The torque sensor that isn’t

A friend pushed back on GearSense the other day, and he had a good point. The fancy new automatic e-bike drivetrains — the ones with two motors and a gearbox running in an oil bath — have a real torque sensor. They feel how hard you’re pushing on the pedals and feed that into how much help you get. GearSense has no such thing. Just two little speed sensors and a cheap brain reading them. So how could it ever do what that does?

I didn’t have an answer in the moment. But it nagged at me, and by the end of the evening — with a bit of help from Bjorn thinking it through — I’m fairly sure we can get there. Not by adding a torque sensor. By realising we don’t need one.

Measure the effect, not the cause

Here’s the trick. You don’t have to measure a force to know it happened — you can watch what it changed.

Start with something simple and true: everyone has a comfortable pedalling rhythm. A cadence they naturally settle into and hold without thinking about it. Mine sits around a certain steady spin. GearSense already watches cadence all day long, so given a bit of riding it can learn your comfortable rhythm — the speed your legs like to turn.

Now, in any given gear, your pedalling speed and your wheel speed are welded together by the gear ratio. That’s not a choice; it’s mechanism. So “the wheel speed that lets Paul spin at his comfy cadence” is a fixed, knowable number — a different one for each gear, but fixed. If the motor simply holds that speed, you pedal at your favourite rhythm and the bike does the rest. Effortless, in every gear, without you asking for anything.

Where intent sneaks in

Now suppose you’re feeling frisky. You lean on the pedals. You push harder than the free ride requires.

What happens? Your cadence climbs above your comfortable baseline. It has to — that extra effort has to go somewhere, and where it goes is into spinning the cranks faster than the motor was asking for. And here’s the thing: while you’re genuinely driving the bike, your cadence can’t rise above what the gear demands unless your legs put it there. So a cadence that’s running hot is a fingerprint. It means: this rider is working. This rider wants more.

So GearSense will watch the gap between your cadence right now and your comfortable baseline, and reads that gap as intent. Spinning faster than usual? You’re asking for more — so it should lift the assisted speed to meet you. You never touch a button. You just push, like you would on any bike, and the bike understands.

That’s the whole magic trick. The expensive drivetrain feels your force with a sensor. GearSense infers it from a subtraction: how fast you’re spinning, minus how fast you usually spin. Two cheap speed sensors and some arithmetic, standing in for a precision torque gauge.

The tricky half, and the old bug that may solve it

Speeding up is easy to read. Slowing down is not — and I want to be honest that this part is still ahead of us, not behind us.

When you ease off the pedals, it could mean one of two completely different things. Maybe you want to slow down. Or maybe you’re happy at this speed and you just fancy a rest while the motor carries you. Your legs look the same in both cases — slowing down. Cadence alone can’t tell them apart.

But GearSense has a second sense that can. Remember the freewheel — the one-way clutch that lets the wheel outrun your legs when you coast? A little while back it was the villain of the story; it was the thing feeding GearSense phantom gears, and we taught it to spot the difference between really driving and just freewheeling.

That same detector is exactly what should settle this. If you ease off and the freewheel disengages — wheel still flying, legs slowing, coasting — that’s “I’m resting, hold the speed.” If you ease off but stay connected, soft-pedalling against the bike with wheel and legs slowing down together, that’s “I actually want to slow down.” The clutch tells us which. The bug we squashed last week may turn out to be the key that unlocks reading intent when you back off. I can’t promise it’s tidy yet — it’ll take real rides to tune — but the shape of the answer is there.

The same story, again

I keep landing in the same place with this project, and it keeps surprising me. The costly, clever solution replaces your whole drivetrain to measure something directly. GearSense sits quietly beside the bike you already own, reads the two humble signals it can already see, and reasons its way to the same answer for a fraction of the cost.

It doesn’t lack the torque sensor. It builds one out of things it already knows: your comfortable rhythm, the gear you’re in, and whether your legs are connected to the road. Three numbers, and a way of thinking about them.

We haven’t built this yet. It’s the next mountain, and there’ll be test rides and dead ends and a few “well, THAT didn’t work” evenings. But for the first time I can see the path from “a gadget that names your gear” to “a bike that understands what you’re asking of it.” And it’s made of cadence, all the way down.

— Paul