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I need an adaptor to take my rear PM brake from 160mm to 180mm rotor.
In my spares box I have a Formula PM adaptor marked F 180, but I can't imagine how they could be front and rear specific.
Have I missed something?
Rear is 20mm bigger so while they work the sizes don't match. For post to post mount 180mm front just means +20mm
Thanks but I'm sorry I don't follow you.
Surely it just needs spacing out 10mm? (half the extra rotor width)
The gap between holes must be the same, as calipers are not end-specific.
In what way do you mean the rear is bigger?
PM brakes as standard assume 160F and 140R (the same calliper will give 160 on your forks, but 140 on your frame), so 20mm adaptor will take the front to 180, but the rear to 160. The F180 you have is equivalent to a R160. The "gap between the holes" is 74mm - the post mount standard (forgetting daft Manitou/Trek mongrels).
I think.
I'm confused. Why not just try it?
This ^
If you don't use any adapter then your front should be fine for a 160mm rotor and rear for a 140mm rotor, unless your frame is designed to be 160mm without an adapter.
Size = Front & Rear
None = F160 R140
+10 F180 R160
+20 F200 R180
+21.5 F203 R183
+31.5 FN/A R203
Don't really understand why we have 203mm and 183mm discs tbh. Would have much simpler with 140, 160, 180 and 200.
But then IS and PM are also silly. Radial mount makes life much easier.