I had wanted to ride Stwlan Dam for years after seeing its tight stack of hairpins in a cycling magazine. Staying in Porthmadog finally put Tanygrisiau close enough to stop making excuses, and the climb lived up to the photographs: short, steep, almost completely traffic-free and unlike anything else I had ridden in Wales.
The useful thing to know before going is that Stwlan is not a 20% average wall, as older versions of this page suggested. The core service-road climb is about 2.8km at roughly 10%, gaining around 275 metres. Different segments start slightly lower down and can stretch the distance beyond 3km, but the character is the same: repeated double-digit ramps, a brief easing in the middle and eight distinctive hairpins below the dam.
Stwlan Dam climb profile
| Detail | Stwlan Dam |
|---|---|
| Start | Tanygrisiau, near Blaenau Ffestiniog |
| Core climb distance | About 2.8km |
| Elevation gain | About 275m |
| Average gradient | Roughly 9.5–10% |
| Steepest sustained sections | Around 12–14% |
| Hairpins | Eight |
| Road | Narrow tarmac service road, closed to normal motor traffic |
| Finish | Llyn Stwlan and the dam |
What the Stwlan Dam climb is actually like
The road starts climbing almost immediately after the access gate. You need to get the bike through or over the barrier, but once on the service road there is very little traffic beyond walkers, cyclists and the occasional authorised vehicle connected with the power station.
I made the mistake of starting too quickly. The opening section sits around 9% for long enough that an enthusiastic first few minutes soon become expensive. The road then begins to reveal the dam itself before reaching the switchbacks that make Stwlan so recognisable.
The first hairpin is one of the sharper ones, while the middle section becomes a little more manageable at around 7–8%. That respite does not last. The final approach steepens again, with ramps around 14% before the road reaches the dam.
Stwlan is a better climb for rhythm than attacking. There is nowhere near enough recovery between the steeper sections to justify going into the red early, and the closed road makes it easy to concentrate on cadence rather than traffic.
Why there is a road to Stwlan Dam
Llyn Stwlan is the upper reservoir for Ffestiniog Power Station. Commissioned in 1963, Ffestiniog was the UK’s first major pumped-storage power facility. Water drops from Llyn Stwlan to the lower reservoir at Tanygrisiau when electricity is needed, then is pumped back uphill when demand is lower.
The station’s four generating units have a combined output of 360MW. ENGIE completed a major refurbishment programme at Ffestiniog in 2026, so the infrastructure behind the climb is still an active part of the electricity system rather than an abandoned industrial road.
That also explains the unusual setting. You are climbing a narrow engineering road into the Moelwyns rather than a public mountain pass, which is why Stwlan feels so different from climbs such as Pen-y-Pass or Bwlch-y-Groes.
Where to start and park for Stwlan Dam
Tanygrisiau is the practical base. There is parking close to the lower reservoir and the start of the service road, so Stwlan can be ridden as a standalone climb if you are travelling through the area rather than building it into a long North Wales loop.
The Ffestiniog Railway also passes through the village, and the lower reservoir makes the start easy to identify. The climbing begins only a short distance from the village road.
If you want a bigger day, Stwlan links naturally with other climbing around Blaenau Ffestiniog, Porthmadog and the wider Eryri area. ProCyclingUK’s guide to the best cycling climbs in North Wales is a useful place to build the rest of the route.
What gearing do you need?
A compact or semi-compact road setup with a 30, 32 or 34-tooth largest sprocket will make Stwlan much more manageable for most riders. Strong climbers can obviously use harder gearing, but there is little advantage in arriving over-geared on a climb that sits near 10% for most of its length.
Shift early before the steeper ramps rather than waiting until cadence has collapsed. The tight hairpins can briefly alter the gradient depending on the line you take, so keeping one easier gear in reserve is useful.
What to carry
The climb itself is short, but the weather in the Moelwyns can change quickly and the descent is exposed enough to feel cold after a hard effort. A packable waterproof or windproof layer is worth carrying even when the valley feels mild.
ProCyclingUK has a full guide to waterproof cycling jackets for UK riding. If you need one for Welsh mountain days, Tredz has a current range of cycling waterproofs.
A spare tube or tubeless repair, pump and small multi-tool are sensible even on a short climb because there are no repair facilities once you are through the gate. Current compact cycling multi-tools are available through Tredz.
Best time of year to ride Stwlan Dam
Late spring through early autumn gives the best chance of dry tarmac, reasonable temperatures and enough daylight to combine Stwlan with other North Wales climbs. Summer does not guarantee good weather here, but it reduces the risk of ice on an exposed service road.
Winter riding is possible in suitable conditions, but ice, heavy rain and strong wind can make the descent much less appealing. Check the forecast for the hills rather than relying solely on conditions down in Porthmadog or Blaenau Ffestiniog.
How hard is Stwlan Dam?
It is hard enough to demand proper climbing gears but short enough that most reasonably fit road cyclists can treat it as a standalone target. The challenge comes from the near-10% average and lack of long recovery sections rather than extreme length.
For riders who enjoy steep Welsh roads, the obvious companion climb is Marchlyn Mawr, another service-road ascent to a pumped-storage reservoir. Stwlan is shorter and more compact, while Marchlyn Mawr turns the same basic idea into a longer mountain effort.
If you prefer organised events, ProCyclingUK also has a guide to the best Welsh sportives for climbing.
Stwlan Dam Strava segment
There are several versions of the climb depending on exactly where the start is placed. The long-established Stwlan Dam segment can be viewed on Strava, but treat the segment as a reference rather than a reason to sprint through the gate or descend beyond your sight line.
Is Stwlan Dam worth riding?
Yes. It is not the longest climb in North Wales and it does not need exaggerated gradient figures to make it interesting. The attraction is the combination of a steep traffic-free road, eight compact hairpins, industrial history and the sudden view of Llyn Stwlan at the top.
It also works unusually well as a standalone target. Park in Tanygrisiau, ride the climb, take in the dam and descend carefully, or use it as the sharpest part of a longer Eryri day. Either way, the hairpins are every bit as good in person as they looked in the photograph that made me want to ride them in the first place.
