Read about this trip in an old Bangkok Post article. The article suggested that the travel time from Huai Nam Dang to Sam mun (25km) was about 4 hours, well our travel time was about 2 hours, I can only think that the road was better for us but to be honest this section was not that great with a lot of loose stuff and exposed rocks to negotiate on the track. If you have the thai for 'Sam mun' written down this will help a great deal with the navigation as there are many sign posts. I still don't have GPS so asking the way is still a big deal but having bilingual kids is a really boon. The one mistake I made in navigation was within the first 15km when on an open hill side I chose the lower road into the village but should have gone uphill with the wind turbines above. Easily corrected we carried on to Sammun had a quick drink stop and asked for directions out of the village. Essentially drive straight through the village and then straight on following the best road. The article refered to above suggested that the second (25km) section section would be slow, steep and narrow taking 4 hours. Well thats in the past, today the second section of 25 km to Wiang Haeng was excellent smooth driving an absolute joy and we did this in an hour. The big danger is you miught go to fast for its still narrow and the drops to the side are very steep, so take your time and enjoy this great track. On a bike you will be in seventh heaven. The only navigation (remember stick to the best road) is to go down at the Bor Nam Yai junction (looks improved), this is when you are just starting to see the rice fields in the valley below. This actually was as the Sam mun villagers described, which is to use when to take the downward right fork. Some steep downhill often on concret a couple of uphills and there you are back on the valley floor. We camped at Haui Nam Dang to see the 'Talay Mok', its pretty busy place and fankly I would not do it again but for the morning view and quick start to the main course of the drive it was worth it. The whole journey took us from 8.30-11.15 including stops and photographs, very different from other information I had found. On a bike you would probably do this in 2/3 on this time (no kids).