How do I measure effective seat tube angle?

Triathlon Forum
1

I have measured the seat tube angle on my bikes. What I’d like to know is how do I measure effective seat tube angle as I ride it. This obviously changes depending where I am on the saddle. While mounted on my bike, what point on my hips would I use to measure?

Thanks in advance,

Dennis

2

Dan explains it really well here: http://www.slowtwitch.com/mainheadings/techctr/bikefit.html
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3

The approx. measurement should be from the BB to center of the saddle. Take a digital photo, print and measure with a protractor is one approach.

At one time Slowman had a nifty chart posted that showed your seat agle based upon seat tip plumblined to fore/aft BB and seat height. You’d have to ask him, but I believe he took it down because is wasn’t quite accurate. Ask Slowman and maybe he’ll redo this chart for us some day.

4

Two methods, one more pragmatic than the other:

  1. Measure the length of the saddle. Divide in two to find center point. Use a string, plumb bob or one of the new laser measuring devices from Craftsmen make a line from the center point of the saddle to the center of the bottom bracket. That is your BIKE’S relative, effective seat tube angle. To find your REAL seat tube angle:

  2. Take a digital photo of the rider on the bike while riding. It is best to shoot the photo about 40 minutes into a hard effort, such as on a Computrainer, etc. Using Scion Imaging software (available free on the Internet) extend a line from the union of the femur and the pelvis to the center of the bottom bracket. That is YOUR effective seat tube angle, or, where you actually sit.

You may be amazed to find numbers like your bike has a 78 degree seat tube angle, your saddle is set at 80 degrees and you actually sit at 82 degrees, 4 degrees steeper than your frame geometry. We see that pretty commonly.

5

Thanks for your reply’s. I kind of had the feeling I may be riding steeper than the bike. Now I can figure out for sure.

Thanks again,

                 Dennis