Luna Because of the presence of the motor taking up space, the point where a chainring would mount onto the motor is pushed out to the right from where a chainring would normally be. Offset is defined as how much distance you are able to bring the sprocket closer towards the frame. This results in a better chainline, less likelihood of chain drop and less wear on the drivetrain.

Here is how we measure the offset.

The measurements are from the outside of the chain ring to the inside of the mounting point (inside face):

First we measured the thickness of the mounting plate, I zeroed the caliper to subtract the thickness and measure total height:

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TOTAL HEIGHT with the mounting plate thickness subtracted

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RESULTS:
The measurements are from the outside of the chain ring to the inside of the mounting point (inside face)
Total diameter includes teeth. Note all teeth will work with chains up to 10spd


Luna
Spyder
Lekkie
28T
Luna
Mini
30T
Luna
One
36T
Lekkie
36T
Luna
Alloy
40T
Lekkie
42T
Luna
Eclipse
42T
Luna
Alloy
42T
Luna
Alloy
44T
BBSHD
Stock
46T
Luna
Eclipse
48T
Lekkie
52T
Eclipse
40T
Offset 7.5mm
7.2mm 8mm 8mm 7.2mm 9mm 20.4mm 24.8mm 9mm 9mm 19mm 24.8mm 18.3mm 8mm
Total
Diameter
131mm 151mm 174mm 180mm 190mm 205mm 216mm


Please note the video above has a typo on the Lekkie in the video, it is 20.4mm
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For an idea of how various rings look side by side:
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Luna BBSHD Spider adaptor is 7.5mm in height but there is no chain ring mounted on it. The chainring mounts in a recessed part of the adapter, so depending on the thickness of the ring the offset will or will not change.
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The distance from the bottom bracket to the chain ring mounting spot on the BBSHD:
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At this point you may be asking yourself how much offset you need for your application. Generally the more the better and just use a chainring spacer along with longer chainring bolts to back it off if it hits the chainstay. However there are a couple situations where you can do without a lot of offset.

One of the most common is using an IGH (Internal Gear Hub), since an IGH does not have an external cassette that physically moves the chainline, as long as it is in a good position it will always have that good position regardless of what gear you have it in.

Another is to rework the rear sprocket, rearranging the sprockets in such a way that you still have the same amount of gear ratio but you aren't shifting through as many gears, meaning that when you do shift into the lowest gears it isn't pushing the chain so far towards the frame that it will create chainline problems.
See this link for our documentation on reworking the rear cassette.


Also it may depend a lot on the frame. For example a frame that uses a 68mm bottom bracket shell but has wide hub where the cassette would be further outbound.

For more information comparing chainrings and how chainline can affect a build see this link.