Norcoroman писал(а):
Если нормальная езда предусматривает кручение педалей назад - то да, самозатягивание.
Честно говоря, первый раз вижу велосипедиста, которого не убедила ссылка на Шелдона Брауна!
Предлагаю почитать на досуге:
http://www.zatumanom.ru/manual/doc/BC-5450DBRUS.doc
Обратите внимание на то, что у левой педали – левая резьба, т.е. её необходимо закручивать на ось против часовой стрелки. Это сделано для предотвращения самораскручи-вания педалей в процессе эксплуатации тренажёра.
http://www.faqs.org/faqs/bicycles-faq/part4/
In the case of the pedal, fretting motion is directional and can cause precession by the "wandering" load whose center of pressure rotates in the crank thread opposite to the rotation of the crank. Even without clearance, elastic deformation of
the crank and pedal spindle cause micro motions that, if not countered by an appropriate thread direction, will unscrew the pedal. The presence of a left hand thread on the left pedal and on many bottom bracket right side bearing cups is proof that fretting occurs.
http://yarchive.net/bike/cassette_freewheel_unscrew.html
If your roll your finger in a ring made by the thumb and forefinger of the other hand, you will notice that it wants to rotate opposite to the progression of the contact point. This is precession and it unscrews such attachments, as would left pedals with RH threads or other such joints that bear radial loads, like the right hand cup of a BB that works in much the same sense as your hub.
http://www.sheldonbrown.com/brandt/left.html
On bicycles, left hand threads are used mainly in three places, on left pedals, right bottom bracket (BB) bearing cups, and freewheel cones, to prevent unscrewing under operating loads. Unscrewing occurs from precession, in which a round object rolling in a circular ring in one direction will itself turn in the opposite direction.
For a pedal, a rotating load arises form downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces.
Precession of right side BB cups is less obvious because the rotating load is only partial. The largest load being chain tension, that together with the moderately large downward force on the right crank and the smaller upward force from pushing down on the left crank, make 3/4 of a fully rotating load. For this reason some right BB cups have used right hand threads and some with left hand threads have loosened. The left BB cup with no significant rotating load has little tendency to turn.
Freewheel cones are more obvious candidates for precession, their load being mainly radial, and rotating continuously in the direction that would unscrew a right hand thread. There are other such but less common threads on bicycles.
Precession forces are large enough that no manner of thread locking glues, short of welding, will arrest them. Mechanical fretting, the micro-motion of tightly fitting parts moving against one another, is the mechanism of this motion. Motion in these joints causes visible fretting rouge, red iron oxide, on the shoulder of the BB cup and on the face of the pedal spindle.
http://knaddison.com/technology/how-to-uninstall-re-install-new-pedals-on-your-bike
One thing to note about bike pedals is that one is regular thread and one is reverse thread. The reason for this is that as you pedal, your left foot would naturally be unscrewing the pedal if it were a regular thread. The story goes that the Wright brothers encountered this problem first and created reverse threads.
http://www.unicyclist.com/forums/showthread.php?s=&threadid=18867
The unscrewing action, when pedaling, comes from the far end of the screw, so it happens the opposite way from the pedaling direction. I'm not going to attempt an ASCII drawing, but basically, picture your crank at 3:00, riding forward, under full power. The pedal pushes downwards. The bottom of the pedal axle compresses against the downward side of the hole in the crank. This tilts the axle, so on the other side of the crank (the inner side), the TOP of the pedal axle compresses against the upper side of the hole in the crank. If you follow this effect around a pedal revolution, you'll see it rotates in the opposite direction from the pressure on the outside of the crank hole. Hence the counterintuitive unscrewing.
http://www.mbaction.com/detail.asp?id=1438
To keep the left pedal and the right BB cup from unthreading by the turning bearing balls or rollers, it's necessary to use reverse threads at these two power transfer pivots.
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