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Measuring frame angles(6 posts)

Measuring frame anglesJBF
Feb 8, 2002 10:07 AM
Does anbody have suggestions on the best way to measure frame angles such as the seat tube and head tube?
Frame anglesNessism
Feb 8, 2002 11:34 AM
It's very difficult to measure angles properly. If you use an angle level, such as sold by Sears, it will only be accurate to + or - 1 degree or so.

For the seat angle you can drop a plumb bob off the top of the seat tube and then measure the setback. Once you have this measurement, along with the seat tube length, you can calculate the angle.
Seat tube angle = 90* - Sin(-1) (setback/seat tube length)

For head angle, I don't have any suggestions.
I use one of these....Jack S
Feb 9, 2002 7:37 AM
you gotta make sure the bike is level to get a decent reading. I do this by making sure the tt is level. This won't work for slooping/compact frames, but none of mine are so I don't care.
re: Measuring frame anglesstella
Feb 9, 2002 7:57 AM
survey and engineering supply stores have plastic good quality protracters. grind the tips off so it will fit inside the juction of the seat/top tube. this will give you a quite accurate angle, but not true centerline. headtube is more difficult as mentioned. if you have a long quill to set into the fork column, mark the centerline on the quill with tape or fine marker and also on the top tube. use your best eye and the protracter to approx the angle. a frame builder and a table/jig is the next step. later.
re: Measuring frame anglesNessism
Feb 9, 2002 12:03 PM
This method will work but assumes the top tube is level with the ground. Many frames have a sloping top tube which is not visible to the naked eye.
re: Measuring frame anglesLone Gunman
Feb 10, 2002 2:48 PM
I would suggest laying the bike over on it's side on top of a piece of blank paper and drawing lines of the angle you want measured and use a protractor to measure the angle on the inside of the frame for seat tube and head tube.