I got a new TT bike in 2017 and replaced my road bike after a crash, shortly after, in 2018. I have raced road cycling and Triathlon for 11 years, and have worked as a bike mechanic in an LBS- here are my thoughts:
Personally, I would keep the defy. You won’t get alot of value for it in the 2nd hand market, but the defy is a perfectly good bike. It can do everything- and if you’re fast- you can go fast on the defy. Shimano 105 is no limiter, and having two bikes- one as a spare- is very useful if one is in the shop. Your budget is plenty to get a nice tt bike if you look on the 2nd hand market. The only exception would be if I were comitting to disc brakes- see point 3 below.
- what are your main races going to be-i.e. whats your main use case for the bike? How many times a year will you race long course? If you are doing mainly group rides, olympic distances, and up to half ironman- a road bike, well fitted with short aero bars- will be plenty. A road bike is simply more versatile. Above half iron it starts to tip toward a full blown tri or TT bike.
If I were keeping 2 bikes, I’d spend more money on the one I spend more time on. In my case, I use my road bike more, so my road bike is nicer than my TT bike. In fact, my TT bike has shimano 105 and microshift parts. It goes plenty fast. Don’t be ‘afraid’ of 105. It’s a good groupset and whoever is saying it isn?'t good enough is a bike snob. Lower level professional bike teams use it- and put many more miles into it than you will.
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how often do you travel with your bikes? That probably influences decisions 3 and 4.
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choose your brake system wisely- I would harmonise brake systems between bikes if I had 2 bikes. Either go both rim or both disc- it just makes things easier from a logistics perspective.
You don’t NEED discs. Discs have more power- but do you have enough traction in your tyres to take advantage of that? Your area has alot of descents- but are they technical, or does it rain alot? If no, I mantain you don’t NEED them. Discs have more power- but are higher maintenance, more difficult to travel with- and right now generally more expensive. Whichever system you choose- know the pros and cons of each.
- Regarding Di2- again, you don’t need Di2. In fact, I would say there is even less of an argument for Di2 than disc brakes in your case.
Di2 makes more sense on a TT bike than a road bike, so if it were a choice of having Di2 on a TT or road- it would be TT, any day.
Di2 is ‘nice to have’- shifting under power is seamless- but I would say the advantages are small. shifting from the base bars as a huge advantage? That’s bollocks. It takes me 0.5 seconds to shift with one hand on the base bar, and I’ve raced in all conditions on my TT bike. If you can’t shift a mechanical TT system with one hand on the base bars, you have bigger problems than choosing Di2 over mechanical!
Also note that Di2 is a commitment to a slightly more high maintenance system. If you travel with your bike, any mom and pop bike store can sort you out with a snapped cable or a ruined derailleur, in 30 minutes. If TSA clamped one of your Di2 wires? Or if a rear derailleur electronic shorted out? Cold weather can drain batteries prior to race day. Faults with Di2 are not uncommon, and they typically take longer and are more complex to fix. Good luck. You better be travelling with spares for an A race if you go Di2. This is not to say Di2 is BAD- it’s perfectly good- but its good to understand what can go wrong.
Di2 has some nice advantages if you’re planning to go race UCI Time trials- but this probably does not apply for your case.
If budget were no object-I’d probably go Di2, thats true. But if not- I wouldn’t be afraid to drop it.
- finally- keep within your reasonable budget. 3-5k can buy plenty of bike. For example, the cervelo p series disc with shimano 105 is just above 3k retail and thats all the bike 99% of people need. Then you could sell the defy and get a cheap disc road bike. If you chose rim brakes for the tt bike and kept the defy - then your options are even wider.
You could always upgrade it to Di2 later if you wanted, but to my mind it’s not a priority if budget is an issue.