Saddle Setback Between Sizes
 

Hey All,

On a road bike, I've heard you never adjust reach by adjusting your saddle setback. I had a general body scan fitting and was told my setback should be about 3.25". My question is, do bikes within one size of each other (say 54 and a 56) differ with how much you'd need to set the saddle for/aft? In other words, say I had to set the saddle way back on a 54 to achieve the right setback. Would the seat need to be set back the same on the next size up? Assuming reach issues were settled all with the stem. It seems to me all things considered, the angles and rear triangle wouldn't make that much a difference setback-wise between sizes?

If the saddle is the same height on the 2 different sizes and the seat tube angle the same ,the saddle set back would be the same. What usually changes is the length of the top tube. Keep in mind that these body scans and other fitting schemes are not the final word on what will work best for you, they are just a good starting point.

Thanks. The defy series between a M and M/L is .5 degree difference in seat angles. So I'd imagine that wouldn't make a huge difference on how far back you'd have to set the seat on the rails?

And yes, the body scan thing I got told me "functional height" was 5'10.5" with shoes on, when all other measurements I've taken and at the doctor I'm 5'9.25" without shoes. No way shoes add almost and inch and a half of height!

Sometimes. Seat tube angles often flatten as the sizes grow.

With a 73cm saddle height there's 1.2cm more setback with the saddle rails in the same location moving from a smaller size with a 74 degree seat tube angle to a 73 degrees.

Sometimes not. The range of angles can be very small on the same frame model - 74 degrees through a 53, 73 through 59, 72.5 for a 61.

Look at the geometry charts and apply the necessary trigonometry.

OK, so roughly speaking, if we're talking the difference of .5 degrees between a M and M/L, it would only be a difference of .6cm in saddle setback? Seems like no matter the size, I need to set that saddle back kinda far.

Saddle setback is measured as a distance from the center of the bottom bracket with the saddle at a set height. The frame makes no difference to the setback of the saddle. It may change where the saddle rails fall on the seatpost, so you may need a seatpost with more or less setback to accomodate your needs.

Since all saddles differ in shape, length, etc.. as well as where the rails reside in relation to the sitting surface, where on the saddle is the measurement to? Where the sitbones should be?

For two frames with different seat angles, the setback will differ by the cosines of the seat angles X the distance from the seat to the BB.

Say one frame is 72.5 degrees and the other 73. Say BB to seat is 75 cm. The difference in setbacks is:

cos 72.5 X 75 - cos 73 X 75 = 22.55 - 21.93 = 0.63 cm. (Multiplying your BB-seat distance times 0.63 and dividing by 75 will get you pretty close.)

Ben

There doesn't seem to be a consensus on that. Some go by the tip of the saddle, some use the center of the rails and others try to determine where the sitbones will be. None of them are all that good, unless you are using the same saddle on all of your bikes

For my own bikes with different saddles, I measure to where the saddle just reaches its widest point.

Too much focus on the numbers.

Ride the bike, carry tools, and make small adjustments as needed, including seat setback, until you get it where you like it...

I agree with dsaul. You were told that the setback is 3.25 inches. The question then is what reference points did the fitter use. For myself, I measure setback from the centre of the bottom bracket to the nose of the saddle. If you measure setback from one bike to another or from one size to another, the only constant and assumption is that you are using the same saddle. Differing frame angles make no difference. If you are using a different saddle from one bike to another, setback may need to be adjusted to take into account the length of the saddle and in particular where on the saddle are you sitting. Different shapes of saddles can have you sitting more forward or more rearward. When I changed my saddle to a noseless saddle, I had to adjust setback. What is important is to understand what the fitter was measuring and then trying to duplicate that measurement.

I agree with giro_man, but I don't use the same saddles on different bikes. I measure setback on mine and on wife's from the BB plumb line to where the sit-bone pressure is. I find this by asking her to point to it - imprecise but she's consistent. Then I mark that point, measure and write it down. Same for my bike.

Interesting thing is, that point is usually just a little forward of the widest point for widely different saddles like Brooks B17, SelleAnatomica, and Specialized Toupe. Measuring to the nose of the saddle is only valid for saddles that are the same.

Worse, with a Selle Anatomica, the length and nose position changes as the saddle stretches and you restore fit by adjusting the tension bolt.

that makes sense. I dont know ehat he used for reference or if he used a bike, I just know that setback number is what the body scan came up with.