http://www.cycle-routes.org/penninecycleway/off_road.htmPaddy O'Kelly has this to say ' First, I think it should be made clearer that the off road part through Wark forest is NOT suitable for road bikes. I did it on 28mm touring tyres and I'd say thats the minimum you should have. As this is the only off road bit for which there is no alternative, this section dictates the choice of bike, in my opinion. Some of our group tried on racing tyres and we had 2 punctures in quick succession'
http://www.cycle-routes.org/penninecycleway/pauls_journal.htmDon’t be dismayed by the surface you meet as you enter Wark Forest. This forest road was built for logging trucks. The surface is solid but consists of sharp, large stones. Setting off at a pace down the hill could lead to disaster or punctures so take it easy. This surface lasts for perhaps 2½ miles before joining tarmac all the way to Bellingham. The first section in Wark Forest could be difficult for road bikes and is the one section where I wished I were using off-road tyres. It doesn’t last long. Once out of the forest the road sweeps along falling and climbing gently for mile after mile until about 3 miles before Bellingham you encounter a steepish climb. Hit the top and you are rewarded with a FAST downhill into Bellingham – look out for the hard right at the bottom!