Wheel and Tyre Fitment: Stiffness
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Stiffness
04.28
| 00:00 | While it's commonly understood that more stiffness is a positive attribute for wheels, it's less commonly understood as to why this is and how important it is. |
| 00:09 | It's also a lot more difficult to measure stiffness compared to the weight of a wheel. |
| 00:14 | Stiffness describes how our wheel resists deformation when force is applied, and this should not be confused with strength, which we'll discuss in the next module. |
| 00:25 | Again, the origin of this force will be the tyre, and this is applied to the wheel through the bead seat. |
| 00:32 | This force is reacted through the mounting of the wheel to the hub, all of which is fundamental to how our vehicle accelerates, brakes and corners. |
| 00:40 | This can cause the wheel to flex under braking and acceleration as the barrel is winding up relative to the centre, or under heavy impact where the barrel moves in towards the centre. |
| 00:53 | In practice though, these tend to not make a significant difference in performance and driver feel. |
| 00:59 | What's more critical is the wheel flexing laterally. |
| 01:02 | Think of this as the wheel bending around its centre. |
| 01:05 | This is referred to as wheel camber compliance. |
| 01:09 | Aluminium wheels, which are most likely what we're all going to be using, have been found to flex enough to lose up to one degree of negative camber per cornering g force. |
| 01:22 | For example, let's say that a tyre needs one degree of dynamic camber, that's mid corner, to generate maximum grip and corner at 1.5g. |
| 01:31 | That means that the wheel could lose up to 1.5 degrees of negative camber. |
| 01:37 | And add to this one degree of body roll, we now have to run 3.5 degrees of static negative camber to make up for this, as opposed to only 2 degrees if we had a perfectly stiff wheel, which obviously doesn't exist. |
| 01:52 | This will result in a clear compromise to straight line traction. |
| 01:56 | This compliance is like that of a suspension bushing, where a lack of stiffness results in inconsistent handling and alignment, as well as less feedback to the driver and less control, which hurts driver confidence and leads to slower lap times. |
| 02:11 | Put simply, we want our wheel to be as stiff as possible. |
| 02:16 | Unfortunately, there's no free lunch, and the greater stiffness generally means greater mass, and this brings us back to our production methods. |
| 02:24 | The common misconception is that forged wheels are stiffer than cast wheels, and this isn't accurate. |
| 02:31 | If the same aluminium alloy is used, the production method, be it cast, flow formed or forged, won't affect the stiffness. |
| 02:40 | We know from our discussion on mass that if the wheel design is identical, it doesn't matter if it's forged or cast, the mass will be approximately the same, and the same goes for stiffness. |
| 02:51 | What's more accurate is that a forged wheel can be stiffer than a cast wheel, since there's more flexibility in the design due to the better strength to mass ratio. |
| 03:02 | Essentially it's easier for the design to be optimised for stiffness while retaining adequate strength. |
| 03:08 | A little known fact is that wheels used for professional racing are often constructed with more mass in order to gain that extra stiffness when compared to the extra lightweight wheels that the same company sells to consumers, since weight is easier to measure and relate to. |
| 03:25 | But this should highlight how stiffness can actually be more important. |
| 03:29 | As a final note, carbon fibre wheels are not only lightweight, but incredibly stiff. |
| 03:35 | You might notice that although we make mention of these throughout the course, they're so expensive and rarely used, it just doesn't make any sense for them to be our primary focus. |
| 03:46 | Before moving on, let's look at the key takeaways from this module. |
| 03:50 | The stiffness of a wheel is critical to performance, maybe even more so than mass. |
| 03:55 | If a wheel isn't stiff enough, it will have excessive camber compliance, resulting in inconsistent handling and alignment and poor feedback and control. |
| 04:06 | This also means we have to run more static camber , which will compromise straight line performance, among other things. |
| 04:13 | The production method does not affect stiffness, all other things being equal, but if the strength to mass ratio is considered during the design, this can be leveraged to improve stiffness. |
