He had a big head of steam riding his bicycle, then he lost his balance and fell over.
ーHe had too much momentum and he fell.
「彼は勢い余って転んだ」
to have too much momentum で「勢い余る」と言えます。
ーHe had a big head of steam riding his bicycle, then he lost his balance and fell over.
「彼は自転車で勢い余ってバランスを失い転んだ」
to have a big head of steam で「勢い余る」を表現することもできます。
ご参考まで!