Technical FAQ: Tour de France tech
TTT gearing at the Tour
I saw that BMC Racing was using 58-tooth chainrings on its bikes in the team time trial at the Tour de France. How did that help them?
BMC Racing’s Simon Gerrans said the team was pedaling at 90 kph (56 mph) at times. That’s pretty amazing. Let’s see how that’s possible.
A 58 X 11 gear is a 139.4” gear with a 700 X 25C tire (or a 138.6” gear with a 700 X 23C tire), and the bike goes forward pi (3.14) multiplied by 139.4, or 438 inches, with each revolution of the pedals. Since there are 5,280 feet in a mile, 12 inches in a foot, and 60 minutes in an hour, then their pedaling cadence (in RPM, revolutions per minute) is given by:
Rev/min = (56mi/hr * 5280ft/mi * 12in/ft)/(60min/hr * 438in/rev)
= 135 RPM
This is a very high, yet conceivable, cadence for short periods of time. (And if the BMC riders were riding 700 X 23C rather than 700 X 25C, they would have been pedaling slightly faster — at 136 revolutions per minute rather than at 135).
On the other hand, if their bikes instead had had a top gear of 53 X 11, then, using the same equation, their cadence at 56 mph would instead have been 148 RPM (since a 53 X 11 gear is a 127.5” gear with a 700 X 25C tire). Almost 150 RPM! That is not a cadence at which they conceivably could have pedaled efficiently for any length of time. So BMC probably gained a bit on the downhills due to those 58-tooth chainrings.
On the flats, pedaling the 58 X 11 gear at their average speed of 34.1 mph would have required a cadence of 82 RPM. In a 58 X 12, instead, that cadence would have become 89.7 RPM, since the gear inches of a 58 X 12 is 127.8” — almost the same as the 127.5” of a 53 X 11. A team on 53 X 11 at 34.1mph would have been spinning at 90 RPM, essentially the same as if BMC had been going that speed in a 58 X 12. This would have reduced the chain friction on the BMC bikes by pedaling a 12-tooth cog where others were pedaling an 11-tooth cog.
Assuming the chain length and tension applied by the jockey wheels is adjusted appropriately, there is a slight reduction in drivetrain frictional losses with a larger chainring and a larger cog. This is because the chain does not bend and straighten as much as it does when coming off of a smaller chainring and a smaller cog when achieving the same gear ratio.
Team Sky bikes
What’s that fin thing on the seat stays of the Sky time trial bikes? It looks like a bat wing.
Those are covers over the brake arms.
About the wings on the Sky TT bikes, Pinarello says that it is, “part of the brake and acts as a cover also.”
Carbon frames vs. steel
I am going to be in the market for a road bike upgrade in the near future and am considering either a locally made, custom high-end steel bike (custom, love the look and supporting local) or a high-end carbon bike (cheaper, lighter and “faster”). With so much marketing about the advantages of aero these days (see the new Cannondale SystemSix marketing and reviews), how much speed would I actually be leaving on the table in terms of aerodynamics between a custom steel bike and a high-end carbon bike, with all other things obviously being assumed equal?
The answer will depend a lot on how you ride. The faster you are, the bigger difference a more aerodynamic bike will make. It also depends on how much you sweat the other details. If you wear a loose jersey vs. skintight clothing, that will make the expense of an aero road bike as good as flushing dollar bills down the toilet, especially if you happen to also ride with long hair flowing down your back.
As you may have seen in my review of the SystemSix, as opposed to the Cannondale marketing material and reviews you mention, I’m unconvinced that the magnitude of calculated gains by the bike will be realized in real life. For instance, Cannondale’s calculation of a 30-watt power savings when pedaling this bike at 30mph within the peloton is based on the assumption that drafting reduces a rider’s drag by 40 percent. My guess is that in the real world, bikes within the peloton get more shielding from the wind than do the riders above them, due to identical wheel diameters and relative uniformity of sizes of the bikes compared to the bodies sitting on top of them, as well as air boundary layers along the road surface reducing the wind they are subject to. I would be very surprised if a guy sheltered in the middle of a peloton rolling along at 30mph would see anywhere near a 30-watt difference on his power meter between riding a standard road bike and a SystemSix, assuming he had the same wheels on both.
If you ride at a mellow pace, and/or you stay sheltered in a pack much of the time, and you wear the same kind of clothes and use the same wheels on either bike, your power savings between the custom steel bike and the aero road bike will be modest. If you pedal at 30mph on flat ground, though, the difference would probably quite substantial. And of course, the speed difference on climbs due to the weight difference will amount to something; it can be found here.
Remember that the front of the bike (fork, handlebars, cable routing) and wheels play a primary role in the bike’s aerodynamics. Since these things could be put on a custom steel frame just as easily as on a carbon frame, you can reduce the aerodynamic difference by forgoing the classic steel bike look.
I researched your prior writing on tire width and was convinced to move from my 23s to some 28s. My bike manufacturer said my frame would take up to a 28 width, so I bought some. I put them on, and while the width fit the chainstays, the tires were too high to clear the seat tube. Arrrgh. Can you please explain the relationship between tire width and tire height?
The tire height will go up linearly with tire width only if the rim width goes up proportionately as well. When the rim width stays the same, the tire’s height will increase more than its width with an increase in tire size.
Cleaning presta valve cores
Thrifty soul that I am, I find it easy to take removable presta valve cores completely apart to clean the sealant out of them. I use two pairs of needle nose pliers to unscrew the stop all the way off of the tiny brass threaded rod, which opens up the inside of the core for cleaning. Hot water may help. After reassembly, use the pliers to crimp the top of the brass rod so you don’t lose the stopper trailside!