Wheel and Tyre Fitment: Dimensions, Measurement & Sizing
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Dimensions, Measurement & Sizing
07.11
| 00:00 | If we're discussing wheels in a performance context, then it's hard to leave tyres out of the conversation as there are so many overlapping factors and both parts need to be considered together for the best results. |
| 00:13 | Tyres though, while generally being the most important part of a racecar, are an extremely complex topic that needs its own course or even series of courses to cover. |
| 00:23 | With that in mind, our discussions around tyres in this section of the course will only be on fitment related topics, mostly around their sizing and the impact this has on our wheel fitment. |
| 00:34 | There's also some fundamental knowledge of tyres that's important to understand if we're modifying the fitment of a wheel and we'll also briefly touch on some performance related topics. |
| 00:45 | To be clear though, we aren't going to discuss any different treads, compounds, constructions or any deeper topics because again, it's such a massive and complicated subject. |
| 00:56 | So, let's get started with the most important aspect of a tyre if we're discussing the wheel fitment and sizing, being the dimensions. |
| 01:03 | Of course, this, among many other aspects of tyres, has a huge impact on performance, but that's not our focus. |
| 01:10 | The standard method of expressing sizes for radial tyres, which are by far the most common, is using the width, aspect ratio, tyre construction letter and the diameter of the wheel. |
| 01:21 | There might also be other letters in front denoting the tyre type or at the end showing the load rating and speed rating but we won't focus on these. |
| 01:31 | Let's use a 275 35 R18 example to explain this. |
| 01:36 | It makes sense to work from left to right, starting with the width, also known as the section width. |
| 01:42 | This value of 275 is in millimetres and describes the distance between the inner and outer side walls of the outside of the tyre, excluding any raised sections for lettering and so on. |
| 01:54 | The important thing to note here is that two different 275 wide tyres could have completely different tread widths and overall widths. |
| 02:03 | Therefore when fitted to the same width wheel, the stretch on the sidewall could be completely different from one to the other. |
| 02:09 | Generally speaking, motorsport tyres tend to have wider tread widths for the same section width, but the section width is still a quick and easy way of understanding the rough size of the tyre. |
| 02:20 | The forge slash just separates the width from the aspect ratio, which is also commonly referred to as the profile. |
| 02:28 | This brings us to something that many people newer to cars don't understand. |
| 02:32 | This profile aspect ratio is a percentage, not a height measurement itself. |
| 02:37 | It's actually the height of the sidewall measured from the wheel rim to the top of the tread, expressed as a percentage of the tyre width. |
| 02:46 | In other words, if we put it into a simple equation, the aspect ratio equals the sidewall height divided by the section width times 100. |
| 02:56 | So, the bigger the aspect ratio, the higher or taller the sidewall is for the same section width. |
| 03:02 | This will also mean a larger overall diameter on the same size wheel. |
| 03:07 | Next, we have a letter denoting the construction type , which will more than likely be R for radial. |
| 03:13 | Finally, we have the wheel diameter that the tyre is made to fit and this is expressed in inches as is the standard. |
| 03:21 | While tyres with different section widths and profiles can be fitted to the same wheel, it's hopefully obvious that we can only use a tyre for an 18 inch wheel on an 18 inch wheel. |
| 03:32 | Measurements for road racing slicks, or in fact nearly all motorsport tyres are a little bit different and are usually expressed using the tread width rather than the section width, then the overall diameter rather than the aspect ratio or profile, followed by the wheel diameter as per usual. |
| 03:49 | Sometimes this is also expressed in centimetres. |
| 03:51 | For example a 27-65-18 wheel will have a tread width of 27 centimetres or 270 millimetres, an overall diameter of 65 centimetres or 650 millimetres and is for use on an 18 inch wheel. |
| 04:07 | There are some other methods of expressing tyre dimensions like those for truck tyres or bi-sply tyres, but what we've just discussed is by far the most common in performance related applications. |
| 04:19 | Before moving on, let's discuss some performance related topics around the size that must be considered. |
| 04:25 | First up, the circumference and therefore the overall rolling diameter of the tyre has an effect on the gearing. |
| 04:31 | A smaller diameter is the equivalent of a higher gear ratio, so it can help with acceleration but the engine RPM will be higher for the same speed. |
| 04:41 | This will also mean the chassis will be lower to the ground for the same suspension settings, but provide more clearance around the wheel. |
| 04:50 | The opposite is also true for larger wheels, with some advantages or disadvantages depending on how you look at it. |
| 04:57 | Different sized tyres can also make the speedometer reading inaccurate. |
| 05:01 | So, for these reasons it's usually best to stay close to the factory specifications or within around 10% depending on the vehicle. |
| 05:09 | Now, let's dive a little deeper into the tyre width, which is the most commonly used metric to compare performance but may be the most misunderstood. |
| 05:17 | Firstly, in dry conditions, a wider tyre generally provides greater ultimate grip. |
| 05:22 | But there's so much more to consider than this, which we just can't cover in this course. |
| 05:27 | So, in short, everything else being equal, a wider tyre does not mean a larger contact patch area. |
| 05:34 | The contact patch might be wider but it'll probably be shorter and naturally this has different effects on lateral acceleration, aka cornering and longitudinal acceleration, aka forward acceleration and braking. |
| 05:47 | What's more important is the pressure of this contact patch and the distribution of that pressure over the contact patch, which is influenced by the tyre size among many other factors. |
| 05:58 | In the dry, lowering the contact patch pressure with more contact patch area will generally help grip. |
| 06:05 | But in some cases, like rain, a wider tyre might be detrimental. |
| 06:09 | In an extreme case, consider an ice rally tyre. |
| 06:13 | These are anything but wide, with the intention being the opposite, to increase the pressure at the contact patch to improve grip. |
| 06:21 | What I'm getting at here is that we shouldn't do everything we can to fit the widest tyre without considering the compromises. |
| 06:29 | Before getting too deep in the weeds, let's summarise the key points from this module. |
| 06:33 | Tyre sizes for radials, which are by far the most commonly used construction in most applications, are expressed in a standard arrangement of section width in millimetres, sidewall aspect ratio as a percentage of the width and wheel diameter in inches. |
| 06:50 | Motorsport tyres are usually a bit different, showing the tread width and the overall diameter in millimetres or centimetres and also the wheel diameter, again in inches. |
| 07:00 | There are a lot of performance factors around tyre sizes so just understand that changes you make need to be carefully considered. |
