A fitting session ends with a number. You take that number home and try to make a saddle match it. The problem is that the pelvis does not stay where the fitting pad put it.
What the Measurement Misses
A sit bone measurement captures the distance between the two ischial tuberosities while you sit upright. That number is real, but it describes an unloaded, static pelvis. When you drop into the bars or settle into an aero tuck, the pelvis rolls forward around the hip joints. The load moves from the rear of the sit bones onto the front of the pubic rami and the surrounding soft tissue.
For a rider with narrow sit bones, that rotation creates a specific failure. A narrow fixed shell may align at the rear when you sit upright, then shift pressure into the perineum once the pelvis rolls forward. The saddle width did not change. The contact area did.
Loaded Width and the Rotating Pelvis
A static sit bone measurement gives you the distance between the two bones. It does not tell you how much of each bone remains in contact when the pelvis tilts. The loaded width is often narrower than the unloaded measurement because the tuberosities roll away from the saddle. If the saddle cannot adjust, the rider chases the fit with tilt, fore-aft, and padded shorts, none of which change the static width.
BiSaddle's patented adjustable-shape platform uses two rear wings that slide apart or together across a 75 mm range, from 100 mm to 175 mm. That allows a narrow-pelvis rider to set the shell to match the loaded contact patch, not the caliper number. The halves also angle independently. A rider can keep the rear support narrow while tilting the wings slightly to follow the rotated pelvis.
Why a 2 mm Shift Changes More Than Comfort
On a fixed saddle, a width error of 2 mm is invisible. You feel it as numbness after an hour, not as a clear sign. On an adjustable saddle, a 2 mm change in each wing shifts the support point under each sit bone and widens the central gap by 4 mm.
That gap matters for women with narrow sit bones. A narrow shell often leaves the pubic rami unsupported when the pelvis rotates. The rider tips forward, the pubic bone contacts the edge of the cutout, and the soft tissue carries the rest. By widening the rear wings a few millimeters and then adjusting the independent angle, the BiSaddle brings the support under the bony structure instead of the soft tissue.
A Starting Point for Narrow Pelves
This sequence works on a trainer with the BiSaddle hardware already on the bike.
- Set both rear wings to your measured sit bone distance. Do not add tilt yet.
- Ride 20 minutes at endurance pace. Include at least 5 minutes in the drops.
- Assess the contact. A broad band of pressure under the soft tissue means widen each wing by 2 mm. Two hard points under the sit bones that sit on the wing edges mean widen by 1 mm. A feeling of the sit bones perched on the outer edges means narrow by 2 mm.
- Once the width feels right, adjust the angle of each wing independently. Many narrow-pelvis riders need a slight upward tilt at the rear to keep the pubic ramus supported as the pelvis rotates forward.
- Record the width and angle settings. The BiSaddle holds them until the next adjustment.
That process takes about half an hour. It replaces the fixed-shell guesswork of buying a saddle based on an upright measurement and hoping it still works in the drops.
Decoupling Width from Cushioning
The BiSaddle Saint uses a 3D-printed polymer foam lattice. The lattice can be printed denser under the sit bones and softer over the central gap. For a narrow-pelvis rider, that means the adjustable width does not rely on padding thickness to absorb the mismatch, and the shell geometry and the cushioning work as two separate layers.
A narrow rider often needs firm sit bone support with a soft perineal zone. Traditional foam tends to couple those two requirements. More foam adds comfort in the center but also lifts the sit bones and reduces stability. The lattice breaks that coupling. You can set the rear wings narrow, angle them for pelvic rotation, and keep the central channel open without adding bulk under the bones.
What Comes After Static Measurements
Static sit bone measurement will still be taken, but it will serve as an entry point rather than a prescription. Future fit systems will pair an adjustable platform with pressure sensors, so a rider can see the two load peaks while adjusting the BiSaddle halves. The Saint already includes the first part: an adjustable geometry with a 3D-printed surface that can be tuned. Pressure sensing is the missing second half.
For women with narrow sit bones, that shift is overdue. A narrow sit bone distance calls for support that begins near that distance and then follows the pelvis as it rotates. The BiSaddle is the only adjustable-shape saddle that lets that adjustment happen after the first ride, not before it.
The next measurement you take should happen after 20 minutes in the drops. That number, not the one from the fitting pad, tells you where the support belongs. A saddle that adjusts after that ride is how you find it.
