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Calculating Seat tube angle

1K views 2 replies 2 participants last post by  bolo yeung 
#1 ·
Hope someone can give some input.

I'm in the process of having custom frame built. My current contact points – from a bike fitting several years ago – were given to the frame builder and from those measurements drawings have been submitted.

I've made a few changes during the process (namely to head tube height to save a stack of spacers beneath the stem) but Im not totally sure of the ST angle (currently 71.5º for a seat height of 775mm and set back of 95.5mm, with head tube angle of 72.5º).

The frame is year round rider so full mud guards with 28mm tyres can be fitted hence the slightly slacker angles.

Can some one confirm if this seat tube angle will equate to the said set back at that seat height???? Sorry if a little vague with info
 
#2 ·
As far as I know, there is NO formula to calculate what you're asking for.

IN THEORY, if you knew your thigh length, the angle your thigh formed when your pedal was at 3 o'clock, and your crank arm length, you could calculate setback which would then be used to derive your seat angle.

In reality, I'll bet no framebuilder goes to that length.

Produce the numbers anyway and I'll see what I come up with.
 
#3 ·
Sorry I didn't explain that very clearly. The current set back I ride with is 95.5 mm with saddle height of 775mm. The drawing the designer has submitted has a ST angle of 71.5 degs. The saddle I use is 279mm long.

To achieve the said SB of 95.5mm is this the correct ST angle. The free version of bikecad doesn't show SB when setting up a drawing otherwise I might work this out :S
 
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