thank you ben, to be honest-it doesent make sense to me- i dont know the basics.. ive tried to find video online but didnt find nothing..Depends on the tools you have available.
If you have a protractor, then yes, you can just let the string hang vertical (with the bike level) then measure the difference. Depending on if you want the actual angle, or effective will dictate the point you measure to.
If all you have is a tape measure, the screenshot you posted will also work. You're basically just making a triangle in that case. If you know the lengths of the legs, it's just simple trigonometry to get the angle.
In both cases, you'll need to subtract your result from 90 to get the measurement we use for bikes, which is off of the ground.
Hope that helps / makes sense.
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Put a drywall square on the ground, line it's edge up with the center of the BB (this ensures that there's less error due to uneven ground). Select some point up from the center of the BB (e.g. 20cm). Now measure over to the center of the seat tube. This gives you 2 sides of a triangle and an angle (90 degrees)
Plug those into the triangle calculator:
Side B = 20cm
Angle C = 90
Side A = 6.12cm
That gives you
Angle B = ~73 deg
Side C = ~ 21cm
Angle A = ~17 deg
If you use an angle finder or plumb bob there's a possibility of error because your floor might not be level. Seeing as you're worried about STA as opposed to being able to put your saddle in the right place, I'm guessing you want to be as accurate as possible (this isn't a criticism)
yes we have agreed on 76, its seems to me that it is 78(using a app in the phone on level ground)Op, you are going to have a very hard time accurately measuring your seat tube angle. What makes you think the seat tube angle is different than agreed upon? Sliding even a minimalist seat all the way forward or back on the rails can change the effective seat tube angle 1.5 degrees. Is your definition of seat tube angle the same as the builder of your frame?
Most importantly, have you spoken with your builder about this?
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Using an app on your phone is not accurateyes we have agreed on 76, its seems to me that it is 78(using a app in the phone on level ground)
yes we have spoke about that, i had to use 30mm setback and put the seat all the way back, its not ideal espically when its a custom frame.
no, no bend at the seat tube.Angle finders only measure actual not effective. Effective is what you are after. Is there a bend in the ST?
I tried using image apps and between getting the phone completely level and the bike completely level and then marking "exactly" the right points, there was a lot of margin for error.
I have tried to use an app to measure it and I could easily get + or - 2 degrees just with super minor adjustments.
When you say you had to use a 30mm setback and push the seat all the way back..what are you feeling that made you have to do that? What is your experience like with 76 degree seat tube angles?