I’ve noticed that my wattage for rpm is a bit lower this year on my fixed gear+kurt kinetic setup. Since the PM offset is the same, and it’s a controlled environment, I’m guessing trainer tire – last year I was using CycleOps trainer tires, and this year, a Conti.
The Conti certainly seems more durable – but the harder tire might have a lower Crr. As I move into doing more sweet spot, I’ll either have to start spinning those 90% FTP efforts at 100+ rpm (which wouldn’t be a bad thing), or slap on a stickier tire.
Has anyone seen any Crr data on trainer tires? A quick Google didn’t yield anything much…
A couple of things to consider here: a) The Kurt Kinetic’s resistance is temperature dependent. In a cold garage, the KK resistance will change from the beginning of the workout to the end as the KK heats up. Therefore in an uninsulated space, your KK resistance will have seasonal dependencies. b) I always test my tire for slippage. Clamp down the KK and push your tire in one direction and then suddenly reverse direction (using your hands). No slippage → you’re set. I don’t think a hard or soft tire will affect your connection to the KK at this point. Note that for me that takes 5 1/2 turns of the KK clamp after first contact with the tire, which is higher than KK’s clamping recommendation.
I put a silver sharpie mark on one of the star points on the KK knob, years ago (it gets an update every now and then). Always two turns after contact gives me no slippage (I’ve been using KK Road Machines for over a decade now).
Temp might be a factor – the bike room at the old house would get chilled down to 65 or so. In the new place, it’s 68-70.
I may just have to put a 15 cog on the fixed hub, or just pedal faster on those 90% efforts.