If you are looking at a frame with an "external headset" then deduct b25mm from the head tube length as that is the approx sprace taken up by a high quality external headset.
So if my Carbon road bike has a Head Tube Length (HTL) of 195mm, does that mean that on a bike (older steel for e.g.) will need a heaed tube lenth of 170mm?
I dont understand how that is equivalent, i tried wiki-ing and i still dont understand the differences in headsets.
Any gurus that can help explain this?