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When riding a road bike with thin tires over small obstacles such as tree roots, there is a risk of damaging the tires.

Cyclists argue that rising off the saddle will all but eliminate the risk of rim damage.

Why does rising save the rims? Do elbows and knees act as shock absorbers?

Here we completely ignore the comfort of the rider. The rider is rising not to spare his body from the impact, but to spare the rims. Why does rising make any difference? The biker did not yank the bike off the road, hence the total weight has remained the same.

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2 Answers 2

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Here we completely ignore the comfort of the rider. The rider is rising not to spare his body from the impact, but to spare the rims.

Actually, it's not quite right to ignore the comfort of the rider. Think of this from the point of view of Newton's third law: if the rider's body doesn't get high impact, then it doesn't give as high an impact on the bike.

Then the question is, why does moving the body off the saddle reduce the impact on the body? It's because what we mean by impact here is the force: when sitting, the rider gets almost immediately the force from the ground, though the wheels, through the bike frame, and the saddle, handlebars and pedals. But when standing only on the pedals (and on the handlebars), legs and arms play the same role as shock absorbers in a car.

The main part of the body, which has the most mass, is suspended on the shock absorbers, and gets little force transmitted through them. This, in turn, means less force being given back to the ground—again through the saddle along with handlebars and pedals, then frame and then wheels.

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The answer to 2, is that the 70kg of the rider is decoupled from the bike mass which effectively pivots at the pedal crank, which is approximately midway between the wheels. Forces acting on either wheel simply pivot the 10kg of bicycle about the crank pivot, significantly reducing the effective moment of inertia.

If the crank was welded, not freely rotating, forces on the rider would be far higher as the mass of bike and rider would still be inertially coupled.

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  • $\begingroup$ Interesting analysis. You're the first to provide a hint that the rider should not only rise, but should also stop pedaling. If the rider continues to pedal, that only increases the impact on the rims. $\endgroup$
    – Sam7919
    Commented Apr 9, 2020 at 23:46
  • $\begingroup$ Well.......For freewheel bikes front wheel yes, for rear, no as the moment for rear impact will not create a reaction against the one way clutch of the freewheel. Even for front impact, the referred inertia of the rider via gearing and lever arm from bottom wheel to crank centre is probably a factor of 10 or so less than sitting down. A full analysis would include jerk, which is even more sensitive to inertia. $\endgroup$
    – cumfy
    Commented Apr 10, 2020 at 0:34
  • $\begingroup$ The answer is likely a combination of your and Ruslan's answers. Ruslan's description fits a dynamic analysis of the problem. Your analysis ("which effectively pivots at the pedal crank") fits a static analysis. I'm not sure how the two ideas blend, or whether they can. $\endgroup$
    – Sam7919
    Commented Jul 16, 2020 at 20:31
  • $\begingroup$ The key observation is that the bike and rider rotate independently about the crank bearing. Inertial acceleration is implicitly dynamic. Possibly you are conflating simplicity with static ? $\endgroup$
    – cumfy
    Commented Jul 19, 2020 at 22:31

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