Joined: May 2008
Posts: 10,106
Likes: 2,757
From: Fredericksburg, Va
Bikes: ? Proteous, '65 Frejus TDF, '73 Bottecchia Giro d'Italia, '83 Colnago Superissimo, '84 Trek 610, '84 Trek 760, '88 Pinarello Veneto, '88 De Rosa Pro, '89 Pinarello Montello, 'Litespeed Catalyst'94 Burley Duet, 97 Specialized RockHopper, 2010 Langster
My educated opinion (mechanical engineer) is that [MENTION=305894]oddjob2[/MENTION] almost said it right. It absolutely does do both functions, radial positioning and retention of the head.
Prevents the chair from rotating when pumping hard in too high a gear and keeps the chair on the bike when the motor leaps up to climb a hill (crating a negative pressure are above the saddle)!