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This video was featured in a leading news website in my country. It's a cyclist riding on a highway with very high velocity - said to be 90 km/h - close behind a truck. The road is flat, so it keeps puzzling me: how could he reach and maintain such a high speed? Is it even possible to do this without any external help, e.g. grabbing the truck? Is it the "air-dragging" effect of the truck that makes it possible?

P.S.: I myself don't have too much experience with biking - I ride a mountain bike, my highest speed on flat surface was about 40 km/h, and I found maintaining it pretty exhausting. Couldn't have kept it for more than a couple of minutes.

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    About 100 years ago a cyclist achieved 100 mph riding behind a train. – Daniel R Hicks Dec 12 '17 at 17:55
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    It seems like if one wished to show how fast one can ride a bike without air resistance, a treadmill would be an even better way (although of course not as dramatic). – Acccumulation Dec 12 '17 at 19:48
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    @DavidRicherby that's one hell of a set up. Only 400x the earth's escape velocity. – Brad Dec 12 '17 at 21:24
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    @axsvl77 I used to be a cyclist like you, but then I took a pedal to the knee. – Aaron Dec 12 '17 at 23:39
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    How has Guy Martin not been referenced yet? 110mph, British bicycle speed record! \\ youtube.com/watch?v=byZTGiWQzd8 – Sam T Dec 13 '17 at 9:39
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In my younger foolish years I actually did this exact scenario, except at a slightly lower speed 85 kph (speed limit was 80 km/hr), but for a distance of nearly 10 km (between two suburbs). In my case I drafted a double trailer gravel truck which was very slow to accelerate making it easy to keep pace. This was only possible due to the massive low pressure wake at the rear of the vehicle.

I was on a road bike with a max gearing of 53-11. When we were at speed I remember spinning like crazy to keep up, typically I would have to do it in pulses where I would bridge up a foot or two to secure a better position in the draft, then take a quick rest. Adrenaline really keeps you motivated in this type of situation. I even remember a few cars driving up beside and staring in amazement or disbelief - I am not sure which one.

The road is flat, so it keeps puzzling me: how could he reach and maintain such a high speed?

It really depends how fast the vehicle accelerates. It needs to accelerate slow enough so that you can keep pace and remain in the draft. At the time I had no trouble hitting 50 kph on my own for short periods so I had the acceleration needed to keep pace until the draft really started to kick in. At higher speeds it is amazing how much the draft really sucks you along, you really don't need to generate crazy power, just have enough leg speed so that you can keep pace in your largest gear.

At 80+ kph you need at minimum 53-11 gearing. Even then I could barely stay on top of the gear, I had to spin incredibly fast. I had a bike computer at the time, but I never checked the cadence, just my speed which was 80-85 kph. At that time (late 1990's) you didn't have many data loggers, which record every moment in detail for posterity like we do today, so I can't offer any more detail than this.

Is it even possible to do this without any external help, e.g. grabbing the truck? Is it the "air-dragging" effect of the truck is what makes it possible?

As long as you keep close to the back of the truck the draft literally sucks you along. Once in the draft you really don't need a ton of power, just the ability to spin fast so you can keep nudging yourself back up into the sweet spot of the draft. I tried to minimize my risk exposure by drafting to the side of the truck, so I had a shot of getting over to the shoulder quickly if the truck suddenly braked. This meant I lost a bit of the draft, but given the insanity of the situation I felt it was a reasonable compromise. Others seem a bit more brave drafting in the dead centre (pun intended).

WARNING: All this is of course incredibly foolish and could quickly descend into disaster. At the time I was young and clueless.

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    I wanted to add that at the time I had also been doing a lot of motor drafting as part training so I was practiced at pacing behind vehicle in the draft. – Rider_X Dec 12 '17 at 21:27
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    Trucks and buses often accelerate at a comparable rate to a cyclist from a standing start, especially when heavily laden. A heavy load also means they can't stop as suddenly as when empty, which is probably a good thing – Chris H Dec 12 '17 at 22:31
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    There's a stretch of road on my morning commute where heavily laden trucks often have to accelerate from a standing start up to the speed limit of 70kmh, providing me with ample drafting opportunities. The big danger is you don't see the massive piece of debris or pothole that the truck drives over until you hit it. But it does provide a hell of a rush on your morning commute. – stib Dec 13 '17 at 2:10
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    Now imagine what would happen if there are any unexpected bumps/dents in the road… – Display Name Dec 13 '17 at 16:42
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    I, likewise, was young and foolish. I drafted a dump truck at about 35 MPH with a stack of cars behind me. I extended the foolishness to the point where I decided I could pass him. I got about 3/4 of the way up the side of the truck before the headwind hit. I was just dumb enough to finish the pass, then immediately found a place where I could get off the road without hitting the brakes (figuring he'd just run me over for being stupid), and pulled over. I lay in the grass for about 20 minutes recuperating, before getting back on and slowly wobbling home. Do NOT try this at home!! stupid – FreeMan Dec 13 '17 at 17:13
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It's all in the air resistance. Flat out is not much more than 50km/h for me on a deserted road. Even 10 metres behind a bus it's much easier (but you still have time to react to braking). I actually found this out when I changed lanes into the slipstream of a bus while riding quite fast and found myself seriously gaining on it.

Entering the zone is quite a surprising feeling when you're not expecting it. You can close the distance betwene the vehicle and you really quite quickly until all you can see is the back of it, without a good view of its brake lights or the road surface appearing from underneath. This leaves you very vulnerable to sudden braking (the driver doesn't know you're there) or road hazards appearing from under the truck (even a shallow pothole hits you quite hard at 50+km/h when you don't have time to even unweight the saddle).

Riding very close behind a vehicle that stops the wind from slowing you down makes a huge difference. It's also dangerous and illegal on the road. But in controlled conditions with communications between the driver and rider, it's possible to beat 100mph (170 km/h) as was done by former motorbike racer Guy Martin when he switched to riding proper bikes.

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    +1 for "proper bikes". Or, "pushbike" as he puts it in his books. – FreeMan Dec 13 '17 at 17:17
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We can perform a few checks to see if 90 Km/h is plausible.

The Wikipedia List of cycling records page states that speeds well in excess of 200km/h have been achieved with a motor powered vehicle with a fairing travelling in front of the bicycle. That tells us that a human can power a bicycle to those speeds on a level surface if air resistance is removed (or greatly reduced).

The rider in the video seems to riding a regular road bike. What cadence would be necessary to travel at 90 Km/h?

Using this calculator assuming 53t chainring, 11t sprocket, 622mm rim and 28mm tire the cadence is ~145rpm. As far as I know that is possible for a reasonably well conditioned rider.

The other thing the rider would have to be able to do is accelerate at the same rate as the truck up to 90 Km/h.

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  • From experience, 145 rpm is difficult to sustain for any length of time, but not impossible. The video quality makes it a bit hard to tell how fast the rider is pedaling. It appears they are periodically coasting, probably to maintain separation from the rear of the trailer. – Tim D Dec 12 '17 at 19:35
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    @TimD - There's nothing in the "rules" saying that you have to use a standard chainring. The bikes I've seen pictured for such record attempts had enormous chainrings. This one has a chainring I'd estimate at about 80t. (And the rider is doing well in excess of 100 rpm.) – Daniel R Hicks Dec 12 '17 at 20:13
  • @Daniel R Hicks I think the OP is asking about the rider in the video specifically, his bike does not seem to have an oversized chainring – Argenti Apparatus Dec 12 '17 at 20:32
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    While I'm familiar with some rules regimes for some forms of land speed racing, I'm not familiar with any rules governing cycling speed records. Not saying there are none, just that I'm unaware of them. Nevertheless, nowhere did I suggest that a particular chainring size was required, or even typical. Certainly were I trying this I wouldn't use a 53T... My comment was on the feasibility of sustaining a cadence of 145 RPM. – Tim D Dec 12 '17 at 20:35
  • @ArgentiApparatus - I never saw a clear image of the chainring in that video. – Daniel R Hicks Dec 12 '17 at 21:15
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Nearly all the power required to sustain a given speed on a bicycle moving through still air is used to overcome two sources of resistance:

  1. Rolling resistance (call it Fr)

  2. Aerodynamic drag (call it Fa)

The rolling resistance depends on a number of factors, including the type of road surface, the type and size of the tires, and the inflation pressure of the tires. Fd is largely a linear function of speed, with a shallow slope.

The aerodynamic drag, by contrast, is a cubic function - it rises with the cube of the speed, but is also a function of air density and temperature, rider size, and rider position on the bike. Above about 20 kph Fa tends to dominate and becomes by far the most significant contributor to the required force.

A fit recreational rider might be able to sustain 200 watts and 30 kph for tens of minutes, whereas a Pro Tour cyclist using performance enhancements might be able to sustain 350 watts and 45+ kph for hours at a time.

In the video, the rider is riding in the low-pressure wake of the tractor-trailer rig. The air surrounding the rider is moving at a speed close to that of the trailer, meaning the rider is moving through the air stream at a significantly reduced (air) speed. The force required to overcome Fd at 90 kph is thus greatly reduced, to the point which allows this rider (about whom we don't know much) to maintain about 90 kph.

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    Close. Drag is a force. Rolling drag is independent of speed; aero drag varies with the square of speed. That means the power to overcome the drag force varies as force * speed, so the power to overcome rolling resistance is linear with speed and the power to overcome aero resistance is cubic with speed. – R. Chung Dec 12 '17 at 19:49
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    @R. Chung is correct: aerodynamic drag is CdA × ρ × v² / 2 (see this answer) – Argenti Apparatus Dec 12 '17 at 20:18

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