Selim vs. sela
Although both words translate to "saddle" in English, usage in Portuguese depends strictly on what is being ridden. The main difference is that selim is used for vehicles like bicycles and motorcycles, while sela is used for animals.
Selim
B2The word selim specifically refers to the seat of a bicycle, motorcycle, or stationary exercise bike. It describes the narrower, mechanical seat attached to a seat post.
O selim da minha bicicleta está muito baixo.
(My bicycle saddle is too low.)
Ele comprou um selim de gel para ter mais conforto.
(He bought a gel saddle to have more comfort.)
Você precisa ajustar a altura do selim antes de pedalar.
(You need to adjust the height of the saddle before cycling.)
O ladrão roubou apenas o selim da moto.
(The thief stole only the saddle of the motorcycle.)
Este selim é muito duro para viagens longas.
(This saddle is too hard for long trips.)
Sela
B1The word sela refers to the traditional leather seat placed on the back of a horse or other mount. It is almost exclusively used in equestrian contexts and is incorrect when referring to a bike part.
O cavaleiro colocou a sela no cavalo.
(The rider put the saddle on the horse.)
Esta sela de couro é muito pesada.
(This leather saddle is very heavy.)
Precisamos limpar a sela depois do passeio.
(We need to clean the saddle after the ride.)
O cavalo derrubou a sela no chão.
(The horse threw the saddle on the ground.)
Ele segurou firme na sela para não cair.
(He held onto the saddle tightly so he wouldn't fall.)
Summary
Use selim when talking about the seat of a bicycle, motorcycle, or scooter. Use sela only when referring to the equipment used to ride a horse or another animal.