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Fork rake question(2 posts)

Fork rake questionScott G.
Jul 22, 2001 9:55 AM

i just ordered a fork with a 4.3 rake. the fork on my rig now has a 4.7.

is this going to effect the bike in a negative way? what will this change in rake do to it?


The steering will be slightly slower....C-40
Jul 22, 2001 10:21 AM
Here's the details of why:

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.