Headset Diameter / Steerer Tube Diameter

Just having my lunch and there is something which is bugging me since last night. Despite searching various forums and I can't get an answer so here goes ( go easy on me if it seems a silly question ). When measuring a steerer tube diameter it usually is 1'' or 11/8'' but when you press in the upper and lower cups surely that reduces the inside diameter by a few mm. In that case surely it stands to reason that the headset diameter should be the diameter of the inside of the cups for a good fit. Am I making it clear :wacko:.
 
Location
Loch side.
Perhaps you are confusing the steerer with the head tube. The latter is part of the frame, the former, part of the fork.
Fork steerers are 1" 1 1/8" 1 1/4" and 1 1/2". Headtube diameters are much larger - 41mm to about 52mm, i.e. up to two inches.

It is another one of those irritating cycling paradoxes. Head tube diameters measured in mm and steerers in inches.
 
Location
Loch side.
Looking at your question again, I think you may also assume that headsets are denoted 1" or 1 1/8". Nominally yes, practically no. The last "Standard" headset was `1" where the denotion 1" would automatically indicate the correct OD of the headset cup as well, all head tubes being standard in those days. Nowadays we have straight, tapered, oversized and stupid-sized head tubes. Thus the steerer could be 1 1/8 tapering to 1 1/2" but the head tube could be 41mm, 46mm, 50mm etc etc. That's one of the reasons for selling half headsets nowadays. You have to mix and match to get the upper and lower sizes right.
 
Perhaps you are confusing the steerer with the head tube. The latter is part of the frame, the former, part of the fork.
Fork steerers are 1" 1 1/8" 1 1/4" and 1 1/2". Headtube diameters are much larger - 41mm to about 52mm, i.e. up to two inches.

It is another one of those irritating cycling paradoxes. Head tube diameters measured in mm and steerers in inches.
Apolgies. Yes got it back to front which I'll put down to tiredness. Cheers for the 2nd reply. Question answered perfectly
 
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