Guidão vs. guiador
Both guidão and guiador refer to the steering part of a bicycle or motorcycle known as a handlebar. The primary difference is regional usage: guidão is the standard term used in Brazilian Portuguese, while guiador is the term used in European Portuguese.
Guidão
B1This is the most common word in Brazil for a handlebar. It refers to the metal bar used to steer bicycles, motorcycles, and similar vehicles.
Segure firme no guidão para não cair.
(Hold tight to the handlebar so you do not fall.)
O ciclista trocou o guidão por um modelo mais leve.
(The cyclist swapped the handlebar for a lighter model.)
Ela pendurou a sacola de compras no guidão da bicicleta.
(She hung the shopping bag on the bicycle handlebar.)
O guidão da moto ficou torto após o acidente.
(The motorcycle handlebar got bent after the accident.)
Você precisa alinhar o guidão com a roda da frente.
(You need to align the handlebar with the front wheel.)
Guiador
B1This is the standard word in Portugal and other European Portuguese contexts. It describes the exact same object as the Brazilian term but is rarely used in Brazil.
Não deves largar o guiador enquanto a mota estiver em movimento.
(You should not let go of the handlebar while the motorbike is in motion.)
Preciso de subir a altura do guiador.
(I need to raise the height of the handlebar.)
O guiador desta bicicleta é muito confortável para as mãos.
(The handlebar of this bicycle is very comfortable for the hands.)
Ele comprou uma fita nova para envolver o guiador.
(He bought new tape to wrap around the handlebar.)
Agarra bem o guiador quando desceres a rua.
(Grab the handlebar well when you go down the street.)
Summary
To sound natural, use guidão when speaking with Brazilians and guiador when speaking with start Portuguese people. There is no functional difference in the object they describe, only in where the word is spoken.