Measuring disc offset?

Bitza · Today, 12:45 PM

Hi an awkward question, what does the offset of a brake disc actually mean? Is it measured from the hub/disc join, out to the side of the braking surface of the disc or to the center line of the braking surface? Not as obvious as I had assumed.

Mr Gazza · Today, 01:04 PM

Usually the disc offset is as you say, from hub face to outside of the disc.
Or the entire thickness of the disc and carrier (Excluding any carrier bobbins that might be above the disc surface.)

Be warned that some sellers, particularly on eBay, don't seem to understand that, and you do get some strange interpretations sometimes!

I sourced some spacers available in bespoke thicknesses should you need any.

Dukedesmo · Today, 01:29 PM

As above, from hub to outside of disc, as per this cross section where, F = offset;

Bitza · Today, 02:57 PM

Thanks for that, just a bit of research for my winter project. Does seem slightly illogical, because as the disc wears the offset reduces, whereas if it was from the center line of the disc it would remain constant.

utopia · Today, 03:06 PM

There are good, sensible reasons why the specification of disc offset makes no sense at all.

As an example of the pitfalls it is worth noting that even this thread has already fallen foul of one ..... probably. As follows ....
Bitza asks "Is it measured from the hub/disc join, out to the side of the braking surface "
A fair assumption would be that he means "out to the nearest (ie inner) side of the braking surface"
And yet both Gazza and Dukedesmo (both knowledgeable and sensible guys) have said "yes, its measured to the OUTER side of the braking surface" A self contradiction .... possibly.

So we have here two possible ways of measuring offset, starting at the mounting face and extending to either the inner or the outer braking surface.
But now consider a rear disc with no floating element, ie cut from a single piece of steel plate. Lets say the material is 6mm thick.
Such a disc is flat and therefore could be said to have zero offset.
Measuring to the inner braking surface also gives zero offset
BUT by the previous definition of measuring to the outer face, it could be said to have 6mm offset.
Furthermore, one could argue (and I would myself) that the most factually accurate way to specify offset is by measuring to the midpoint of the two braking surfaces (ie the centreline), in which case our 6mm flat disc has 3mm offset.

I am not aware of anything like a British Standard specification or other industry code for this, but even if there is one I would not trust it to be universally applied.
So the answer is that the only safe way to proceed is to specify (in clear detail) the method of measurement .... preferably in pictorial form as mere words can very easily fall short (as observed earlier).

ps.
In support of my assertion that offset should be measured to the centreline of the disc, consider that in either of the two other methods, the offset changes as the disc wears thinner (an odd state of affairs) whereas by measuring to the centreline it remains constant throughout the life of the disc (as Bitza has commented while I was typing this).
Measurement to the centreline also makes much more sense when determining whether a disc will run central in a (fixed) caliper.

But of course this would mean that a flat, 6mm disc has a 3mm offset, which does seem odd at first sight.

So perhaps one could argue that measurement to the inner braking surface would cause less confusion.
Or would it ?

According to our information thus far, its "normal" to say bugger that, lets use the least logical method and measure to the outer surface.

Today, 12:45 PM	#1
Bitza Silver Member Join Date: Jul 2006 Location: Poole Bike: M900ie Posts: 518	Measuring disc offset? Hi an awkward question, what does the offset of a brake disc actually mean? Is it measured from the hub/disc join, out to the side of the braking surface of the disc or to the center line of the braking surface? Not as obvious as I had assumed. __________________ Bitza

Today, 01:04 PM	#2
Mr Gazza Lord of the Rings Join Date: May 2013 Location: Norwich Bike: M900sie Posts: 5,990	Usually the disc offset is as you say, from hub face to outside of the disc. Or the entire thickness of the disc and carrier (Excluding any carrier bobbins that might be above the disc surface.) Be warned that some sellers, particularly on eBay, don't seem to understand that, and you do get some strange interpretations sometimes! I sourced some spacers available in bespoke thicknesses should you need any. __________________

Today, 01:29 PM	#3
Dukedesmo Registered User Join Date: Sep 2010 Location: Leics Bike: M900 Posts: 2,907	As above, from hub to outside of disc, as per this cross section where, F = offset; __________________ M900, 916, LeMans II.

Today, 02:57 PM	#4
Bitza Silver Member Join Date: Jul 2006 Location: Poole Bike: M900ie Posts: 518	Thanks for that, just a bit of research for my winter project. Does seem slightly illogical, because as the disc wears the offset reduces, whereas if it was from the center line of the disc it would remain constant. __________________ Bitza Last edited by Bitza; Today at 03:01 PM..

Today, 03:06 PM	#5
utopia No turn left unstoned Join Date: Jun 2010 Location: leicester Bike: M750 Posts: 4,563	There are good, sensible reasons why the specification of disc offset makes no sense at all. As an example of the pitfalls it is worth noting that even this thread has already fallen foul of one ..... probably. As follows .... Bitza asks "Is it measured from the hub/disc join, out to the side of the braking surface " A fair assumption would be that he means "out to the nearest (ie inner) side of the braking surface" And yet both Gazza and Dukedesmo (both knowledgeable and sensible guys) have said "yes, its measured to the OUTER side of the braking surface" A self contradiction .... possibly. So we have here two possible ways of measuring offset, starting at the mounting face and extending to either the inner or the outer braking surface. But now consider a rear disc with no floating element, ie cut from a single piece of steel plate. Lets say the material is 6mm thick. Such a disc is flat and therefore could be said to have zero offset. Measuring to the inner braking surface also gives zero offset BUT by the previous definition of measuring to the outer face, it could be said to have 6mm offset. Furthermore, one could argue (and I would myself) that the most factually accurate way to specify offset is by measuring to the midpoint of the two braking surfaces (ie the centreline), in which case our 6mm flat disc has 3mm offset. I am not aware of anything like a British Standard specification or other industry code for this, but even if there is one I would not trust it to be universally applied. So the answer is that the only safe way to proceed is to specify (in clear detail) the method of measurement .... preferably in pictorial form as mere words can very easily fall short (as observed earlier). ps. In support of my assertion that offset should be measured to the centreline of the disc, consider that in either of the two other methods, the offset changes as the disc wears thinner (an odd state of affairs) whereas by measuring to the centreline it remains constant throughout the life of the disc (as Bitza has commented while I was typing this). Measurement to the centreline also makes much more sense when determining whether a disc will run central in a (fixed) caliper. But of course this would mean that a flat, 6mm disc has a 3mm offset, which does seem odd at first sight. So perhaps one could argue that measurement to the inner braking surface would cause less confusion. Or would it ? According to our information thus far, its "normal" to say bugger that, lets use the least logical method and measure to the outer surface.