Wheel and Tyre Fitment: Measurements
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Measurements
05.30
| 00:00 | This section of the course isn't about learning anything new. |
| 00:02 | We'll just be condensing the knowledge from the previous modules into three easy to follow steps that we can follow when determining what size wheels to fit to any vehicle. |
| 00:13 | So, if there's anything here that has you scratching your head, then jump back to the relevant module for a refresher. |
| 00:20 | This process will have some variability for different applications and depending on what tools you have available, but we'll cover that in our discussions. |
| 00:29 | The first step is going to involve measurements to understand what's possible in terms of fitment, essentially how much space there is to work with. |
| 00:37 | But we also want to know other key bits of information like the factory wheel specifications for example. |
| 00:44 | This is so we can understand the track width and scrub radius as we could end up changing these and they tend to have significant knock on effects. |
| 00:52 | Note that this is making the assumption we're working on a vehicle based on a factory car. |
| 00:57 | And even if the suspension has been modified, this is still a sound approach. |
| 01:02 | But this won't always be the case, like for vehicles with completely custom suspension geometry. |
| 01:08 | If that's the case, some extra thought will need to go into factors like the scrub radius. |
| 01:13 | Before getting started, it can be very helpful to search online for the factory specifications for the wheels, tyres and lug pattern. |
| 01:21 | Or it might even be noted in the service manual if it's still collecting dust in the glove box. |
| 01:26 | If we still have the factory wheels, it's worth understanding all of their dimensions. |
| 01:31 | Either by taking measurements or checking the inside around the mounting pad for markings. |
| 01:36 | Just to be completely sure we know what we're working with. |
| 01:40 | As we can't always trust online information. |
| 01:43 | From here, the first and simplest thing to consider is the mounting method. |
| 01:47 | In other words, what's the lug pattern and centre bore requirements of the wheels based on the hub. |
| 01:52 | If we don't already know the lug pattern, we can measure it and the centre bore. |
| 01:57 | It's also worth measuring the diameter of the studs or the bolts so we have all the information. |
| 02:02 | The same thought process applies to centre lock wheels regarding the hub size, tapers and pattern of the drive pins. |
| 02:10 | Following this, all measurements are going to be in the interest of understanding our spatial constraints and clearance. |
| 02:16 | Attached to the hub, the braking package is a critical factor. |
| 02:20 | With the disc diameter having the most influence over the clearance in the radial direction. |
| 02:25 | And the caliper size and location relative to the hub being the determining factor in the axial direction. |
| 02:32 | Measuring the disc diameter is simple if we don't already know it. |
| 02:35 | And it's also possible to use a straight edge and rulers to understand where the brake caliper is located relative to the hub. |
| 02:43 | Test fitting a wheel of known specifications, whether that's a factory wheel or not, will help us understand roughly what will clear the brakes. |
| 02:51 | But with all the different centre disc designs, pad heights, spoke shapes and even barrel profiles, even a wheel of identical specs can see significant differences in clearance. |
| 03:03 | In some cases, for standard factory brake setups on more common cars, the supplier will be able to confirm if the wheel will fit or not. |
| 03:12 | But more often than not, extra actions will be required to make sure of this, like using a template of the wheel profile for example. |
| 03:20 | Test fitting a wheel of known specifications, ideally with a tyre fitted, can also provide good reference for checking clearance to the suspension components, the inside of the wheel well and the guard. |
| 03:33 | With our knowledge of wheel dimensions, we can get a good idea of how certain changes in the dimensions will influence the clearance and therefore what will fit. |
| 03:42 | What's critical here is that we consider this over the full range of suspension travel and steering. |
| 03:48 | And this will require removal of the spring and bump stop to cycle the suspension. |
| 03:53 | This allows us to take into account camber gain as well. |
| 03:56 | Although ideally we'll take these measurements using the same static camber we intend to run with the new setup. |
| 04:03 | But we'll also need to consider any deflection in the suspension and even the chassis and leave an appropriate amount of clearance for this. |
| 04:12 | In most cases a few millimetres should suffice but it really depends on the car and application. |
| 04:19 | If we don't have access to any wheels for this, then there are some tools available that are adjustable to simulate the fitment of different sized wheels or alternatively, a straight edge and a ruler will be able to get the job done with some extra effort. |
| 04:33 | Just remember that the less accurate our measurements are and the more potential for variability with tyre size or even suspension bushing compliance, the more clearance that we should leave. |
| 04:44 | This really sums up all the general considerations around measuring for wheel fitment, so let's recap the key points. |
| 04:51 | The first thing we want to do is understand our starting point or the current wheel setup. |
| 04:56 | While it's helpful to find the factory specifications online, taking our own physical measurement of the wheel and hub gives us the confidence that we've got things right. |
| 05:05 | From here, we need to understand the amount of brake clearance required as well as the clearance to the suspension components in a wheel well and fender. |
| 05:14 | But it's critical this is considered throughout the entire range of suspension and steering travel. |
| 05:20 | We'll always want to allow for some extra clearance as things move with comp iance, but this will depend on the vehicle. |
