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A BASIC QUESTION ABOUT FORK RAKE(4 posts)

A BASIC QUESTION ABOUT FORK RAKEfroryde
Jun 11, 2001 6:16 PM
Just a basic question about fork rake - if my frame is designed for a 45mm rake and I run a 43mm rake, how would (and how much) that affect handling? Thanks!
Minimal effect (nm)Kerry Irons
Jun 11, 2001 6:33 PM
here it is again...C-40
Jun 11, 2001 7:58 PM
Rake is the horizontal distance the front axle is in front of the steering axis (a line through the center of the steering tube). Trail is the horizontal distance the tire contact point is behind 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.
What do you think about the handling now?Spoke Wrench
Jun 12, 2001 7:58 AM
A straighter fork is going to give you more trail, more stability, and slower handling. 2mm isn't very much, but there will be people on this board who will say they can sense the difference. Personally, I wouldn't worry about it unless I thought the bike was already too slow handling.