Cinder tracks were fine, but if you compare cinder and track, logically one would gain some yardage on tartan, although not that much, admittedly. If one looks at a runner like Bob Holt (Hercules Wimbledon) he was running around the time (70s and 80s) when he could make a direct comparison between running on cinder tracks (Wimbledon Park, Motspur Park, etc) and tartan tracks (Crystal Palace, West London stadium, Highgate, etc..) -
Bob Holt always thought tartan conferred benefit, but he was a 'form runner' and so tartan suited his metronomic style, as it did his brother, David - who famously went on to qualify for the G.B. team for the 1972 Munich Olympics in the 10,000 at Crystal Palace (on tartan). Bob's best on tartan was 28:39, and 13:48 (5,000). The disadvantage in comparing times with Bob, is that when he ran races such as the Surrey Championships, Motspur Park, the quality of the field was not as strong, so the times were not generally as fast.
In my own case - as a runner of modest club ability in the UK, in the late 70s, I ran 15:17 for 5,000 on cinders at Wimbledon Park (Melbourne Trophy, August 1978), and then did not better that until I went to Mansfield State College (U.S.A.) on a running scholarship in 1980 - 1982. I ran 14:42 for the indoor 3 mile race on a 200m track at Bucknell university in February 1981. . That 14:42 was only a few seconds better, intrinsically, than the outdoor 15:17.
Many experts have postulated that Ron Clarke's 27:39 on cinders back in the 60s would have been worth around 20 seconds faster, but this is something which is difficult to establish, because there are so many factors which come into play. Jim Ryun was another runner who ran very fast on cinders for the mile, and I do not think ran significantly faster on tartan in his day.
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