Duplicating saddle position when the saddles are not the same

Seat height and setback must change if saddle length and shape differ

Dear Lennard,

I recently had a professional bike fit done for a new bike I’ve ordered. The fitter set my seat height at 819mm from the bottom bracket, and it seemed fine on the fit bike. But when I got home, I checked the seat height on my bike, and it is 849mm—30mm higher! I moved my seat down to the number on the fit sheet he emailed me, and it felt horrible! It is so low that I feel like I can’t produce nearly as much power.

The fitter measured my sitbone width and put me on a 14cm SQ Lab 612 saddle. The saddle on my bike is a 143mm Specialized Power, so it seems like my Power saddle fit is fine.

Is there something I am missing here?

How could that 819mm seat height have felt fine on the fit bike and felt so bad on my bike? I wasn’t pedaling very hard on the fit bike, and the resistance was not set very high on it. But the seat height immediately felt terrible on my bike before I started pedaling hard.

I even used a plumb bob to check the saddle setback. The fitter had the saddle setback at 93mm, and I made it 123mm with the Power saddle than with the SQ Lab saddle, because the Power is 30mm shorter. It still felt just as much too low when I made this adjustment.

Any insights here?

Mike

Dear Mike,

Absolutely I have some insights here! The difference you feel is highly dependent on the saddle difference. Your instinct was correct to make the 30mm correction to the saddle setback based on the length difference between your saddles. But there are more things to correct for than just setback. I did some calculations with the specs on those two saddles, and I think you’d be surprised at how close the functional seat height is on those two setups, even though your tape measure says they are vastly different.

There are eight variables to consider in the differences between the two saddles. In this list, I define “ASH” as the “Actual Saddle Height” from the sit bone contact to the center of the bottom bracket.

1. saddle length (ASH is larger if the length is greater)

2. saddle curvature in the X-Y plane (ASH is less if the curvature is concave—the rear of the saddle swoops up)

3. saddle curvature in the Z-Y plane (ASH is less if the saddle curves down toward the sides)

4. saddle tilt angle (ASH is less if the entire saddle is level or tilted up vs. if only the saddle nose is level)

5. saddle rail flex (ASH is reduced the further the saddle is slid back on the rails—more rail length behind the clamp to flex)

6. saddle shell flex (ASH is reduced the further the saddle is slid back on the rails—more shell length behind the clamp to flex)

7. foam compression (ASH is reduced the softer the saddle)

8. seatpost flex (flex is greater with more seatpost extension as well as with a lower seat tube angle—increases setback, not ASH)

First, I assumed that your sitbones would choose to be the same distance from either saddle tail, since both saddles are essentially the same width. I estimated that horizontal distance forward of the center of the sitbone to be 30mm from the back of the saddle. That’s the L-30 term in the equations, where L is the saddle length.

Item 1 is a big deal here. The SQ Lab 611 saddle is 271mm long, while the Specialized Power saddle is 240mm long. That extra 31mm of length of the 611 saddle makes it effectively higher where you sit on it for the same height measured from the BB to the top center of the saddle.

Item 2 also comes into play, because the sit bone contact on the Power is lower (by about 7mm with the nose level) than the top rear rim of the saddle due to its concave curvature, whereas the convex curvature of the back few cm of the 611 saddle means that we add about 2mm for that.

Item 3 affects the effective seat height because the sitbone contact point on the Power saddle is lower (by about 7mm) than the center point down the midline of the saddle between the sitbone contacts due to the saddle curving down to the sides. The 611 saddle, by contrast, is almost flat; the sit bone contacts are perhaps 2mm lower than the center point down the midline of the saddle between the sit bone contacts.

In the attached calculations, the numerical values of the quantities in items 2 and 3 make the difference between *ASH, which doesn’t take into account saddle curvature and is only calculating *ASH based on lengths, and ASH—the actual distance from the sitbone contact point to the center of the bottom bracket, which does take into account saddle curvature both in the X-Y plane and the Z-Y plane (side to side).