TL;DR - New bike, new cranks, new crank length. Higher cadence.
All of my bikes have had 170mm cranks, which work pretty well for me, however, I build a new aero bike and decided to hope on the short crank bandwagon and got some 160’s.
Initial impression was kind of amazing. It’s much easier to get over the top of the stroke which means more comfortable in an aero position.
Got it set up on the trainer as I dial in fit, and I’ve been doing intervals on it.
The cadence change is immediately noticeable and sort of bothering me.
Does this make sense in ERG? I was sort of under the impression that for riding with gears it would make a difference and I would have to shift in order to keep the cadence I’m used to but it seems like intervals are a bit harder than they used to be mainly due to having to keep the cadence up higher than I’m used to.
Anyone have this experience? Did you get used to it? I’m seeing like 5-8 rpm higher for the same wattage, which doesn’t sounds like a lot but when I’m deep into an O/U it starts to hurt lol.
You know how high cadence VO2 intervals start to hurt as you struggle to hold the cadence and it slowly comes down over the course of the interval? It’s sort of like that only on a smaller scale.
I can go back to 170, but I’m enjoying the benefits of the shorter cranks so if I can acclimate I’m willing to keep at it.
If you’re using erg mode, why don’t you just slow your cadence? The trainer will then just increase the resistance a bit to keep you at the prescribed power.
See this is what I’m getting at. My understanding is that cadence changes are accounted for by the ERG however, what I’m experiencing suggests otherwise.
Everything you’re saying makes sense and I think is a common experience when switching cranks. For example at say 250 watts and 90rpm, you have to put 6% more force into the pedal with the shorter crank to get the same watts output. If you want to keep force on the pedal the same, you have to spin 6% faster.
Shifting gears and such with Erg mode will just change how the flywheel on the trainer feels. (how much momentum you have).
I went from 175 to 165 a while back and had a similar experience, but now I don’t even think about it.
Ok, so I’m not crazy lol. It sort of feels like doing high torque intervals, so while the wattage remains consistent, the effort feels slightly exaggerated.
I went from 170-162.5 and recently to 160 (based on bike fit recommendations), and have found that my cadence settled back to my natural preference over time - albeit with improved efficiency as I have overcome a hip issue as a result.
It takes weeks for the central nervous system to fully adjust so give it time (and don’t forget to adjust seat height and setback accordingly).
By putting shorter cranks on, you have opened up hip angle at the detriment of torque production (As you are now applying Force (N) x 0.16 rather than x 0.17, a not insignificant 6% drop)
As your input torque is now lower, the only way you can achieve the same power output is by pedalling at a faster cadence when using gearing. In ERG mode, either by gearing or force increase.
So in ERG mode, if you were at 200W @ 95RPM on 170mm cranks, you were generating 20.11Nm of torque and therefore applying 118.3N of foot force.
Now that you are on 160mm cranks, to generate the same torque and therefore same power at 95RPM, you will need to apply 125.7N of foot force.
It is not that the effort feels exaggerated, it is literally harder.