So after reading people's recommendations, I have decided to train for my half Ironman (HIM) on my road bike. I live around nothing but steep hills so it's probably the best for me anyway. However, I have a brand, new tri bike that I would like to race with for the advantages of using a tri bike during a race. My question is, at what point in my training and how close to race day, do I switch my training to my tri bike?
Great question! So you need to answer a few more questions before diving into the answers. First, is the course you are racing similarly to where you train? Meaning, are you training for a flat half or a hilly half? You say you have plenty of hills. That is good for many aspects of training, LT work at short intervals at a high heart rate or watts if using a power meter! Good force and strength work on the hills for sure. This will increase power. Working different muscles while in the hills is a benefit as well if you get out of the saddle for some climbs and stay seated for leg strength for most of it. The downside is that climb on a tri bike is a bit tougher with the seat angles. So using a road bike for all of these situations is great! But you say that you want to race on your tri bike. So again, is it a hilly course or a flat course? Either way you will want to get on that tri bike and get time on that as well. Just like learning any skill it must be specific. You need to think about getting your bike handling skill down first! Riding a tri bike is much more challenging then on a road bike. Some skills to work on might include, turning in and out of your tri bars. How comfortable are you at turning at high speeds in the aerobars? If the course has a lot of turns, you will want many hours of practice on that particular skill to become comfortable. So 1 or 2 sessions a week working in this area would be key. Another reason to ride your tri bike often is to get all of your core muscles used to being in the aero position for extended periods of time. I am assuming you will be aero for most of your ride since you want to ride the tri bike. If your back is tight or sore after getting off the bike your run will surely suffer, so training time in the aero position is essential! So your long rides should be on your tri bike.
There is also the theory of specificity that comes into play. As stated in an article written by Stegeman, J. (translated by J. S. Skinner). (1981) Exercise physiology. Chicago, IL: Year Book Medical Publishers. (p. 267) It was stated that the body adapts to adequately cope with the specific forms of exercise stresses which are applied. The adaptive process does not include any capacity that extends beyond the specific training stress. Thus, there is no basis to expect training effects from one form of exercise to transfer to any other form of exercise. Training is absolutely specific (Noakes, 1986). This would mean that training on a tri bike would be of great importance if you want to ride well in your race, on your tri bike.
Another source showed that running off the tri bike showed huge gains in run speed. Dan Empfield show that a study conducted by Ian Garside, Garside utilized triathletes, but: All participants were naive to training and racing on bicycles with steep seat tube angles (>76-degrees); all participants used a 73-degree frame geometry as standard. As opposed to most of the testing up to this point, Garside's protocol called for the tests to be conducted, as fast as possible under race-like conditions. The test called for triathletes to ride a 40km simulation on both a 73-degree set-up and then on an 81-degree set-up, each followed immediately by a fast-as-possible 10km run on a treadmill. The authors noted the improved bike/run performance in the field, based on anecdotal testimony from athletes purporting to have experienced improved performance. But, they noted that prior to this study, No empirical evidence exists. Frankly, the results were groundbreaking, for three reasons. First, these triathletes absolutely blew away their "duathlon" performances in the steeper configuration. The average time it took subjects to complete the 40km/10km brick was about 1:50 at 73 degrees of seat angle, and it was a full 5+ minutes faster at 81 degrees. This is a huge gain in run performance!
So clearly it is in any triathletes best interest to train on a tri bike if that is what you are going to race on. So get in some base miles and climbing time on the road bike, but get in long rides, some intervals and runs off the tri bike and your race should have good results!