Wheel and Tyre Fitment: How to Measure a Wheel

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How to Measure a Wheel

08.25

00:00	It's an instrumental skill to be able to measure a wheel and determine exactly what the specifications of it are.
00:06	This can come in handy if we have a set of existing wheels we want to run or are able to test fit and then want a reference for dimensions of a new set of wheels.
00:15	Armed with the knowledge from the dimensions and sizing module, check back if you need a refresher, this will make this practical work logical and simple.
00:24	Before we even start we need to check over the wheel for any markings that could save us the time and effort.
00:30	The wheel dimensions are often written on the mounting pad.
00:33	If this is a single piece wheel, that's great, but if we're working with an older, multiple piece wheel, we can't really trust them as they could have been rebuilt to different specs.
00:44	We'll work through this in the same order that wheel dimensions are expressed, starting with the diameter.
00:49	We'll be using a Worker Motion RS11 two piece wheel as an example throughout this module to help convey the practical skill.
00:57	If there is a tyre fitted to the wheel, then again we've gotten off easy.
01:01	Clearly, the wheel diameter will be written on the sidewall of the tyre.
01:05	Otherwise, this is actually a bit tricky to measure unless you have a huge pair of measuring calipers.
01:11	Remembering that the actual diameter is at the bead seat, not the overall diameter, so using a measuring tape or ruler, we can measure to the approximate location on the outside of the wheel, finding that our example wheel is 17 inches.
01:26	In this case an approximate measurement is fine, because the wheel sizes step up in inches anyway, so it'll be pretty clear of what the diameter is.
01:35	Next, up is the width, and while we might be able to get a rough idea of this with the tyre fitted, it's really ideal to have the tyre offs so we can directly measure this with a ruler or tape measure between the flanges.
01:48	In most cases this will be a whole number of inches, 9 inches for example on our Worker Motion.
01:54	Our other wheels could be an inch and a half measures, like 8 and a half inches, or in some cases it could also be down to a quarter of an inch as well, like 8 and a quarter or 8 and three quarters.
02:05	Offset is next and this is a bit more tricky as it's not something that we can measure directly.
02:10	We first have to understand where the centreline of the wheel sits.
02:14	The trick here is not assuming that the flanges are the same thickness.
02:18	On our example wheel the outboard flange is 11mm thick and the inboard flange is 14.5mm thick.
02:26	This means that the centreline of the tyre when mounted will be slightly offset to the outside of the wheel.
02:32	The width measurement of 9 inches between the flanges is about 229mm, so dividing this by two and adding 14.5mm of the inboard flange thickness shows us that the true centreline of the tyre will be 129mm from the inside of the wheel.
02:52	From here we can place the wheel face down on a flat surface, carefully of course if we don't want to scratch the lip.
02:59	The next step is measuring the backspacing of the wheel, which might also be something that we're interested in anyway.
03:06	To do this we can lay a straight edge between the flange of the back of the wheel and then use a ruler to measure down to the mounting surface.
03:13	On our wheel this measures at 157mm.
03:17	Then we can just subtract the centreline measurement of 129 from this backspacing measurement of 157 to get an offset of positive 28mm.
03:27	Note how this method will give us the correct offset where a negative value means there's less backspacing than the wheel centreline and vice versa for positive.
03:37	So, our wheel specs are therefore 17 inch diameter, 9 inches wide and with a positive 28mm offset, or in shorter terms 17x9J ET28.
03:48	Sometimes the offset specs from the manufacturer will be slightly different to these, but this is fairly normal as they might just have a slightly different method of measuring or just more accurate measurements.
04:00	In the end it's not a problem because the difference will be negligible.
04:04	Moving on, the same method stands for multi piece wheels, as long as they're assembled.
04:09	But if not then we need to understand how to measure the wheel halves.
04:13	Getting the initial measurements between the flange surfaces is simple with the same method of using a straight edge and a ruler.
04:20	Following this we really need to know how the wheel goes together.
04:23	By this I mean, is the face sandwiched between the halves? If so, then the width will need to include this, but if not then it's simply the sum of both the inner and outer sections of the barrel.
04:35	The same idea applies to the offset.
04:37	Without the wheel assembled, we'll need to measure from the mounting surface to the surface of the face that sits flush with the inner lip.
04:45	Then subtract this measurement from the total width of the lip to find the backspacing.
04:50	From here it's just the same method as before of understanding the flange thickness to find the distance to where the true centreline of the mounted tyre will be from the inside flange, and then subtract this from the backspacing to get the offset.
05:05	But the diameter, width and offset aren't the only dimensions on the wheel that are important to its fitment.
05:12	Clearly the PCD and centreboard need to be correct to even mount the wheel.
05:17	Measuring PCD is fairly straightforward on a lug pattern with an even number of holes, as we can just measure between the centre of the two opposite holes.
05:26	Since it's a bit tricky to get a measurement to the centre of the hole, it's usually more accurate to measure the minimum distance between the holes and then add the hole's diameter to this.
05:37	For example, on a 4 lug wheel, if the measurement between the holes is 85mm and the hole diameter is 15mm, then the lug pattern is 4x100.
05:48	For PCDs with an uneven number of holes, specifically 5 stud, we can't do this as the holes aren't directly opposite each other.
05:56	For this we need to find the distance between the centre of the two holes that are closest to each other.
06:02	Either measuring this directly, or from the minimum distance between them plus the diameter of the hole.
06:08	Then we can multiply this value by the constant of 1.7 and we'll get the PCD, or a value very close to it, which will indicate it is the closest common PCD.
06:19	Back to our Worker Motion RS11 example wheel, the measurement between the edges of the holes is about 52mm.
06:27	Add the hole diameter of 15mm to this so we can understand the distance between the hole centres , which is 67mm.
06:35	Multiplying this by 1.7 gives us 113.9mm, which is really close to the common PCD of 114.3mm or 4.5 inches.
06:47	So, since we have 5 holes, the pattern is 5x114.3.
06:52	The same method can be used between the holes furthest apart, but use a constant of 1.05 to find the PCD.
07:01	Finally, the centre bore, which is best measured with calipers if we can fit them inside the wheel to get an accurate measurement.
07:08	Otherwise, you'll need to use a ruler and for our example wheel this measures at 73mm.
07:14	That covers all the critical dimensions so let's recap the main points.
07:18	It's good to know how to measure the dimensions of an existing wheel to understand its specs or to use it as a reference for other wheels.
07:25	Sometimes this will be stamped on the back of the wheel but this should be taken with a grain of salt for multiple piece wheels.
07:32	If the tyre is fitted that'll tell us the diameter from the marking on the sidewall.
07:36	Otherwise we can measure it and an approximate reading will tell us all we need to know.
07:41	The width is best measured with the tyre off and it's the distance between the inside of the flanges.
07:47	With the offset we first need to measure the overall wheel width and then the backspacing and then subtract half of the wheel width from this.
07:55	Multiple piece wheels are the same given they're assembled, otherwise we need to consider how they go together and stack the measurements of the individual parts.
08:05	The centre bore is simple and best measured with calipers as is the PCD for a lug pattern with an even number of holes.
08:12	But for 5 lug wheels we need to measure the distance between the centre of the closest holes together and multiply this by 1.7 to get a rough idea of the PCD.