Single-speed indoor sprockets solve one problem and expose another.
They simplify the trainer bike. They reduce drivetrain noise. They make it easier to leave a bike on the trainer without worrying about cassette matching. Then the road kicks up and you still need a lower gear. The descent opens and you still need a harder one.
That is the point of virtual shifting in Strappo. It is not a novelty layer. It is how a quiet indoor setup still behaves like a bike.
The Old Setup: Cassette on the Trainer
The traditional direct-drive setup uses a cassette on the trainer. Your bike shifts like it does outside: the shifter moves the derailleur, the chain moves across sprockets, and the gear changes.
That setup is familiar. It is also a little fussy.
The cassette on the trainer has to match the bike well enough for the drivetrain to behave. Swap bikes and you may need to think about speed count, chain width, cassette range, indexing, and whether the trainer setup still lines up cleanly. A bike that shifts perfectly outside can still sound rough indoors if the trainer cassette is not a good match.
For riders who leave one bike on one trainer, this can be fine. For riders who share a trainer, swap bikes, or use a trainer-specific setup, it becomes another piece of maintenance.
The New Setup: Single-Speed Indoor Sprocket
A single-speed indoor sprocket replaces the cassette with one fixed chain line. The drivetrain gets quieter because the chain is not moving across a stack of sprockets.
The tradeoff is obvious: the physical drivetrain no longer gives you a full set of gears.
Virtual shifting fills the gap. The app changes the simulated gear ratio while the chain stays on the same sprocket. The trainer still controls resistance. You still get harder and easier gears. The physical drivetrain stays quiet.
How Strappo Uses Virtual Shifting
For Shimano Di2 setups, Strappo can read D-FLY button events and use validated controls as virtual shift inputs. The important word is validated. You choose the controls in Shimano E-TUBE. Strappo listens for the D-FLY channel presses. If the tested control still moves the derailleur, Strappo refuses to arm the virtual drivetrain.
That matters because the safest virtual shifter is one that does not secretly shift the physical drivetrain.
Strappo also supports controller input through the Open Bike Control shape used by BikeControl. That gives riders a practical path for shifter-style buttons, including same-iPhone setups where local network transport works better than CoreBluetooth discovery.
Cassette or Virtual Shifting?
Both setups can make sense.
Use a cassette if your trainer bike is already dialed, you like the mechanical feel, and compatibility is not giving you trouble.
Use virtual shifting if you want a cleaner indoor setup, a quieter drivetrain, easier bike swaps, or a single-speed trainer sprocket that still gives you usable gears on real gradients.
The difference is not about whether shifting matters indoors. It does. The difference is whether the shift has to move the chain.
FAQ
Does virtual shifting replace ERG mode?
No. Virtual shifting changes the simulated gear ratio during SIM riding. ERG mode controls target power during workouts. Different jobs.
Can Di2 virtual shifting move my derailleur?
That is the failure case Strappo guards against. It validates D-FLY button presses and refuses to arm virtual shifting if the tested control still produces fresh gear-position movement.
Why support BikeControl?
BikeControl gives riders a practical controller path for shifter-style inputs. Strappo listens for Open Bike Control-style actions and routes them through the same ride-control path as other controller events.
Is a cassette still supported?
Yes. A standard cassette setup remains valid. Virtual shifting is for riders who want the clean single-speed indoor setup without giving up shift control.
Keep the indoor setup clean. Keep the bike feeling like a bike. Shift the simulated drivetrain while the route keeps doing what real roads do.