Wheel and Tyre Fitment: Mass
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Mass
05.21
| 00:00 | In this section of the course we're going to be discussing how different factors around wheels affect performance. |
| 00:06 | We'll be looking at this from a general perspective of performance, essentially with the aim of maximising the car's speed around a course. |
| 00:14 | This performance can be condensed into three key elements, being acceleration, braking and cornering. |
| 00:21 | These elements are based on longitudinal and lateral acceleration, or put simply, acceleration back and forward and side to side. |
| 00:30 | Let's start with mass. |
| 00:31 | If we consider the Newton equation where acceleration equals force divided by mass, we can see that to maximise our acceleration, we want to maximise the force and or minimise the mass. |
| 00:45 | There are of course a lot of other things to consider, like weight distribution and balance, but in general, minimising the mass is going to be beneficial. |
| 00:54 | This is even more important for wheels on a few different levels as their unsprung mass, meaning they're mass that isn't supported by the suspension springs and they also rotate with the drivetrain. |
| 01:07 | The tyres provide the force for acceleration, braking and cornering and from a very simplified view we want to maximise the tyre contact patch and keep it as consistent as possible. |
| 01:20 | Minimising the mass of the wheels and therefore unsprung mass reduces the kinetic energy of the wheel, which allows the suspension to control the vertical load easier and therefore the contact patch better. |
| 01:33 | Another way to look at this is that more unsprung mass requires more spring rate and damping to control , which hurts grip. |
| 01:41 | Less unsprung mass is the opposite, allowing for flexibility and options for suspension setup as well as less trade offs between comfort and grip. |
| 01:51 | This means we can generate more grip or tyre force while also having less weight, both leading to better acceleration in all elements of performance. |
| 02:01 | Further to this, any mass, be it sprung or unsprung, contributes to load transfer, which is something we want to reduce in the interest of grip. |
| 02:10 | So, in short, less mass means less load transfer. |
| 02:14 | From a rotating mass standpoint, a lighter wheel will have less rotational inertia. |
| 02:20 | This means a lighter wheel will be easier to accelerate rotationally. |
| 02:24 | The diameter of the wheel also has a significant impact on the rotational inertia, where more mass further from the centre of rotation on a bigger wheel will increase the rotational inertia. |
| 02:37 | Think of a flywheel on an engine for example. |
| 02:39 | A lighter flywheel will allow the engine to rev up quicker. |
| 02:43 | A wheel is no different, as when in gear it's part of the driveline and connected to the engine. |
| 02:50 | With that said, a lighter wheel with less rotational inertia will not only be easier to accelerate, but also decelerate in braking as well. |
| 02:59 | The last thing we'll discuss might be the least understood, and that's the gyroscopic effect of the wheel. |
| 03:06 | This is the tendency of a spinning wheel to resist changes in direction. |
| 03:10 | That's why a rolling bicycle is more stable than a stationary one. |
| 03:15 | The more inertia a wheel has, the higher the gyroscopic effect, and this makes the steering feel heavier with less feedback. |
| 03:23 | A lighter wheel with less inertia will provide sharper and quicker steering, handling, control and feel. |
| 03:30 | This might sound like something that wouldn't make much difference in practice, but a light wheel could actually reduce the steering resistance by 50% or more when compared to a heavy wheel. |
| 03:40 | The final topic to cover here is the manufacturing process and the material. |
| 03:46 | And this has a joint influence over mass, stiffness and strength so we'll discuss this more over the next few modules as well. |
| 03:54 | Steel wheels are almost always going to be by far the heaviest. |
| 03:57 | They're just not a good choice if lightweight is on your priority list, which it should be. |
| 04:03 | Contrary to popular belief though, forged aluminium isn't significantly heavier than cast aluminium. |
| 04:09 | It's just that it has a better strength to mass ratio, meaning that a lighter wheel can be designed with the same strength. |
| 04:16 | This is why the equivalent size forged wheel is usually lighter than a flow formed wheel, which is usually lighter than a cast wheel. |
| 04:24 | Or if the mass is the same, the forged wheel will be strongest, then the flow form, then cast, of course given that the design is considered. |
| 04:34 | Lastly, carbon fibre has an incredible strength to mass ratio and is usually significantly lighter than an equivalent forged aluminium wheel. |
| 04:43 | Let's wrap up here with a summary of the main points. |
| 04:47 | Lightweight wheels are beneficial for acceleration, braking and cornering. |
| 04:52 | Provided that a wheel is adequately stiff, strong and safe, it's always beneficial to minimise wheel mass, especially as it's unsprung and rotating, and the gyroscopic effect resists steering. |
| 05:05 | When aluminium wheels are designed correctly, forged wheels are generally the lightest, followed by flow form, then cast. |
| 05:14 | But carbon fibre wheels are really the ultimate in lightweight. |
