Ideally, a triathlete should have both a road bike and a time trial bike. A lot of triathletes, however, only have a time trial bike. It makes sense, since the vast majority of triathlons are raced on time trial bikes. What bothers me, however, is when the only bike in the stable is an uber-expensive time trial bike, because, ideally, a triathlete should have both a road bike and a time trial bike. I realize triathlon is an expensive sport and even entry-level bikes are expensive. If you already have a time trial bike you don’t need a road bike and if you already have a road bike you don’t need a time trial bike. However, the triathlete with nothing but the uber-expensive time trial bike, for the same amount of money, could have opted for a less expensive, although still excellent, time trial bike, and also purchased a basic road bike (I will discuss budgeting and budget priorities in another blog). If you are just starting out I recommend starting with a road bike and, after you get some riding experience, purchasing a time trial bike. If you have to choose one bike, even long-term, I still recommend a road bike.
It is important to understand that time trial bikes and road bikes are built for different purposes. A road bike is made for all kinds of riding. A road bike can climb, descend, and handle quickly and precisely. It is made for riding on the bar tops, the hoods, and in the drops. A time trial bike is made to optimize riding in the time trial position. It is faster than a road bike, but only if you ride it in the time trial position. If you have to come up out of your time trial position to the pursuit bar, you will go slower than you would on a time trial bike. The saddle position does not distribute weight in an ideal manner for climbing, descending, or quick handling. Time trial bikes are built for stability, which is really good for going really fast in a straight line, but they turn slower.
Time trial bikes are primarily built for riding really fast, in a fairly straight line, on fairly flat roads, in the time trial position. A time trial bike can climb, descend, and turn, but not as well as a road bike. Furthermore, a time trial bike is only faster than a road bike when ridden in the time trial position. This is the position that optimizes aerodynamics, comfort, and power. Sure you can sit up on the pursuit bars, but you will generate less power, you will be less comfortable, and you will not climb and descend nearly as well as your would on a road bike.
A lot of triathletes take the view that since they race on their time trial bike they should train on their time trial bike. All the time. For everything. This is fine if you ride on your own on flat roads where you never have to stop, but if you ride in a group, maneuver around hazards (including other riders), climb, and descend, and, as a result, spend relatively little time in your time trial position, you are not effectively training to race in your time trial position. First, you are not actually riding in your time trial position. Second, the hip angle in your time trial position is the same or close to what it should be on a road bike. If you ride on the pursuit bars you are not training the right muscles in the right way. Sure you have to climb, descend, and turn when you race, but if a race has so much climbing and so much descending and is so technical that you would spend the majority of your time on your pursuit bars then you would not spend enough time in the time trial position for a time trial bike to be advantageous. In such a race a time trial bike, because of its inferior handling, climbing, and descending abilities, would be slower than a road bike. Few races have so much climbing, descending and technicality. Ironman France and Alcatraz are the notable two and having done the latter I only recall one short section where I would have ridden on my time trial bars. Even many of the professional athletes at Alcatraz opt for road bikes.
The rule of thumb here is that if you are planning a race where you will spend most of your time on your time trial bars, ride your time trial bike. If not, ride your road bike. The same goes for training, which is why, ideally, you want both a road bike and a time trial bike. Many of my Chico Tri Club friends, who do a lot of group rides, fast group rides (like the Wednesday night “world championship” Fast 50), practice criteriums, and road races, train and ride on their road bikes much more than their time trial bikes. We generally encouraged our newer tri club members, who were often newer cyclists as well, to start with a road bike so they could learn the basics and learn to ride in a group so they could become involved in the larger cycling/triathlete community. Do not go out and blow your entire bike budget on an uber-top end time trial bike if it is going to be your only bike. Get a slightly less-uber time trial bike and leave some room in your budget for a basic road bike. If, however, you can only have one bike and you have to choose, I recommend a road bike (a basic one if you are just starting out). You can take advantage of much more versatile training and add clip-on aerobars for triathlon. I raced a road bike with clip-ons during my first year and Gloria one for four years. Although less optimal than a time trial bike, a converted road bike still works.