Why don't Tour de France riders always ride their TT bikes?

Question

When the tour riders are doing time trials, they ride their super aerodynamic, aggressively positioned bikes for maximum efficiency.

However, on normal stages, there are many cyclists who could also benefit from the greater aerodynamic efficiency. Any riders at the front of the peleton or any cyclists on the breakaway would gain a great deal from having their TT bikes.

I can't even think of any downsides, given that with electronic gears they can have duplicate shifter buttons on the aero bars and the bullhorns.

Why don't they use their TT bikes all the time?

Answer 1

There are 2 good reasons for this.

First, a TT bike is the most efficient machine for flat, fast course racing, where there are a minimum of hills. It is not the most efficient machine when it comes to climbing. The body position is far too aggressive for even professional athletes to use in a non-TT style event. A bike with more a more upright body position, a more stable weight distribution, a lighter frame, and far, far lighter wheels, is a more efficient choice for that style of ride.

Secondly, a TT bike sacrifices bike handling for that extreme power and aero efficiency. The TT stages are controlled events, where there are either one rider, or one team on the course at a time, and so limitations on stability and handling are acceptable. Take the same bike in a 150 rider peloton, where riders constantly jockey for position, and where even on a stable bike losing focus for 1 second can cause a major crash, and all of a sudden, those same sacrifices in bike handling and stability are no longer a benefit for the rider or the team. They become a hazard to the team's chances of winning. And not just for the stage or the day, since a bad crash can take a rider out for the season. Or God forbid, permanently.

For these reasons, the UCI has seen fit to ban the bikes in non-TT events, in order to ensure rider safety. And for once, their decision makes sense.

Answer 2

The main reason is because they are not legal.

UCI rules states that (Technical regulations for bicycles, a practical guide to implementation, comments on article 1.3.022):

Only the traditional type of handlebars is authorised for use in massed-start road races [...]. The attachment of any additional handlebar component or extension is prohibited.

They are not legal mainly for safety reasons. Normally, the brake levers on a TT bike are positioned at the "normal handlebar" while the gears are positioned at the extension bars. The result of that configuration is that you can not brake while in the more aerodynamically position, which makes it extremely dangerous to ride such bikes in the peloton. It is also easier to wobble around, you don't have the same stability when riding in the aerodynamically position.