Wheel and Tyre Fitment: How to Measure a Car for Fitment

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How to Measure a Car for Fitment

10.33

00:00	Measuring a car for fitment isn't just a straightforward method that involves direct measurements.
00:05	What I mean by this is that some factors might take a bit of imagination to visualise what will fit based on the clearance available and how the suspension moves.
00:15	So, to some degree this is something that takes some experience and you'll get better at it the more you do it.
00:21	In saying that, there are some methods that can help paint a picture, so let's cover them.
00:27	One of the easiest ways is fitting a wheel and a tyre with known specs to a vehicle and using this as a reference.
00:34	Naturally, if it fits and presents no issues, then we know that any wheel of the same specs should work in terms of diameter, width and offset.
00:42	However, different spoke and barrel profiles can cause clearance issues, especially with the brakes.
00:48	For an example let's look at a 1991 Porsche 911 of the 964 generation.
00:54	This one in particular is what's referred to as a narrow body, basically meaning the guards aren't as accommodating for wider and lower offset wheels compared to turbo models.
01:05	The wheels currently on the car are Porsche Fuchs, albeit in a different size to what would have originally come on the car.
01:12	The front wheels are 17x7 ET40 with a 225 45 R17 tyre, and the rears are 17x9 ET50 with a 20mm spacer and running a 255 40 R17.
01:27	There are a few different wheel options available from factory on these cars, but the widest and lowest offset original specs would be 17x7 ET55 front and 17x8 ET52 on the rear.
01:42	So, the current front wheels are 15mm lower offset, meaning the front track is 30mm wider and the scrub radius is also 15mm greater.
01:53	The entire wheel is essentially shifted out 15mm.
01:57	The current rear wheels are 1 inch wider with an effective offset with the spacer 22mm lower.
02:03	All of this adds up to the outboard flange being around 35mm further outboard, and the inner flange being also 10mm further outboard, or 10mm less backspacing.
02:16	So, more clearance to the inner wheel well but less clearance to the guard.
02:21	The track width on the rear is also now 44mm wider than the factory setup.
02:26	This increase in rear track width relative to the front track width will likely move the balance of the vehicle further towards understeer.
02:35	Although, this is probably minimal and not much of a concern on a street car like this.
02:40	Early 911s are somewhat known for having sunk rear fitment, where the factory wheels don't fill out the guards, which is likely what prompted the initial change in this case simply for looks.
02:52	Just from looking at the vehicle on the ground we can see that the current wheels fill out the guards nicely with a relatively small amount of space to the guard.
03:00	This is a street car and a classic one at that, so the new setup must look good.
03:05	The intention is for a relatively flush fitment where the outer lip of the wheel meets the width of the guard.
03:12	But above this it's critical that the car still drives well with no clearance issues that could cause damage or just be an annoyance.
03:20	We still need to consider the brake clearance and also how the scrub radius compares to the factory wheel, and if we want to make changes to this.
03:28	But we'll discuss this more in the next module.
03:31	After supporting the car on axle stands we can disconnect the anti roll bars and disassemble the front suspension to remove the spring and bump stop and then reassemble it without these parts.
03:43	With the safe spec wheels fitted again, we can move the wheel through the full range of suspension and steering travel and start to see how much clearance is left to the suspension, guard and wheel well and understand how changes to our wheel dimensions would affect this clearance.
03:59	For example, on the front axle there's about an inch and a half of clearance between the inner flange and the coilover and inner wheel well, and about half an inch to the guard on the outside.
04:10	17 inch wheels clear the brake safely in the radial direction and the caliper is relatively far back from the hub mounting surface, helping with clearance in the axial direction.
04:20	Although the design of these Fuchs does come within a few millimetres.
04:24	The lower ball joint and the outer tie rod for the steering are well clear of the barrel in the 17 inch wheel as well.
04:30	The current rear setup with the much lower effective offset has inches of clearance to the inner wheel well and the suspension, but the outer lip of the wheel is relatively flush with the guard already.
04:42	If we didn't have a test wheel like this, then there are alternative methods.
04:46	For example, we can use a straight edge and a ruler to take physical measurements or there are tools designed specifically for this.
04:54	These bolt onto the hub just like the wheel and are adjustable to simulate different wheel specs.
05:00	Some will even include the size of the tyre or allow for a tyre to be fitted.
05:04	If you're not doing this regularly, then the purchase price of these probably won't be justified, but they do make the process easy so you can try a bunch of different wheel sizes quickly before buying the real thing.
05:16	They are also very easy to see around so you can check for clearance.
05:20	The downside is that they won't actually be the exact shape of the wheel spokes and barrel, so checking tight clearances around the brakes isn't really useful.
05:30	Alternatively, if you have the skills in CAD or know someone who does, it's possible to get a cross section drawing from the wheel supplier and scale this to size.
05:39	We were able to get these profiles from Rotiform for their ROC-H wheels, an 18x8.5 ET49 on the front and 18x10 ET49 for the rear.
05:52	Comparing this to the current wheels, clearly we've moved up an inch in diameter, but they're also 1.5 inches wider in the front but 9mm greater offset.
06:02	That's about 10mm more on the outside flange.
06:05	Since we had around half an inch or 13mm of space to the guard before, we won't be able to run a very wide tyre if we want this to fit.
06:14	And no we're not going to stretch a tiny tyre onto the wheel just to clear the guards.
06:19	But in this case a 225 wide tyre is relatively wide on a 7 inch wheel.
06:25	The same width could be used on an 8.5 inch wide wheel, but we'll probably use a 235.
06:31	The small increase in stretch will help with clearance and likely some steering feel and response with no reduction in grip.
06:39	The inside flange will also be around 28mm further inboard, which should be fine as we had about an inch and a half of clearance to the coilover.
06:48	Again depending on the width tyre we use, things might get tight at the front of the inner wheel well at full lock.
06:55	The 10mm increase in offset will reduce the front track width by 20mm and the scrub radius will now only be 5mm from the original factory setup, as opposed to the 15mm from the current setup.
07:08	This should tend towards a more stable setup with less pull on the steering wheel with bumps in the road.
07:14	After tracing the outlines in CAD we exported DXF files to get plasma cut from cheap steel, including a hub piece that could be tack welded to the spoke and barrel profile.
07:26	All of this so we can use this as a reference to check if a certain size wheel will fit.
07:31	With the templates bolted to the hub and the suspension at full droop, we can see that there's still plenty of clearance to the coilover and other suspension parts.
07:41	This method also gives us a clear indication of the brake clearance as well, as we have an accurate profile of the face in the barrel and in our case there's absolutely no concern here.
07:52	Moving the suspension and steering through its travel, we can see that the fitment of these wheel specs in this design looks promising with no clear indications of contact at any point.
08:03	Moving back to the rear, the 18x10 ET49 has essentially the same offset as the current 17x9 ET50.
08:11	So, this means we're moving the inner flange half an inch or about 13mm inboard and the outer flange 13mm outboard.
08:21	The current setup is basically flush with the outer guard, so another 13mm will surely be too wide.
08:28	But removing the 20mm spacer would mean that the outer flange of the new wider wheel would be 7mm further inboard, which wouldn't be too much of a compromise to looks.
08:39	The inner flange would then be 33mm or so further inboard, which there is plenty of room for and we'd be back to the factory specs for the rear track width.
08:48	So, in the end there would be a small increase to the front track width relative to the rear, leading towards a bit more oversteer.
08:56	But given the nature of the street car, which won't often be driven at the limit, this won't make any practical difference or likely even be noticed.
09:04	Without being too repetitive we also completed the same test with the plasma cut templates to confirm we have clearance for what we just discussed.
09:13	The final thing to mention is around measuring the PCD and the centre bore.
09:17	These are easy to find for most vehicles with a quick search online, otherwise some methods are just the same as measuring a wheel.
09:25	Calipers can give us a quick centre bore measurement in a lug pattern with an even number of studs or holes can be measured directly between opposite studs.
09:34	Again for 5 stud hubs, like on our 911, measuring between the centre of the closest two studs and then multiplying this by 1.7 we'll find the PCD.
09:45	76mm by 1.7 in our case gives us about 130, meaning a 5 by 130mm PCD and we have a 71.6mm centre bore on the front.
09:58	To summarise what we've covered, when measuring a wheel for fitment, using an existing wheel with a known spec can give us a good idea of the clearance and then we can use our knowledge of wheel dimensions to understand how much room we have for changes.
10:12	Without a wheel we can still take measurements with a straight edge and a ruler, using the hub as the reference point.
10:19	Past this there are actually specialised tools for this process that are adjustable and allow you to simulate different wheel sizes, just be sure to also consider the tyre and brake clearance.