I thought I had gotten dialed in to my new bike so I was very frustrated when I found it difficult to hit my normal 90+ rpm, in fact I shortened my cranks to 165mm from 172.5mm so I was expecting a cadence of over 95 rpm. I even chalked it up to being overly fatigued.
I checked online and came across a Bike Fit Adviser video where he suggested that moving the saddle aft always has more benefits that being too far forward. Well I moved the saddle aft about 10mm and behold, my cadence is back up to 95+ rpm and I don’t feel like I’m struggling against the pedals anymore. Another thing to note is that when my saddle was forward my left knee kept wobbling at the top of the pedal stroke, this doesn’t happen after moving my saddle aft.
Does anyone here know why moving a saddle aft helps with pedal stroke? What exactly are the benefits?
Did you make adjustments to your seat height and fore/aft when you went to shorter cranks? You might have just needed to go up and the back is doing that for you in a roundabout way
Indeed, with fore-aft saddle position adjustments, you typically need to do a related change in saddle height.
It is roughly 2:1 so if you move the saddle 10mm aft, you should drop the saddle height about 5mm. This assumes the starting height was “correct” and the goal is to maintain the leg extension after the fore-aft change.
This change is needed due to the roughly 73* seat post angle (typical road bike).
I increased the saddle post height when I got the short cranks, but I didn’t move the saddle back because I was concerned that reach might be an issue for me.
the 7.5mm shorter cranks effectively moved the saddle forward by that same amount for your knee of spindle measurement. If you moved the saddle up 7.5mm, then the additional setback was ~3m, so another 5mm would have gotten you closer to your previous knee position
There is a lot of black magic and strong opinion among fitters about proper fore-aft position. K.O.P.S is great… no, it’s the devil… and other sorcery about how to position yourself. Ideas about power delivery and cadence run the full range and it is all a bit maddening if you dig in too deep.
Personally, I start with K.O.P.S. and adjust to suit the needs and preferences of each rider.
With your shorter cranks, even with your saddle 7.5mm height increase (which also moves you back about 4mm at the same time), you “effectively” ended up more “in front” of the pedal spindle by about 3mm after all the changes are done.
That may have lead to a more closed leg angle at the bottom of the stroke for extension. It also leads to a different angle through the majority of the stroke. That may account for you knee popping out, but it’s not the only possible cause.
woops i think i got those numbers backwards in my math
I also moved to 165mm cranks(from 170mm) and it a took a bit of time to get my natural cadence up and I was already a spinner(90-110rpm). I used to sit around 90rpm and now ~95rpm feels normal for ~200w tempo