Like you, we are enthusiasts too. Daily drivers, autocrossers, HPDE, national championship winning race cars. We understand what is important for both the street and competition driver. Specific features that make your car work better and facilitate your fun factor and success in competition. Our experience both in casual HPDE and high level motorsports has given us insights in how to go beyond existing industry norms and standards with features like our dual valve, knurled bead and higher load ratings. But we don’t neglect the inspiration factor. No matter how strong, affordable and functional wheel might be, it has to look great. Inspiring performance and inspiring to look at. We hope you pause to turnaround and take a look at your car as you are walking away, just to check it out. We do.
Real world design
The most efficient form for a race wheel ends up being an organic looking mesh structure that looks a bit like the webbing inside a bone. Many small spokes and branches like a cross between a bicycle wheel and tree branch. Unfortunately, such a shape is really hard to make, impossible to clean and well, ugly. These spoke shapes are what AI will come up with if we let our fancy software optimize purely for strength targets and minimal mass. The good thing is we can then learn from these shapes an incorporate various features in a much more user friendly, easier to manufacture and hopefully, prettier design. In wheel to wheel racing, car to car contact is a reality. This is why all 949 Racing wheels have their valve positioned to protect against being damaged. This also helps protect the valves if you get a bit too close to a curb when parking on the street. Some small design features seen in this market space serve no function and are only added to add “surface detail”. Every radius, every chamfer, ever millimeter of a 949 Racing wheel is built towards a functional goal.
Our basic approach is to start with a big brake template that we need the wheel to clear. We may make a 3D scan of the brake and wheel well to determine the envelope we have to work with. Next comes fatigue life targets. FOS “Factor Of Safety” refers to how much stronger a structure is than the minimum required strength. Strength in this case is load x duty cycle. How big a hammer and how many times you hit it. We aim for an FOS of 2 at 10% above industry standard load. So if the DOT/JWL standard is say, 690kg at 500k cycles, we bump that load 10% (760kg) and test for 1M cycles. Once we have reached our strength targets, we work to optimize weight by reshaping and reducing mass where we can without affecting strength. But the starting point is always strength and fitment. The final weight is the product of those two.
Weight matters. But not as much as stiffness. The reason most buyers ask about weight first is that its the only quantifiable value that most companies offer. No brand offers a stiffness index. Like shopping for professional services, most of us have idea how to differentiate one lawyer, doctor, real estate agent, plumber from another. So we ask for a referral from a friend because there are no other metrics. When it comes to wheels, we have style, fitment, price and weight usually. No other metrics. But what if we added stiffness as a metric to choose wheels for your project? Wanting better answers, we engineered and built a massive test rig to compare our wheels to samples from other brands. More data makes better wheels.
Everything else being equal, a stiff wheel that weighs 10% more will drive better and be faster on a race track than a lighter wheel that flexes 10% more. In practice, we can engineer a wheel to gain more percentage of stiffness than it gains in weight. A simple gusset here, chamfer there, a tweak in radii rewards us with big gains in stiffness and fatigue life with only a minor weight increase. The result is a light weight, purpose built, high performance wheel that we can offer a lifetime structural warranty on.
What we are ultimately focused on from a structural perspective is the highest possible strength to weight ratio. A super light wheel is pointless if it flexes so much that your tires wear uneven or the wheel wears out too soon. On the other end of the scale, a super strong and stiff wheel is overengineered if the same values can be designed at a lower weight. We do our best not to get into the common engineers trap of optimizing something that isn’t even needed.
What is tilt casting?
Aluminum alloy wheels are made by either casting or forging. Cast wheels are either gravity, low pressure or tilt cast. Gravity casting makes up probably 90% of all aftermarket alloy wheels. Low pressure a smaller percentage and tilt cast perhaps 1-2%. Flow forming gained prominence in the mid 2000’s and is now pretty standard for any performance oriented wheel. Most of the high end performance and race oriented aftermarket wheels offered in North America are gravity cast & flow formed. Our upcoming Coto and 6UL wheels are tilt cast & flow formed.
Gravity, low pressure and tilt casting can all produce a strong wheel. Basic strength being the same, gravity will be the heaviest, low pressure a bit lighter, tilt cast lighter still, then forged. We can use as a rough guide, a composite “strength” index that combines elongation, Youngs modulus and ultimate tensile strength. Assuming the same mass wheel; low pressure offers an increase in overall strength over gravity casting, perhaps 10-15%. Tilt casting another 20-25% over low pressure. Forging another 20% over tilt casting. If you’re thinking that a tilt cast wheel can be 40-50% stronger than the same weight gravity cast wheel, you get the idea. Strength being equalized, tilt cast will be about 10% lighter than gravity cast.
Due to the different flow path of molten alloy within the mold, gravity cast wheels will have slightly stronger hubs and spokes than a low pressure cast wheel of the same dimensions. Low pressure cast wheel thus have slightly stronger barrels than gravity cast wheel of the same dimensions. Manufactures can compensate for gravity wheels slightly weaker barrel with flow forming. So gravity can match the overall strength of the low pressure with only a slight weight penalty.
How are wheels cast?
To get a better picture of the difference in materials properties when used in alloy wheel casting, it’s important to have a grasp of how wheels are cast and the typical challenges faced in that process.
With gravity and low pressure casting, the mold is stationary, wheel face down. Molten alloy is poured into the mold and solidifies as it cools. In low pressure, it’s injected from the bottom near the hub first then flows upward. In tilt casting the flow is from the barrel down to the hub. The tilt cast mold starts on it’s side, locked together with the crucible containing the molten alloy. The crucible and mold are then rotated in unison allowing the molten alloy to flow smoothly into the mold. The common analogy is tilting a beer bottle and glass together slowly to eliminate foam vs just pouring straight into a glass sitting on a table. This is an oversimplification but the concept is the same. This method used in tilt casting both reduces unwanted turbulence and allows for more rapid cooling of the alloy. Reducing turbulence helps eliminate oxides and inclusions caused by “foaming” of the liquid alloy flowing through the mold cavities.
All else being equal, the faster the molten aluminum cools and solidifies within the mold, the tighter the grain structure. Tighter grain is denser and stronger than a less dense material. Tilt casting allows faster relative flow with less turbulence compared to gravity or low pressure.
Tilt casting also reduces what is called Cold Joint Defect. This is where the molten alloy flow from one part of the mold meets another part of the flow on the far side of the inlet. The alloy cools as it flows so the coldest part of the flow is where these two rivers meet at the end of mold filling. Gravity and low pressure require compensations in the mold design which can result in added weight and limitations on final product shape. Tilt castling allows shallower draft angles and increased detail in the mold, which gives me and John a bit more freedom in the design (me) and engineering (John) of the wheel.
Gravity casting tooling costs quite a bit less than low pressure tooling which makes smaller volume production runs more cost effective. Gravity casting is a bit slower process than low pressure casting but both are significantly faster than tilt casting. Tilt casting tooling and the actual production process is more expensive than either gravity or low pressure.
These days, most OEM wheels are still gravity cast. Many higher end optional OEM wheels are now being tilt cast as it offers auto manufacturers a more cost effective option than forged.