Wheel and Tyre Fitment: Offset

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Offset

06.09

00:00	Out of all the dimensional features of a wheel that we've discussed so far, offset might have the biggest effect on performance and more specifically, the handling of a vehicle.
00:10	It's probably the area that causes the most confusion as well, and that might be due to this aspect being described by other terms like inset, outset and backspacing.
00:20	Even though these are technically different dimensions, they all allow us to understand the same thing, that being the location of the mounting face of the wheel compared to the centreline.
00:31	To be clear, by mounting face, we mean the face of the wheel that sits flush against the hub when the wheel is mounted.
00:40	Let's get a better idea of what we're talking about by looking at a side view of the wheel.
00:45	The centreline of the wheel is halfway between the inboard and outboard bead seats.
00:50	The offset is the distance of the mounting face from the centreline, so if the mounting face lies directly on the centerline, this is what we call zero offset.
01:00	If the mounting face is further towards the outside of the wheel than the centreline, this is positive offset.
01:06	As you might have guessed, if the mounting face is further towards the inside of the wheel than the centreline, then this is what we call negative offset.
01:15	Your offset will generally be expressed in millimetres and sometimes has the letters ET in front of it, for example ET35 or ET-10, where ET actually stands for the German word for offset, "Einpresstiefe".
01:31	High and low are also used to describe offset, where lower is more in the negative direction and high is naturally more positive.
01:40	What's important to understand is that the mounting face actually stays in the same place, as it mounts to the hub, which isn't being changed.
01:48	So, if the wheel width remains the same, this means that for a wheel with a more positive offset, the outer flange will be closer to the mounting face and therefore hub, meaning it'll sit further inboard.
02:01	Whereas with a negative offset wheel, or a wheel with lower offset, the outer flange will be further from the mounting face and therefore hub and therefore be further outboard on the car.
02:12	Using the same logic, the centreline will also be further inboard at a higher offset and further outboard for lower offset.
02:21	What this means is that the track width, which is the distance between the centreline of the wheels on the same axle, will be wider with a lower offset or narrower with a higher offset.
02:33	Since the centreline moves, the scrub radius also changes.
02:37	This is the distance from where the steering or kingpin axis touches the road to the centre of the contact patch.
02:45	Again, a more negative offset means a more positive value for scrub radius.
02:51	These both not only have a significant impact on how the wheels fit the vehicle and clearance to the guards with wheel travel and steering, but also the handling of the vehicle.
03:02	We're going to cover track width and scrub radius and the effects they have on performance in much more detail in their own dedicated module soon.
03:11	The other thing we need to note is the terms inset, outset and backspacing, as they're often used to describe the same idea.
03:20	Inset and outset are simple.
03:22	Inset is positive offset due to how this moves the wheel inboard and outset is negative offset due to how this moves the wheel further outboard.
03:31	On occasion you'll even see a negative inset value just to make things confusing but at the end of the day, this will just be the same as offset.
03:42	Backspacing is slightly different though.
03:44	This is the distance from the mounting face to the inboard flange, which is more typically expressed in inches and not as commonly used nowadays.
03:54	Don't confuse a lot of backspacing for lots of space behind the wheel because it's actually the opposite.
04:00	Higher backspacing means the inside of the wheel will be further inboard so less clearance to the wheel well and suspension, all other things equal.
04:10	Basically, if the width remains the same, a wheel with more backspacing has a higher or more positive offset.
04:18	The final thing we need to mention is spacers.
04:21	These are machined aluminium pieces with matching lug pattern holes that slip over or are fixed to the lugs of the hub.
04:29	The purpose of these as the name suggests is to space the wheel outboard and they're available in different thicknesses.
04:37	The outcome is the same as lowering the offset of the wheel where the thickness of the spacer can be subtracted from the wheel offset to understand the equivalent setup.
04:48	It's important to consider the length of the lugs to ensure there's still plenty of thread engagement for safety.
04:54	For reference here in New Zealand, the law for road cars is about 80% of the diameter of the stud or bolt.
05:01	So, for example, for a M12 with a 1.5 pitch, it's 6.5 full rotations and for 1.25 pitch, it's 7.5 full rotations.
05:14	Otherwise, although spacers get a bad rap, they're commonly used with success in motorsport and road applications with the same considerations as lower offset wheels.
05:24	To summarise, offset is the distance of the mounting face from the wheel centreline.
05:30	Positive offset, also called inset, is when the wheel mounting face is closer to the outside of the wheel.
05:37	Meaning that the wheel will sit further in on the car.
05:41	Negative offset, also called outset, is where the mounting face is closer to the inboard flange, making the wheel stick out further.
05:50	Backspacing on the other hand is the distance from the mounting face to the inboard flange, so a higher backspacing is like a higher offset.
05:59	This has a significant impact on wheel fitment but also track width and scrub radius, which we'll discuss soon.