Wheel and Tyre Fitment: Strength
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Strength
06.09
| 00:00 | Strength, or more specifically yield strength, describes how much stress a wheel can take before it permanently deforms, bends or cracks. |
| 00:09 | This isn't specifically a performance factor, but rather safety is the primary concern. |
| 00:15 | Just like with stiffness and mass, strength is part of the equation and rarely comes down to the manufacturing process and design. |
| 00:24 | The fact is, every quality wheel should be able to handle the tyre forces from acceleration, braking and cornering without failure. |
| 00:33 | But when things get interesting, like contact in wheel to wheel racing or drifting, or hard impacts in rally or off road racing, or even just hitting a big pothole on the road, it's not unheard of for wheels to crack, bend or just simply break. |
| 00:49 | After this, in the best case scenario, the car will still be able to drive while the tyre deflates. |
| 00:54 | But worst case, if the wheel really starts to fall apart, we could be in for some expensive damage and a bad crash. |
| 01:02 | So, the most critical factor here is impact strength. |
| 01:06 | In other words, how high of an impact force can the wheel withstand. |
| 01:10 | The scary fact is that in most countries, the standards for testing are voluntary, meaning that it isn't required for the supplier in order to sell wheels. |
| 01:20 | That said, Germany's TUV and Japan's JWL are two examples of countries with good standard requirements. |
| 01:29 | Moving on, another easy factor to understand is load rating. |
| 01:33 | In motorsport it's common to corner weight the car, and this tells us exactly how much load that wheel is subject to under static conditions. |
| 01:42 | If we've got a street car that hasn't been corner weighted, then there should be a sticker somewhere on the vehicle showing the gross vehicle weight and also the axle weights. |
| 01:52 | In this case we'd take the heaviest of the axle weights and divide by two to get a good idea of the worst case static load on the wheels. |
| 02:01 | For example, let's say the load rating for a wheel is 500kg. |
| 02:05 | This doesn't mean at 500kg the wheel will fail, this just means that the wheel is rated to carry the equivalent of 500kg static load safely. |
| 02:17 | And this should take into account an increase to this under driving conditions. |
| 02:21 | This also doesn't mean that a vehicle that weighs 2000kg could use this wheel, as that would assume that the load is perfectly evenly distributed, which is almost certainly not going to be the case. |
| 02:33 | Load rating isn't actually a value that determines strength, but it's still worth understanding and can be useful when comparing different wheels if these values are significantly different. |
| 02:45 | In other words, if one wheel has a much higher load rating than another, then there's a good chance it is stronger. |
| 02:52 | As we've discussed over the previous two modules, mass, stiffness and strength all go hand in hand. |
| 02:59 | Focusing on aluminium wheels, for the same size wheel, any differences in these attributes will come down to the design and manufacturing method used. |
| 03:08 | More specifically, relating to strength, the grain structure of the metal is best in a forged wheel compared to a flow wheel, which is generally better than a cast wheel. |
| 03:18 | In basic terms, this is because the grain structure becomes aligned with the shape of the wheel and this is hugely beneficial to strength. |
| 03:27 | Having this inherent strength gives us more flexibility in the design of the wheel and again this is why forged wheels can be designed for a lighter weight or more stiffness, while retaining enough strength. |
| 03:40 | So, it's not accurate to say that forged wheels are stronger, stiffer and lighter than cast wheels, as they'll often be the same strength, but lighter, or the same weight, but stronger. |
| 03:51 | It really comes down to a case by case basis, with every design being different. |
| 03:57 | This is where the problem with replica wheels comes into play. |
| 04:00 | As we've touched on already, if a wheel design is copied, but the manufacturing process is not, we could run into problems. |
| 04:08 | Let's consider the Volt Racing T37 by Rays Engineering for example. |
| 04:13 | This is one of the most popular aftermarket wheels ever made and as a result has been imitated to death. |
| 04:19 | The genuine wheel is a forged wheel made in Japan through an extensive process involving a lot of quality control. |
| 04:27 | The replicas though are simply cast. |
| 04:30 | If they've managed to copy the shape perfectly, then they might weigh the same and be just as stiff. |
| 04:36 | But the genuine wheel is only able to weigh what it does, because it's forged and therefore strong enough to warrant that design. |
| 04:45 | So, a cast replica wheel will very likely not be nearly strong enough for safe use. |
| 04:51 | This comes down to the safety factor of the wheel, which explains the maximum stress a wheel can handle when compared to the normal stresses that the wheel will see. |
| 05:00 | Of course, the cast replica wheel would still be able to handle normal stresses like driving down the road and supporting the weight of the vehicle. |
| 05:08 | But the safety factor is lowered, meaning the maximum stress it can handle will be less. |
| 05:14 | All this means it's much more likely to fail if we hit a pothole, run a kerb or just from the general abuse of motorsport. |
| 05:22 | But let's wrap up this discussion and cover off the main points before we fall too hard down the real vs rep rabbit hole. |
| 05:29 | Strength is most important for the wheel to survive heavy impacts, usually from unintentional circumstances. While the load rating doesn't tell us everything we need to know, it still provides some valuable information. |
| 05:42 | The manufacturing process has a significant impact on the strength and this can be factored into the design while being considered alongside mass and stiffness. |
| 05:52 | In the end, strength is not so much about performance, but critical to safety. |
| 05:57 | And replica wheels that have copied the design of forged products but using casting as the manufacturing method can spell trouble. |
