RE: Rake (was:Re: [CR]postscript)

(Example: Racing:Jean Robic)

From: "Mark Bulgier" <mark@bulgier.net>
To: classicrendezvous@bikelist.org
Subject: RE: Rake (was:Re: [CR]postscript)
Date: Thu, 24 May 2001 09:58:50 -0700


I think it's been shown by finite element analysis and empirical measurement that the flex in forkblades occurs predominantly above the curved part, making the curve pretty unimportant in the comfort equation. It's just a matter of leverage: the bumps push on the fork tips; an inch above the tips they only have an inch of leverage to flex the blade. At the crown the leverage is 14 inches or so. And a good deal of the total fork flex isn't even in the blades, it's in the steerer above the crown.

Yes the blades are bigger at the top, precisely to deal with this extra leverage, but they aren't bigger in proportion to the increase in leverage - they still flex more near the top.

Blades with lots of curve can theoretically be a *little* more comfortable, but only to the extent that they are *longer* than the straight line from tips to crown. And in one special case - a bump so large that the contact point with the tire is on the extension of the line from crown to tips: there, a straight fork would have no bending leverage on it at all and would be merely in compression (which gives negligible deflection, at the level of force involved here). A curved fork would have at least a bit of flex in that scenario - still not much though, again because of the short lever arm. Bumps that big usually dent the rim anyway, if not throwing you off the bike.

Forks with lots of rake are also likely to be longer (as in taking long-reach brakes) - which will do more to add comfort than the rake per se. Often these forks have thicker-wall blades, and more ovalized (stiffer front-to-back) though, and can actually be less comfortable than a light fork with thin-wall, less-oval blades and much less rake.

All in all, of the factors that affect comfort, amount of rake and how it's achieved are near the bottom in importance.

Mark Bulgier
Seattle, Wa USA