Sorry for the long silence.
I was researching the Caerphilly events for an article in a local magazine, and my best guess is that the hillclimb started at the junction of Mountain Road and Corbett's Crescent (CF83 1HP) and followed the Mountain Road to its highest point.
I'm not entirely convinced, though... The information I have (from
http://www.silhouet.com/motorsport/t...tml#caerphilly) indicates that the course was 1,194 yards, with an average gradient of 1 in 8.6. That would put the total climb at 416 feet, give or take 6". The elevation at the start, according to Google Earth, is 452', so that would put the elevation at the finish at 868'. Google Earth puts the highest point on the road at 804', some 60' short.
My problem is that I can't find another route which would match these figures--There's another potential start point--junction of King Edward Ave with Mountain Road (CF83 1HE) the elevation of which is 388', which works for the 416' climb, but the length of the course increases by another 420 yards. Without clearer evidence, I can't be sure of the race's location.
It seems to be clear that the races took place along Mountain Road at some point--the event was cancelled after 1924, when a similar event at Box Hill, Tring ended in near tragedy, and Parliament outlawed events on public roads; Mountain Road is the steepest public road in the area, and it is named (though ambiguously) in some of the reports.
Jury's still out, then. If I get more concrete information, I'll post it here...