Chopper Motorcycle Frame Neck Tube Adjustments!
Frame Neck Tube Angle Question:
Hi! I have a question. I got my frame plans, tube bender, frame jig, started to bend up a few tubes to get started and, presto. Going from a standard rake to a 8 inch over forks!
How do you adjust the neck tube angle or the degrees of the the neck tube to compensate for different length forks? Is there a graft or scale or is it try it and see till you get it right?
Chopper Frame Neck Tube Answer:
This is a difficult question, adjusting the frame neck tube angle. We'll try our best to explain and advise what I recommend. For it all to work you must be well versed in rake and trail.
Yes the plans can be adjusted for a raked 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 frame neck tube and 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 below. 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. A 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 in regards to your question on frame neck tube adjustments. 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 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. Let us know with any other questions or concerns.