Indoor training, part 6: The flywheel effect

I’ve had the Minoura LiveRide LR340 turbo trainer for a while now and I’ve used it with both Hyro, my cyclocross bike with 700C wheels, and Bino, my 20″-wheeled folding bike. A few buckets of sweat later, shared between these two bikes, I started noticing a few differences.

At this point, I’ve documented numerous times the more rapid rate of tire wear from the smaller wheels – and the larger resulting mess that comes with it. It turns out that’s not the most telling difference between the two bikes.

With Bino, I find I can crank up the resistance and use much heavier gears. I regularly dip into the higher end of the gear ratio spectrum. I’ve gotten to the point where I can sprint in the 50×12 top gear combo at the final flat-out interval of a workout…and push as hard as an indicated 60.8 km/h on my cyclocomputer.

I have no idea how I’d sprint to this speed on a folding bike. My limit on the road is around 44 km/h, at which point I’d be pretty spun out.

Even in my 50×12 top gear, it’d take my legs a pedaling cadence of 154 RPM to get to this indicated road speed. Yes, that’s not sustainable for long periods, and most likely very hard to do out on the open road due to aerodynamic drag, but I’m mentioning it to illustrate my case.

On Hyro, though, it’s a different story. I use nowhere near half the LR340’s total resistance range, and despite using the same cassette and chainrings as Bino, I usually never breach the halfway point of the cassette. Beyond 50×19 or 50×17 are gear combos that are too big for me to push my pedals to on a turbo trainer. My indicated road speed also peaks at a significantly lower 52.5 km/h.

V-max on Hyro while on a turbo trainer.

It’s entirely possible that the two bikes are giving me slightly different workouts, and I pin this down to their rear wheels acting as flywheels of different sizes.

With Bino, the smaller 406 mm rear wheel acts as a flywheel that is much lighter and quicker to spin up with pedaling the cranks. The consequence is it takes much more resistance from the turbo trainer to give the training load called for by sprint intervals. Hyro’s 622 mm rear wheel, on the other hand, has more mass and needs more energy to get going…but requires less out of the turbo trainer to give roughly the same training load.

In practical training terms, I think that indoor training with Bino is more of a test of souplesse, or pedaling smoothness. With the smaller wheel size, I find it is much easier to accelerate and decelerate simply by changing my pedaling cadence. Peculiarly, with a change of bike, I find I could get the same kind of training coaches used to recommend getting a roller trainer for. Bino also has a slight edge for really high cadence work. Mounting Hyro’s bigger rolling stock on the LR340 will allow me the low-cadence training that replicates endurance climbing efforts, as well as getting used to holding low positions for faster riding.

Without hard data, and sharing just my palpable differences training between the two bikes, I feel like I may just be blowing a load of hot air. For that reason, I would love to quantify all this difference with a power meter. Unlike heart rate or speed, where external factors such as aero drag or physical condition can affect readings, power meters are more “insulated” and are better at actually quantifying your training load and output – a watt is a watt is a watt. Unfortunately, while they have been coming down in price almost constantly since 2012, they’re still too rich for my blood.

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