Wheel and Tyre Fitment: Diameter

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Diameter

06.55

00:00	In this section of the course we're going to cover all the key dimensions and measurements of a wheel that we need to understand and consider when sizing them for a vehicle.
00:10	While some of these are fairly basic concepts, they can have many lead on effects that are often overlooked, so we'll also be discussing the key impacts of each dimension on performance.
00:22	Before we start, wheel dimensions are usually expressed by the standard method of diameter times by width and then offset, for example 15x8J ET-10, but we'll explain this in more detail as we go.
00:37	We'll start with the most basic and usually the first mentioned dimension when discussing wheel sizes being the diameter.
00:45	This measurement is almost always expressed in inches and could be anywhere from 10 inches on the likes of historic minis and formula cars, all the way up to 18 or even 20 inches in motorsport applications or even bigger on street cars.
01:01	What's important to understand is that this measurement isn't the overall diameter of the wheel, but rather the diameter of the bead seat, which to reiterate is where the bead of the tyre will be located when fitted to the wheel.
01:14	This is something that's gradually gotten bigger on OEM road cars as the years have passed, where in the 60s and 70s you'd be lucky to see anything larger than 15 inches, but now on luxury SUVs and even performance cars, 20 inch wheels have almost become the norm.
01:31	With that in mind it's understandable that for older vehicles it's common to step up a size or two in wheel diameter, which is something that's less common on modern vehicles and sometimes owners might even choose to reduce the wheel diameter.
01:46	This brings us to the practical considerations around wheel diameter and the effect that it has on performance.
01:53	First, let's think about the overall rolling diameter of the wheel and tyre.
01:57	Clearly if the wheel is bigger then the tyre needs to be of a lower profile to maintain the same rolling diameter and vice versa.
02:05	We'll discuss tyre sizing in more detail soon, but for clarity, the profile is essentially the size of the sidewall, where a lower profile means a smaller sidewall.
02:16	We'll assume the intent is to keep the same rolling diameter as this opens up another can of worms, affecting the gearing of the vehicle as well as making the speedo inaccurate, but again we'll come back to this later in the course.
02:30	Tyres are the most important factor when it comes to performance and there's a lot more to it than just using the widest tyre or the stickiest compound.
02:38	The size and stiffness of the sidewall plays an important role in the handling, response, grip and also comfort.
02:45	Generally speaking, a stiffer sidewall will be more responsive and provide better control, all other things being equal.
02:52	But if it's too stiff, the spring rate of the sidewall is essentially too high and under damped and the comfort and contact patch and therefore grip can suffer.
03:03	It's often assumed that a lower profile tyre will have a stiffer sidewall, but this is definitely not always the case.
03:11	Tyre manufacturers can change the sidewall stiffness to make up for its size.
03:16	Basically, each tyre is different.
03:18	So, it's just something to keep in mind and at the extreme ends of the spectrum, it's safe to assume that a very low profile tyre will be excessively stiff and harsh, limiting grip whereas a very large sidewall will usually flex more, limiting response.
03:34	Remember, this is a very simple view as tyres are a complex subject that would require its own course to cover.
03:41	Moving on, another main consideration is around the internal clearance of the wheel for the brakes.
03:47	As we touched on earlier in the construction module, different centred disc designs will have different clearance for brakes.
03:54	But clearly the diameter of the wheel will have the most significant impact on this.
04:00	Larger brake discs offer many advantages in performance applications around brake torque, cooling and thermal capacity.
04:07	So, maximising their size, particularly on the front, is desirable.
04:11	For these reasons it's common to move to a larger wheel diameter to allow for a larger centred disc but larger, more open faced wheel designs can also help with brake cooling to some degree.
04:24	The profile of the barrel will also be a determining factor when it comes to brake clearance.
04:30	More specifically where the drop centre is.
04:33	For a reverse mount wheel with the drop centre towards the inboard flange, there will be more clearance towards the front of the barrel.
04:41	In saying that, a step lip wheel with a deep lip could have the entire drop centre outside the face, meaning that it won't be an issue for brake clearance.
04:52	Again it's a case by case situation that depends on the wheel and also where the brake disc and caliper are compared to the hub mounting surface on the vehicle.
05:02	At this point you might also be asking, what about weight? As wheels and tyres form part of the unsprung and also rotating weight for the vehicle, this has a huge impact on the suspension operation and maintaining the contact patch, as well as accelerating and braking.
05:20	Basically, a heavier wheel and tyre package with more rotational inertia is almost always a disadvantage.
05:27	And to clarify, more inertia comes from having more weight further out.
05:33	A larger diameter wheel will weigh more and have more inertia than the equivalent smaller diameter wheel.
05:39	But we also have to consider the tyre, as a lower profile tyre will usually weigh less and therefore with the same overall diameter, have less inertia.
05:50	In practice, this tends to have a negligible effect if everything else is kept the same, but it's still worth keeping in mind if changes are being made.
05:59	Finally, it should be noted that the same diameter doesn't necessarily need to be run on the back wheels as the front.
06:05	This is very common in drag cars but also in drifting and road racing like Japan's Super GT cars, where the drive wheels are of a smaller diameter or the larger sidewall tyre.
06:17	They'll need to be the same diameter on each side though except for some forms of dirt oval track racing, but that's usually different sized tyres rather than wheels.
06:28	The important things to remember here are that the diameter is measured to the bead seat not the outside of the flange and changes to the diameter of the wheel will require changes to the size of the sidewall, which can have significant effects on performance.
06:44	The other key practical consideration around the diameter of the wheel is the clearance for the brake calipers and rotors.