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There are countless videos showing BMX/freeride/downhill etc. riders jumping and dropping from heights. For the unprofessional spectator, they seem impossible to survive. From the physical point of view, the bike reaches the ground with a given kinetic energy which is dependent on the height of the drop and on the combined mass of the rider and the bike. Where is all this energy dissipated? I assume that most of this energy is absorbed in the bike, and some by the rider. How is this energy distributed in various components of the bike?

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3 Answers

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Landing big jumps is all about dissipating the inertia that pesky old gravity has created on your trip back to earth. The better you dissipate that inertia, the better the chance that you wont kill yourself.
There are several factors in play here:

  • The Transition of the Landing.
    The landing is almost always sloped downwards. Combine forward momentum with a downslope and you've got a significantly softer landing than you would landing on flat ground. That's one of the major reasons you see riders get hurt when they overshoot a landing transition.
  • Suspension.
    A little bit of suspension goes a long way, and a lot of suspension goes even further. The amount of impact damping that even an inch of give will provide is enormous. Think about it in terms of what it feels like to fall backwards onto a mattress vs falling backwards into tall grass or snow, distance being equal. The give of the soft surfaces decreases your velocity and dissipates your inertia which is what keeps you from cracking your head open as you would on concrete. Big-hit bikes can have 8 or 10 inches (or more) of suspension travel. That's a lot of travel to dissipate an impact. Even the tires provide a little bit of squish which is especially important for BMX riders (and it's also why rigid mountain bike riders tend to run larger volume tires).
  • Proper Landing (how you leverage your bike and the bike's suspension)
    Notice how most riders will land rear wheel down first, especially on flatter landings which are common in trials and street BMX. This further reduces impact because the rider can, in a way, leverage the give of the front and rear of the bike in succession. This is effective for both rigid and suspended bikes. By landing on the rear wheel first, you can use the bike as a lever of sorts - absorbing part of the impact and decreasing your body's velocity before the front wheel lands. This is achieved if your weight is balanced correctly by bracing against the pedals and handlebars. The effect is even greater for a bike with suspension. As the rear wheel hits, the rear shock soaks up what it can, then the front wheel comes down and the fork soaks up even more. Compare this to landing totally flat (both wheels at the same time) where the bike will provide roughly only as much suspension as the averaged travel of the combination of the front and rear shocks, or for a rigid bike, only as much as the tires would give (ouch!). Landing rear wheel first doesn't mean you get double the travel, but it certainly gives the bike more time to dissipate the force of the landing.
  • Knees and Elbows (even more suspension)
    Really, this includes most of your joints - any part of the body that can flex and move to soak up the impact. You don't see riders taking big jumps and landing seated. That's because they're using their arms and legs to take up as much of the impact as they can that the bike couldn't.

When you combine all of these elements in harmony you've got a really substantial amount of movement, and although it only takes a fraction of a second to land a big jump, it's enough time to slow down a rider's mass and keep them from becoming a grease spot at the bottom of The Tooney Drop.

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This does a good job of explaining mountain bike style drops, and the concepts are similar but applied slightly differently for BMX and trials drops. Using the same "proper landing" style but exaggerated these bikes drop significant distances with little speed onto flat landings. –  Glenn Jan 27 '12 at 21:05
Also it goes without saying joint is a part of big drops. –  Glenn Jan 27 '12 at 21:06
All are excellent points but one was missed. The bicycles geometry plays a big role too and is why we still ride the style of bikes we do today. The double triangle of bicycles was designed to absorb vibrations/impact from the road and be transferred away from the rider. When you work it out, the impact force is re-directed around the triangles and focuses the forces on the main tube welds more as compression than flex. The shocks/knees/elbows/tires and extreme laid back geometry of big hit bikes allow for much more force before you reach catastrophic failure. –  Chef Flambe Jan 27 '12 at 21:25
@Glenn edited to try to better explain how these factors work with trials and BMX bikes. –  joelmdev Jan 27 '12 at 21:28
+1 The Red Bull rampage is held in Utah not just for the terrain, but also because the soft soil at the bottom of the drops effectively gives an extra inch of travel, allowing for much larger drops to be performed. –  cmannett85 Jan 29 '12 at 10:19
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You mentioned KINETIC ENERGY, which obviously have to go somewhere. Sometimes you have reception, and the bike comes at speed, but sometimes, like in bike trial, the bike lands "flat" on plain concrete. Sometimes, too, freeriders land on flat concrete at speed, and at least the vertical component of the drop's kinetic energy disappears.

I would say there are only three places where this energy can go:

  1. Most of it is neutralized by the decelerating forces created by the rider. The more techique and style, the greater amount of energy can be absorbed. Usually it means extensor muscles performing an excentric contraction (applying a force while being stretched, so as to deccelerate/oppose the joint movement). This implies energy expenditure by the muscle cells, which come from food calories. If the drop is high, most trial riders prefer to land rear first, so they have more time to act the same force, and more muscle groups to act during each portion of the landing (this is very fast, and has to do with well rehearsed skills).
  2. In a bike with suspension, A LOT of kinetic energy might "disapear" inside the dampers due to high-speed viscous flow of oil, which increases the oil temperature. Modern big travel extreme free-ride suspensions have a lot of oil inside, working with lower flow speeds (larger bores, larger valve holes) so that the oil doesn't reach too high temperatures.
  3. Finally, the deformation of tire/terrain interface might absorb a lot of energy, and decrease the peak-decceleration (impact) of a landing. Good landing examples would be soft beach sand, grass, and some types of mud.

It is important to mention that rigid elements of bike (frame, wheels) don't take up the kinetic energy, only transmit forces to somewhere else. Also, just to add to what @jm2 said, the joints only transmit the forces and (fortunately) don't take up any significant amount of energy: the landing kinetic energy is couteracted by muscle contraction acting through the joint.

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As already stated by jm2...there are many reasons riders can take bigger drops. However as your question was how is it distributed...

Look at the swing arm for an example...vertical impact of the bike causes the rear lower stay to move upwards from the pivot point at the crank. That movement (force) is redirected to the upper rear stay and transferred to the shock that absorbs the majority of the force before finally transferring the last amount to the seat tube at a perpendicular angle to the rider not up through the rider.

That's why the full force isn't placed directly on the riders legs.

The geometry is what's splitting the lion's share out to 10 inch shocks which allow the rider to drop 20 feet without destroying the bike first and then being left with a much smaller amount of energy to suck up in his/her legs and arms.

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