Shorter crank arms, saddle height and set-back?
 

About a year or so ago, I decided to try 165mm crank-arms (Shimano GRX 46/30T, previously 172.5mm). [I have short legs for my height, but also large (size 47) feet, so it wasn't clear it would be an improvement. I also have sub-optimal range of motion in my right ankle and possibly hips.] I've raised the saddle by about 7mm, which has also resulted in a small additional set-back.

I've read several times now that when people go to a shorter crank-arm length, they often find they have to further increase saddle set-back.

Is this true? It seems counter-intuitive to me. If anything, I would have expected that they might have to slam the saddle further forward to counter the small additional set-back that by necessity results from raising the saddle at an angle, in order to preserve the initial reach.

So, which is true?

My take on crank length changes is that you are still pedaling around the same center point, so there is no reason to change your saddle height or set back. Any change you make to satisfy one part of the pedal arc is just going to make another part twice as far off.

I don't see any reason for any to have stated the setback must change. Although you do have to change your saddle height, the change to reach is miniscule.

Perhaps it was just those that were wowed by the Benson&Hedges commercials of the mid 60's... "A silly millimeter longer"

We have one that says change nothing, another that says change height but not setback. Do we have one for change setback but not height, or change both?

How much shorter did you go?

From OP:

​​​​​​​Hang on, let me get my calculator.

7.5mm

Look at it this way. You just reduced your total leg extension by 7.5 mm, or a half centimeter and some change. Unless you were overextending to finish your stroke with the 172.5s, I'd raise the saddle.

As for setback, think of it this way. The power phase of the pedal stroke is from 45 to 135 degrees. You moved the point where you apply force back, but the location of the fulcrum remains the same. Regarding setback, use your judgement. I'd try back a couple millimeters just to see what happens, but it probably won't make much difference.

What might be slightly interesting in this context is that this is a custom bike, where everything seemed perfect, except the builder's recommended saddle height. I've always had to keep it a bit lower to be comfortable, but had the sensation that it was too low. However, it caused pain when I raised it to the recommended position. Now it is within 1mm of the recommended position (which is as close as I can measure it, realistically). If I had added an additional 7.5mm to the recommended height to compensate for the shorter cranks, it would have been impossible to ride with stability.

Some set back could be good, when the pedal is at the 3 o'clock position your foot has moved 7.5mm back, raising the height negated some of this but hardly any if you only went up a tiny bit. The old standby of dropping a plumb from your knee to the spindle is still a good indicator of how your seat setback is and if you should move it.

I've been using the position of the greater trochanter of the femur in line with the frame's seat tube and BB.

That's essentially the same reasoning as KOP's. I don't agree with KOP's other than to serve as a starting point that might vary widely on ones preference and desires and things like whether or not the rider finds that they have to constantly push or pull their butt back into position on the saddle.

If it works well for you in all cases, then that's good. It doesn't work well for me.

Then don't.

It's your job to figure out whether that instability is from too much leg extension or something else going on with your balance or posture. Maybe your legs would prefer to make up the extension with setback. Maybe they're happy the way they are. They're your legs. If something doesn't work for you, stop doing it.

Regarding it being a custom frame, don't put too much stock in the builder's specification. They're working off charts that convert your measurements into bike dimensions. They're not perfect or omniscient.

This is exactly in line with this well-respected UK fitter's advice

https://bikedynamics.co.uk/FitGuidecranks.htm#1

"The usual assumption is that as cranks shorten, the saddle height can go up by a similar amount. This sounds entirely intuitive as saddle height is often defined as the extension of the leg and shorter cranks will result in less extension through the bottom of the stroke. My experience of fitting many people before and after crank length changes is that this is not always the case and depends how you set your saddle height in the first place. An ideal saddle height is in a narrow window or 'sweet spot' between being too high and too low. If you have found this spot, either through trial and error or a good Bike Fitting, the likely scenario is that changing your crank length will result in no requirement to change the saddle height. Essentially the sweet spot is in the same place"

Given your restricted mobility I would try starting off from your original saddle height i.e. equally less leg extension and compression. Also I don't see how crank length has any direct relation to saddle setback.

Interesting topic. Following.

Did you have any pains that prompted you to go to shorter crank arms? Right now I'm dealing with back issues and sciatica and wondering if shorter crank arms might help.

No (apart from pedal strikes). Wife has knee pain, so we got her a shorter crank too.

OK, thanks. Pedal strikes have never been a problem for me, but come to think of it, toe overlap is a problem on my road bike.

Is it an actual problem or just a potential problem?

I just can't ever see toe overlap as a problem since on most bikes when going at anything more than 2 mph, turning the wheel far enough to strike the toes will be a wreck in progress even if there is no toe overlap.

My son tried to start off from a red light once with his wheel still turned and toe overlap reared it's ugly head. But that was more funny than it was dangerous. At least from a viewers perspective! <grin>

Potential problem in certain situations like very tight turns at low speeds. There is this very tight turn I sometimes have to make all while climbing a hill. With my size 12's, I need to be very careful where my pedals are.

Nothing like sciatica but having moved two bikes to 165 from 172.5 and a 175 I do believe it has helped with the arthritis that has found my lower back. Trying slight shifts in saddle position I am surprised that I am more or less settled on BB to saddle nose and saddle height no changes.

For shorter cranks, you will need to increase the saddle height accordingly (plus just a little more) then push saddle slightly forward to achieve same SSB.

I have size 12 shoes and short legs (therefore smaller bike -- 54), so toe overlap is an issue.

Maybe. Rather you CAN increase the saddle height. But really, your knee flexion will be the same on the upstroke, so you don't really need to.

1. Shorter crank arms will drop the foot at 12 o'clock so the rider will have less knee flexion by just installing shorter crank arms, but, the byproduct of this will be an increased knee flexion at 6 o'clock.
2. To accommodate the same 6 o'clock knee angle, you will need to raise the saddle height to get the rider back to the same 6 o'clock knee angle position as before, which will open the knee angle at the 12 o'clock position even more.
So not sure how you are calculating the knee flexion (12 o'clock position) will be the same?

I meant your knee flexion will be less on the upstroke even if you don't raise your saddle. True that knee flexion will be more at the bottom of the stroke, but is that really a problem? It's on the upstroke that is the issue.