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Going Threadless...and carbon...(4 posts)

Going Threadless...and carbon...Kellum1969
May 29, 2002 9:26 AM
Well, I've decided to swap out my AL fork with a threadless carbon fork. Once question: I'm not all too familiar with "rake". How do I measure it, and do I want the same rake as my current fork, or do different rakes offer different advantages (and disadvantages)?
re: Going Threadless...and carbon...woodes
May 29, 2002 10:37 AM
I'm not sure of the answer, but the issue comes up often in the tech forum at the archives there and good luck!
Fork Rakechare
May 29, 2002 10:50 AM
you will want to keep the same for rake you currently have w/ the old aluminum fork. basically the fork rake is specific for the size frame you have. the rake is usually a little greater for smaller frames to keep the front tire out of the way of the down tube. back in the days of steel forks, the rake was usually a bit longer too in order to give a smoother ride. i don't know the choices you have w/ the new carbon fork, but i'm guessing either a 43 or a 45.
if you like to measure the rake of the old fork, you may want to bring it into a local shop and have it measured, it's a bit difficult to explain for me w/o diagrams.
May 29, 2002 2:37 PM
Fork rake isn't specific to frame sizes. Fork rake is used in conjunction with the head tube angle to produce a specific amount of trail. Increasing rake decreases trail and quickens the steering response (and vice versa). Smaller amounts of trail are generally used on large frames with longer wheelbases to speed up the steering.

The amount of trail considered "appropriate" on stock frames varies. Trek uses less trail across the entire size range than Colnago, for example. Neither is right or wrong, but the Colnago geometry (with more trail) is better suited to long road races rather than criteriums.

Here's the technical stuff:

Rake is the perpendicular distance between two parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance between the tire contact point on the road and a line through the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Both rake and head tube angle affect the amount of trail. Steepening the head tube angle or increasing rake will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/tan H)-(rake/sin H). As an example if R = 33.65cm, H=73, and rake is 4.0cm, trail = (33.65/tan73) - (4.0/sin73). This calculates to 6.1cm or 2.4 inches.

Changing the rake from 4.0 to 4.5cm yields a trail of 5.58cm or 2.2 inches.

Changing the head tube angle to 74 degrees, with the 4.0 rake yields a trail of 5.49cm or 2.16 inches. Thus, a .5cm increase in rake will have a similar effect to increasing the head tube angle by slightly less than one degree.