A bike that spends the winter on a turbo trainer sees no rain, no grit and no road salt. It ought therefore to age better than a bike ridden outdoors. On paper. In practice, plenty of machines come back out in March with a stretched chain, two hooked sprockets and a notchy headset that did not exist in November, despite never having left the garage.
The trainer does not remove wear, it relocates it. It swaps abrasion for two other forms of attack: corrosion from sweat, and localised wear caused by constant effort on two or three sprockets. Both are invisible during the session and remind you they exist on your first ride outside, when the new chain skips under load. Here is what actually wears, how fast, and what to check before you start the season.
Sweat is not water
In an intense indoor session with no proper airflow, it is common to lose between 0.5 and 1.5 litres of sweat an hour. A good part of it lands on the bars, the stem, the top tube and the head tube — precisely the areas nobody wipes.
That liquid is anything but harmless. It contains something like 1 to 3 grams of salt per litre, essentially sodium chloride, and it is slightly acidic. Chloride ions attack the protective oxide layer on aluminium and start pitting corrosion. Worse: once the droplet has evaporated, the salt stays behind as crystals, and turns aggressive again as soon as ambient humidity rises. A stem left dirty in January carries on working away in February.
Two habits remove 90% of the problem: a floor fan aimed properly, which evaporates sweat before it runs, and a protective cover or simply a towel laid from the bars to the seatpost. They are the two most worthwhile trainer accessories by a distance.
What rusts first
The order is nearly always the same, because it follows the path the drops take:
- The headset. It sits directly under the bars. Sweat runs down the steerer, gets past the top cap seal and ends up in the upper bearing. A cartridge headset bearing that grinds at the end of winter on a bike that has not been ridden is almost always this.
- Stem and bar bolts. Steel in aluminium, plus a salty electrolyte: galvanic corrosion does its work. A bolt that rounds its head on removal in April is a bolt nobody cleaned in January.
- Cable ferrules and outer entry points. On a mechanical drivetrain, salt water enters at the top of the outer and corrodes the inner cable inside. The symptom arrives later: a derailleur that shifts up well and down badly.
- Bottle cage bolts and the seat clamp. No mechanical consequence, but a good indicator of the general state of things.
- The bottom bracket on internally routed frames. Whatever runs down inside the frame ends up there.
Drivetrain wear under constant effort
Outdoors, you shift constantly: accelerations, false flats, descents. Over an hour the chain visits eight or ten sprockets and the load is spread around. On a trainer you can spend forty-five minutes on two or three sprockets, at a sustained 200 to 300 W, at a steady cadence and without ever backing off.
The chain stretches slightly more slowly than in muddy conditions, for want of abrasives. But the cassette takes the whole of that wear in one place. The classic result: a cassette with three hooked teeth and the rest almost new. It runs perfectly well on the trainer, and makes a brand-new chain skip on the first hard acceleration outdoors.
The measurement is the same as anywhere else, with a chain checker at £9 to £13: you replace the chain at around 0.5% elongation on an 11 or 12-speed drivetrain, and at about 0.75% on a more tolerant 8 to 10-speed. Go past those thresholds and you pay for the cassette too. It is the same reasoning as for a road bike, set out in our guide to when to replace a bike chain.
The "nothing wears out indoors" idea comes from a correct observation (the chain stays clean) and a false conclusion (so it is not working). A clean, dry chain wears too: the lubricant evaporates, sweat drives it out, and the pins run dry under high torque.
Direct drive: the trainer cassette is a consumable
On a direct-drive trainer the rear wheel comes off and the cassette mounted on the trainer takes everything. It wears exactly like the one on a bike, except that nobody thinks to check it because it is not on the bike.
Two points to watch:
- The cassette lockring torque. It works loose under the jolts of a sprint. The torque figure is generally engraved on the lockring itself, around 40 Nm on most common standards.
- The trainer's own freehub. It has its own bearings and its own grease, with a manufacturer's service schedule. A click that turns dull, or a tight spot when freewheeling, means work is due.
And one frequent mistake: fitting a cassette to the trainer with a different number of sprockets from the one on the bike, then swapping between the two without touching the indexing. The derailleur cannot be right on both.
