We recommend you consult Ordnance Survey Map (OS-214) or view this gps.com map
There is a lot of climbing, around 4,400ft over 45 miles if you do the whole route. It's probably worth spending the whole day cycling the route, so you can enjoy the views and stop for a good look around Llanidloes. The route is based around Sustrans Route 81 from Llanidloes to Caerwys, but uses two B roads to avoid riding Route 81 both ways. Note that although Route 81 follows the valley alongside the A470 it winds it way up and down several spurs, so there is much more climbing than you might think! Route 81 is generally very well signposted, so easy to follow. However, be aware that at a couple of points in the ride there are branches of Route 8 and 818 that go off to the West.
If you start at Rhayader there is a pay and display car park just by the turn off for the B4518. Turn right out of the car park and the climbing starts immediately. Follow the road through Harmon and Pant y dwr ignoring any turnings to the left and right. After several ups and downs you go past a big landfill site and there is a long steep descent into Llanidloes.
Turn right towards the town and then left to cross the bridge over the river. After a short distance turn right up the B4569 (signposted Trefeglwys). There is a steep climb up through the woods, leading onto a ridge with excellent views. There are several ups and downs before a final long descent to Trefeglwys. There are now fewer hills and a nice undulating ride into Caersws, where there are local shops.
Note that the turn off to route 81 back to Llanidloes goes off to the right just before you cross a small stream a few hundred yards before reaching the village. There is a sign, but it is easily missed when travelling from this direction. The route now follows single track roads, which run parallel to the A470 in the valley below. The route climbs up and descend several steep spurs before reaching Llanidloes. There are several restaurants and cafes, but I visited The Great Oak, which serves vegetarian food and epic slices of cake. Once in the town you need to turn right next to the Market Hall down a narrow street (Shortbridge Street, well signposted). At the end of the street turn left (signposted). After a short distance Route 8 splits into two branches, make sure that you take the left hand one.
The route continues with several steep climbs to Llangurig, where you emerge onto the A44 at a T junction. Turn left (signposted Rhayadar) and then after a few yards take the road to the right (signposted). Navigation is now straightforward. Just stay on the single track road until a long descent takes you to a T junction. Turn left and you will soon be back in Rhayadar.
Route supplied by Ian Barton - thank you!