The chassis was flatter than the BT18, the suspension was revised and the body was completely rebuilt.

This drove the rear wheels through a Hewland Mk.4 five-speed manual transmission.

The chassis was constructed out of a tubular space frame, with a lightweight fibreglass body.

This meant the car was very light and nimble, weighing in at only 900 lb (410 kg).

[4][5] Private drivers like Peter Westbury with his Felday Engineering Ltd. had much success with the BT21 in 1967.