The Physics of Bicycling

  1. Steering
    1. Why not just turn handlebars?
      1. DEMO
      2. Attempt to turn bars while riding, no leaning.
      3. Can’t do it, fall over to other side.
      4. DEMO
      5. with bike with tied handlebar.
    2. Centrifugal force
      1. Objects going in circle appear to be forced outward.
      2. DEMO
      3. bob on string.
      4. DEMO
      5. Bike merry-go-round, bike upright.
    3. Leaning: Compensate by falling the other way. Gravity opposes centrifugal.
      1. DEMO
      2. Merry-go-round with tilt.
      3. DEMO
      4. Bike with tied handlebar.
      5. Faster requires more lean
        1. DEMO
        2. Speed up merry-go-round, flops over.
        3. DEMO
        4. Increase angle, stable.
      6. Too much lean, falls down on other side: Must balance carefully.
      7. Increase speed, same circle, more lean. DEMO with string and bob.
      8. Fixed speed, tighter circle, more lean.
      9. Similar to airplane banking.
  2. Balancing
    1. By shifting weight?
      1. Need to shift center of mass. DEMO with ruler, mass.
      2. Falling right, shift hips left, but shoulders go right. No net change.
        1. DEMO
        2. while riding balance bike.
        3. Center of Mass demo would be nice.
    2. Steer into fall
      1. Fall right, steer right, centrifugal force to left.
        Fall left, steer left centrifugal force to left.
      2. Alternate explanation, steering underneath the fall. DEMO with balancing ruler on palm.
    3. Net result: bike nominally going straight actually making tiny turns right and left. See this in tracks made by sand.
    4. Faster easier than slower: smaller turns needed to get same size centrifugal force. Alternately, smaller radius turn moves bike underneath fall faster.
    5. Edge aversion difficult. Normally create lean for a turn by steering the other way. DEMO with "short prof step"
    6. Diversion falls
      1. DEMO
      2. with bike and 2x4.
      3. Railroad tracks, storm drains
  3. Angular momentum
    1. Most physicists think that angular momentum in the wheel is responsible for balancing.
    2. DEMO
    3. angular momentum with lead wheel and rotating chair.
    4. NOT TRUE! Very little to do with actual behavior. Proved by construction of bike with counterrotating wheels that behaved virtually the same.
  4. Trail
    1. Good bike design aids process.
    2. Bike almost works automatically:
      1. Lean right, bike turns right.
        Lean left, bike turns left.
      2. DEMO
      3. with bike on table.
    3. ???DEMO
    4. shoving bike. Counting time to fall while moving and without moving.
    5. No hand riding…lean bike by shifting hips, bike responds by turning. DEMO???
    6. Why auto? Trail.
      1. DEMO
      2. effect of trail with mock wheel. Turn wheel around, effect is backwards.
      3. Trail on real bikes not immediately obvious. Headset at angle, but fork is J’d. DEMO Show trail with bike on table.
      4. DEMO
      5. unrideable bike.
        1. Ok in straight lines.
        2. Have to force handlebars to go around corners.
    7. Amount of trail
      1. Short trail: quick handling, feel road imperfections. Racing and road bikes, generally expensive bikes.
      2. Medium trail: intermediate response, some insulation from road. Mountain and touring bikes.
      3. Long trail: sluggish response, insulates entirely from road. Feels easier, but actually harder. Cheap "department store bikes"
  5. Braking
    1. Friction: Friction between brake shoes and wheel slow down wheel, and friction between wheel and road slows down bike.
    2. Frictional strength depends on weight. DEMO with weighted blocks
    3. Heavy people need more force to stop them, but have more weight, so frictional forces are stronger.
    4. Many people taught to brake primarily with rear brake. Bad idea.
      1. Right hand is stronger, so traditionally right brake lever controls rear wheel. Weaker left hand controls front brake.
      2. Bad idea
        1. Rear wheel bucks up when braked.
          1. DEMO
          2. Balance bike with hand on top bar, push backwards on front an rear wheels.
          3. Car brakes suddenly, front car slams down, rear lifts up.
        2. Converse acceleration front wheel bucks up…drag racers pull wheelies. Easy to see this on a steep slope, say like Marin which has 22% grade. Every time push hard on pedals, front wheel lifts off ground.
        3. Connection with braking
          1. Brake with rear wheel. Bike slows, but rear bucks up. This lessens weight on rear wheel. Frictional forces DECREASE. Bucking forces decrease. Brakes strong enough to lock rear wheel, rear wheel skids. Not in itself too dangerous, recover by releasing brake.

            DEMO First off bike, then on.
          2. Brake with front wheel. Bike slows, rear bucks up, front bucks down. Frictional forces INCREASE. Bucking forces increase. Get thrown over front wheel if not careful.

            DEMO First off bike, then on
          3. Seems like rear brake is better. This is an illusion
            1. Rear brake is much weaker than front. Stop much slower
            2. Best braking with both brakes, front on stronger, just below being pitched over.
            3. Takes experience.
  6. Speed
    1. Speed on bike limited by friction.
    2. Above ten miles an hour, air friction dominates.
    3. Air friction increases sharply. Requires four times as much work to double the speed.
    4. Personally
      1. I can produce steady state about 225W Like two 100W light bulbs. DEMO with light bulbs.
      2. Peak to 300W for a few minutes. Max out at 350-400W for brief spurts.
      3. Professional bicyclist can put out double…400W steady state.
      4. Untrained cyclist can put out half…100W.
      5. My average speed on a very long ride this summer was 18.3mph.
      6. Taking into doubling rule, Professional cyclist only goes about 24mph. Doesn’t seem like much, yet impossible.
      7. Untrained cyclist goes about 12mph.
    5. Drafting to beat air resistance.
      1. Stay within 1ft of leading wheel
      2. Amazingly strong effect.
    6. Tandems!
  7. Helmets
    1. Really important to wear helmet. Always been glad to have helmet after I’ve crashed.
    2. How do helmets work?
      1. Shield? NO!
        1. Military use hard shell helmets, for shrapnel.
        2. Metal helmet do no good. Hitting head against metal just as bad as hitting head against concrete.
      2. DEMO
      3. Eggs against sheet. Also ping-pong ball.
      4. Helmet crushes, gradually slowing head. Don’t reuse helmet after crash, can’t crush again. DEMO Old helmet.




  1. LeMonde, Miyata, Bridgestone, Tandem, Unrideable
  2. Piece to tie bike handlebar
  3. Bob on string
  4. Bike merry-go-round
  5. Meterstick, hand weight
  6. No such Center of mass demo
  7. Short prof step
  8. 10ft 2x4
  9. Rotating chair, lead bike wheel (A+30+15 with wheel, no wieghts)
  10. Trail fork.
  11. Friction demo (A+12+5)
  12. Board with 5 switchable 100W lights
  13. Eggs, sheet, ping-pong ball
  14. Old crushed helmet.