|Fork Rake||Jack Daniels|
Feb 8, 2002 9:05 PM
|I am ready to buy a fork for my frame I just purchased. The fork for the frame is suppose to be 45 degrees. Would I notice a problem if I used a fork that was 43 or even 40. Would it make a difference?|
|re: Fork Rake||Toboggan|
Feb 9, 2002 4:01 AM
|It will make a big defference, especially if your frame was designed for a 45 degree rake fork.
Draw an imaginary straight line through your frames headtube to the ground. The rake of a fork positions the bottom of the front wheel (the point where it contacts the road) closer to or farther forward of this imaginary line based upon who much rake it has. The farther the center of your front wheel gets away from this imaginary line the more stable and slower turning your bike will ride. As you move the front tire more towards the rear of the bike and closer to this imaginary line your steering will become quicker and the bike less stable. When building a bike I would always recommend going with a fork that is the same rake as recommended by the manufacturer unless you have a lot of experience and a real good reason for not doing so.
|rake not in degrees...||C-40|
Feb 9, 2002 6:26 AM
|Rake is the perpendicular 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 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.
There is no such thing as a rake that a frames in "supposed to have". To get determine the appropriate rake, the frame size, head tube angle and intended use of the bike must be considered.
The amount of trail that builders consider appropriate on stock frames varies substantially. A stock Trek may have a 55mm trail while a stock Colnago is likely to have 65mm of trail, in the same frame size.
|rake not in degrees...||EEEEESH|
Feb 10, 2002 5:39 AM
|For a given frame with a given headtube angle and a given fork length the rake of the fork will determine the trail and hence the ride quality. Many frame manufacturers in fact suggest a given fork length and rake for a given frame to provide an intended trail and thus a certain ride. Assuming the guy already knows what frame he wants, the only factors left to determine trail are fork length and rake.|
Feb 10, 2002 6:34 AM
|Didn't intend to imply that head tube angle could be changed, just that it must be known in order to determine the appropriate rake. I copied one of my previous posts which was intended to show how rake could be varied to produce the same trail, even with different head tube angles.
As you noted, there are some variations in fork length, which almost no one bothers to check (but should) before purchasing a new fork.