Wheel and Tyre Fitment: Wheel & Tire Sizing

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Wheel & Tire Sizing

06.41

00:00	With a clear understanding of what we can fit and other potential considerations like the factory wheel specifications and resulting track width and scrub radius, we can start to make some decisions around what size wheels and tyres we should run.
00:14	The reality is, is that there's no simple rule to follow here, as every vehicle and application would be different.
00:21	Our focus has been primarily on performance in this course, but aesthetics will always be important, and we clearly want our car to look good if we're spending money on a new setup.
00:32	All dimensions really need to be considered together, but we'll discuss the diameter first.
00:38	One constraint here will be the diameter of the braking package, as we clearly need a wheel that will be big enough to clear this.
00:45	Another constraint is the overall rolling diameter of the tyre.
00:49	More often than not, we'll be aiming to keep this approximately the same size as the factory setup, but we also know that this can be changed in the interest of gearing and grip, though this does come with some mass and rotational inertia considerations, as well as changes to the ride height and the available space we have in the wheel well.
01:08	Anything past this in terms of tyres is out of the scope of this course, as the contact patch discussions get very complex very quickly.
01:16	With a target rolling diameter and the required brake clearance, it's just a matter of sizing the wheel diameter to achieve a suitable tyre sidewall profile.
01:25	We don't want a super low profile or an excessively big profile either, as both of these come with compromises to performance.
01:33	There'll almost always be a balance for wheel size, where say a 17 inch or 18 inch wheel will work, anything bigger will end up with too low a profile tyre, and anything smaller won't clear the brakes.
01:46	The stiffness of the sidewall will depend mostly on the model of tyre used, but assuming that it's the same, generally speaking, the 17 will offer more comfort and damping to maintain the contact patch, whereas the 18 inch wheel will likely provide a bit more feedback and sharper response.
02:03	In the end, small changes like this rarely come down to preference in terms of acidics and what's available.
02:10	Moving on to the width and offset, the constraints are the clearance to the suspension, wheel well and fender, but we also need to keep in mind the track width and also the scrub radius for the front axle.
02:22	One way to approach this is by understanding the widest wheel that will fit, within reason.
02:28	By this I mean a wheel that will still clear all the components by a safe margin and not protrude outside the guard.
02:35	It's also not just down to the width, but more how that width is distributed about the mounting face, and this will in turn define the offset of the wheel.
02:45	For example, let's say the factory wheel is 8 inches wide with zero offset, and when fitted we have at least 1.5 inches of clearance to the suspension and or wheel well.
02:57	Then we could safely increase the backspacing by 1 inch.
03:01	So, we could increase the wheel width by 1.5 inches over the factory wheel to 9.5 inches wide.
03:08	However, since half an inch more has been added to the inside of the wheel, the offset would now be a quarter of an inch greater, as this is the distance from the mounting surface to the centreline.
03:20	Since the offset was zero before, it would now be about positive 6mm, and this would decrease the track width and the scrub radius.
03:28	Depending on the car, a relatively small change like this probably wouldn't make much of a difference in the grand scheme of things.
03:35	But had we added 2 inches to the outside of the wheel, and taken an inch off the inside with a very low offset wheel, things would be significantly different, with the offset decreasing by about 38mm.
03:48	Whereas if we just added half an inch to the inside and outside, the offset wouldn't change.
03:55	This isn't to say that changing the track width or scrub radius is a bad thing, it just needs to be considered carefully.
04:02	Also remember that although wider tyres most often result in more lateral grip in the dry, all other things equal, there will often be a compromise in the wet or snow, and make the steering heavier, placing more stress on all the components.
04:17	With an idea of what width and offset we want to run, the next key thing to determine is the width of the tyre.
04:24	Generally speaking, a tread width matching the width of the wheel, or about half an inch less, will be the most efficient combination of wheel and tyre width in terms of grip, and provide the best feedback, response and control, all leading to faster lap times.
04:40	A wider tyre can of course provide more grip, but usually at the expense of some steering feel.
04:46	And if we do have room for a wider tyre, then we probably should be stepping up to a slightly wider wheel.
04:52	The width of the tyre will also have a significant impact on the clearance to the fender and wheel well, so this too needs to be allowed for.
05:01	On that note, with the width and overall rolling diameter, we can understand the profile dimension of the tyre, meaning we know all the measurements and can check the availability in that size.
05:12	In cases where that size isn't common, and expensive, or simply not available, we might need to make compromises to our wheel size to find something that works.
05:23	With all these changes to the wheel dimensions, we must consider the mass and rotational inertia and the effect this will have.
05:31	All this effort maximising the width in the interest of grip could be weighing the car down, affecting not only the straight line acceleration, but also braking and cornering.
05:41	Excessive unsprung mass will also have a negative effect on the contact patch, so it's all a balancing act.
05:48	Besides the size of the wheel though, the design and corresponding manufacturing processes will have equally significant impact on the mass, stiffness and strength.
05:59	So, it pays to be aware of what you're buying because simply put, the cost of quality wheels is usually justified to some extent.
06:07	In summary, there's no real best wheel and tyre size combination for every circumstance, but some options are better than others and some just won't fit at all.
06:17	There will always be constraints limiting what size we can and can't fit, like mounting methods and brake size, as well as clearance to the suspension, wheel well and fender.
06:27	It makes sense to let these constraints guide our choices, while also taking into consideration factors like track width, scrub radius, mass and tyre size availability.