What seat angle go you ride? Seems a fairly simple question. Let's start with an even more basic and measurable question. Once we determine where your saddle is in relation to the bottom bracket, and we draw a straight line from the BB to the saddle, we can determine what your seat angle is, right?
Even a question that straightforward can yield a variety of answers.
Last Spring I published a chart with the aim of making it easy to find your seat angle.
The chart from that article is reproduced here:
|
|
-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 |
But it isn't quite as simple as I made it out to be. Back then, and for quite some time how, I've been operating on the basis of an arbitrary determination, that when considering seat angles the point to which we all should measure—or at least the point at which I've been measuring—is 14cm back from the nose of the saddle. In restrospect, I'm not telling enough of the story.
Why this point? Because it's roughly the midway point on most saddles. That explanation is not imbued with enough precision, in retrospect.
I asked several well-known bike fitters, "Once you determine a person's saddle position in relation to his bottom bracket, what determines his seat angle? Will Pennino from Synergy Bike Fit replied, "BB to the midpoint of the saddle." Seems reasonable. It's basically what I've been doing.
Another famous fitter essentially refused to answer. He wrote, "...what I think you are really asking is how can one locate the saddle and reproduce or compare that data on another bike."
But, that is not what I asked. This is the question I asked, the one he answered: "Let us say that you've positioned someone atop a Serotta Size Cycle. So, here's a saddle, sitting there in space, versus the bottom bracket. How is it that you would determine the seat angle of this position? You would measure center of the BB to... where? Where on the saddle? ... I'm prepared for any tangent you'd like to take on this, and will follow along with it, but one question I'd really like to have answered is, where on the saddle do you measure for the purposes of determining the seat angle?"
He did go off on a couple of tangents, which are all basically addressed in the article I wrote earlier in the year, referenced above. But he didn't answer the question. This tipped me off that even though there are quite a few fairly high-priced and well-schooled bike fitters out there, a lot of them haven't thought this question through.
What just about every good fitter agrees upon is that there are four elements that determine what your seat angle will be. Mind, not what your bike's seat angle will be, but what your seat angle will be—your "apparent seat angle," we might say. These four elements are as follows:
1. What is your bike's seat angle?
2. What sort of seat post are you using? Set-back? Straight?
3. What is the profile of your saddle in relation to its rails and its nose?
4. Where do you sit on the saddle?
The first two questions have been analyzed quite a bit, and in any case are discussed in the article referenced above.
The third point is less often considered. Gerard Vroomen of Cervelo says that a person's seat angle ought to be measured through the center of the rails, and assumes the use of a set back seat post because this is still probably the most common style of post.
Okay, this raises some questions. First, the middle of the rails is... where? Where is it versus the saddle's nose? It's 15cm behind the nose of the saddle, and I know this because I measured it on a Selle San Marco Aspide. But wait... it's 16cm behind the nose of a Fizik Arione Tri. It's also 16cm behind a Selle San Marco Rolls saddle, but the center of the rails on a Selle Italia Flite saddle sits 17cm behind its nose. So, no consensus.
By the way, how much are we really talking about here? What does this look like on a saddle? I've stuck two dots on a Fizik Arione Tri, and taken a picture. These two dots are positioned 14cm and 17cm back from the nose of the saddle respectively (which defines the maximum range of where bike fitters might choose in determining seat angles). If you're, say, 5'6" tall, and you have this saddle on your bike, and your saddle is positioned appropriately for you, then measuring from the bottom bracket up through these two dots yields about 3 degrees of difference. If you're taller, then correspondingly less, shorter, correspondingly more. In other words, if I measure your seat angle through the rearward dot, and it comes out at 76 degrees, then by measuring at the forward dot it's roughly 79 degrees. This is why it's a little silly to get too hung up on whether 77 degrees, or 74, or 81, is too shallow or too steep. It's hard to find a consensus on where to measure, and changing the reference point even a little makes a pretty big difference in the seat angle measured.
As to that question of reference point, let's return to it. I suspect a reasonable consensus might be that 16cm denotes the distance a typical saddle's rail center sits behind its nose. This is about in the middle of where a lot of popular saddles sit, and the Arione Tri is probably the most popular of the batch right now.
Then there's the question, how much set back, quantitatively, is there in a set back seat post? Anywhere from 1cm to 3cm based on a random sampling of seat posts in my garage. Again, if we take a mid-point and assume it's 2cm, then we're at that point I've been using over the past couple of years, that is, 14cm behind the nose. In other words, Gerard Vroomen's idea of a seat angle being measured through the center of the saddles rails, then steepen it 2cm to adjust for a set-back seat post, is in line with what I've been doing.
What does this mean in practice? Okay, you set the saddle up in relation to the BB, and this is the point that conforms to your appropriate fit. This is one of your "points in space." Let's say you're going to get a bike made, a custom bike, just for you, and you want to know it's seat angle. If you measure 14cm back from the center of the nose, and draw a straight line from the center of the BB through this point, then your custom bike's seat tube ought to measure exactly that angle, assuming you intend to use a set-back seat post (like Cervelo does, which is probably one reason Gerard likes the idea of the set-back seat post assumption). But, if you intend to use a straight seat post, with the clamp right over the centerline of the post, then you need to measure to a point 16cm back from the nose of the saddle. This will give you a seat angle two degrees shallower, more or less.
How convoluted can this get? From Gerard Vroomen: "I measured [Ivan] Basso’s P3 SL using the 16cm from the tip of the saddle rule (taking into account that the 2cm are cut off so I actually measured 14cm to get to the same spot on the saddle) and to that spot, his seat tube angle is 77.2 degrees."
Basso cut off 2cm from the saddle's nose so that he could ride UCI legal. Hence Gerard's explanation. In other words, Basso's seat angle on his well-fitting P3 is 77 degrees. We know this even though the P3 frame doesn't really have a measurable seat angle. Were he to ride a bike with, say, an Easton post (these have a bit more set-back) Basso would ride a 78-degree seat angle, were he to ride a straight-tubed frame like Cervelo's Dual. But, he'd ride at 75 to 76 degrees were he on a frame with a straight seat post. It just depends on what sort of seat post you use.
Then there's the question of where he rides on the saddle. Gerard says this is largely determined by the flare of the saddle. This is at least partly true. When in the aero position, laying flat, with a pivoted pelvis, the saddle's flare limits your rearward positioning on the saddle, because your thighs will be impeded by the saddle's flare. The lack of a flare, or a more gentle flare, means more fore/aft availability (you can shift forward or backward on the saddle while you're riding in the aero position).
Consider the two saddles pictured. The one on the left is the Fizik Arione Tri, the other is Selle Italia's Flite. It is evident that the Fizik is a longer saddle—by 2cm—and as such one might make the assumption that this extra length, plus its flat profile across it's top (it doesn't have the little scoop in the back) means it's a good saddle for moving fore/aft. However, the flare starts early and is more angular than the Flite, which means the Arione is more of a nose riding saddle. This fits with the style of its champion, Gilberto Simoni, who time trials right on the nose of this popular saddle.
When one considers where one is actually riding, this accentuates the difference between the seat tube angle on one's bike versus the seat angle actually being ridden. As we have seen, 1cm more or less equals 1 degree. So, you take a Selle Italia, if one rides rearward on this saddle one might be 5cm, or 8cm, behind one who nose-rides an Arione. So, perhaps we're talking 5 to 8 degrees of seat angle swing just by virtue of one's position on the saddle.
What are good examples of variable riding styles? Bjorn Andersson and Kenny Glah are rear-saddle riders, Lance Armstrong is right on the saddle's middle, and Peter Reid and Tim DeBoom tend toward the front. But they're not as far forward on the saddle as Simoni, Ivan Basso, Natascha Badmann and Lori Bowden, who are all nose riders. Probably the archetype for nose riding was Mike Pigg, who certainly did not contact the saddle with very much of his anatomy. I suspect that Mike Pigg and Kenny Glah, both top competitors in the same era, rode bikes with saddle positions that, on paper, were within two degrees of seat angle of each other, yet these guys probably rode apparent seat angles at least 7 or 8 degrees apart from each other, maybe as much as 10 degrees.
In the best of all worlds, one would find one's apparent seat angle through some sort of variable position simulator (fit bike); then select a saddle that allowed one to ride with a flattish back comfortably; then calculate where on that saddle one would ride; then place under the saddle a theoretical seat post of a desired sort; then be able to determine the seat angle of the ideal bike. Whatever happens underneath the rider, however—the style of saddle, and of seat post—does not change the reality that there is a specific apparent angle versus the bottom bracket that is formed when one finds that rider's ideal apparent seat angle. How would one measure that apparent angle?
I suspect that the answer is in finding that place on the body that corresponds with the proposed seat angle on the bike. In other words, one might start at the bottom bracket, draw a line up through the seat tube, through the saddle at a point 16cm behind the nose if using a straight seat post, and up through a particular point on the body. What would that point be? The greater trochanter would be nice, since it's an easy spot to find. But, on a couple of people I measured the greater trochanter seems to be a bit forward of that line. Perhaps some point using some made-up triangulation with the trochanter and the PSIS.
I don't know. That's the subject for a later article. Perhaps this is number-2 of an eventual trilogy of Slowtwitch articles on this subject. Stay tuned.
