Iride01 

That's the way I took it since that's the way saddle height was defined later in the post.

Now I need something to go with my coffee. I think the muffins my wife bought are a little stale. Thanks for the chit chat!

smontanaro 

Understood, but my knee will also be in greater flexion at the top of the pedal rotation. There is probably a happy middle somewhere.

smontanaro 

No, however... If I'm going from a common 170mm crank length to 175, leaving the distance from the saddle to the pedal the same means that my foot will be 1cm higher at the top of the pedal circle. I will probably compromise a bit to start, raising my saddle a tad, then listening to see if my knees tell me I've got it wrong.

big john

I can't really help with that. I've relied on the saddle to pedal at the bottom and never considered where it ends up at the top. I've also never changed crank length and I'm almost 69.

I do move around on the saddle when riding which changes the relative position a bit, I'm sure. Like when on a flat section, I will slide back and try to get aero (which isn't very aero). Or I move around when climbing.

Good luck with the knee issue! I've been very fortunate with my knees and I attribute at least part of that to spinning instead of mashing years ago.

hsea17

I've just ordered crank arms on 170mm from my bicycle dealer. Going from 172.5mm and when get them dealer told me to set up the saddle with 2.5mm . I want to try due to arthritis in one knee, second knee have started show similar symptoms I was diagnosed with arthritis some 12-13 years ago but "knock on wood" as you I have been fortunate to keep going even with some slow run /jogging using Kinesio taping now on both knee. However will be interesting to see if shorter cranks will make any difference when cycling.
hsea17

Road Fan

Skip, it looks like you face a trade off between "tighter" knee closure angle due to higher TDC foot position with the longer crank arm, and more rocking of your hips on the seat as your leg stretches down an extra few mm to reach the pedal and finish a good power stroke. I doubt there's a generic solution to that dilemma. In my self-fitting I strive for ust enough pedal extenstion that I'm not driving my foot into the pedal at BDC. Usually this results in low hip rocking with 172.5s or 170s. I find 170s a little more comfortable overall.

Another question is, which choice makes it easier to pull your foot back at bottom, and to (perhaps more affected) push your foot over the top for a little earlier start at the power stroke. I could see the longer arm and tightened knee angle affecting how the start of the downstroke works.

bruce19

I will be 77 on Juneteentth. I also have not changed anything since I began riding in my early 30's. FWIW people should be advised that you measure seat height from pedal spindle not BB.

Road Fan

Actually one can do it either way, but you hav to be careful to make corrections wo there isn't a 17 cm problem built in to you r new bike "fit."

bruce19

Only if crank arm length remains the same.

Road Fan

Maybe I'm not clear on what we are talking about. You can set and adjust saddle height on a given bike based on measuring from any fixed point on that bike. As long as you do the numbers consistently, it will work. If the crank is changed, one needs to think about how to compensate so your leg extension remains the same.

But I don't see how it can be true that measuring from the BB axis to the saddle top is the only valid way to address saddle height.

bruce19

Maybe I'm the one who was unclear. I'm saying that if you measure saddle height from BB axis to saddle top your fit will change if you change crank arm length. FWIW that's what bike fitters have told me as well.

Road Fan

That is certainly true. I've changed from 170 to 172.5 and backwards several times, and I do adjust my saddle height to compensate. Once when I forgot to reset the height I had a lot of pain until I figured it out - the saddle was effectively higher than my butt wanted it to be.

Trakhak

Earlier in the thread I mentioned that I've dropped my saddle height as I aged from my 60s into my 70s. Someone responded that his height has changed but not his leg length.

Did a search. He was right. Evidently the majority of people lose height in the spine but not the legs. Now my saddles are feeling as though they're too low.

bruce19

I think I was the one who posted that. When I was a college QB I was measured at about 5'10 1/2". My inseam was 32.5". Now at age 77 I am 5'8 1/2" but still have a 32.5" inseam. Why? Well age, of course. But, I also have had some spine issues and in 2013 I crashed my bike and broke my neck. This a fusion of C1 & C2 and the loss of height. So, in my case, yes it's about upper spine.

hsea17

Well, at least I'm not the only one who shrinks. In military service as a 20-year-old I was measured at 5'77 now plus 40 years later I'm barely 5'65.
Btw, I've clearly been measuring seat height wrong my whole life, measuring from BB to the center of the saddle
hsea17

Calsun

The trend is toward shorter cranks on bikes with owners replacing 165mm with 155mm cranks. With adjustments of seat height I always take a tool and ride for at least 20 minutes before making an adjustment. I need to give my body time to warm up and my tendons and ligaments and muscles to stretch out.

terrymorse 

And as with many of the bike fashion trends, there is little to no data to suggest a benefit from using shorter cranks.

There is one caution to heed: if you do switch to shorter cranks, you will need to install lower gearing.

easyupbug 

Living with arthritis my knees give me all the data I need to stay on my bikes, my 165mm is not a fashion trend but a necessity.

terrymorse 

How does a shorter crank help your arthritic knees?

easyupbug 

May not apply to anyone else as I am in my 70s but after experimenting with a couple of my bikes the results had me ask my Orthopedist. She believes that at my fit recommended saddle height (set at the bottom of the pedal stroke) the releaf I get is that at the top of the pedal stroke my leg is not as bent so my knees arenot seeing as much compression of the knee cap or the knee moving to the outside with the 7.2mm shorter crankset.

terrymorse 

Thanks, that makes sense. The medical/physiology folks would call that "reduced knee flexion", I think.

smontanaro 

That was my thinking when I originally started the thread. While fuller range of motion would be preferable, our knees will tell us what works.