Tour de France riders today take on the only individual time trial in this year's race, a 27.2km (16.9-mile) loop that starts and finishes in Pau, so they'll be swapping to TT bikes for the day.
Time trial bikes tend to have steeper frame angles than road bikes, the idea being to pivot the rider further forward around the bottom bracket for a more aerodynamic, flat-backed riding position.
Canyon's Speedmax CF SLX time trial bike, for instance, as used by both Movistar and Katusha–Alpecin, has a seat angle that's a whopping 80.5° (in all frame sizes) whereas the Ultimate CF SLX road bike's seat angle is about 73.8° (depending on the frame size).
A time trial bike's head tube is also shorter, as a rule. It's 128mm on a medium sized Speedmax CF SLX compared with 155mm on an Ultimate CF SLX.
The front end of most high-end time trial bikes is also lower because the stem is integrated into the head tube so it doesn't add any height. Nicolas Roche's Cervelo P5, above, is a good example.
Time trial bikes come with frame tubes and features that use airfoil shaping to slip though the air with the minimum of drag.
The Merida Time Warp TT belonging to Rohan Dennis (who has now abandoned this year's race), for instance, is typical of the genre in its use of a super-deep head tube and a seat tube that is cut away around the leading edge of the rear wheel. The seat tube is shaped for aerodynamics too, as are the seatstays which join the seat tube low to minimise the bike's frontal area.
The down tube is dropped and is cut away around the trailing edge of the front wheel in order to manage airflow in that area, while the fork legs are narrow and deep too.
Rather than trailing away gradually, frame tube profiles are typically cut off square at the back. This maintains most of the aero performance of a deeper tube while saving weight and staying with UCI equipment rules.
"The [current] iteration of the Time Warp TT is around 400g lighter than its predecessor [and offers] an overall advantage of nine watts," says Merida. "This has been achieved through computer modelling and wind tunnel testing."
Virtually all bike brands say that their time trial bikes have been designed and validated in this way.
The Pinarello Bolide's down tube has been designed with a concave flatback intended to work aerodynamically with a water bottle.
Even the fork dropouts are airfoil shaped to reduce drag.
Designers hide many components from the wind in order to improve aerodynamic efficiency. Fork crowns and stems are typically incorporated into a bike's existing silhouette so as not to increase the frontal area.
Merida claims that the covers over its brakes (the rear brake is located on the underside of the chainstays) reduce the aerodynamic drag by the equivalent of three watts, for example.
The Shimano Di2 Junction Box lives in a small compartment in the Pinarello Bolide's down tube so as not to cause any extra turbulence.
It is tucked in the end of the aerobar on Sunweb's Cervelo P5s...
...and the water bottle and cage are designed to integrate almost seamlessly with the frame.
You'll rarely see external cables or brake hoses on a pro-level time trial bike. Any that you do spot are likely to be well managed to avoid any unnecessary drag, as on this Dimension Data bike.
BMC houses Hidden Brake Booster Technology inside the stem of its Timemachine. According to BMC, "[This] increases the brake lever pull to calliper ratio ensuring the brake pads always provide sufficient clearance at times of wheel flex."
The idea of aerobars is that they allow the rider to achieve an aerodynamically efficient position that has in many cases been honed in the wind tunnel, while being comfortable enough to stay in that position for the duration of the stage or race.
The horizontal distance between the centre of the bottom bracket and the tips of the extensions (including controls/levers) must not be more than 75cm in most cases, although it can be up to 85cm for riders 1.90m and taller – so no Superman positions.
Most riders use standard bars that are adjusted to suit their dimensions. These are Enve SES Aerobars on Roman Kreuziger's BMC Timemachine, for instance. The extensions can be set further forward or back to get the right position.
The height of the armrests and extensions can usually be set with the use of spacers. Elie Gesbert of Arkea Samsic rider has his armrests positioned low, but that isn't the best position for everyone.
This Cofidis rider uses a big stack of spacers, for example.
Similarly, different riders have different aerobar width preferences. This Bora–Hansgrohe rider has his extensions positioned close to one another...
...while Movistar's Imanol Erviti has a wider setup. This doesn't simply come down to aerodynamics; some riders find that a narrow position constricts their chest and affects their breathing.
2018 Tour de France winner Geraint Thomas uses the Talon TT aerobar from Most, Pinarello’s in-house brand. It is 3D printed from titanium and made especially to fit the Team Ineos rider.
"The handlebar was manufactured with Powder Bed Fusion technology, where titanium powder is locally melted in a layer-by-layer fashion to create the final product," says Pinarello.
3D printing allows the various parts of the bar to be made in about two days. That might sound like a long time but complex components can be made using this technique, Pinarello doesn't need to invest in new manufacturing tools, and the design can be changed easily to suit different riders. All of this helps to keep costs down.
The base bar measures just 32cm wide from centre to centre.
Bahrain-Merida's Rohan Dennis uses a Vision Metron TT base bar with extensions that support the full length of his forearms. Rather than positioning his forearms horizontally, Dennis has them angled, his hands significantly higher than his elbows.
UCI rules state, "The handlebar extensions plus any accessories (controls, levers, etc. and all their mounting accessories) must be within ±10 cm from the level defined by the middle of the elbow/forearm supports."
We didn't measure but Dennis must be close to the limit here. That material covering on the topside of the aero extensions is the CPC material that Prologo uses on some of its saddles, by the way, Prologo being a Bahrain-Merida sponsor.
"The maximum inclination of each elbow rest (measured on the support surface of the arm) is 15°," says the UCI.
We've marked a 15° angle on this bike. Again, Dennis is cutting it very fine!
You see grip tape used extensively on base bars, shifters and aero extensions to stop riders' hands slipping, particularly when sweaty.
This is the end of Geraint Thomas's base bar, for instance.
The Dimension Data mechanics here have added a foam ramp here to smooth the transition from the bar to the lever and covered it in grip tape. It's easy to apply and effective.
This Cofidis rider uses Fizik handlebar tape instead, perhaps for a little extra comfort.
The bars on the Canyon Speedmax CF SLX bikes ridden by Katusha–Alpecin and Movistar use these grips from Ergon.
If it's a flat course, most riders will go with a disc wheel at the rear. Disc wheels weigh more than standard spoked wheels – this Zipp wheel on one of the Katusha–Alpecin bikes is light for a disc wheel at just under 1kg – but they offer amazing aerodynamic efficiency.
You'll see all kinds of wheels up front but they'll all have deep-section rims, the only exception being in blustery conditions when more depth can lead to tricky handling. Nicolas Roche's Shimano Dura-Ace C60 front wheel, for example, has a 60mm rim depth.
Many teams use tri-spoke front wheels to reduce drag. This is a Pro 3-spoke Textreme wheel on Geraint Thomas's bike, for instance.
Arkea-Samsic and Total Direct Energie riders are using the new FFWD Falcon that has just two spokes which are shaped to NACA airfoil profiles. The rim varies in depth from 48mm at the shallowest point to 90mm at the deepest. The idea, of course, is to reduce drag, and FFWD reckons that its two spoke design outperforms even a tri spoke in that respect.
Multiple shifters are possible with electronic systems. This means that riders can shift while their hands are positioned at the ends of the aero extensions, and also from the base bar when they're riding out of the saddle or cornering.
The Most Talon TT bar on Geraint Thomas's bike has buttons for Shimano Di2 built in, the ones on the left allowing him to change between chainrings, the ones on the right allowing him to move up and down the cassette.
He has just one button on each of his aero extensions, one moving the rear derailleur inward and the other moving it outward. He can't change between chainrings from the extensions.
Nicolas Roche uses Shimano's R9160 Di2 Remote Triathlon Shifters. The design is a little different but they work in the same way.
Bahrain-Merida's Rohan Dennis opts for shifters with two buttons each at the end of this extensions. These allow him to control both derailleurs while in his aero riding position.
Campagnolo's bar end controls, seen here on a one of Cofidis's Kuotas, are a little different, featuring a 'Back to Zero position' system. One lever controls each derailleur. You push the lever up to move the chain in one direction and down to move it the other. The lever always returns to its central position.
SRAM uses what it calls Blips for EPS gear changing on TT setups (and as remote shifters on road bikes). The Katusha–Alpecin mechanics have simply taped one in position here.
Riders tend to push bigger gears on their time trial bike than they do on their standard road bike. Many find it possible to produce more power this way, and they don't need to be ready to respond to unexpected accelerations like they do when riding in a group.
The gears chosen will obviously depend to some degree on the stage profile, but 56-tooth and 58-tooth outer chainrings are common on flatter courses.
Movistar's Imanol Erviti, for example, has a 58/46-tooth chainset fitted here, with an 11-29-tooth 12-speed cassette. That's by no means unusual.
Some riders use a regular saddle on their time trial bike. Geraint Thomas, for example, has the same carbon-railed Fizik Arione model on his Pinarello Bolide TT as he does on his Dogma F12. He knows what he likes and he's sticking with it!
Other riders switch saddles, though, for ones that are designed specifically for time trialling. Sunweb's Nicolas Roche goes for a Pro Aerofuel saddle.
The geometry of a time trial and the low riding position adopted on aerobars means that many riders feel more comfortable with a saddle that has deeper padding at the front end. Many time trial saddles have a shorter than normal nose so as not to restrict movement.
The Aerofuel has a recess that's designed to reduce pressure when in an aero position while those stripes on the cover are designed to stop the rider gradually creeping forward while pedalling, avoiding the need for periodic repositioning.
It looks like this Katusha–Alpecin rider has taken matters into his own hands and chopped the nose of his Selle Italia saddle off square.
Mat has worked for loads of bike magazines over 20+ years, and been editor of 220 Triathlon and Cycling Plus. He's been road.cc technical editor for eight years, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a past winner of the Cycling Media Award for Specialist Online Writer.