Just put a 180mm disc on the backwheel of a bike and so fitted an IS to PM 180mm adaptor.
Put it all together and caliper is about 5mm too far out so only gripping outer edge of disc.
Am I being very stupid?
What have I missed?

is it the right way round?
Is it 185?
Pics may help

Avid discs are 185 I think, is that what you have?

If you are using an Avid brake it may be expected a 185mm disc but if you’ve fitted a 180 that might explain the problem. But it would only be about 2.5mm out, might be enough for it not to work well.

Thanks for thoughts but it is a 180mm disc with 180mm neatly stamped on it

Is it avid though? I thought the bloody stupid trialign thing messes up their sizing?

I assume its a rear adapter? They are different to the front ones on spacing if I recall. Ie a 180 front caliper is the same spacing as a 160 rear.

Balls!

I’m guessing the adapter is upside down and the calliper will grip the disc much further round but 5 mm further on. It’s easy done!