Saddle fore-aft compensation in relation to crank length change

Question

I've found a few questions which answer the question of saddle height changes in relation to a change in crank length, however I haven't found any questions relating to compensatory changes in fore-aft when crank length is changed.

If one changes their crank length from, say, 175mm to 165mm, then their saddle height should (in theory) move up approximately 10mm to compensate (when measured from the bottom bracket centre; the height from the pedal at full extension would therefore stay the same as it was before).

What does one do to saddle fore-aft to appropriately compensate for the crank length change, if anything?

EDIT: Just to expand upon/clarify what I am looking to evaluate; there will of course be a change in the effective setback which comes as a result of saddle height changes, which can be calculated using trigonometry.

However, the intention of my question was more to clarify whether a change in setback would be appropriate due to the change in effective forward "reach" of the leg at the front (3 o'clock) position of the crank given it is hypothetically now 10mm shorter. Likewise the rear (9 o'clock) position is 10mm further forward than it was.

To take the saddle height part of a crank length change once more; as a general rule of thumb, you change the saddle -> BB center measurement by the same amount that the crank length has changed (10mm shorter cranks = 10mm higher saddle relative to the BB center, with a tiny adjustment for relative setback if you wish).

Is there a similar rule of thumb for setback given the same crank length change?

The answer may be that no change is necessary/appropriate but reasoning would be appreciated!

Answer 1

There is no rule of thumb for this that I've ever seen.

Saddle fore-aft adjustment is used by different types of cyclist in different ways. Performance-oriented cyclists and people that fit bikes for them typically have ideas about optimal geometric relationship between the knee, femur, pedal, and/or BB center. Cyclists primarily interested in comfort will tend to use saddle fore-aft to strike the best possible balance between their upper body posture, their weight distribution (more offset back means less weight supported by the hands/arms), and possibly performance.

In your example of going from 175 to 165, the answer really depends on what the old saddle fore-aft position was doing for you. If you're of the mind where all you're trying to do is position your knee over the pedal spindle, and you liked how that relationship was before, then yes you'll need to go back approximately 10mm. In doing so your upper body will be more stretched out, and you may find other changes are necessary (i.e. shorter stem). If the goal was simply to get a crank length that's more proportionate to your leg length, and you like your upper body position and don't want to change it, then you're likely better off keeping the setback as it was, although since your seat is going down around 10mm you may find you need to do the same to your bars.

Reducing crank length should always come with a raising of the seat if the seat height was correct before. Raising the seat always offsets you back due to the angle of the seattube. Thus, it follows that if you wanted to keep your upper body posture the same or as close as possible, moving the saddle clamp fore-aft position forward some amount would always be necessary, along with lowering the bars.

Ultimately for saddle fore-aft, there is no bedrock of truth once you dig through enough dogma. It's too complex and it has too many subtle interactions with other aspects of fit, performance, and handling. Just do what feels good.

Answer 2

In theory, trigonometry could help you do this. You can measure the distance from the center of the BB to the top of the saddle, which is the hypotenuse of the triangle. You know the bike's seat tube angle from the geometry chart, which gives you one of the angles. The side adjacent to the STA is the one you're looking at.

My bike had a saddle height of 640mm with 170mm cranks (which is very low, because I have short legs). It's custom, so it has a seat angle of 76 degrees. I changed to 165mm cranks, so the new hypotenuse is 5mm longer. From basic trigonometry, with 170mm cranks,

cos(76 deg) = adj_before / 640
adj_before = 640 * cos(76 deg)

adj_after = 645 * cos(76 deg)
delta = 645 * cos(76) - 640 * cos(76) = 5 * cos(76) = 1.21mm

Dave Rome at the Escape Collective wrote an article on what bike fitters wished all cyclists knew. Here, one fitter reported that

“The tolerances for pairing a human with a bicycle are actually quite small. It can be plus or minus 1.5 mm for saddle height, and 3 mm for saddle setback. I work with a wide range of clients, and it never stops amazing me just how consistent these tolerances are. One rider could be 6’7″ (200 cm) vs another at 4’11” (150 cm) – sprinters to ballerinas – and we can still detect compensations as we deviate away from the optimal by as little as 3 millimetres.”

I actually didn't bother to adjust my saddle setback with that change. Fitters may deal with more experienced clients on average, so they could be more sensitive to saddle position than your average cyclist. However, adjusting saddle position is easy on most saddles, so you might as well do it if you change your crank length by 10mm.

For others' reference, the question about changing saddle height with crank length is here.

Astute readers will realize that your feet will be a bit closer to the BB at the 3 and 9 o'clock positions, and that your feet won't come as high at the top of the pedal stroke. In the latter case, the hip angle at the top of the stroke becomes shallower. I don't believe any corrections should be made to the pedal stroke to account for this.

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If you're using Google search as a calculator, remember that the default setting for angles is in radians, not degrees, so hit the Rad | Deg switch once before you enter your own numbers. If you attempt to check the math without doing this, you'll get the wrong answer - but you should feel free to check the math anyway.

For reference, pi radians are equal to 180 degrees.

Answer 3

My bike fitter (using a RETÜL system) didn’t touch the seat rail clamp at all when I moved to 7.5mm shorter crankarms (172.5mm to 165mm). He did raise my saddle considerably (I think even more than the 7.5mm).

However, as you point out, due to “effective setback” I’m now slightly further back. On the old crankarms he actually wanted to move me slightly more forward but we were already on the maximum the seatpost allowed.

So from personal experience it looks like shorter crankarms mean you should move your saddle up and back.

Bike is a performance oriented road bike.

Answer 4

I agree saddle hight should be lifted to save the knees but I think for and aft should be down to reach to the bars and whatever is comfortable.