Effect of Seat Angle on Top Tube Length

Can anyone please explain to me the effect of seat tube angle on effective top tube length?

My understanding is that a (hypothetical) seat tube angle of 90 degrees would mean the seat tube points directly vertical (assuming that the measurement is taken between the chain stay and seattube?). Reducing this angle would thus shift the angle of the seattube backward, in turn lengthening the effective top tube.

or am I talking shit…?

You’re about right, although the angle is normally measured from horizontal, not the chain stay. By effective top tube you mean horizontal distance from seat to head tube, yar?

Yes and this measurement will vary depending on leg length.

Have a look at the Zesty (I think) the Med and small are similar length but the seat post angle is different allowing for a short Effective TT and longer wheel base.

So are quoted effective top tubes taking into account seat tube angles?

If I refer to my 456, it has a effective tt of 609 mm, and a seat angle of 71 degrees. Is the slack angle the reason for the long effective tt? Or do you need to factor the SA and Effective tt into the equation when sizing up a frame?

ie if 2 bikes had the same effective tt length, would the one with a 71 degree SA actually be longer than one with a 74 degree SA?

It’s the horizontal distance from the centre of the top of the head tube, to where a line running up through the centre of the seat tube and extrapolated beyond would cross that horizontal line from the head tube. The higher your saddle and the lower your bars, the longer the seated reach you’ll experience compared to the nominal effective top tube length. The slacker the seat tube, the more this will be exacerbated.

When standing up (which is what matters – this is a mountain bike!) it’s the reach and stack that are of interest. The slacker the seat tube, the shorter the reach for a given effective top tube length.

ie if 2 bikes had the same effective tt length, would the one with a 71 degree SA actually be longer than one with a 74 degree SA?

Other way round!

There are many factors affecting frame design wheelbase and length of stays,distance from dropout to bottom bracket etc.You can have a long toptube and shallow or steep angles depending on how you like your frames built or what you want it for .You may want 72 parallel on a touring frame but 73/74 on a race frame both possible with the same top tube length by changing other measurements

Just sit on it and see if it’s comfy. 😉