Gear ratio and speed calculator

Ratios, gear inches, metres per pedal stroke and speed at cadence. And the part other gear calculators skip: which gear to sit in on a trainer, and why it is different for wheel-on and direct-drive.

## Three ways of saying the same thing

Gear ratio is chainring teeth divided by sprocket teeth. A 50 on the front and a 17 on the back is 2.94, meaning the wheel turns just under three times for every turn of the cranks. Simple, and useless for comparing bikes with different wheels.

Gear inches is the ratio multiplied by the wheel diameter in inches. It is a survival from the penny-farthing, where the gear genuinely was the size of the wheel, and it persists because it lets you compare a road bike with a mountain bike in one number. A 50/17 on 700x25C is about 78 gear inches.

Development is how far you travel per turn of the pedals — the most intuitive of the three. That same gear moves you 6.2 metres per revolution.

All three say the same thing. Use whichever you think in.

## Why this matters on a turbo and not just outdoors

Most gear calculators are for choosing a cassette. On a trainer, gear choice does something different, and which kind of trainer you have decides what.

On a wheel-on trainer, the roller is driven by your tyre, so your gear sets how fast the roller spins. These trainers have a resistance curve that rises steeply with roller speed, which means:

• a bigger gear puts you higher up the curve, where there is plenty of resistance and small changes in effort produce large changes in power
• a smaller gear puts you at the bottom, where the resistance is low and you may not be able to reach your target watts at all

If you cannot hit your intervals and the flywheel sounds slow, you are in too small a gear. Most wheel-on trainers want something around a 50/15 to 50/17 for threshold work.

On a direct-drive trainer in ERG mode, the trainer holds the power whatever you do, so gear does not change your watts. It changes how the session *feels*, because it sets the flywheel speed:

• a bigger gear spins the flywheel faster, giving more stored momentum and a smoother, more road-like feel, but making the trainer slower to respond when the target changes
• a smaller gear gives a lighter, more responsive feel, and makes the dreaded ERG spiral of death less likely — that is the one where your cadence drops, the trainer adds resistance to hold the watts, your cadence drops further, and within five seconds you have ground to a halt

The usual advice is a small-to-middling gear for ERG intervals, and it is good advice. Something like the small ring and the middle of the cassette.

In simulation mode gear works exactly as it does on the road, because the trainer is pretending to be a hill.

## Picking gears for the road

The question worth asking is not "what is my biggest gear" but "what is my smallest, and is it small enough for the worst hill I ride?"

Work backwards. Decide the slowest you are willing to grind up something steep — call it 6 km/h — and the lowest cadence you can turn without hurting your knees, say 60 rpm. Put those in the calculator and find the gear that gives it. On most modern road bikes that means a compact chainset and a cassette with at least a 32 on the back, and there is nothing to be ashamed of in any of it.

At the other end, a 50/11 at 100 rpm is over 60 km/h. Unless you are descending or sprinting in a race, you will almost never use it, which is why most riders would be better served by a smaller big ring than they think.