Why a 9F could not compete with Mallard in wheel rotation speed

PETER KELLY asks if a reader could compare the rotational speed of a 9F’s 5ft diameter wheels at 90mph, with Mallard’s 6ft 8in wheels at 126mph (issue 281). While I would not call myself a mathematician, the maths are actually quite simple: 90mph is 1.5 miles per minute, or 7920ft per minute.

The 9F driving wheels are 15.708ft in circumference, so at 90mph, the wheels are rotating at 7920/15.708 = 504.2rpm; 126 mph is 2.1 miles per minute, or 11088 feet per minute. The A4 driving wheels are 20.945ft in circumference, so at 126mph, the wheels are rotating at 11088/20.945 = 529.4 rpm.

So, the 9F has some way to go to match Mallard’s rotational speed in 1938. In fact, it would need to be doing almost 95mph to match it!

Ron Head, Didcot, Oxfordshire