Audax riders cover a particular route within a pre-selected time limit. Although they prefer bikes similar in design to road bikes, an Audax bike may have more space under the fork crown and rear brake bridge to allow for fatter tyres, which add to comfort. A triple chainset is a practical addition, since using low gears on hills conserves energy.
It used to be an Audax rule that all bikes were equipped with mudguards and lights, irrespective of whether there was night-time riding on the route. Now this requirement is left up to an event organiser's discretion. However, mudguards can add to your comfort if the roads are wet.
A pannier rack allows all kinds of bags to be carried, from a single large saddlebag to high volume touring panniers.