Bicycles play a significant part in the plots of Pigeon Post and The Big Six, and are mentioned in other books.
Swallowdale
When the able seaman and the boy are exploring, they see a road through a gap in the stone wall: Then three natives on bicycles passed the gap, going the other way .... (SD4)
Peter Duck
A telegraph-boy had come along the quay on a red bicycle .... (PD1)
Pigeon Post
They trek to Tysons with their gear on Dromedaries (bicycles) or the handcart (PP7). Later Dick cycles back to Beckfoot to test the gold dust (PP33).
Coot Club
Joe cycles to Horning on Bill's bike (CC11,12). Bill cycles to Potter Heigham (CC15). At Beccles a boy on a red Post Office bicycle is too late to give them a telegram from home (CC23).
The Big Six
The detectives go out on bicycles to find out whether other boats have been cast adrift: Tom on his bicycle, Bill on his (Bill is the only one of the Death and Glories with a bicycle), Joe on Dick’s bicycle, and Pete on Dot’s (BS25,26).
George Owdon and Ralph Strakey have bicycles (BS9,32). So do a number of adults: Mr Tedder the policeman (BS19), the old parson and Mr Jonnatt. The detectives are particularly interested in bicycles with Dunlop tyres as the chap what put them shackles down our chibley have a bike (with) Dunlop tyres (BS19). George Owdon’s cycle has Dunlop tyres; and so do the cycles of Mr Tedder, Mr Jonnatt, the district nurse, the milkboy and the parson. Some have Palmer or John Bull tyres, but as Mr Bixby at the bicycle shop says They mostly use ‘em (Dunlops) (BS20). Frank Farland says Almost all bicycles have Dunlop tyres (BS32). Pete says hotly of Mr Tedder And he got Dunlop tyres himself (BS20), and thinks that no matter what the others might say, Mr Tedder's bicycle had Dunlop tyres (BS21).
The Picts and the Marytrs
The postman rides a bicycle (PM7). Billy Lewthwaite rides on Nancy’s bicycle to get petrol for Rattletrap (PM25).
We Didn’t Mean To Go To Sea
They see Dutch girls on bicycles in Flushing (WD23).