Information on the tubing is a bit spotty at best but generally it seems to follow these lines for Koga-Miyata:
EDIT 2020-jan-16:
Miyata confirmed most of what I mentioned below.
Seatposts seem to all be
26.8mm.
Steel tubing:
Hardtlite® FM-1: Chrome-Molybdeum Splined Double-butted/Triple-butted (STB) tubing.- Highest quality grade,
- Comparable to Columbus SLX, roughly comparable to Columbus TSX or Reynolds 753
- Dimensions:
- Wall thickness at lug: Between 1.0 and 0.8mm
- Wall thickness mid-tubing: 0.55mm
Hardtlite® FM-2: Chrome-Molybdeum Double-butted tubing on the main frame.- Comparable to Reynolds 531 or Columbus SL
- Sometimes triple butted on their racing frames, though I suspect those would later be called FM-1 models
Hardtlite® FM-3: Chrome-Molybdeum (non-butted) tubing.- Non-butted, wall thickness 0.8 mm for seat tube and top tube, 1.0 for down tube and head tube.
- Simplest grade, comparable to Columbus Cromor/Aelle or perhaps straight gauge Reynolds 531
Hi-Manga HM-2: Special Manganese steel tubing (complete frame).- Not a lot of information on this but they managed to make some pretty lightweight frames with this, especially in double-butted varieties.
There is also HM-1 and HM-3 as well as FM-A1 but no real information on it.
When they switched over to aluminum frames (Alloy 7005), which were double/triple butted frames as well, you would see less and less Miyata frames.
Alloy tubing:
Carbon fiber:
- Carbolite: Honestly I don't know much about the different varieties of Carbolite. From what I've read some of the early models used carbon fiber with a thin alloy tube inside, often lugged, later models are completely built out of carbon fiber. If anybody can tell me a bit more about this I will gladly add it.
This image shows the Special Alfrex alloy 6000 0.7mm thick tube on the inside with all the different layers of carbon.
(Original scan by Elevation 12000)