Wheel and Tyre Fitment: Structure

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Structure

09.23

00:00	To start things off, we're going to cover the fundamentals of wheels.
00:04	This is going to include their basic structure and construction, as well as the different materials and manufacturing processes used to make them, and all the different ways they can be mounted to our car.
00:15	Since wheels are such a commonly modified part, and the majority of enthusiasts have had at least some involvement with them, it's fair to assume that you'll probably be familiar with some of these concepts.
00:27	But even if you think you're a bit of an expert, it might surprise you what knowledge might actually be misinformed, as well as some special considerations that aren't commonly known.
00:37	Regardless, going through the entire course is necessary, so we can ensure that we're all on the same page, before we dive into some more technical detail or practical skills.
00:48	First, we need to get a simple definition out of the way, and that's the word wheel.
00:54	Although this is frequently used to refer to the whole assembly, including the tyre, in this course we will be using the term wheel, only to describe the metal part that mounts to the hub of the vehicle, also commonly referred to as the rim, without the tyre mounted.
01:10	With that said, let's begin with the core structure of the wheels, basically what each part is called, and its function.
01:18	An automotive wheel is made up of two main sections, the barrel and the centre disc or face.
01:23	Both of which include various other features.
01:26	The barrel is the cylindrical part of the wheel where the tyre mounts, but if you've ever paid much attention to this part of the wheel without the tyre fitted, you'll know it's not a uniform shape, and there's a lot of features to it, all with a particular purpose.
01:40	Starting from the outside and working in, we have the inboard and outboard flanges, which are the small returns around the circumference of the edge of the barrel, and prevent the tyres from coming off the wheel.
01:52	If you've ever formed sheet metal, you'll know that adding bends or flanges goes a long way in stiffening the part and adding structure.
02:00	The flange and other features in the barrel do the same for the wheel.
02:04	Just inside each flange is the bead seat or bead rest.
02:08	As the name suggests, this is where the tyre bead is located on the wheel, and is also where the width and diameter of the wheel are measured.
02:17	But we'll be covering these measurements soon.
02:19	Inside this is a mounting or retaining hump or bump, which offers some security in keeping the bead on the bead seat between the flange and the hump.
02:30	You're probably familiar with the loud bang that comes with fitting a tyre to a wheel and first inflating it.
02:37	This is when the internal pressure gets high enough that the bead of the tyre is forced over the bump and into the flange.
02:44	The tyre pressure holds the bead of the tyre against the outer flange, creating the seal.
02:49	A bead lock wheel is a more secure option, most commonly used in off-road and drag racing, where the tyre pressures are generally a lot lower and de-beading a tyre is more likely.
03:01	In this case, the retaining hump is replaced with a flange, and another outer flange is then bolted to the barrel to clamp the tyre bead and hold it in place.
03:12	This can be used on just the outer bead or on both beads for a double bead lock.
03:16	The middle section of the barrel is where we see some variation and this is most commonly called the drop centre, or in some cases, the tyre well.
03:26	As the word drop implies, this area has a smaller diameter than the rest of the barrel, which allows the tyre to be installed onto the wheel.
03:34	You might be wondering why any of this is important when it comes to buying wheels, and surely it doesn't make any practical difference.
03:42	It actually does as the drop centre will almost always be in the way and must always be the smallest diameter point of the inner barrel, so depending on its size and location, this could be a potential issue when it comes to clearing brakes.
03:55	The location of the drop centre also moves depending on the wheel design, more specifically, where the centre disc, which we'll discuss shortly, is mounted inside the barrel.
04:06	For a wheel with a more face forward design, where the disc is closer to the outside of the wheel, the drop centre will also be further outboard.
04:15	Likewise, for a more inboard disc on a wheel with a bigger lip, the drop centre will be further inboard.
04:22	The lip being the section of the barrel on the inside diameter of the wheel in front of the disc that's visible with the wheel and tyre fitted.
04:30	A step lip rim, where the start of the drop centre is forward of the centre disc, will also mean that the disc is smaller since the lip steps down.
04:40	When the drop centre is pushed right to the back of the wheel with a bigger lip, this will typically be referred to as a reverse mount, as the tyre needs to be mounted from the back of the wheel rather than the more traditional approach from the front.
04:55	In this case, the disc can be larger, which is just a consideration for acidics.
05:00	Another drop centre related consideration is around the use of TPMS or tyre pressure monitoring systems.
05:07	There will be a hole through the lip of the wheel into the drop centre for the valve stem so the tyre can be inflated.
05:13	The TPMS sensor is almost always integrated into this.
05:18	If the drop centre isn't deep or wide enough, or the sides aren't angled correctly, then the wheels won't be TPMS compatible, simply because the TPMS sensor won't fit in the drop centre or the tyre won't be able to be installed over it.
05:34	Let's move on to the centre disc or face, which is the circular section of the wheel that mounts to the hub and supports the car and allows the wheel to rotate while the car moves and also allows torque to be transferred.
05:48	Starting from the centre and moving outwards this time, the central section of the disc will have a centre bore hole of a certain diameter and often some other features allowing fitments of a centre cap to cover this.
06:01	Surrounding the centre bore will be the lug holes that allow the wheel to be fixed to the hub and held on the centre bore using studs and nuts or lug bolts.
06:10	The centre bore can be used to locate the wheel on the hub of the vehicle with a spigot, but we'll come back to this in a coming module.
06:18	An alternative method, which is popular in motorsport is what's referred to as a centre lock wheel.
06:24	This still uses a spigot to locate the wheel through the centre bore, but only uses a large single nut to fix the wheel to the hub.
06:32	Dowels or pins from the hub protrude through the holes in the back of the wheels to allow torque to be transferred.
06:40	We'll be diving deeper into these mounting options in a dedicated upcoming module.
06:45	Next, the spokes extend from the centre section out to the barrel and connect to the barrel through various methods we'll be discussing in the next module.
06:54	The thickness of the centre section of the disc behind the spokes is commonly referred to as the back pad.
07:01	Lastly, we need to discuss the different disc types before moving on.
07:05	Every wheel design will have a differently shaped face and spokes and some will naturally leave more clearance to the brake callipers than others.
07:14	The width, diameter and offset, all of, which we'll be covering in more detail soon, will also have an impact on this.
07:22	Also common for the same wheel design to be available with different discs for this reason.
07:28	The key difference between the discs is the position of the hub mounting surface to the connection point between the spokes and the barrel, sometimes referred to as the disc type.
07:39	This is really influenced by the thickness of the back pad and the shape and angle of the spokes.
07:45	If we look at a cross sectional view of a wheel disc, when the mounting surface is much further inboard than the connection between the spokes and the barrel, which is what we'd usually call a high disc, then we'd generally have more brake clearance but a relatively small lip.
08:02	The opposite would also be true.
08:04	With a low disc we'd typically have less brake clearance, but a bigger lip.
08:09	This is sometimes used by wheel manufacturers to ensure fitments for a wide range of vehicles and something that we need to consider for not only brake clearance, but also aesthetics.
08:20	For example, Work Wheels, a popular Japanese manufacturer, have a range of disc types but the three main ones are A, O and R.
08:30	With O having the least brake clearance but the biggest lip, R having the most brake clearance but the smallest lip and A sitting somewhere between.
08:40	With this, we could have the same wheel width and offset with a completely different brake clearance and lip size and therefore look.
08:48	To summarise what we've covered in our first discussion, wheels have two main parts.
08:54	The barrel, which features the flanges, bead seat, retaining humps and drop centre and the centre disc, which mounts to the hub using the centre bore and lug holes and also includes the spokes.
09:06	Bead locks and the use of TPMS sensors come with some extra considerations for the barrel design and the centre discs are often available with a range of different disc heights for the same wheel designs and even size.