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Sometimes on a highway outside my city, Nairobi, I have been passed by fellow cyclist cycling behind trucks. As much as it is similar to holding onto the back of the track this is different since you are basically pedaling either faster or slower to keep up with the speed of the truck while watching its brake lights and indicators just in case it brakes so that you can brake too.

One time I tried this when I was on a fairly steep climb with a relatively fast truck. It was fun trying to maintain my speed up the hill and towards the end the truck was accelerating. I was so obsessed with keeping the pace on my road bike till I passed the place I was to branch off the highway by like 3 kilometres. When I narrated this to a friend a fellow cyclists he laughed at how I had managed to cycle at 40 kph for over 30 kms. Though I didn't feel the strain immediately but there was an effect later and I had to keep practicing daily to remain fit though these days I just do 25 kph.

But my friend warned me that if I was going far and the truck accelerated faster and left me behind or it branched off too soon than my destination I would be exhausted drastically battling with the wind. Anyway personally I feel like I can cycle behind a truck as a way to train. I don't know what others here on this platform would advice me on this.

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    Note that the winner's average speed on the tour de france is 40kph. The shortest stage last year was 117.5km. So, when lockdown ends, we expect to see you there. – Strawberry May 9 at 18:01
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    @Strawberry 40kph is not crazy for a group ride, so doing it behind a truck is presumably achievable, meanwhile OP reports doing 25kph solo, which is fine and perfectly normal. No need to disparage OP – Swifty May 9 at 18:20
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    @Swifty No disparagement here – Strawberry May 9 at 18:21
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    @Strawberry my apologies, though it's easily misconstrued – Swifty May 9 at 19:51
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    It's only a bad practice if you don't want to flatten your face into the back of a truck. – Richard May 11 at 10:14
38

It is really dangerous to follow a large vehicle closely. It is possible behind a directeur sportif in a passenger car which is smaller: you can see through it, and the driver is a cyclist who knows you and knows about you.

You cannot see what is coming in front of the truck. If they choose to brake hard, watching the brake lights will be of no use; you have simply no chance to brake so fast as many cars can (the truck can be unloaded or transporting light goods). You do not see what kind of obstacles or potholes are coming, how the truck will avoid them, and so on.


Note that I was assuming a road bike without any front suspension fork.

Ride On says "Cars will generally stop faster than bikes due to their more efficient brakes and lower centre of gravity," and they indicate a bicycle stopping distance of 31.5 m from 40 km/h. However, take into account that this is from a prepared rider who knew he had to slam the brakes, not just noticing a brake light. A passenger car will stop at something like 26 m.

Even for motorbikes it is dangerous, and the stopping distance can vary with the experience of the rider by a factor of 2. [1, 2]

And of course, even a car tailgating another car will often crash into the preceding one. The two-second distance is often required by law. That is 22 m at 40 km/h and 28 m at 50 km/h. That is not compatible with drafting behind the vehicle in many, if not most, traffic rules.

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    I had a mate follow a truck on one of our regular training rides some years back. He and the truck disappeared up the road ahead of the rest of us. Speeds would have been 50-75km/h. A few minutes later we found him unconscious, bleeding from the head and underneath the stopped truck. He was very fortunate to survive after a stay in hospital but he was close to dying that day. It might have been a famous scene in the movie "Breaking Away" but it's a very risky thing to do. – alexsimmons May 8 at 20:25
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    Bikes don't brake anywhere near as quickly as cars. Put some numbers in this page – Fred the Magic Wonder Dog May 9 at 0:03
  • @FredtheMagicWonderDog I intentionally did not base my answer on that as there are so many factors that can be disputed. I do not know if any study compared bicycles and trucks. The stopping distances in controlled tests may be comparable and highly variable (truck load, ABS, type of bike, rider skill). If you know of such data, you can enter your own answer. – Vladimir F May 9 at 7:20
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    "The two-second distance is often required by law." When I took Driver's Ed class, I was taught the 2 second rule. Decades later I became an instructor and later worked for the same Driver's Ed training company that taught me decades earlier. By then, they had switched to teaching the "4 to 6 second" rule. – TOOGAM May 9 at 13:06
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    I would put more emphasis on debris/potholes/etc... If there's a branch, loose rocks, etc... on the road the truck will just plow right through without any indication that something is coming. Any unlucky cyclist behind will hit those obstacles head on, with no chance to react as they appear from beneath the truck. It's a guaranteed accident, and given the speeds involved, it will hurt, or kill. – Matthieu M. May 11 at 14:14
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I once tried to cycle behind a truck out of sheer exhaustion and curiousness to see what it feels like doing that. It was easy at first since all you had to do was wait downhill for a truck to come by. I had seen other cyclist do it and others even hold onto the back of the truck and get pulled by the truck uphill.

So for my case I just wanted the truck to be my pace setter so I could climb the hill with motivation. Towards the end of the climb the truck was gaining speed and I was sort of lagging behind. Another truck was behind me too and it was catching up with me and I realized I had to give way. Before I could do so a car was overtaking the truck behind me when a bus appeared from the other side. I would say it was my lucky day because all I remember was waking up in a hospital bed with shocked faces of my parents. Me and my bike survived but the lesson of not risking cycling behind a truck was big. I can't think of a better answer than that.

Overtaking vehicles can crush on you when they are confronted by another oncoming vehicle before they can overtake fully the vehicle ahead of you. Drivers are known to sandwitch their vehicles esp cars between trucks when there are many trucks following each other. The driver will see you when it is too late to evade you.

Another important thing is that you may be arrested by highway patrol traffic police over endangering yourself and other road users. I don't know if you have such a thing in your locality.

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    Sorry to hear about your incident. It sounds like you were "taking the lane" quite legally, and another car failed to manoeuvre safely. The fact you had a truck in front of you seems circumstantial. I do hope the car driver was charged for causing your injuries. – Criggie May 10 at 10:14
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    Here in Africa we have many hit and run incidences and am sure mine was one of them because there was no trace of the car driver. They said I was found by the wayside unconcious – WaSiro May 11 at 6:49
7

This is called "Draughting" or "Drafting" depending on your locale, when done as described. If done on a track it can be motor-pacing or similar. There are even world speed records for bikes that are shielded behind a vehicle.

Like anything in cycling, safety is relative.

You can choose to ride behind a large solid vehicle, knowing that:

  • This may be illegal in your location, by your local road laws as "following too close" or "tailgating"
  • your vision is obscured and your line of sight is shortened
  • Your reaction time is minimised
  • your visibility to other road users is almost zero from some angles
  • you're breathing really hard right right behind a vehicle's exhaust
  • if you fall for any reason, any following vehicle may not be able to avoid you
  • at significant speed, the vehicle can flick up road debris faster than you can react.

On the positive side:

  • the "assist" is significant - you can often go much faster for the same effort.
  • the shape of the vehicle can provide shelter from wind and rain. Busses and square trucks tend to be best. Vans are middling, and cars are mostly useless.
  • There's a motivation to "hold the wheel" which can feed the desire to stay in the wind shadow. If there's an undulating road, the tow vehicle will slow for climbs and corners, and possibly get away from you on straights and downhills. So you push extra hard to stay in place.

It's up to you if this is cheating - in pro sport its absolutely cheating, unless the tow vehicle is another cyclist in which case its "tactics" or "racecraft".

If you get a Strava PR or KOM with this technique - you're still riding the bike and making a big effort, so it's not really any different from a powerful tailwind.

Here's someone clearly draughting on Strava. Is it fair?
enter image description here

As for training - it's not a free ride. You have to be super-attentive and you're riding really hard to stay with the tow car. So yes that's sprint training. It's not interval training or similar. So there's no more training here than a normal hard ride. However it is practice for riding at speed - good luck taking a drink at these speeds!

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    And if the driver has to slam the brakes for any reason you'll end up against the rear end. – Carel May 8 at 13:24
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    Thanks for that insightful answer. I really appreciate the effort you put up answering me this way. – JacksiroKe May 9 at 1:56
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    @JacksiroKe don't get me wrong - I'd totally draught behind a bus if everything were right. But if one thing were wrong (dense traffic, rain, late, getting into dusk, I'm tired, etc) then its not worth doing. Vehicles tend to accelerate faster than bikes, so it can be hard to keep up with surges. It might be more practical for you to try group rides - a paceline is a safer place to try these things, and with fellow riders you're all on the same wavelength. – Criggie May 9 at 4:09
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    Buses have a couple of advantages over trucks: a big wind shadow, helped by going low to the ground, and a rear engine, so you can hear when they're about to brake. Of course there are bigger trucks but you can feel the wind coming underneath. It's still rare to be able to take much advantage. – Chris H May 9 at 14:00
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    I'd say personal records are fine but king of the mountain with draughting is cheating. It's like wight lifting on the moon. "What's your problem? I still pushed 200kg!" – Peter - Reinstate Monica May 11 at 7:41
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This is an immensely dangerous practice. The kinetic energy of the smallest car is easily capable of killing a human. A truck would be able to instantly kill a cyclist even at a crawl, let alone at higher speeds. Many cycle deaths in London have been due to cyclists being too close trucks. Truck drivers do not always have the best visibility around the truck or at the back, so this adds to the risk of cycling anywhere near a truck. A bicycle and a truck is an extremely toxic and dangerous combination.

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  • A truck can have blind spots that hide cars. I've known multiple people that had a truck clip their passenger vehicle. I've also heard from truckers that they sometimes can't even tell their trailer hit something until they stopped and saw the scrape marks. – Nelson May 11 at 8:39
  • Surely it is the kinetic energy of the cyclist that is the problem – and quite bad enough – when the driver slams on their brakes and the cyclist crashes into the vehicle? – PJTraill May 11 at 20:06
4

Long ago I used to ride a small relatively underpowered motorcycle (usually 1960's or 1970s 175cc Jawa) about 70 miles between two cities, typically one return journey a week*. Top speed on the level was maybe 50-55 mph. Large trucks would "illegally" exceed the motorcycles speed.

On a number of occasions I tried 'slipstreaming' behind these trucks.
The gains are significant - you can back well off on the throttle, the bike feels much more comfortable and it's warmer (and smellier if a cattle truck). Also - following distance needed to be 1 to 2 metres and the utmost of vigilance was required to stay in the slipstream without hitting the truck. It was hard enough to do that I inevitably lost contact after a few minutes and the truck would pull away, uncatchable.

People here have mentioned relative braking deceleration of bicycles and trucks. Even with superior braking reaction time is an important issue. I tried plugging in various decelerations and reaction times into standard equations of motion and it suggests that at bicycle speeds and typical reaction times and following distances, the risk of rear end impact should be small. However, alexsimmons gives a real world example of a friend being hospitalised, and I was always completely "on edge" and aware that I was dicing with danger, or death.

As well as the rear impact risk when reaction time is adequate, you will often be unlikely to be able to see the truck brake-lights clearly, or at all. If you are out on the edge of the truck where the brake lights are visible you may also be visible to the truck driver. This is a "two edged sword" - some drivers will take extra care if they are aware you are there, while others, sadly, may actively try to trap you. Add to this the other risks of pot holes and unseen objects suddenly "appearing" and it adds up to being a significant;y dangerous activity.

If you like to live dangerously, don't mind dying young and can tolerate being hospitalised with a range of possible damages, then this may be "fun" to try. Most people's risk - reward metrics would suggest that tail-gating trucks is lots of fun but not worth the gains.

____________________________________

*After some while the young lady who was a significant factor in my making the occasioned weekly visits moved to my city and the visit rate dropped and we usually went by car. I survived the tailgating experiences and we have now been married 45 years.

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No,you can get arrested, hurt and disrespected [that is, increasing the level of overall disdain and disrespect that many drivers have for cyclists.]

Never try or even think about it.

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  • Upvoted for "disrespected." – barbecue May 9 at 19:44
  • Welcome to Stack exchange - this answer has been flagged as low quality because its very short. Could you expand it using edit Consider covering points like "where in the world you might get arrested and for breaking what relevant law" and "how one might get hurt, how to reduce or eliminate those risks" and develop "what does disrespected mean in this context ?" "Never try" is obviously a statement from authority... please say why. You can learn more about how SE is organised by reading the tour – Criggie May 9 at 23:33
  • @barbecue do you know what is intended by "disrespected" I can't understand the use in this context; it may be regional. Could you please define and give a relevant example? – Criggie May 10 at 10:15
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    @Criggie Disrespected as in others having a low opinion of you, being regarded as foolish or stupid, and particularly with regard to increasing the level of overall disdain and disrespect that many drivers have for cyclists. A lot of hostility towards cyclists and cycling infrastructure comes from the actions of a few cyclists who flout the rules of the road, making it harder for all cyclists. – barbecue May 10 at 15:15
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The energy win is of no discussion.

In a lot of situations, it may be even safer to ride behind a big truck or bus. Where I live, car drivers tend to ignore cyclists, but are pretty much aware of the presence of a large truck or bus. The truck/bus driver stands higher and has better headlights so he can deal with the road situations better than a cyclist. He is usually sober and the cyclist sometimes is not.

The sudden braking is really not an issue - these vehicles tend to brake slower than the cars and they don't run at very high speed in the first place. Even if they stop (and you fail to), you will hit the vehicle at rather small speed difference. Not good for sure, but pretty much better than colliding with a standing object.

The only thing I fear when I tailgate a truck is some object on the road (stone, brick, hole) that the truck can run over with no problem, but I may not see it in time. Busses mostly have low enough clearance so their drivers avoid such problems better.

Exhaust fumes may or may not be a problem. These vehicles have their exhaust pipe either on the left or the left upper back (busses), so the fumes may not reach the cyclist at all. And the cyclist have a choice to follow another, more favourable vehicle. Road dirt and dust in a dry weather, on the other hand, ...


edit: p.s. as of popular demand, I should add that the practice of closely following a motor vehicle bears risks that are different and can be higher than the risks when cycling alone. The risks are even higher when you use the vehicle drag to cycle faster than you are used to (e.g. faster than you can cycle for an extended period on an empty level road).

Doing so on a lane that is not the rightmost that is used by the motor transport is not acceptable for me.

Use common sense! (Even if this is the narrow meaning of the "common sense" that is compatible with cycling on the public roads in your area.)

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  • Potholes appearing suddenly from under a vehicle are an issue even hanging back further. And it's all very well saying cars will notice the bus and not you, but if they try to pull out of a turning fast, right behind the bus because something else is coming, they're heading straight for you – Chris H May 9 at 14:10
  • @ChrisH No strategy is perfect. But in your case you still can see them. – fraxinus May 9 at 14:33
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    Exhaust fumes absolutely ARE a problem. A moving vehicle has a low pressure region immediately behind it where fumes can accumulate, and you're basically sitting in it. The same phenomenon is why RVs don't have rear windows, and why passengers are not supposed to ride in enclosed truck beds. – barbecue May 9 at 17:39
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    @fraxinus The most visible fumes are actually much less hazardous than the completely invisible ones. Some nitric oxides have color, but not all. Carcinogenic VOCs are not visible, though may have an odor. Road dust also floats around, which can include heavy metals, organics, asbestos, and more.But all of this pales next to carbon monoxide, which is deadly, but also completely colorless, odorless, and tasteless. Without a meter to detect it, the only way you know your'e exposed is when you start having symptoms like dizziness and weakness, neither of which lends itself to save cycling. – barbecue May 9 at 19:43
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    Dangerous advice. Sudden braking is definitely an issue - the danger is not only from impacting the truck, but also more from falling down and then being run over by the car (or worse: another truck) that may be tailgaiting you. And that "a truck driver... can deal with the road situations better than a cyclist. " may be true, but I don't see how this helps the cyclist. – sleske May 11 at 12:50
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I say its fine as long as the vehicle in front is not carrying any goods and you're keeping a safe distance/speed.

By safe distance & speed I mean the speed you use when going uphill, so that's about < 15km/h and close enough to be able to turn away from it if happens to brake suddenly.

About the goods, i see trucks here in Nairobi carrying pipes & sand/rocks etc. I wouldn't slip stream a truck carrying such goods in case they fall off.

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    15 km/h would give more time to react to hazards, however the benefit of draughting at this speed is minimal because the air resistance is low. It seems like extra risk, for low reward. If you keep a gap from the truck too, the benefit reduces further. Keeping a safe speed and distance from trucks is smart, but to me that means not draughting – Swifty May 9 at 15:18
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    Why would a rider bother drafting at 15 km/h? Most of us can cruise at double that without assistance. The point of drafting is to get an assist from the wind. Good point about the vehicle's load and safety. I'd also avoid vehicles with livestock because they tend to drop fluids. – Criggie May 9 at 23:39
  • I would do 15 kph if am acting a movie but on a normal occasion it's better at something twice that – JacksiroKe May 10 at 4:41
  • @Criggie, exactly. But: there's also the ancient technique of not only drafting but acutally grabbing a trailer (when I was a kid, it was sufficient concern to warn us not to hold on tractor trailers - meanwhile, tractors often go 60 km/h so the idea doesn't appear in the first place) – cbeleites unhappy with SX May 11 at 14:14
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    @cbeleitesunhappywithSX yep thats called "sketching" or "skitching" and is frequently illegal in various locations. – Criggie May 11 at 20:43

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