Wheel and Tyre Fitment: Width

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Width

07.16

00:00	Like the diameter, the width of a wheel is also measured between the bead seats rather than the overall width to the outside of the flanges.
00:08	We'll be covering how to measure wheels in a specific module, so don't worry too much about the intricacies of this process for now.
00:16	The width of wheels commonly used for performance applications ranges anywhere from around 5 inches all the way up to 13 or more in the most extreme cases.
00:27	When expressing wheel size, there's usually a letter, most commonly J, that's used after the width.
00:35	This doesn't need too much attention, it's really just showing the profile or shape of the flange that the width is being measured between.
00:43	It's more common to run a staggered setup when it comes to wheel width compared to diameter.
00:50	What this means is that the driven wheels are typically wider, allowing for a wider tyre to be used.
00:56	A side note here, enthusiasts will often increase the width of their wheels at one end of the vehicle relative to the other in the interest of filling out the guards for aesthetic reasons.
01:07	The increase in tyre width at one end relative to the other of course changes the balance of the vehicle, making it tend more towards understeer or oversteer.
01:18	What's important to understand here is the practical considerations around changing the width of wheels.
01:24	For now, we'll assume the offset of the wheel remains unchanged and we'll discuss this in an upcoming module.
01:32	But basically, that just means any changes to the width of the wheel will be equal on the outside and inside of the wheel.
01:40	So, the track width and scrub radius will remain the same, but again we'll cover these in more detail soon.
01:46	We're also not going to consider changing the width of the tyre here, just the wheel, and obviously there will be limitations around what will fit on a vehicle.
01:56	It might come as a surprise that just changing the width of a wheel with everything else the exact same can actually have a significant impact on the performance and resulting lap times.
02:08	The reason for this is very similar to what we discussed earlier regarding sidewall stiffness.
02:14	Recalling that a stiffer sidewall helps improve response and control, but if it's too stiff, it can not only be uncomfortable, but the spring rate of the tyre can be too high for the damping, causing the contact patch to suffer.
02:28	A good rule of thumb to follow here is to use a wheel that's at least the width of the tread of the tyre, or up to half an inch wider.
02:36	This will result in a slight amount of stretch and therefore preload on the sidewall, which generally speaking will improve the tyre's response.
02:45	However, excessive stretch from too narrow of a tyre on too wide of a wheel will cause the sidewall to become overly stressed, and this can deform the carcass of the tyre, resulting in uneven pressure across the contact patch.
02:59	This all adds up to unpredictable grip and poor ride quality.
03:03	On the other hand, if the wheel is too narrow for the tyre, the sidewall bulges, and this generally leads to vague and sloppy handling, and can also result in uneven contact patch pressure, and therefore less than optimal grip.
03:18	Just a note here, the tread width of the tyre is not the nominal width that's written on it.
03:23	For example, a 205 55 R15 tyre could have a tread width of only 7 inches, whereas 205mm is actually closer to 8 inches.
03:35	So, ignoring other factors like wheel clearance for example, this tyre would be best fit to a 7 or 7.5 inch wide wheel.
03:44	In many OEM configurations, we might see a 205 fit to a 6.5 inch wide wheel, but OEMs need to consider other factors like protection from snow chains or curbing, and of course cost, all at the compromise of absolute performance.
04:02	There are a few different ways to look at changing the wheel and tyre width from a factory setup.
04:07	If the car came with a 285mm width tyre, with a 10.6 inch tread width, on a 10 inch wide wheel, it would likely produce more grip and have better response on an 11 inch wide wheel.
04:23	Alternatively, we could keep the same 10 inch wide wheel, and optimise the grip using a smaller 275 width tyre.
04:31	Although, that said, there will likely be a slight reduction in the total grip, for a slight gain in response.
04:39	So, we're probably better off increasing the width of the wheel, but all of this will really depend on what size tyres are available, which we'll be diving deeper into later in the course.
04:51	Keep in mind that different tyres have significantly different tread widths, even though they might have the same nominal size.
04:59	Motorsport tyres for example, tend to have wider tread widths than their road going counterparts.
05:05	Manufacturers for racing tyres will actually often have a recommended rim width for each tyre, so this can help guide us in the right direction.
05:14	This isn't a tyre course, but it is important to understand that the combination of tyre and wheel width is more important than the tyre width alone, and should be considered before stuffing the biggest tyre we can in our wheel well.
05:29	The other factor that needs to be mentioned is the weight of the wheel.
05:33	Obviously, all other things equal, an increase in wheel width will result in more weight, which is unsprung and rotating, so is especially important to minimise.
05:43	This probably won't offset the advantages we just discussed, but can be an issue in extreme cases, so it's more just something to keep in mind.
05:52	Finally, if we're planning on using a much wider wheel than factory, to allow for a wider tyre in the search of more grip, then we need to keep in mind that there are some drawbacks.
06:04	We won't be diving into the complex subject of tyre forces in this course, but to put it extremely briefly, this will result in heavier steering and more stress on all of the components involved.
06:16	This will still be the case even if the centreline of the wheel remains in the same place, by keeping the offset the same, and therefore track width and scrub radius.
06:27	But again, we'll shed some more light on these topics in upcoming modules.
06:31	Summing up this module, the main takeaways are that the width of the wheel is important to ensure that the tyre is supported properly.
06:40	There will be constraints limiting the size of wheel we can use, such as the clearance of the fenders and wheel well, or even the governing bodies for our form of motorsport.
06:50	We don't want to go too wide and run into clearance issues or excessive weight, and using a wide tyre, while potentially offering more grip, will also put more strain on the steering and suspension.
07:03	Generally speaking though, a good rule of thumb is to use a wheel width that is at least equal to the tread width of the tyre, if not up to half an inch wider.