Climb the Rhigos mountain from Hirwaun in the Cynon Valley (Official Climbs #98), over the top to Treherbert at the head of the Rhondda valley. Then turn around, and go back over the top to return to your starting point. The gradient averages at around 5%, but there are several sections of 20%, especially as you tackle the hairpins near to the summit.
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Download FulGaz now and ride Rhigos Mountain (OC-98) on your home trainer. You'll then feel every change in gradient and see every turn in up to 4K video.