How Strong is Your Wheel?
On test standards for the aftermarket automotive Lightweight High Performance”LHP” wheel industry and choosing wheels
Stronger is heavier
Designing and engineering LHP wheels is about balancing strength versus weight. Given the same spoke design, manufacturing process and size, the heavier wheel will generally be the stronger wheel (stronger quantified as greater resistance to bending, longer fatigue life or both). Not too many HPDE drivers would want a wheel 10 lbs heavier than OEM no matter how strong it is though. This is the tradeoff. More mass = more strength, everything else being equal.
For street going vehicles that are not used in competition or any activity where the driver is required to wear a protective helmet, we believe the widely adopted test standards in the alloy wheel aftermarket industry adequately represent the loads, stresses and fatigue life requirements for wheels. Where we feel those standards can be improved upon is precisely those conditions that they were not created for: competition or any high performance usage that requires the driver to wear a protective helmet or additional safety gear.
In short, we feel the aftermarket wheel industry is lagging behind the reality of how these alloy wheels are now being used. We feel it’s time to change that.
Brief history of autocross, HPDE and wheel to wheel racing in the US.
We want to discuss this as it is vital to understand the relationship with the usage factors of HPDE and racing vs the standards LHP wheels are built to. The story goes that the first wheel to wheel race happened when the second automobile was built. Racing goes back that far, 1895. Autocross was a postwar phenomenon, growing rapidly in the mid 1960’s. Most enthusiasts don’t know that autocross is the #1, four wheel motorsports activity in the US, ranked by participation. Trackdays, or HPDE (High Performance Driving Event) came on the scene only in the late 90’s. Up until then, HPDE events were rare, once a year events for marque-based clubs, generally only in major urban areas. In the late 1990’s a few enterprising marque clubs began allowing non marque cars, perhaps Hondas in to a Alfa club track day for example. Other groups began promoting HPDE events that did not have any other marque requirements other than a drivers license and basic tech. It’s not uncommon now for virtually every state in the US to have at least one HPDE event per weekend, weather permitting. Some southern states may have 2-3 events on a given weekend. The participation rate is growing closer to that of autocross every year. Auto manufacturers, tire companies and the aftermarket have adapted by producing ever faster cars and equipment designed to perform and withstand the rigors of track use. The wheel industry went the other way, using test standards created before HPDE events existed and making ever lighter wheels to satisfy consumer demand for racing inspired designs. Breaking an alloy wheel in an autocross was and still is, exceedingly rare. Cracking or breaking an LHP wheel on track became more common as the number of drivers on tracks around the country grew. The awareness of LHP wheel fatigue life has grown with the social media boom.
Is JWL/VIA good enough?
Even though the JWL standard is thorough for passenger car wheels on public roads, it is inadequate for competition use. To the best of our knowledge, 949 Racing was the first wheel manufacturer of any size to release a lightweight tuner wheel with 10% greater test load on all three dynamic tests comprised of Radial Fatigue, Cornering Fatigue and Impact test. We call this proprietary standard that surpasses industry standards HP10/10. HP for High Performance, indicative of the intended usage profile, “10” for 10% greater Impact and Radial test loads and “10” again for Cornering Fatigue test load. We combine impact and radial load values as our experience has shown that the more critical dynamic test for race track use is cornering fatigue. To our knowledge, four other wheel companies have adopted similar internal standards to surpass the JWL standard. Two companies were seemingly directly inspired by our new standard, replicating it verbatim a few years after we first shipped HP10/10 wheels in 2015. Two other companies increased load cycles for their proprietary standards before 2010. It is good to see our industry showing an awareness and action in response to the higher stresses LHP wheels are now seeing in comparison to nearly 30 years ago when JWL standards were created.
It is our belief that a 10% greater test load has a greater bearing on increasing fatigue life than 10% greater test cycles. In laypersons terms, increasing test cycles means you hit it just as hard, but hit it a few more times. Take a wheel with a FoS of 2, double the number of test cycles. Most likely it will still survive the test. Take that same wheel and double the load at the standard number of test cycles. In theory, and in practice, it will fail much sooner. What is “FoS?, read on.
JWL vs track day (bro)
A cheap gravity cast, T-4 solution treated OEM 18×8 JWL approved wheel might weigh 27 lbs. The fancy low pressure cast, flow formed T-6 heat treated aftermarket 18×8 with the same JWL approval might weigh only 19 lbs, about 33% less. Not accounting for the slightly greater strength to weight ratio of pressure vs gravity cast material, the FoS of that far lighter wheel is likely to be lower than the heavier one. But we all want the lightest wheels right?
A VIA certified wheel rated at say, 620kg (1,364 lbs) could have a FoS in a street environment of say 2. Or roughly twice as strong as it needs to be. The greater the safety factor at 620kg rating, the more load and duration it will survive. We understand that track driving and autocross put far more load into an alloy wheel than street driving does. We do have significant and useful data on how many hours from which load cycles can be extrapolated. On a typical 2.5 mile road course, the baseline JWL dynamic test cycle equates to a minimum 55-75 hours use, depending on the diameter of wheel/tire combination. How many and how long the curves are on a given track affect this estimation. This assumes that at no time the wheel was subject to any load beyond the baseline. Spin off course, hit a kerb, drop a wheel off the edge of the pavement or simply turn harder than our ~1g baseline and that duty cycle value plummets. That is how a wheel that one might expect to last “forever”, doesn’t.
JWL standards require cast wheels to be tested at 2x the number of load cycles as forged wheels. Wheel manufacturers design a wheel in CAD (Computer Aided Design), simulate loads and perform virtual testing in FEA (Finite Element Analysis). Then they make samples and perform physical tests. FEA is good enough now that an engineer can virtually guarantee a new design will pass JWL/VIA on the first try. What they cannot tell you is precisely how many cycles beyond the test standard it will run before it ultimately fails. Only a predicted range. Most non-engineers would be surprised to learn that Fatigue Analysis science is as much parsing metadata on actual test results and those statistics as it is actual metallurgy. A fatigue life estimate is just that, an estimate. That is the nature of materials fatigue science, more statistics and probabilities than absolutes.
This leaves the automotive aftermarket in a quandary on how to design and engineer a wheel that will meet the modern high performance enthusiast customer expectations without being unfashionably heavy. Should the industry standard VIA certification be used or something new? Should manufacturers simply build stronger and heavier wheels? Our review of these standards comes to a few conclusions. Not all will agree with us but most will agree that any standard that is specifically tailored for street cars in 1981 may not be ideal for your trailered race car thirty years later in 2011, particularly when you spent weeks searching for the absolute lightest wheel available in your size. It’s not too difficult to make a wheel that is pretty much indestructible but no one wants a 35lb “race” wheel.
Casting, heat treating
Most consumers by now understand the basic difference between gravity casting and pressure casting. Gravity casting basically pours molten aluminum into a mold with the face of the wheel at the bottom. Pressure casting injects the molten aluminum under pressure which results in fewer voids, tiny air pockets in the material once its cooled. It also compacts the grain somewhat, similar to what forging accomplishes, albeit to a much lesser degree than forging.
Some cast wheels are heat treated to T-4 condition. Most LHP wheels available now are T-6 heat treated. Heat treating increases tensile and yield strength. Effectively making aluminum more “springy”, allowing it to flex more before it stays bent or cracks. Un-heat treated aluminum is far more brittle than any alloy in T-6 condition.
What is the best test standard?
This is the million dollar question. While 10% more test load may not seem like much, it significantly increases the FoS. We recognize that everything else being equal and expressed as percentage, test load matters more than test cycles in the context of the usage environment, namely race tracks, apex kerbs and the occasional off track excursion. We also recognize the dynamic cornering fatigue test is the most relevant of the three dynamic tests of JWL, TUV and SAE. So our focus going forward is increasing test load to between 10% and 20% greater than the JWL standard just for cornering load. This may still not result in an indestructible wheel, and it certainly won’t reduce weight but it more acutely addresses the actual usage environment a wheel sees on that noisy, low car with big sticky tires. We are labeling our newest wheels with “VIA HP10/10”, “VIA HP10/20” and so on. So you know what you are getting and how to compare our wheels to other options on the market. Our 15×10, 15×11, 15×12 4×100 pcd wheels first shipped in 2015 are HP10/10 but not labeled as such.
How long should your LHP wheel last?
For the amateur performance car enthusiast, the idea that any OEM component could “time out”, crack or fail outright as a result of HPDE use often comes as a surprise. This reality does not make it any less true. Components most of us would expect to last a lifetime on a street car, might begin to show signs of excess fatigue within the first year of use on track. Ball joints, subframe mounts, hub flanges, hub bearings, control arms, engine mounts, miscellaneous brackets around the car and yes, those pretty new LHP wheels you saw on a pro race car last week.
Most drivers, us included, expect to get a minimum of one year out of an HPDE wheel, and hope to get more like 6-10 years. But things happen on track that can cause a wheel to fail during the second decade, the second year, the second session or even second lap. That’s one of the many reasons we all wear helmets on track. There is a fundamental risk in driving 80mph next to other adrenaline junkies jockeying for the same piece of track in production cars not specifically engineered for that purpose.
Wheel standards: SAE, DOT, JWL, TUV, VIA and other acronyms
The JWL standard was created in 1981. It has been revised and updated a few times since then, mostly to add wider and larger diameter wheels to its test criteria charts. VIA is an independent council in Japan that actually performs the lab tests of sample wheels submitted by the manufacturers. Privately owned but VIA registered labs in other countries also perform VIA certification. This was a huge leap forward over basic pre-1981 industry standards by adding a radial load test, cornering fatigue test (most relevant to sport cars) and impact test. The JWL impact test actually strikes the aluminum flange, not the mounted tire. The pass/fail for passenger cars is whether the tire holds air. As such, the impact test is more an evaluation of the flange shape and basic material strength than the spoke or overall wheel capacity. Around 2012, SAE adopted a slightly modified version of the JWL standards and test methodology. The outlier in all this is TUV (Technical Inspection Association) Germany. TUV has far higher dynamic radial, dynamic cornering and impact test requirements than even JWL. Assuming the same strength to weight ratio, a TUV will usually have 15-25% greater mass (heavier) than a JWL certified wheel.
Factor of Safety
Die vs billet forging
Stiffness vs weight
You might ask, but will a stiffer wheel make me faster? The answer is an unequivocal yes. Just as wheel width has been repeatedly demonstrated to have a greater influence on lowering lap times than wheel weight, stiffness is more critical than a few ounces of weight in lowering lap times. If you are mulling this over and realizing a much stiffer wheel might allow you to run less camber on your performance car to achieve the same optimized contact patch loading.. you are getting the picture.
So racers want and need stiffer wheels right? But exactly how much weight penalty are racers willing to accept for improved wheel stiffness? Without a clear understanding of how much stiffer a wheel might be than a different design, most consumers are in the dark here. A few simple tools to ascertain the relative stiffness of two different LHP wheel designs of the same size: Look at JWL load ratings and total weight of the wheel. While spoke design and layout have a significant effect on relative stiffness, most LHP wheels are pretty well optimized. More often than not, the slightly heavier option will be stiffer and result in better performance.