Tehran sits against the Alborz mountains like a city pressed up to a wall, and from the northern suburbs you can look up at snow peaks while the traffic below never seems to stop. It is a huge place, with several million people in the city and many more in the metropolitan area, and Damavand, the volcanic cone that is the highest peak in the Middle East at about 5,600 metres, can be seen on clear days. The north is wealthier and greener; the south is older, dustier and busier. It is not a city for relaxed riding: lanes are suggestions, motorbikes slip through everything, and the pollution can be harsh.
For the Chalus road, though, leaving Tehran is the first test and a good one. You head north toward the mountains, and the route climbs steadily through the Alborz foothills and up to a high crossing, where the road enters a long tunnel before dropping toward the Caspian Sea. The Chalus road is one of the most famous mountain routes in Iran, built in the 1930s under Reza Shah, and it remains a popular weekend run for Tehranis who want to escape the heat and reach the sea.
Weather is the main point. In summer, the north of Tehran is warm and dry, while the road over the mountains can be cool, foggy and busy. In winter, snow and ice close sections or require chains. The best times are late spring and early autumn, and early starts to avoid the Friday traffic, when it seems half the capital drives to the Caspian. Fuel is easy in Tehran and along the main routes, and petrol is cheap by international standards, though rules for foreign vehicles and travellers vary, so check before you go.
Take time for breakfast before leaving: warm barbari bread, cheese, walnuts, sweet tea and a little jam is the classic combination. Tehran has fine museums and gardens if you have a free morning, including the Golestan Palace and the Grand Bazaar. But for a rider, the real city is the one you leave behind, climbing into cool air and a world of mountains and mist.