Angle Finder
by Dan Empfield 3.31.04
(www.slowtwitch.com)

How do you know what seat angle you're riding? There are two considerations.

First, you can't rely on the seat angle advertised on your bike's brochure. There is too much variability associated with what kind of seat post you're riding (straight or set-back) and where your saddle is on its rails. On that latter issue, the rails give you between 35mm and 40mm of fore/aft adjustability. As you'll see by the chart below, there are two to three degrees of seat angle inherent in this range, depending on your seat height. So, a 76.5° bike is really also a 75° bike and a 78° bike just by moving the saddle back and forth. And that's without changing the style of seat post.

The other consideration is, where do you sit on the saddle? Saddles are generally between 28cm and 30cm long. Even if you limit fore/aft movement on top of the saddle to the middle 8cm—leaving 10cm on either end of the saddle unburdened—that's 5° or 6° of swing in relative seat angle.

However, none of that answers the question. How do you figure out your seat angle? Below is a chart created to give you a little help. There needs a bit of explanation. There are two measurements on your bike you need to take, but in each case all you need is a measuring tape and a plumb line (a weight hanging from a string).

First, measure the distance in centimeters from the center of the bottom bracket (the center of your crankset) up to the top of your saddle, and choose a spot right in the center of your saddle's top.

Second, drop a plumb line from the nose of the saddle and see where the line falls relative to the bottom bracket. That line will pass over a point even with the bottom bracket, or in front of it or behind it. You'll need to note where that plumb line passes, and measure its relationship to the bottom bracket.

Take me as an example. My saddle height is about 78cm. Closest to that measure in the chart below, on the first column at left, is "77cm." The nose of my saddle sits in front of the bottom bracket—on my tri bike—a total of 2cm. Looking down from the 2cm column, the box that intersects these two measures shows that my seat angle is 80.5° (green cell below).

 -5 -4 -3 -2 -1 BB +1 +2 +3 +4 +5
65cm
 75° 76° 77.5° 78° 79° 79.5° 80.5° 81°
68cm
 75.5° 76.5° 77.5° 78.5° 79° 80° 81° 81.5°
71cm
 75.5 76° 77° 78° 79° 80° 80.5° 81.5° 82°
74cm
 76° 77° 78° 78.5° 79° 80° 80.5° 81.5° 82.5°
77cm
 76.5° 77° 77.5° 78.5° 79° 79.5° 80.5° 81° 82° 83°
80cm
 76.5° 77° 77.5° 78.5° 79° 79.5° 80° 81° 81.5° 82° 83°
83cm
 76.5° 77° 77.5° 78.5° 79° 79.5° 80.5° 81° 81.5° 82.5° 83.5°
1 Plumb line
1 Saddle height

All the numbers above are rounded off to the closest half-degree. If your saddle height falls in between the 3cm jumps between rows, just look at the rows above and below where yours would fall. For example, if your saddle sits 78.5cm above your bottom bracket, and 2cm behind the BB, you'll see that the 77cm row shows 77.5° of seat angle and the 80cm row shoes 78.5°. So, your seat angle would be within a half-degree of 78 degrees.

All these numbers are plus or minus half a degree. They weren't generated via trigonometry, but by placing the saddle at these precise points versus the BB and measuring using my digital angle finder. In each case I measured to a place 14cm behind the saddle's nose. Yes, this is arbitrary, however you have to pick a spot and that's the spot I'm now using to measure my seat angles.

One of the reasons for this is that 14cm behind the nose is a spot USA Triathlon uses to determine a specific measure for reasons of rules conformity. It seems therefore appropriate to ask everyone to remember only a single place on the saddle for purposes of measurement.