The String And Compass Method
For any transition you cut, you will probably want to use the String and Compass method. Using it you can make transitions very smooth and precise.
Here's how it works, when cutting a transition, the curve always follows that of a perfect circle (unless you want an irregular ramp). So, to draw the transition, you just need to figure out how large of a raduis you want for your curve and where to start/end the transition. These factors will depend on the ramp you are building.
The following is directions for cutting the transition for a 4' halfpipe, After the directions I will explain how to modify the steps for other ramp transitions.
Watch The Video!
Click here to see a sped up video of me demonstratind the String and Compass method
Step 1: Attach a 2x6
Here I will demonstrate how to cut the transition for a 4' tall mini halfpipe with a 6'10" transition.
First, draw a line (or just two points), that go along the width of the plywood that are 18" into the plywood.
Next, attach a 2x6 (or a 2x4 or just anything that is at least 8' and straight) to the (3/4)" sheet of plywood. Use the line you drew to make sure the 2x6 is completely vertical.
Again, the 2x6 goes 18" into the plywood, as I have pre-calculated (I will teach you how to do this later).
NOTE: In the picture above where it says 6" it should day 5.5", I forgot that 2x6's were actually only 1.5" by 5.5". At this point i'll mention that I am doing this on a sheet of plywood standing up, this is for video purposes only, I recommend you do it laying flat on the ground.
Step 2: Attach the Pencil
Next, draw a point along the 2x6 that is 5.5" off the ground (marked incorrectly as 6" in picture above). You will start the transition 5.5" off the ground because for this halfpipe there will be a flatbottom that is 5.5" high.
Now measure 6' 10" along the 2x6 from the point you just drew and put a screw into the side of the 2x6 there, attach a string to the screw. Here I use fishing wire (2 lb test line), Any string with minimal give/stretch will work.
At the other end of the string you want to connect a pencil (I use a marker for video purposes only). Take your time doing this so that when you hold the pencil comfortably it draws on the point you made 6" off the ground exactly.
Step 3: Draw the Transition
Now the String and Pencil does the work for you. VERY SLOWLY draw the transition, trying your best to keep a constant amount of tension on the string and keeping the pencil at a 90 degree angle with the plywood.
You are just about done, if you havn't already, draw a line from your point 5.5" off the ground to the ground, Then use a jig saw to cut the transition you have drawn. Use the piece you cut out to trace the second rather than drawing both using this method. Here is a picture of both transitions drawn just to show that using these measurements you can fit two sides on a single piece of plywood.
Transitions for Other Ramps
So now you should know exactly how to draw the transition if you were to make a 4' high halfpipe with a 6" flatbottom, some tips for the other ramps on this site...
- 4' Quarterpipe: The quarterpipe transitions to the ground, so you start the transition from the ground instead of 6" up. Since you are starting from the ground you can afford to start less than 18" into the plywood and still fit two sides in each piece of plywood, this way the sides will have a larger flat top on them.
- 8' Halfpipe: In the plans I show a picture of how to align two pices of plywood needed for each side of the ramp, use those measurements. Drawing the transition for this will be very difficult since you have to syncronize two pieces of plywood, I would recommend finding other resources to help you with this. Maybe someday it will be covered on this site.
- Funbox: Like the quarterpipe you will be starting from the ground instead of 6", It should be pretty easy to fit 4 sides out of a piece of plywood so just draw the transition in a way that allows them all to fit. Since the funbox is 2' high and the plywood is 4' wide, you need to measure your stopping point (another way is to cut the plywood in half the long way (hot dog way), then make two pieces out of each half). As I mention in the plans a 6' 10" transition works well.
A tool I use to figure out what kind of transitions will fit in a 4x8 sheet of plywood can be found at http://www.engineeringcalculator.net/RampCalc.html I strongly recommend anyone building a ramp to use this tool.
* My quarterpipe plans specify the flat space at the top be exactly 10", so if you are following the plans exactly you will need to figure out the measurements to allow this. Someday I will change the plans to have a platform built in which allows for more wiggle when it comes to the size of the flat left at the top of the sides, which is an easier way to build the quarterpipe, you can easily figure out how this would be done by looking at the mini halfpipe plans.