You know what matters?
Drafting.
Especially in the persistent north-easterly that has scoured Western Europe this week.
Drafting takes the work out of riding with friends and is the basis of all major road-racing tactics. A bicycle race is an opaque mystery until you understand drafting.
The breakaway, the domestique, the catch, the sprint train, the summit finish, and the very peloton all owe their existence to drafting.
One of Jo Burt’s amazing illustrations in Paul Fournel’s collection of essays, Vélo, published by Rouleur.
Even the attacks on cobbled sections or short climbs in the Spring Classics are explained by drafting. No-one but Fabian Cancellara can ride away on smooth, flat ground – because of drafting. But on cobbles or climbs, riders are forced to spend proportionally more of their power on resistances other than air drag, so the strength of the best riders momentarily cannot be nullified by drafting. That’s when they make their moves.
Magazine articles have been written about the merit of drafting, paceline schemes, and even, ironically, the unwritten rules of sheltering on a stranger’s wheel.
But I’ve read nothing on that most basic requirement: how to find and stay in the draft. So here goes.
Assume a spherical cyclist in still air
Drafting is simple on a day with no wind. Riding in line behind the rider in front, you gain more benefit the closer you get to the wheel.
But the closer you get, the greater your risk of a hospital stay. Touching wheels often means hitting the ground, if only for the following rider. The leader, whose rear wheel cannot steer, stays upright. Such is the swift, tough, and fair justice of the road.
A larger leader pushes more air out of the way. But if the leader is too tall compared to the follower, the best draft swirls uselessly above the follower’s torso. This is not always apparent to the follower, because the head is both the wind detector and the highest part of the rider. So when following a very tall rider, your head may be in the draft created by the bulk of their body, but your torso, hips, and thighs may be too low for the best draft.
Nonetheless, the draft does narrow and dip as the air closes behind the leader, so the leader should be a bit taller than the follower. But built like a barn door. Broad-shouldered. Fat. Tree trunks for legs.
You can read a cyclist by his legs.
[…]
The slender leg, with a little bitty calf, belongs to the climber. The good-sized thigh – that’s a sprinter. The long and harmonious legs – a rouleur (a racer on the flats). A short femur? He’ll be swift. Rounded buttocks? He’ll start strong. Slender ankles and knees mean class.
Contrary to what’s usually claimed, fat calves aren’t all that useful: the essential energy of the cyclist is concentrated in the back, the buttocks, and the thighs.
So wrote Paul Fournel, this time in Need for the Bike (Besoin de vélo in the original French). Fournel had much to say about drafting too, because “the bike is the school of the wind.”
The role of the bicycle in drafting
You can’t do much about the size of the rider in front, but you can ride a bicycle with a low bottom bracket. A lower bicycle is a net win for drafting, especially if you are tall or cannot hold a racing crouch for long.
Also beneficial is a frame with a short front-centre: the distance between the bottom bracket and the front axle. That gets your body closer to the rider in front for any given safety gap between the two bicycles.
A smaller front wheel achieves the same result. Annie Leibovitz photographed six-time world champion Rebecca Twigg on such a funny bike for Vanity Fair in the run-up to the 1996 Atlanta Games. Twigg specialised in the individual pursuit, a solo discipline that hardly benefits from the small front wheel. The team pursuit, however, gains from bicycles with short front-centres and short chainstays that pack team members closely together in the paceline.
Contrariwise, long chainstays reduce the benefit others get from sitting on your wheel. Keep that in mind the next time you hear someone say a crit bike should be short.
In summary, for best drafting, choose a bicycle with:
a low bottom bracket
a short front-centre
a small front wheel.
And for best drafting as a member of a team or group:
short chainstays
small wheels.
Or, in a race where you don’t want others to benefit from your draft:
long chainstays
a large rear wheel.
Not that we really have a choice of wheel size any more. Moultons as racing machines are dead, although the world speed record in an upright riding position remains the 82.5 km/h set by Jim Glover in 1986 over a flying 200 metres on a faired Moulton. Today’s UCI demands “two wheels of equal diameter”, ruling out funny bikes, and small wheels have higher rolling resistance – a hot topic of optimisation today.
Besides, benefits from bicycle design are small compared to the enormous gains of optimising your drafting position.
Finding the draft in a crosswind
A crosswind subjectively blows from a three-quarters angle instead of head-on.
The resulting yaw angle isn’t simply the difference in your heading and the heading from which the wind blows. It also depends on the wind speed and your ground speed, as Jo Burt again illustrates:
(Just kidding about Jo Burt.)
If you tied a balloon to your bicycle with string, the balloon would point exactly opposite the subjective wind direction.
Average yaw angles are lower than those that were used to justify the introduction of toroidal-bulge aero rims several years ago. Still, yaw angles are rarely zero. So for optimum drafting, you must usually ride at an offset to the rider in front, as rider 2 demonstrates here:
The difficulty lies in finding this optimum offset.
Why not move around and simply feel through the pedals whether the draft has improved? Because your substantial inertia removes any immediate need to pedal harder or softer, leading to a long lag in perception. And the faster you are riding, the greater is this lag, since your kinetic energy equals ½mv² or half your mass times your velocity squared. The squared term is why it takes several hard pedal strokes – almost a sprint – to halt a widening gap at 50 km/h, whereas at low speed an extra nudge on the pedals adds the necessary energy.
For example, consider a cyclist with an all-up mass of 80 kg riding at 49 km/h (13.6 m/s). Their kinetic energy is 0.5 × 80 × 13.6² = 7410 joules.
At 50 km/h their kinetic energy would be 7716 J, so the cyclist has to add over 300 J of energy (that’s an extra 300 watts for 1 second or 100 W over 3 s) merely to go from 49 to 50 km/h – and that’s over and above the incessant, great opposition of the universe to any cyclist travelling at such speeds.
By contrast, adding the same 1 km/h at 19 km/h to take speed to 20 km/h would require a kinetic energy increase of just 120 J (or 120 W for 1 s) for this cyclist: barely a third of the requirement from 49 to 50 km/h.
The important point is that this works in the other direction too: at high speed, when the draft improves or deteriorates, the change is masked by the large energy surplus or deficit that must accumulate before your speed changes noticeably. That is, the gap to the wheel in front does not change rapidly, and so you are not immediately required to pedal harder or softer to maintain position, and therefore it is difficult to detect the change of draft at the pedals in time for it to guide your positioning on the ever-changing road.
This longer delay in response as speed goes up exacerbates the elastic-band effect of the peloton precisely when those at the back most need a good draft: when the pace picks up. This too is the justice of the road.
This is why you have ears
I said earlier that the head is the wind detector. More specifically, the ears do that job, because the rush of air is noisy: white noisy despite its dark violence.
There are two ways to use your ears to draft. The first is by comparing the noise in each ear. You tilt and rotate your head in clean air until it points directly into the subjective wind, which you know has happened when both ears hear equal volume and character of wind noise.
It may help to imagine your aero helmet as a weathervane that must point directly into the wind. Adjust your neck angle in two planes to achieve this. It’s not enough to point the top of your head into the wind, because the helmet then interferes with the airflow over your ears. You must look up while rotating your head left or right.
I confess I have sometimes pretended to look at something beside the road while doing this. Now you know all my secrets.
The other method, which is more useful because it works in the dirty air of the draft, is to monitor the overall volume of the wind noise as you ride along, without needing to move your head. If the wind gets louder as you drift to the left or right of the rider in front, you’re losing the draft. If it gets quieter, great. Hunt the quiet!
One more thing
But wait a second. That’s your head. The bulk of your body is behind your head, and you must take this into account when listening to the wind.
The best place for your head is often at the front of the drafting bubble, just emerging from the draft into the terrible wind. That position ensures your bulkier shoulders, torso, hips, and thighs are in the best draft behind.
Sometimes, in bad crosswinds, your head must be entirely outside the narrowing cone of the draft in order to prioritise the rest of your body. This is a psychologically difficult place to be in a psychologically difficult enough scenario. So? The road favours the brave.
Below, rider 4 is positioned in the quietest part of the draft according to his ears, but that isn’t the best draft for his body. In fact, the wind is cutting straight through his body. Be careful out there.
Rider 6 has her head bumping into the noisy wind beyond the draft, but that ensures her body is tucked into the best draft. Good job.
Deaf cyclists can’t draft, maybe
Some alarming studies have suggested that long exposure to the wind noise of cycling may damage hearing. There are even products to reduce this noise. May I suggest putting another rider between yourself and the wind instead? A nice side effect is saving a hundred watts.
A friend once mentioned that he finds the best draft by detecting changes in temperature. I presume the observed difference is not strictly temperature but the perceived temperature after the wind-chill factor. From this I conclude that deaf cyclists might have a chance after all.
But those who have ears to hear, let them hear.
Set gadgets to show north-up for situational awareness
IRL we steer bicycles. Consequently, the wind comes from a different direction after every turn in the road.
To help with these constant wind calculations, I set my Garmin map orientation to north-up rather than track-up. This makes it easier to mentally superimpose the prevailing wind’s known true direction on my track after the next corner.
Yes, this does mean I might be riding down the screen and turning left on the road when the pink line turns right on the screen. What can I say? I’m so talented I can flip a map in my head while chewing gum.
The point of all this is really only apparent when riding in a group. You then need to know to which side of the group you should drift before the next turn. Let someone else ride in the wind.
Conversely, if by error you find yourself pulling at the front, use your situational awareness to put the riders behind you in a safe spot on the road or in the gutter, depending on whether they are friends or foes.
Beautiful, beautiful echelons in stage 14 of the 2018 Tour de France.
If mental trigonometry at VO₂ max sounds tricky, check out some of the Garmin or phone apps that live-update the wind speed and direction for your location as you ride. Presumably some of these apps take the next obvious step and display your subjective wind direction with an arrow on the screen, depending on your ground speed. But I haven’t tried them. And they cannot fully replace your ears, because local topographic features influence wind direction at ground level.
When to take a break from thinking about the wind
All of this listening, calculating, drifting around the group, and drafting is hard to sustain non-stop. So when should your legs work harder to give your mind a break?
The answer lies, as so often, in the formula P = Fv or power equals force times velocity. That is, a cyclist’s power is equal to the force opposing their movement – mainly aerodynamic drag – multiplied by their speed.
So if you feel the wind tugging your moustache while climbing a steep hill at 10 km/h, don’t worry too much. The force of the wind is robbing you of less energy than you might think.
But if you’re doing 50 km/h and the same blast of air hits you, the appropriate response is panic. The same wind force in your face is now responsible for 5 × greater power consumption. Whoa. You desperately need to get out of the wind by any means possible. Tuck harder unless your chin is bouncing off the handlebar, find a wheel to follow, draft a bin lorry, anything.
It follows that the time to relax your guard is not simply when the wind feels tolerable but when your speed is low. Conversely, drafting and aero efficiency remain important in a tailwind or while pedalling down a hill.
In other words, take a break when your ground speed is low. Your air speed lies about power consumption.
In summary
follow the wheel closely. A wheel’s length is fairly safe after 10,000 hours of practice
draft someone a little taller and much wider than you if you have the choice
find the yaw angle by listening to the wind
ride at the offset that matches the yaw angle and your following distance
in strong crosswinds, account for your ears being ahead of your body
draft more diligently at higher speeds, because the power consumed by the same drag force is greater at higher speeds
choose a bicycle with a low bottom bracket the next time.
Ask Cecil A Throckmorton
Dear Cecil: How tightly should I screw the preload cap on my Hollowtech II cranks before tightening the crank bolts? Shimano says 0.7–1.5 Nm but my torque wrench doesn’t go that low.—Madison
Dear Madison: I assume you are using the Shimano TL-FC16 tool, below, which is about 40 mm in diameter. That diameter was carefully chosen to feel natural and comfortable at the intended torque. Shimano’s torque specification works out to about 5 kg of tangential force at the circumference of this tool, shared between your forefinger and thumb. I would describe that as firm finger tightness.
Imagine closing a bottle of Coke to put it in your bag, knowing you’ll want it again in a minute. You’re not cranking it down to put in the fridge overnight. But you don’t want it to spill, so you give the top a good firm tweak. That kind of tightness. Hold the tool between the pads of your forefinger and thumb and tighten firmly. You probably won’t need to use the heel of your thumb.
The large mechanical advantage of the cap’s greased threads converts this light torque into an axial preload of about 500 N or 50 kg of force. Greased plastic? Yes please:
Make sure the spindle and its splines, the mating splines in the left crank, and the bores and faces of the bearing ‘top hats’ are clean and greased too. And use your torque wrench on the clamping bolts in the left crank.
∞
Dear Cecil: How tightly should I screw the external bearing cups for my Hollowtech II cranks into the frame? Shimano says 35–50 Nm but my torque wrench doesn’t go that high.—Caleb
Dear Caleb: I assume you are using a standard wrench that puts your hand ~200 mm from the bottom bracket. Pull the handle up with the force you would use to lift a heavy suitcase off the floor. To facilitate this, position the handle so that it ends up horizontal at the end of the stroke.
This is tight enough that you might grunt if no-one was watching, but not so tight that you risk hurting yourself if you’re leaning over the saddle with the bicycle resting on its wheels. Which is a reasonable position to adopt for the job. Leave the repair stands to bike shops.
Greased threads ensure this torque generates a high force against the frame’s bottom bracket shell, thereby resisting creaks. Creaking is proof of motion that erodes the threads over time.
Greased threads? I know they come from the factory with blue or grey gunk already applied, but yes please:
∞
Dear Cecil: I’ve been road riding for 15 months and now every ride feels the same. Something has to change. What do I need to try gravel?—Anon from Portland
Dear Anon: Loose aggregate is strictly optional, but you will need $500–600 of custom frame bags and a non-standard bottle cage on the other side of the down tube. A hiking GPS is more soulful, especially if it’s powered by a hub dynamo. Just make sure the hub isn’t a Shimano. Those are for German commuters.
∞
Do you need advice, technical or existential, about a cycling matter?
Send your question to bicyclingage@gmail.com with Dear Cecil in the subject line.
Mr Throckmorton regrets that he cannot answer all questions and may edit others for any reason whatsoever.
Also
The fourth edition of Bicycling Science is now shipping. It has “new information on aerodynamics, rolling drag, transmission of power from rider to wheels, braking, heat management, steering and stability, power and speed, and other topics”. A copy will join my first three editions soon. Maybe I’ll attempt a short review.
The 14 April Nerd Alert Podcast, “Tires for Roubaix”, had a fascinating interview with Jan-Niklas Jünger, a Continental product manager with high technical understanding, about the state of bicycle tyre tech today. From 33:55 to 1:08:49 and well worth your half hour.
Suggested listening: Richard Thompson – I Ride in your Slipstream
In recognition of the torque questions above, issue #3 of Bicycling in the Age of Shopping will discuss using a torque wrench properly.
Know someone who might find this interesting? Send them to https://bicyclingage.substack.com/about.
Ride bike!
Samuel