Is polyester or cotton better for cycling?

Polyester for most rides. It wicks sweat and dries quickly, while cotton absorbs sweat and stays wet, causing rubbing and chills on descents.

Is merino better than polyester for cycling?

Merino resists smell and stays warm when damp, which makes it excellent for multi-day trips and cold rides. Polyester is cheaper, tougher and dries faster.

Why does polyester cycling kit smell?

Synthetic fibres tend to hold on to body odour. Wash promptly, skip fabric softener, and use a sports detergent or vinegar rinse.

Can I cycle in a cotton t-shirt?

For short, easy rides, yes. For long, hot or cold rides, cotton holds sweat and loses warmth when wet, so a wicking fabric is better.

Polyester or cotton for cycling? And where merino fits

The short answer

For cycling, polyester beats cotton on almost every ride that makes you sweat. Polyester moves sweat off your skin and dries quickly. Cotton absorbs sweat, holds it and stays wet, which means more rubbing and getting cold when you stop or descend. Merino wool is the third option: comfortable over a wide range of temperatures and good against smell, but more expensive and less durable.

How each fabric handles sweat

Polyester absorbs very little water. Sweat spreads across the fabric and evaporates, so the shirt stays light and dries fast. This is "wicking".
Cotton absorbs a lot of water into the fibres. That feels soft and cool at first, then heavy and clammy once you are sweating, and it dries slowly.
Merino absorbs moisture into the fibre while still feeling fairly dry against your skin, and keeps you warm even when damp.

Heat

On hot rides you want fast evaporation, which cools you. Light polyester, especially with mesh panels, does this best. Lightweight merino is good too. Cotton feels cool for the first ten minutes and then becomes a wet layer that cools unevenly and rubs.

Cold

The danger in cold weather is sweating on a climb and then freezing on the descent. Cotton is the worst here: wet cotton loses most of its insulation. Merino stays warm when damp. Polyester dries fast, which helps, and works well as a base layer under a jacket.

Smell

This is polyester's weakness. Synthetic fabrics tend to hold on to body odour more than natural fibres, and some polyester kit smells even after washing. Merino is famously resistant to smell. Cotton is in between. Some polyester fabrics have anti-odour treatments, which help to varying degrees.

Durability and cost

Polyester: tough, cheap, holds its shape, dries fast after washing.
Cotton: cheap and durable, but stretches and shrinks.
Merino: expensive, less resistant to abrasion and snags, and needs gentler washing. Blends with synthetics improve durability.

Side by side

Drying speed: polyester fastest, merino middle, cotton slowest.
Warmth when wet: merino best, polyester fair, cotton poor.
Smell resistance: merino best, cotton middle, polyester worst.
Durability: polyester best, cotton good, merino weakest.
Price: cotton and polyester cheap, merino expensive.
Looks off the bike: depends on the cut more than the fibre. A matte polyester tee can pass for cotton; a shiny race jersey cannot.

Which to choose

Most trail and road rides: polyester or a poly blend.
Multi-day trips or if smell drives you mad: merino or a merino blend.
Cold rides: merino or polyester base layer, never cotton.
Short, easy rides and the café after: anything, including cotton.

For the question of jersey vs t-shirt, which is about cut as much as fabric, see cycling jersey vs t-shirt.

Looking after polyester

Wash cold, and do not leave sweaty kit in a bag overnight.
Skip fabric softener. It coats the fibres and stops them wicking.
A sports detergent or a vinegar rinse helps with lingering smell.
Hang dry. Heat is bad for both the fabric and any print.

What I chose, and why

The Watts tee is 100% polyester, light (4 oz), with a soft, cotton-like feel. I chose it so it wicks and dries like riding kit but looks like an ordinary t-shirt. The trade-offs are the ones above: it can hold smell more than merino, so wash it after every ride, cold, inside out, no softener, hang dry, and never iron the print.