Sit bone experiments for saddle fit

As the WW200 approaches, I've been puzzling over saddle fit as I get quite uncomfortable after the first 3 hours or so on the bike. I read that "you should be as comfortable after 5 hours as after 5 minutes", so I did some Googling and experiments.

There are a few mentions on the web of the Specialized Body Geometry saddle fitting system, otherwise known as the "assometer". This is a pad on which you sit in order to get a sit bone measurement, which is then mapped to recommended saddle widths using a scale on the back.

All very good, and although the scale is no longer available on Specialized's website, I found this in a forum:

"The code on the back of the assometer specifies a 130 saddle for a sitbone center to center width of 100 mm or less, a 143 saddle for a sitbone center to center of 100 mm to 130mm and a 155 for a sitbone center to center of 130 or more."

Right. Except that apparently Specialized USA end up selling almost everyone a 143 saddle.

But most "racing" saddles (ProLogo, Fizik etc) come in sizes from 130-138mm.

This raises some questions, like:

- Do Americans have consistently wide "asses"? What about us Europeans?
- Is this fit system remotely personal, or just a sales tool?
- Do racing saddles fit anyone?

I read that sit bones get narrow as the pelvis rotates down, which would explain how the thinner racing saddles fit the pros, but little quantitative information.

So I got out the play-doh, rolling pin, clingfilm and tape measure and established that:

- Measured sitting against a wall, my sit bones are 110mm apart, centre-centre.
- Measured in a cycling position (plasticine on chopping board on chair) my sit bones are less than 80mm apart c-c.
- Pros typically ride with their chin significantly less than 40cm above the bars (more like 35cm) when on the hoods* whereas mine is more like 45cm.

From this I conclude that:

- I have a more relaxed cycling position than the pros (no surprise)
- There is a huge variation is sit bone spacing depending on riding position, which will affect saddle fit.
- Sitting up, I am marginal for a 136mm saddle**, as I don't feel very stable; my sit bones are on the sloping sides of the saddle, not the central part. Deep in the drops, it's significantly more stable, and it would probably be perfect if I rode a bit more aero.

Hope this will be of some use to someone, just sharing!

* Photoshop measurements compared to wheel diameter.

** Prologo Pass NAS borrowed from CSS. It's easier to feel my bone position on this than my Selle Italia, for some reason.