Is cycling behind a truck a bad practice? 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.
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 &quot;Cars will generally stop faster than bikes due to their more efficient brakes and lower centre of gravity,&quot; 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.