Wheel and Tyre Fitment: Choosing the Correct Sizing Wheel & Tire

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Choosing the Correct Sizing Wheel & Tire

11.52

00:00	Now, we know what size wheel and tyre will fit, it's not always going to be a matter of biggest is best.
00:05	It's going to depend heavily on the application of the vehicle and how it's been modified from the factory configuration.
00:12	In this module we'll look at our Toyota GT86 endurance race car as a case study.
00:18	Although the drivetrain is a far cry from the factory arrangement, the suspension is still based on the original McPherson strut in the front and double wishbone in the rear, albeit with a lot of adjustable components.
00:30	With almost three times the factory power, grip under acceleration out of the corner is the main concern and we're going to be limited by the standard bodywork as the car will not have wider guards fitted.
00:43	It's also important to note the details of the braking package, which involves six piston calipers and 370mm discs in the front with four piston calipers and 340mm discs in the rear.
00:57	Let's start with the overall diameter first, but to some degree all the dimensions need to be considered together.
01:03	By this I'm referring to the rolling diameter of the tyre, not the wheel, we'll get to that in a moment.
01:09	From factory these cars use a 17x7 ET48 wheel and a 215 45 R17 tyre.
01:17	Although some models come with an 18x7.5 ET48 wheel and a 215 40 R18 tyre, which is what we'll make our comparisons to.
01:28	Note that this is the same for the front and the rear for what's called a square setup, which is great for balance and the near 50-50 weight distribution of this car.
01:37	We'll be maintaining this moving forward.
01:40	If we can target the same rolling diameter as the factory specification of between 625 and 630mm or a few percent either way, this is ideal as the suspension geometry and chassis clearance are largely based on the original setup.
01:56	It's unlikely that we'll ever make the rolling diameter significantly smaller although slight decreases can help with acceleration, which is really not something we need help with on this car.
02:07	Increases in rolling diameter while potentially increasing the contact patch and therefore grip, usually present issues with clearance in the wheel well, which is limited in this case as the area of the car hasn't been modified.
02:20	But still ground clearance, mass, inertia and gearing all need to be taken into account.
02:26	So, let's assume we're targeting approximately the same rolling diameter.
02:30	If so then with regards to the wheel, increases in diameter will mean a lower profile tyre and vice versa, where a smaller wheel means more sidewall.
02:41	Unfortunately, there's not a magic number we can apply here for the ideal profile as every tyre is different.
02:48	In short though, for most road racing applications we recommend avoiding an overly large and balloon looking sidewall as this will generally not be stiff enough for most modern performance tyres.
02:59	Similarly, a very low profile tyre should be avoided as it will be too stiff and the ride and contact patch will suffer.
03:07	Probably the biggest factor for changes in wheel size is brake clearance though.
03:12	Put simply, a 17 inch diameter wheel just isn't big enough to clear the braking package.
03:17	A good minimum value for clearance between the caliper and the wheel is around 3-5mm.
03:24	In most cases this will be enough to allow for some relative movement between components with stress and heat.
03:31	18 inch wheels will provide adequate brake clearance while still allowing for a reasonable sidewall with their original rolling diameter.
03:39	Recalling that there was a factory offering for this car with a 215-40 R18 tyre.
03:45	Moving up to a 19 inch would require a lower profile tyre though with some significant compromises in performance, not worth any future upgrades in brake size.
03:56	Moving on to wheel width and offset, since these dimensions are both in the axial direction, they really need to be considered together.
04:04	This along with the face design could have an influence over the caliper clearance.
04:08	However, the main constraints here are going to be the clearance of the chassis, suspension and guard as well as the scrub radius.
04:16	In terms of clearance we must understand this throughout the entire suspension travel.
04:21	There are a few different areas we are looking at.
04:23	On the inside of the wheel, the suspension arms will be the key limitation.
04:28	This is usually the coilover for a McPherson strut like the front of our GT86, or the upright in some double wishbone setups like in the front of our Honda CRX.
04:38	But other members like the tension control arm or caster arm could also be limiting when the steering is turned.
04:45	Side note here, adjustable aftermarket coilovers like in our GT86 will typically run a smaller diameter spring and perch, which generally provides more clearance on the inside of the wheel.
04:57	Clearly any increase to the backspacing of the wheel will extend into this area, so we need to be aware of how much space we have and what we can get away with.
05:06	The inside of the wheel well also needs to be considered and some vehicles will naturally offer more space here than others.
05:13	For our GT86 there's enough clearance for an inch more backspacing without any issues and still leaving plenty of clearance.
05:21	This extends around the wheel to the outside of the well and the clearance to the guards.
05:26	Not only do we want to avoid contact with the guard like any other parts of our vehicle, but as you might remember from our legalities module, there could be legal limitations around this as well.
05:37	It's very common for there to be some rubbing between the tyre and the guard when wide wheels and lower ride heights are used in conjunction, which is usually the case in motorsport.
05:47	But that doesn't mean that it's okay and it should be avoided as the only outcomes are negative and result in damage to the tyre and the guard.
05:56	In saying this though, it's also possible and very common to extend the guard, increasing the space.
06:02	This could be as simple as a light roll or pull if we only need a small amount of extra room and this takes some care and skill to get good results.
06:11	Alternatively, the guards can be removed and replaced with wider versions or cut and modified with flares or over fenders for significant increases.
06:20	There's a lot more to it than this, but that's well outside the scope of this course.
06:25	On our GT86 we aren't going to use wider guards, but we have lightly rolled the flange of the guard flat for some extra clearance.
06:33	Compared to a factory wheel, we're able to bring the outer lip out about 1.5 inches without causing any clearance issues with the guard or other parts of the chassis throughout the complete suspension and steering travel.
06:46	It's important to note though that if the scrub radius is increased, the wheel will sweep through a larger arc, meaning it moves forward and backwards in the wheel well when turned, requiring more clearance.
06:58	On the topic of scrub radius, changes to the wheel width and offset aren't all about clearance.
07:04	And again, we're not just trying to fit the widest wheels with the widest track width under our car.
07:10	When choosing what offset we want to run, we need to consider how we wish to change the handling of the car, if at all.
07:17	This is a better approach than maximising the wheel width while using offset to alter how this width is distributed on each side of the mounting face, and then dealing with the changes it makes to the handling.
07:30	So, essentially we can use our knowledge of scrub radius and track width to alter the handling of the vehicle within this window that the clearance allows for.
07:39	If we don't want to change the handling at all, then obviously we should keep things the same.
07:44	Most specifically, the offset as this moves the scrub radius and the track width.
07:49	Changes to the tyre width also have a significant effect on the grip, and changes to the wheel width relative to the tyre will influence the handling.
07:58	In most cases though, relatively small changes will have a minimal impact, say within 10-20mm or so.
08:05	You'll probably feel the difference, but your lap times probably aren't going to change too much.
08:10	Again on our GT86 we have room for about an inch and a half more width on the inside and the outside.
08:17	Ideally we'll keep the relative track widths on the front and the rear and the scrub radius unchanged to maintain the renowned handling of this chassis.
08:26	We'll leave balance changes to be made with the spring rates, anti-roll bars, alignment and slight geometry adjustments.
08:33	And saying that, some extra width to make use of this free space will generally help with lateral grip.
08:39	With this in mind, we've decided on the Enkei Raijin.
08:42	A relatively lightweight, stiff and quality wheel that doesn't break the bank.
08:47	In the 5x100 lug pattern for the GT86, this was available in 18x9.5 ET45.
08:55	Meaning the outer flange is 28.4mm or just over an inch further outboard, and the inner flange is 22.4mm or just under an inch further inboard.
09:06	The track width is only 6mm wider on both axles and the scrub radius has only increased by 3mm.
09:13	So, this should keep things relatively unchanged while filling out the extra space and allowing for more rubber.
09:19	On that note, the last thing to mention is tyre availability.
09:23	Put simply, performance tyres will be limited for smaller wheels.
09:26	There won't be many options at all for 14 inch wheels.
09:30	Things get a bit better for 15 inch wheels and 16 inch wheels would have more selection again.
09:36	But the majority of performance tyres will be offered in 17, 18 and 19 inch wheels.
09:42	So, it might be worth considering this if you're working with a smaller size wheel and potentially stepping up a size if it doesn't cause too much compromise in other areas.
09:52	Since the width and clearance will also concern the tyre size and how it fits on the wheel, we also need to consider tyre availability.
10:00	Before we make any final decisions, we need to be sure that we can get the tyres we want and the size we want at a reasonable price.
10:08	For the GT86, which runs on racing slicks, 18 inches tends to have the biggest available range.
10:14	We typically run a Michelin Sport GT slick in a 27-65-18, which has a recommended rim width range between 9.5 to 11.5.
10:25	This means the new tyres are wider than factory by 50mm or so in the interest of lateral grip for cornering, and more crucially for this car, also have a larger rolling diameter by about 20mm for longitudinal grip in acceleration and braking.
10:41	This increase in diameter means 10mm more at the top and the bottom, so it could fit in the wheel well without any issues.
10:49	And the compromise of 10mm more to the ride height for the same suspension geometry was justified in this case.
10:56	Note that the car has been lowered significantly from the factory ride height while the suspension has been corrected.
11:03	In summary, when choosing the correct wheel and tyre size, keeping things the same as the factory setup is usually going to be the safe option.
11:11	But in some cases we're going to want to make changes in the interest of more grip, better handling and more brake clearance.
11:18	In terms of wheel diameter, this is a balance between the brake clearance and the tyre sidewalls.
11:23	A smaller sidewall can be an advantage, but generally we need to have a wheel big enough for our brakes.
11:30	In terms of wheel width or offset, the key point here is how we can use our knowledge of scrub radius and track width to alter or maintain the handling of the vehicle within the window that the clearance allows.
11:42	It's still important to keep in mind tyre availability for that wheel size and that will work with that fitment.