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I have a fron disk hub which has FTF 62mm, offset 7.7 (Novatec d791sb)

According to this:

Wl = (F.T.F/2 + Offset) = 38.7mm
Wr = (F.T.F/2 - Offset) = 23.3mm

But when I look at the picture, it seems to me that Wl should be smaller Wr because left side contains disk.

enter image description here

Left side is disk side. So Why is wl > Wr?

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The offset can be positive or negative, depending on which flange is closer to the centre of the hub. In the formula you've quoted, a positive offset indicates that the right flange is closer to the centre.

This is typically the case on a rear wheel, where the freehub body occupies more space on the right side than the disc does on the left. In this case, the flange centre is "pushed" to the left, which is denoted by a positive offset.

On a front wheel there's no freehub body to get in the way, and the disc "pushes" the flange centre to the right, hence a negative offset.

A picture is worth a thousand words:

enter image description here

So for your hub, if you want to get the correct numbers out of this formula, you just need to negate the offset: -7.7 mm.

Note that this is just a matter of convention, and you could just as easily put the minus sign in the formula instead, so that a positive offset indicates a left-biased hub, and vice-versa.

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