By breaking in your Brooks B17 etc. leather saddle, you create the "valleys" in the leather for your sitbones and the crotch area of the pelvis bone.
Therefore you need enough time for the leather to deform at these three areas. There is various information on the net. My experience is that about 500km of riding (so about 25h at 20 km/h) gives a good enough break-in.
However, I agree with "Fred the MagicWonder Do", if it is not roughly comfortable on day 1, i.e. roughly the width is not good enough, it never will be.
So from these, the biggest deciding factor of saddle comfort is total effective sitbone width. This is measured in cm, and the width you need depends on two things:
- Base width = width of your sitbones (in cm, typically 10-14 cm)
Plus additional width required for your riding position (in cm, typically 0-4 cm)
- triathlon +0cm
- sporty +1cm
- trekking +2 cm
- city +3 cm
- fully upright +4 cm
If in doubt, go for wider. Narrower causes pain.
You can measure your sitbone length with cardboard paper (like an amazon shipping box).
See the info at SQ Lab for the measurement (you don't need the special device, you can do it on a simple chair).
SQ Lab saddle width measurement
In theory, if the B17 is wide enough for you, after break in, it will be even more comfortable. If it is not wide enough, try Brooks B67.
If I remember well, the max effective sitting width of a Brooks B17 saddle is 12 - 12.5 cm, but you need to check. If your total effective width is above that, you will find it uncomfortable after break in, because your sitbones will be over the metal part of the saddle, where it cannot deform. So what counts for leather saddles is not the total width, but the parts that can deform, inside the metal U-frame. Also, for other saddle brands, this is what you need to check.
The big part of men find comfortable a Brooks B17 width, and these people write in the reviews how comfortable it is. Those who complain, probably have more distance between the sitbones, and/or ride more upright, and the B17 is not wide enough for them.
I have a wider than average sitbone width (12.5 cm), and I ride in a trekking position (14.5 - 15.5 cm), and no surprise, the B17 was uncomfortable for me. The B67 was better. In the end, I decided on a 15 cm SQ Lab 604 saddle, as it is water proof, and I often bike in rain/cold(wet). I don't like that when it rains, I stop, stand on my feet, the leather takes up water, and keeps me wet the whole day.
The other deciding factor is crotch depth - whether there is room for the crotch part of the pelvis bone, or the bone and genitals hit hard plastic/metal.
- Anatomic, or leather saddles have/create room for the crotch.
- This part of the comfort you cannot feel on the first ride on a leather saddle, only after break in.
- It is independent from saddle size.
- For 1-2 hour rides, or triathlon positions ("sitting on the legs"), the crotch comfort doesn't matter that much. It matters usually after 50-60 km of ride. When my crotch hurts, I simply cannot ride more. When the saddle is right, my buttocks are tired, but don't hurt, and I can do 100+ km.
As there is less weight on the crotch, and when it hurts, unconsciously you will stand more on your legs, it takes more time for the "crotch-valley" to form in the saddle, maybe double time. At the beginning it hurts, and as time goes by, and as the leather deforms slowly, it will be better and better.
An anatomic saddle, which has the valley for the crotch built-in, only needs some play with angle of the saddle, otherwise it should not hurt from day one.
Note that if you change the angle of the leather saddle after break-in, it can be the the crotch-valley is not deep enough, and it hurts again, until it deforms according to your new position.
Otherwise, here's a link for saddle break in:
Breaking in a Leather Saddle
This is the most imformative page I've found about Brooks saddles:
Leather Saddles - SWHS.home.xs4all.nl