How strong is your wheel?

How strong is your wheel?

© copyright 949 Racing 2019

On test standards for the aftermarket automotive lightweight tuner "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 requirements 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 the paradigm.

How long should your LHP wheel last?

Every model of aftermarket wheel ever sold has had at least one crack or break.  The more popular the wheel, the more it is seen shared on social media and the more likely you are to see one or more cracked/broken wheels. The average HPDE driver is not aware that all pro race teams have stringent service life standards, more akin to highly stressed aircraft than road going cars. This means carefully recording hours of use and then scrapping them at a certain duration whether they show signs of failure or not. Most pro teams do not use wheels for more than one year. Note: This is a good reason not to buy used wheels from pro race teams.

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.

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 saying 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. A perfect storm of goal misalignment but a great opportunity to make consumers aware of the relationship between usage, fatigue life, test standards and expectations.

Wheel standards: SAE, DOT, JWL, TUV, VIA and other acronyms

The US government does not require aftermarket alloy wheels sold in the country to meet any performance standard.  It only requires wheel to have DOT (Department Of Transportation)  marking for dimensions, load capacity (not test load rating), date and country of manufacture, etc.  The applicable performance standard in the US was and is, SAE J2530 (Society of Automotive Engineers). Early versions of this SAE standard were rudimentary.  To this day, few, if any LHP wheel manufacturers use the SAE standard. In the late 80's Japanese made, performance oriented and race inspired wheels entered the US and worldwide markets in great numbers. With them came the JWL (Japan Wheel) test standard. This has now become the defacto standard that the entire tuner wheel segment uses, even for brands sold in other countries. An LHP wheel made in Vietnam, by an Italian company and sold in France will most likely have a JWL/VIA stamp and no others.

 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.

Is JWL/VIA good enough?

Generally speaking,  the accepted industry standard in the US is JWL/VIA even for wheels manufactured  outside of Japan. Some consumers will know to look for that VIA stamp next to the valve.

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.

 Factor of Safety

"FoS" in engineering vernacular. A degreed engineer might cringe but in laypersons terms, this is "how much stronger the part is than it needs to be". FoS is roughly equivalent to yield strength.  Example, an FoS of 2 means stresses are allowed to reach 50% of the components yield strength. It's far more complicated than that but key to understand are the inputs: The part (wheel), the load and the duty cycle.  How hard you hit it and how many times you hit it in other words.  Structures undergoing high loads but not under significant restrictions to save weight,  can have FoS of 5 to 20. Yes, that's 20x stronger than it needs to be. Most bridges you drive on have a minimum FoS of 7 but could be as high as 20. These are structures and components  intended to last virtually forever, or least decades. Aircraft parts on the other hand, are subject to the most severe weight restrictions. Thus it is common for aircraft parts to be engineered with FoS as low as 1.5. Yikes! How do planes keep from falling apart? The aircraft industry and owners keep very careful track of the number of hours on every single component in the aircraft. If the engineer estimates it will last 100 hours and has a FoS of 1.5, that bit will be swapped out long before it reaches 100 hrs duty cycle. It is near impossible to get FoS data from auto manufacturers for OEM wheels but  its assumed to be 2 to 3. How do we know that? We can extrapolate that from currently available data.

JWL vs track day (bro)

Let's start with an 18x8 +40 5x1143 pcd wheel, low pressure cast, flow formed weighing 19 lbs. Max load rating of JWL standard for this diameter/PCD of 620kg. The dynamic Cornering Fatigue test mounts a tire, inflates to max psi, tilts 30° and rolls on a drum to simulate the road. The test load of 620kg is applied vertically, in the same radial plane as the drum. 620kg is 1364 lbs. If we are simulating the average car that a 5x114.3 pcd 18x8 would be on, we can use 3800lb as a reference. The engineers reading this have already done the rough calc in their head. Yes, that's barely 1g simulated. Why so little? Because the standards were created when even the highest performance street cars struggled to turn at .85g on street legal tires. That was 30 years ago.  Some LHP wheel buyers  30 years ago might have been autocrossers but HPDE events weren't even a "thing" yet. Now your parents Camry generates .85g on its all season tires.  Your high performance prepped street car generates 1.4g on street legal tires without breaking a sweat. Mount slicks, add a little downforce with bolt on aerodynamics and a street going production car can see 1.6G sustained cornering force. Perhaps you do a half dozen track days in your 300whp sports coupe every year, averaging maybe 85mph laps.  That load and duty cycle is worlds past what JWL standards were intended to simulate. The world has changed, wheel industry standards have not kept up.

A cheap gravity cast, T-4 solution treated OEM 18x8 JWL approved wheel might weigh 27 lbs. The fancy low pressure cast, flow formed  T-6 heat treated aftermarket 18x8 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.

Manufacturing techniques

Cast or forged? It may come as a surprise to some consumers, but the stiffness and weight of forged and cast alloy material is basically the same. The difference is forged materials have higher yield and tensile strength. This means forged material will flex further before it stays bent and bend further before it cracks. This allows designers to use a little bit less material in a forged wheel  to match the fatigue life of a cast wheel. Or use the same amount of forged material to achieve greater fatigue life than a cast wheel.  There is no magic that allows a forged wheel to be significantly lighter and have greater fatigue life than the best cast wheels. Weight or fatigue life, pick one.

Flow Forming

Flow forming, also marketed as flow forging, roll forming, rotary forging, MAT  is a process where only  the barrel portion is hot forged.  This process results in a barrel material almost identical in tensile and yield strength to a pure forged wheel.

Die vs billet forging

Die forging is the process where the final spoke shape is created by a die under tremendous pressure and heat. By forging the spoke shape with a die, the crystalline structure or "grain" of the material is aligned with the shape of the part. Die forging is also known as near net forging. Meaning that die pretty much makes the final wheel.  The little remaining machine work is just to cut the lug holes and back pad.  A billet forged wheel starts with a featureless forged puck, with no wheel design, spokes or ports. This puck can either be forged into a puck shape by a die, or cut into a puck shape from a larger block of forged material.  In either case, the grain structure in this billet blank is aligned in one direction like the longitudinal grain in a pine 2x4. It is tricky to explain how this grain structure impacts design and fatigue life without a bunch of charts, images and technical explanations. One can imagine the die forged as a tree trunk with a branch, grain unidirectionally aligned to its specific shape. It is difficult to break that branch off where it meets the root because of that grain structure blending  deeply into the trunk.  The billet forged wheel  is more like a trunk and branch shape cut from a larger piece of wood without the grain matching the structure.  Anyone reading this with a knowledge of woodcraft understand this second "tree" will be much easier to break the branch off of than the real tree with structurally aligned grain. That perhaps oversimplifies but the analogy is relevant.  So everything else being equal, a billet forged wheel requires more material to match the fatigue life of a die forged wheel.  In the US, there are precious few die forged LHP wheels on the market. A far greater number of forged LHP wheels are of the billet forged variety.

Casting, heat treating

Most consumers by now understand the basic difference between gravity or "tilt" 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.

Stiffness vs weight

This is a conversation that very few amateur racers have but large budget pro race teams have carefully mapped out in simulations and data collection. Wheel stiffness plays a huge role in the suspension tuning process and significantly impacts the way a high performance cars feels. Most consumers assume that rigid feeling wheel they pulled out of the box is not flexing at all during high cornering loads. In fact however, all wheels have considerable flex during high load conditions. In cornering, this constant rotating bending moment actually reduces camber, the lower part of the wheel being pulled out of alignment with the hub as it rolls. This is not marketing speak, it is the very basis of the JWL cornering load test and key to a better understanding of the subject.

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.

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 15x10, 15x11, 15x12 4x100 pcd wheels first shipped in 2015 are HP10/10 but not labeled as such. Our little company has occasionally generated friction with some members of the public by refusing to spoon feed answers. This generally occurs when we believe someone is simply asking the wrong question to effectively solve their problem. 10 years ago, last year and last week questions have been asked of us. There is an old saying, often attributed to the wrong person "Give a man a fish, feed him for a day. Teach a man to fish, feed him for a lifetime". We would much rather share the information with enthusiasts to help them decide for themselves than simply "do what everyone else does" when we feel the status quo is not in anyone's best interest.

© copyright 949 Racing 2019