He's describing the type of bicycle he was riding. The main body of a bicycle (bike) is called the 'frame'. Thirties means 1930s. See one here:I left at 9am and arrived at Waterloo at 9.35, beating car and tube travellers.
And I was on a heavy, Thirties loop frame. However I arrived shaken and nervous.Even the most well-meaning motorist is unaware of bicycles and does not see them.
I was on a heavy, Thirties loop frame
What is it?