wheels and well-boosted hydraulic steering and a set of $875.00 aluminum flow formed wheels will work as well, and don’t put the engine in the rear like that dopey beetle you drove….and voila, problem solved, and have more fun driving.

  6. One of the better automotive engineering/technical articles published online ever.

    FYI, the spindle/bearing/hub compliance on a typical passenger car during cornering is also around 1 deg per G. Most door-slammer race cars with sticky tires are way overloading the OE wheel bearings and flexing them too much. So lots of potential gains to be had in terms of camber/toe compliance with stiffer wheel bearings or stiffer structures on both sides of the wheel bearings.

    Thank you Billy/Mike/MotoIQ for doing these fantastic articles!

    1. Hi Bill, do you have data for the hub deflection? I was wondering because I often fight this when working on production-based racing cars due to its effect on piston knockback on the brakes.

      1. No data, only memory. I had a friend 15 years ago who did a year-long benchmark project to measure competitor suspension compliance for a Detroit OE, he probably did 20 cars total. I saw him measure the data on a big flat-plate rig embedded in the floor. He removed the wheel/tire and replaced it on the hub with a stiff alignment-type stand, this allowed visual access to most of the suspension components behind. He put little dimensional measurement balls at all the connection points between parts. Then used optical 3D measurement to measure the position of everything before and after various load cases (simulated 1G cornering, simulated 1G braking, etc.). This allowed measurement of total deflection at the hub and how much individual components were contributing to it. Sort of a poor-man’s K&C machine, but it also gave all the intermediate contributions rather than just the end result.

        He said there was pretty consistently ~ 1 deg/G camber flex in the upright/bearing/hub. I don’t remember if he had a measurement point at the caliper mounting ears, this would be relevant to your brake question.

        Also, he didn’t know if camber/toe stiffness was consistent among competitors because it was a targeted parameter for suspension design, or if the stiffness just consistently worked out that way after all the other needs (cost/reliability/weight, etc.) had been addressed. Yes, he was relatively isolated from the suspension groups he was feeding the data to.

    1. No, and they do not sponsor the site either. This was an independent test that I only agreed to do once they met all of my criteria for a proper (and expensive) back to back test. Expensive because I didn’t want to conduct the test with wheels with differing widths (even 0.5″), differing offsets, 100* weather, different times of the day, etc…

  7. Moto IQ has become more and more paid spots basically masquerading as reviews. You all have to make a living, I know, but there is a lower percentage of real content vs. paid content these days. I read the site less and less…

    1. The concept of stiffness and its effect on camber compliance applies to all wheels. There are a lot of concepts and take-aways from the article that can be used to make better, more educated purchases of ANY wheel.

    2. What do you oppose about this article? I felt Billy did a great job:
      – of explaining the science/physics behind the benefits of low mass wheels
      – some cool real world example demo videos,
      – showing the material property differences
      – how stiffness affects camber performance (which I never thought of nor seen anyone else talk about)
      – executed a well thought-out test plan
      – even if you didn’t believe the lap times, the last picture doesn’t lie.

    3. As far as I know, they didn’t pay Billy anything. I don’t even think he got wheels out of this or anything at all other than he did it for his curiosity. I know they didn’t pay us anything to do it.

  8. I am curious about what you consider “real” content, I mean there are only so many times that you can kick a dead horse. on the subject of suspension setup, I would consider that subject covered by moto IQ already. Yes, i would love to see more garage tips articles, as well as maybe an engine building series of articles. The question then remains what else can they write about? Product reviews, event reviews and project cars? If there are more then please chime in to add more but from the sounds of it people are considering product reviews to be “paid” articles. Then there are project cars which often have sponsored parts in them, so people will also consider those to be “paid” even though on top of all the other costs of running this site there are costs to the parts. Finally there are event reviews, personally this is something i would love to see more on from the motoIQ staff. since these events are usually heavily sponsored by companies, would the general consensus be that they are paid articles? I dont know but i would like to see MotoIQ do a break down of shows such as Import Alliance and Gridlife. Another article series i would like to see is a MotoIQ write up on all of the different tracks around North America like Frank has been doing for the west coast.

    So what else would you consider “real” content?

    1. I had a column called “Fast Lap with Billy Johnson” and I put a lot of time into those track notes; something I get paid to do when I coach but am giving the information out for free. Those are some of the least popular and lowest viewed articles that i’ve written, despite having more time invested in writing them.

      I guess some people just want negativity and criticisms of products rather than focusing on the positives. I agree that there is some loss of a review being ‘genuine’ when there are no negatives to a given review or car, so it’s a tight rope to walk.

  9. Too bad there isn’t much options in 17″ or 18″… Never would have guessed good rims/wheels did THAT much of a difference!
    Also found the post about tyre size very informative, thank you for sharing 🙂

  10. As far as I know, they didn’t pay Billy anything. I don’t even think he got wheels out of this or anything at all other than he did it for his curiosity. I know they didn’t pay us anything to do it.

    1. They paid for my flight and hotel, but I did not get paid or wheels for conducting this test. I refused to write the article or do this test unless it was going to be a PROPER test. It took 6-8 months to get everything in order to the point I was satisfied that we had everything to do an accurate, scientific back-to-back test to evaluate and quantify the performance of the CF wheels over AL on a car.

      Unfortunately every magazine test I’ve seen being conductive is highly flawed and introduce variables far greater than the actual performance difference between whatever they are testing (car A vs car B). It costs money to do proper testing and not many people want to find out the truth, but rather write stories based off the results of poor test procedures.

  11. My concern more than anything, (great article by the way) is kinetic energy.
    Let’s take this Porsche GT3 for instance, which hits a pothole at 80mph causing a X kilo-newton of force to bend or deform the rim. (I use kilo-newton for the sake of the explanation).
    While the OE alloy wheel would bend or break, (not sure if those GT3 wheels are cast or forged, it doesn’t matter really)…..the OE wheel would be damaged while the Carbon wheels wouldn’t?!?! Or to a lesser degree….
    Well, it is impossible to eliminate kinetic energy, therefore that force would have to transfer thru the wheel bearings into the hub carrier, hub, into the shock…..somewhere that shock, SOMETHING would give up. It could be the spindle, a control arm, or even the damper…..causing to crash.
    At that point you ask yourself: yes I spent $12K on bling bling wheels that CAN take the force and the abuse, but I crashed because something else gave in.
    I have a really hard time believing that the carbon wheels would suck up/ absorbing all the kinetic energy when hitting a pothole, (or something of that nature). They can cope better, yes….but to eliminate kinetic energy, not a chance.

    1. the way I see it, if you can afford to drop 12k on a set of wheels, you can afford to replace wheel bearings or hubs or whatever… also the tire absorbs a lot of kinetic energy, one of the reasons I don’t like running rubber band no sidewall tires…

  12. Great article! I love reading stuff like this. More technical articles like this are needed in our overly “showy” scene. I like to know what actually makes a part better and how it works.

  13. Very interesting post indeed. You are right, this never crossed my mind. But, thinking about it, I wonder how many of us in REALITY actually drives/pushes our car hard enough to cause the wheels to flex to cause camber changes like this given our car is properly equipped for it’s purpose. Also, most of us that goes to the track knows better not to equip ourselves with sub-par quality/cheap wheels to begin with if serious about time. Lastly, there has to be more information regarding the wheel size compared to weight of car and suspension geometry to cause x amount of flex during cornering. Lots of cars out there running larger/wider wheel sizes vs stock. I.e…if i ran stock wheels w/ some track tires and a fully built suspension, the flex would likely to occur or be more than me having an over sized forged wheel.

    1. You’ll notice the difference and improvement in steering feel due to the reduced gyroscopic effect when simply turning the wheel at speeds above 20mph. As mentioned in the article, Forged wheels are not stiffer and will not flex less than a cast wheel, and high quality and expensive light weight aluminum wheels will flex a lot since stiffness is relative to weight. The lighter the (even high quality) wheel, the more it will flex.

  14. 3 questions.
    I live in Wisconsin and drive an NSX.
    1 – How do carbon fiber wheels hold up with salted streets? I’ve owned carbon fiber bicycles, but never rode them outdoors in winter. 30 years ago aluminum rims rusted in winter in the salt, but between anodizing and coatings I don’t see much of that today.
    2 – How much weight would these save compared to the standard 2017 NSX aluminum wheels?
    3 – How aero are your carbon fiber wheels? the look more aero than my NSX wheels. As a former Cat 1 cyclist, aero wheels are near and dear to my heart.

    I’ve been thinking about putting all weather tires on the aluminum wheels and the high performance summer-only tires on carbon fiber wheels.

    1. 1 – Read Page 8: they meet all OEM standards for durability, corrosion, etc..
      2 – I do not know how much 2017 NSX wheels weigh.
      3 – I am not aware of any work that has been done in terms of designing the wheel for aerodynamic purposes,.

      Your NSX will be faster with far better steering feel and communication with the CF wheels.

  15. I enjoyed this article a lot. A lot of food for thought when choosing wheels for the track. Thank you Billy for covering all the basics and thoroughly explaining the details by having pictures and videos.
    I myself have used the TE37 for almost 2 decades and I wonder how they would compare.

  16. The strength comparison chart is misleading. Yes, unidirectional carbon fibers can be 3 times stringer than steel. But a commonly used carbon fiber prepreg laminate is equal to the strength of steel at the best. In practice, considering the way CF wheels are designed and mage the strength of the material is equal to aluminum. This is why the weight advantage is only 50% on CF wheels vs Aluminum. But CF has substantially better fatigue resistance so the maximum allowable stress is higher. Also, CF wheel production is difficult to control. Some wheels will come out very strong and some will fail easy.

    1. The material graphs are provided as an example of why carbon fiber composites are becoming the material of choice for applications where weight, stiffness and strength are a priority. The data is from a simple single axis material coupon load test and obviously does not provide the full picture of the multi directional loading that complex structures are subjected to in the field and engineers design them to. The intent of the graphs is to easily show people in very simple terms carbon fiber has some characteristics that are superior to metals and we have data to support that conclusion.

      No one has claimed wheels should be 3x lighter if design in carbon composites because of the material graphs.

      “In practice, considering the way CF wheels are designed and mage the strength of the material is equal to aluminum. This is why the weight advantage is only 50% on CF wheels vs Aluminum.”

      -If your statement was correct the part would be the same weight as aluminum…

      Carbon fiber has material properties that are better than metals and when combined with sate of the art manufacturing method has resulted in significant weight savings. 50% is a significant improvement over the status quo and provides a demonstrable performance improvement. Simple material properties aside, we have listed many industry examples where products are benefiting from the superior properties of carbon fiber composites. Many consumer product and aerospace have used carbon fiber composites superior material properties for stiffness, strength and fatigue to reduce weight and improve performance. Road bikes, airplanes, sail boats, F1 chassis, golf clubs, wings, road cars are all made lighter and stiffer from composites. All these applications benefit from the weight saving achieved by using composite materials to increase performance and efficiency. There’s a reason all of these examples have switched to carbon fiber construction.

      Carbon Revolution wheels are not pre-preg and modern and state of the art composite manufacturing is very repeatable. Carbon Revolution wheels are made under a state of the art quality system (TS16949) and OEM’s like Ford, Renault and Ferrari are not going to sell consumer products with a high degree of structural variation. But let’s not speculate about the repeatably of composite manufacturing …

      There are countless real-world parts made of carbon fiber that are repeatable and have been in market for many years: BMW i3 and i8 chassis, Composite drive shafts, Downhill skies, Mountain bike and road bike fork, frames and wheels, Formula one chassis and suspension links, and many more…

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