Figuring Out Fork Length For A Custom Long Bike

by Evil
(Albuquerque, NM)

I’m planning a pseudo-long bike build. I want to have all the details worked out before I start getting dirty. I’m going to run a 19inch front wheel and the down tubes are going to be +4. I’m guessing the rake will be in the high 40's. Bikes I liked have been running about 12 to 14 inches over forks. It’s important to me that the frame sits level. I however have no idea how to figure this out before I order the frame and I don't want to order a custom frame only to have to cut it up when I can have them do it for me. So my question is how do I determine the rake and fork length so I can order the frame and forks? I have asked several people this question and I don't know if it’s a guessing game or a secret. I asked a guy who was riding a long bike and even he acted like he had eyeballed it. I don't want to guess. I want to use math. My bikes aren't just to look at.

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Aug 23, 2012
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thanks
by: Anonymous

i agree on the trail issue. i have taken that into consideration. i have read that a trail between 4 to 6 inches is best so i am hoping to split the difference and hit 5 inches of trail. im not building my own frame for this project. im having it built. i spoke with the frame company and let them know my concerns about having a sloppy handling long front end. i suggested sending the frame company my front end and front wheel with the tire on it and asked if they could set up the rake on the frame using my components and they agreed that it would insure a proper rake and trail doing it that way. they agreed to send me a pics one they have it mocked up so i can approve the overall shape.

Aug 23, 2012
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Answer to figuring out fork length
by: Custom Choppers Guide

Hello,

It definitely is a math problem, not a guessing game, nor is a secret. You just need to determine several items and then make sure that the design for your bike is sound so that you have a nice stable ride. Without attention to the rake and trail dimensions, you could potentially create a bike that will “Flop” at high speeds and could injure or kill you, so this is very very important. There is a tremendous amount of information on Rake and Trail and I will try to explain briefly. I recommend you do your research before making the actual frame.

The plans can be adjusted for a different rake angle on the frame. You will need to make adjustments in order to make it work. The length of the tubes will change which attach to the neck of the frame. The trail will only get longer as you increase the rake angle. See attached photo showing the new rake angle with a 45 degree rake instead of the stock 30 degree rake. I hope the visual helps show the changes you will need to make.

The Trail is determined by drawing a vertical line down from the center of the front wheel and a line following the rake angle of the frame. The axle plate can be used to adjust the location of the front wheel's center. By adjusting this point forward or rearward the trail will grow or shrink. The trail must be positive as shown in the attached sketch. Negative trail is dangerous and will result in poor handling at high speeds and will result in the bike flopping and will cause severe injury or death. Minimum of 3" of trail is required.

Using a piece of kite string you can easily tape to the appropriate locations and make rough measurements of the rake and trail. Be sure to measure rake and extend line from the frame's neck, not the fork's T. The frame's neck is the pivot point that will determine the trail. Also be sure to measure vertically down from the center of the front wheel, not the axle plate pin or attachment point to the fork. Feel free to let us know with questions. Also feel free to send a photo of your bike from the side view and we can do a rough measurement for you if you are having trouble.

The only conditions you'd have to modify the axle plate in order to maintain proper trail would be if you have an extreme rake angle or a triple T that is not parallel to the rake angle. These are special cases and the Noor1 Springer fork plan is parallel to the rake angle.

Design wise, you also have the option to make the axle plate a little longer. Your trail is 7.8" with a 45 degree rake in attached photo. The axle plate now has a lot of freedom to grow to add some aesthetic style. Just make sure you stay above 3" in order to maintain control on steering and reduce possibility of flop. Now the next step is to work back from here and determine your fork lengths and angles if you are doing an offset angle. Let me know with any other questions or concerns.

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