Firstly, a discussion about seat widths versus pelvis widths. Many years ago, when I didn’t know much about pelvic anatomy, I naively assumed that there had to be link between ‘sitbone’ separation width and seat width. To that end I used to sprinkle talcum powder on a flat wooden seat and then have customers carefully lower their backsides onto the seat in their undies, and then gently stand up again, leaving what amounted to a pressure map in the talcum powder. I then tried to correlate measurements I took from that talcum powder impression with seat widths. What came out of that exercise is that there is no clear correlation between sitbone separation width and the width of a comfortable seat for that person. There is a sort of / kind of /more or less correlation around 75% of the time, with the remaining 25% being at odds with my original assumption. Sometimes wildly at odds. Being curious I was motivated to find out why and the answer lies in the shape of the human pelvis as well as the multiplicity of seat shapes, lengths, widths, tumblehomes and so on.
I need to clear up a common misunderstanding. When people talk about ‘sitbones’ this is often taken as synonymous with the ischial tuberosities which it isn’t, and I’m as guilty of sloppy language like this as anyone. One of the things I like about writing about bike fitting is that it forces me to crystallise and tidy up my thinking, and the sitbone / ischial tuberosity conflation / confusion is an instance of this. Have a look at the pic below.
This is plastic copy of a human pelvis sitting on an SMP seat as viewed from the side. The large bony protusion at the rear is the ischial tuberosity. This is not what a cyclist sits on………….unless they ride a recumbent, or possibly a Harley. We sit on the bone at the base of the pelvis as you can see above. This is the ischiopubic ramus. You will see the slight downward protrusion approximately half way along the ischiopubic ramus. The area behind that is the inferior ischial ramus [which I'll call IIR] and the area in front of that is the inferior pubic ramus [which I'll call IPR]. We bear our weight on a bike on the the ischiopubic ramus and it is the relative loading of the IIR and IPR that plays a large part in comfort.
So why isn’t there a clear correlation between pelvis width and seat width?
The picture below is the same plastic pelvis sitting on an SMP seat as viewed from the rear. Looking closely at it gives us the major clue.

As you can see, the ischiopubic ramus narrows from rear to front. What that means is that maximum width of the pelvis is less important when it comes to determining seat width than which part of the ischiopubic ramus is bearing the majority of weight and what width the pelvis is at that point. And that will depend on the how far the pelvis is leaning forward. As the pelvis leans forward , weight is being borne less on the IIR and more on the IPR where the separation width of the left and right ramen (plural of ramus) is narrower. Which means in turn that if you have a wide pelvis but are flexible enough to be able to roll your pelvis forward well and extend your back well, you may be able to comfortably use a narrower seat than would be expected. Conversely, a rider with a narrower pelvis who is inflexible and sits with a more upright position may need a wider seat than would be obvious at first glance. Naturally, there are any number of permutations of morphology, function and seat choice in between those extremes.
The problem with this is that there is still no definitive way to say “this saddle is the best choice of the SMP range for this rider”. When I recommend a particular SMP to a rider it is usually on the basis of having observed how they function on a bike as well as their size and their global function off the bike. It is an educated guess, not a ‘fact’ and as such, I occasionally get it wrong. What I propose to do is pass on some observations gained during the successful, and occasionally unsuccessful experiences my fit clients have had with SMP’s.