Rake and Trail

One of the things that gives choppers their cool look is a raked out front end. The rake is the angle of the front fork from the body. Think of it this way... if you draw a line directly down from the top of the frame to the ground perfectly perpendicularly this is the 90 degree angle of your right triangle. From where that line (or plane) hits the ground to the spot beneath the front wheel is the second side of your right triangle. The line that goes from the top of the frame to the wheel is your hypotenuse.

So before I knew what kind of front end I needed to buy and what size to get we had to figure out what the rake was on the current frame. If a chopper has a front end that is too long the bike (or trike) won't be able to make a U-Turn. Those of you with big cars with a crummy turning radius can relate to that.

Now there is physics and trigonometry involved in the front end rake of a chopper. Due to the pressure of the vehicle and the resistance of the ground the front wheel wants to spin not round and round north to south but round and round horizontally. For a chopper to be safe the physics must be right so that at both low speeds and high speeds the front wheel does not have too much side to side wobble, nor does the bike (or trike) have the propensity to try and flip ass over front end.

Years of practice, and eventual mathematical computation has taught chopper builders that the ideal rake is between 30-45 degrees. Old bikes where the rider sat upright had a smaller degree of rake on the front end. New high speed "crotch rocket" motorcycles often times have less than 28 dgrees of rake on the front end. The typical Harley Davidson motorcycle has between 28-32 degrees of rake.

C gets out his old school engineering tool, the plumb bob, and hangs it from the neck of the trike frame. I stick a piece of doweling in the neck for visual effect.

As the plumb bob hangs perfectly perpendicular to the ground due to gravity we have the back leg of our right triangle. Based on the readings we learn that this trike frame has a front end rake of 53 degrees. That is too much. We will have to adjust the rake.

However, making a chopper safe to ride involves more than just what angle the front end rakes out from the frame. I can use a right triangle calculator (because I suck at math) to figure out how long a front end to order, but we also have to worry about the trail.

Trail is the horizontal distance from where the steering axis hits the ground (our perpendicular line) to where the front wheel hits the ground. The trail is calculated in inches. The greater the amount of trail the more stable the front wheel is but it also makes the wheel harder to control. If the trail is too small (or negative) the chopper will have little to no wobble at low speeds, but be difficult to handle at high speeds. If the trail is too great the chopper will handle sluggishly at high speeds but will wobble at low speeds. To have a safe ride that is easy to handle 2-4 inches of trail is ideal.

To figure out the trail of the front end once you know the rake you need more than geometry (like we did in our right triangle exercise). It requires trigonometry.

Now, as you recall I mentioned above, I suck at math. Luckily, not only now thanks to the internet do we have online triangle calculators, but we have rake and trail calculators.

We knew we would want a maximum rake of 45 degree angles. Besides, that would be easier to build than a 42 or 44 degree angle. With a simple right angle triangle ruler we had a measure for where the neck should go.

Step #1 level the trike

Step #2 measure 45 degrees and mark frame

Step #3 cut down neck angle (since we had to replace the neck piece entirely it was cut off)

Step #4 weld new neck on at proper angle

This is not the final finished piece, but it is as far as we got that day. There will be more welding to reinforce the strength of the neck. VW Trikes have a tendency to pop wheelies so we want to make sure the neck is strong enough to handle it.

Just behind the frame you can see a piece of sheet metal up against the wall. We will use metal from that to blend the center piece between the tubes of the frame to make it cleaner and stronger. Eventually most of this will be covered by the gas tanks.